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.

Synthesis and Biophysical Characterization of Fingolimod Derivatives as Cardiac Troponin Antagonists.

Calcium binding to cardiac troponin C (cTnC) in the thin filaments acts as a trigger for cardiac muscle contraction. The N-lobe of cTnC (NcTnC) undergoes a conformational change in the presence of calcium that allows for interaction with the switch region of cardiac troponin I (cTnISP), releasing its inhibitory effect on the thin filament structure. The small molecule fingolimod inhibits cTnC-cTnISP interactions via electrostatic repulsion between its positively charged tail and positively charged residues in cTnISP and acts as a calcium desensitizer of the contractile myofilaments. Here we investigate the structure-activity relationship of the fingolimod hydrophobic headgroup and show that increasing the alkyl chain length increases both its affinity for NcTnC and its inhibitory effect on the NcTnC-cTnISP interaction and that decreasing flexibility completely abolishes these effects. Strikingly, the longer derivatives have no effect on the calcium affinity of cTnC, suggesting that they act as better inhibitors.

Missense mutations in the central domains of cardiac myosin binding protein-C and their potential contribution to hypertrophic cardiomyopathy.

Myosin binding protein-C (MyBP-C) is a multidomain protein that regulates muscle contraction. Mutations in MYBPC3, the gene encoding for the cardiac variant (henceforth called cMyBP-C), are amongst the most frequent causes of hypertrophic cardiomyopathy. Most mutations lead to a truncated version of cMyBP-C, which is most likely unstable. However, missense mutations have also been reported, which tend to cluster in the central domains of the cMyBP-C molecule. This suggests that these central domains are more than just a passive spacer between the better characterized N- and C-terminal domains. Here, we investigated the potential impact of four different missense mutations, E542Q, G596R, N755K, and R820Q, which are spread over the domains C3 to C6, on the function of MyBP-C on both the isolated protein level and in cardiomyocytes in vitro. Effect on domain stability, interaction with thin filaments, binding to myosin, and subcellular localization behavior were assessed. Our studies show that these missense mutations result in slightly different phenotypes at the molecular level, which are mutation specific. The expected functional readout of each mutation provides a valid explanation for why cMyBP-C fails to work as a brake in the regulation of muscle contraction, which eventually results in a hypertrophic cardiomyopathy phenotype. We conclude that missense mutations in cMyBP-C must be evaluated in context of their domain localization, their effect on interaction with thin filaments and myosin, and their effect on protein stability to explain how they lead to disease.

Discovery of a novel cardiac-specific myosin modulator using artificial intelligence-based virtual screening.

Direct modulation of cardiac myosin function has emerged as a therapeutic target for both heart disease and heart failure. However, the development of myosin-based therapeutics has been hampered by the lack of targeted in vitro screening assays. In this study we use Artificial Intelligence-based virtual high throughput screening (vHTS) to identify novel small molecule effectors of human ß-cardiac myosin. We test the top scoring compounds from vHTS in biochemical counter-screens and identify a novel chemical scaffold called 'F10' as a cardiac-specific low-micromolar myosin inhibitor. Biochemical and biophysical characterization in both isolated proteins and muscle fibers show that F10 stabilizes both the biochemical (i.e. super-relaxed state) and structural (i.e. interacting heads motif) OFF state of cardiac myosin, and reduces force and left ventricular pressure development in isolated myofilaments and Langendorff-perfused hearts, respectively. F10 is a tunable scaffold for the further development of a novel class of myosin modulators.

Basic science methods for the characterization of variants of uncertain significance in hypertrophic cardiomyopathy.

With the advent of next-generation whole genome sequencing, many variants of uncertain significance (VUS) have been identified in individuals suffering from inheritable hypertrophic cardiomyopathy (HCM). Unfortunately, this classification of a genetic variant results in ambiguity in interpretation, risk stratification, and clinical practice. Here, we aim to review some basic science methods to gain a more accurate characterization of VUS in HCM. Currently, many genomic data-based computational methods have been developed and validated against each other to provide a robust set of resources for researchers. With the continual improvement in computing speed and accuracy, in silico molecular dynamic simulations can also be applied in mutational studies and provide valuable mechanistic insights. In addition, high throughput in vitro screening can provide more biologically meaningful insights into the structural and functional effects of VUS. Lastly, multi-level mathematical modeling can predict how the mutations could cause clinically significant organ-level dysfunction. We discuss emerging technologies that will aid in better VUS characterization and offer a possible basic science workflow for exploring the pathogenicity of VUS in HCM. Although the focus of this mini review was on HCM, these basic science methods can be applied to research in dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM), arrhythmogenic cardiomyopathy (ACM), or other genetic cardiomyopathies.

Conversion of a Cardiac Muscle Modulator from an Inhibitor to an Activator.

The binding of calcium to cardiac troponin C (cTnC) enhances the binding of troponin I (cTnI) switch region to the regulatory domain of cTnC (cNTnC) and triggers muscle contraction. Several molecules alter the response of the sarcomere by targeting this interface; virtually all have an aromatic core that binds to the hydrophobic pocket of cNTnC and an aliphatic tail that interacts with the switch region of cTnI. W7 has been extensively studied, and the positively charged tail has been shown to be important for its inhibitory action. Herein we investigate the importance of the aromatic core of W7 by synthesizing compounds that have the core region of calcium activator dfbp-o with various lengths of the same tail (D-series). These compounds all bind more tightly to cNTnC-cTnI chimera (cChimera) than the analogous W-series compounds and show increased calcium sensitivity of force generation and ATPase activity, demonstrating that the cardiovascular system is tightly balanced.

Discovery of novel cardiac troponin activators using fluorescence polarization-based high throughput screening assays.

The large unmet demand for new heart failure therapeutics is widely acknowledged. Over the last decades the contractile myofilaments themselves have emerged as an attractive target for the development of new therapeutics for both systolic and diastolic heart failure. However, the clinical use of myofilament-directed drugs has been limited, and further progress has been hampered by incomplete understanding of myofilament function on the molecular level and screening technologies for small molecules that accurately reproduce this function in vitro. In this study we have designed, validated and characterized new high throughput screening platforms for small molecule effectors targeting the interactions between the troponin C and troponin I subunits of the cardiac troponin complex. Fluorescence polarization-based assays were used to screen commercially available compound libraries, and hits were validated using secondary screens and orthogonal assays. Hit compound-troponin interactions were characterized using isothermal titration calorimetry and NMR spectroscopy. We identified NS5806 as novel calcium sensitizer that stabilizes active troponin. In good agreement, NS5806 greatly increased the calcium sensitivity and maximal isometric force of demembranated human donor myocardium. Our results suggest that sarcomeric protein-directed screening platforms are suitable for the development of compounds that modulate cardiac myofilament function.

In situ FRET-based localization of the N terminus of myosin binding protein-C in heart muscle cells.

Cardiac myosin binding protein-C (cMyBP-C) is a thick filament-associated regulatory protein frequently found mutated in patients suffering from hypertrophic cardiomyopathy (HCM). Recent in vitro experiments have highlighted the functional significance of its N-terminal region (NcMyBP-C) for heart muscle contraction, reporting regulatory interactions with both thick and thin filaments. To better understand the interactions of cMyBP-C in its native sarcomere environment, in situ Foerster resonance energy transfer-fluorescence lifetime imaging (FRET-FLIM) assays were developed to determine the spatial relationship between the NcMyBP-C and the thick and thin filaments in isolated neonatal rat cardiomyocytes (NRCs). In vitro studies showed that ligation of genetically encoded fluorophores to NcMyBP-C had no or little effect on its binding to thick and thin filament proteins. Using this assay, FRET between mTFP conjugated to NcMyBP-C and Phalloidin-iFluor 514 labeling the actin filaments in NRCs was detected by time-domain FLIM. The measured FRET efficiencies were intermediate between those observed when the donor was attached to the cardiac myosin regulatory light chain in the thick filaments and troponin T in the thin filaments. These results are consistent with the coexistence of multiple conformations of cMyBP-C, some with their N-terminal domains binding to the thin filament and others binding to the thick filament, supporting the hypothesis that the dynamic interchange between these conformations mediates interfilament signaling in the regulation of contractility. Moreover, stimulation of NRCs with ß-adrenergic agonists reduces FRET between NcMyBP-C and actin-bound Phalloidin, suggesting that cMyBP-C phosphorylation reduces its interaction with the thin filament.

Sarcomere length affects Ca2+ sensitivity of contraction in ischemic but not non-ischemic myocardium.

In healthy hearts, myofilaments become more sensitive to Ca2+ as the myocardium is stretched. This effect is known as length-dependent activation and is an important cellular-level component of the Frank-Starling mechanism. Few studies have measured length-dependent activation in the myocardium from failing human hearts. We investigated whether ischemic and non-ischemic heart failure results in different length-dependent activation responses at physiological temperature (37°C). Myocardial strips from the left ventricular free wall were chemically permeabilized and Ca2+-activated at sarcomere lengths (SLs) of 1.9 and 2.3 µm. Data were acquired from 12 hearts that were explanted from patients receiving cardiac transplants; 6 had ischemic heart failure and 6 had non-ischemic heart failure. Another 6 hearts were obtained from organ donors. Maximal Ca2+-activated force increased at longer SL for all groups. Ca2+ sensitivity increased with SL in samples from donors (P < 0.001) and patients with ischemic heart failure (P = 0.003) but did not change with SL in samples from patients with non-ischemic heart failure. Compared with donors, troponin I phosphorylation decreased in ischemic samples and even more so in non-ischemic samples; cardiac myosin binding protein-C (cMyBP-C) phosphorylation also decreased with heart failure. These findings support the idea that troponin I and cMyBP-C phosphorylation promote length-dependent activation and show that length-dependent activation of contraction is blunted, yet extant, in the myocardium from patients with ischemic heart failure and further reduced in the myocardium from patients with non-ischemic heart failure. Patients who have a non-ischemic disease may exhibit a diminished contractile response to increased ventricular filling.

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 ß-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.

Drugging the Sarcomere, a Delicate Balance: Position of N-Terminal Charge of the Inhibitor W7.

W7 is a sarcomere inhibitor that decreases the calcium sensitivity of force development in cardiac muscle. W7 binds to the interface of the regulatory domain of cardiac troponin C (cNTnC) and the switch region of troponin I (cTnI), decreasing the binding of cTnI to cNTnC, presumably by electrostatic repulsion between the -NH3+ group of W7 and basic amino acids in cTnI. W7 analogs with a -CO2- tail are inactive. To evaluate the importance of the location of the charged -NH3+, we used a series of compounds W4, W6, W8, and W9, which have three less, one less, one more, and two more methylene groups in the tail region than W7. W6, W8, and W9 all bind tighter to cNTnC-cTnI chimera (cChimera) than W7, while W4 binds weaker. W4 and, strikingly, W6 have no effect on calcium sensitivity of force generation, while W8 and W9 decrease calcium sensitivity, but less than W7. The structures of the cChimera-W6 and cChimera-W8 complexes reveal that W6 and W8 bind to the same hydrophobic cleft as W7, with the aliphatic tail taking a similar route to the surface. NMR relaxation data show that internal flexibility in the tail of W7 is very limited. Alignment of the cChimera-W7 structure with the recent cryoEM structures of the cardiac sarcomere in the diastolic and systolic states reveals the critical location of the amino group. Small molecule induced structural changes can therefore affect the tightly balanced equilibrium between tethered components required for rapid contraction.

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 ß-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.

The regulatory light chain mediates inactivation of myosin motors during active shortening of cardiac muscle.

The normal function of heart muscle depends on its ability to contract more strongly at longer length. Increased venous filling stretches relaxed heart muscle cells, triggering a stronger contraction in the next beat- the Frank-Starling relation. Conversely, heart muscle cells are inactivated when they shorten during ejection, accelerating relaxation to facilitate refilling before the next beat. Although both effects are essential for the efficient function of the heart, the underlying mechanisms were unknown. Using bifunctional fluorescent probes on the regulatory light chain of the myosin motor we show that its N-terminal domain may be captured in the folded OFF state of the myosin dimer at the end of the working-stroke of the actin-attached motor, whilst its C-terminal domain joins the OFF state only after motor detachment from actin. We propose that sequential folding of myosin motors onto the filament backbone may be responsible for shortening-induced de-activation in the heart.

A Potent Fluorescent Reversible-Covalent Inhibitor of Cardiac Muscle Contraction.

Compounds that directly modulate the response of the cardiac sarcomere have potential in the treatment of cardiac disease. While a number of sarcomere activators have been discovered and extensively studied, very few inhibitors have been identified. We report a potent cardiac sarcomere inhibitor, DN-F01, targeting the cardiac muscle thin filament protein troponin complex. Functional studies show that DN-F01 has a strong inhibitory calcium-dependent effect on cardiac myofibrillar ATPase activity with an IC50 value of 11 ± 4 nmol/L. DN-F01 is shown to bind to a cardiac troponin C-troponin I chimera (cChimera) with a K D of ∼50 nM using fluorescence spectroscopy, indicating that troponin is the likely target for DN-F01. NMR titrations of DN-F01 to C35S and A-Cys cChimera show covalent and noncovalent binding of DN-F01 bound to the calcium-saturated cChimera.

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

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.

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 (cTnISP), 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 cTnISP 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.

The Role of Electrostatics in the Mechanism of Cardiac Thin Filament Based Sensitizers.

Heart muscle contraction is regulated by calcium binding to cardiac troponin C. This induces troponin I (cTnI) switch region binding to the regulatory domain of troponin C (cNTnC), pulling the cTnI inhibitory region off actin and triggering muscle contraction. Small molecules targeting this cNTnC-cTnI interface have potential in the treatment of heart disease. Most of these have an aromatic core which binds to the hydrophobic core of cNTnC, and a polar and often charged 'tail'. The calmodulin antagonist W7 is unique in that it acts as calcium desensitizer. W7 binds to the interface of cNTnC and cTnI switch region and weakens cTnI binding, possibly by electrostatic repulsion between the positively charged terminal amino group of W7 and the positively charged RRVR144-147 region of cTnI. To evaluate the role of electrostatics, we synthesized A7, where the amino group of W7 was replaced with a carboxyl group. We determined the high-resolution solution NMR structure of A7 bound to a cNTnC-cTnI chimera. The structure shows that A7 does not change the overall conformation of the cNTnC-cTnI interface, and the naphthalene ring of A7 sits in the same hydrophobic pocket as that of W7, but the charged tail takes a different route to the surface of the complex, especially with respect to the position of the switch region of cTnI. We measured the affinities of A7 for cNTnC and the cNTnC-cTnI complex and that of the cTnI switch peptide for the cNTnC-A7 complex. We also compared the binding of W7 and A7 for two cNTnC-cTnI chimeras, differing in the presence or absence of the RRVR region of cTnI. A7 decreased the binding affinity of cTnI to cNTnC substantially less than W7 and bound more tightly to the more positively charged chimera. We tested the effects of W7 and A7 on the force-calcium relation of demembranated rat right ventricular trabeculae and demonstrated that A7 has a much weaker desensitization effect than W7. We also synthesized A6, which has one less methylene group on the hydrocarbon chain than A7. A6 did not affect binding of cTnI switch peptide nor change the calcium sensitivity of ventricular trabeculae. These results suggest that the negative inotropic effect of W7 may result from a combination of electrostatic repulsion and steric hindrance with cTnI.

Cardiac myosin regulatory light chain kinase modulates cardiac contractility by phosphorylating both myosin regulatory light chain and troponin I.

Heart muscle contractility and performance are controlled by posttranslational modifications of sarcomeric proteins. Although myosin regulatory light chain (RLC) phosphorylation has been studied extensively in vitro and in vivo, the precise role of cardiac myosin light chain kinase (cMLCK), the primary kinase acting upon RLC, in the regulation of cardiomyocyte contractility remains poorly understood. In this study, using recombinantly expressed and purified proteins, various analytical methods, in vitro and in situ kinase assays, and mechanical measurements in isolated ventricular trabeculae, we demonstrate that human cMLCK is not a dedicated kinase for RLC but can phosphorylate other sarcomeric proteins with well-characterized regulatory functions. We show that cMLCK specifically monophosphorylates Ser23 of human cardiac troponin I (cTnI) in isolation and in the trimeric troponin complex in vitro and in situ in the native environment of the muscle myofilament lattice. Moreover, we observed that human cMLCK phosphorylates rodent cTnI to a much smaller extent in vitro and in situ, suggesting species-specific adaptation of cMLCK. Although cMLCK treatment of ventricular trabeculae exchanged with rat or human troponin increased their cross-bridge kinetics, the increase in sensitivity of myofilaments to calcium was significantly blunted by human TnI, suggesting that human cTnI phosphorylation by cMLCK modifies the functional consequences of RLC phosphorylation. We propose that cMLCK-mediated phosphorylation of TnI is functionally significant and represents a critical signaling pathway that coordinates the regulatory states of thick and thin filaments in both physiological and potentially pathophysiological conditions of the heart.

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

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.

Thioimidate Bond Formation between Cardiac Troponin C and Nitrile-containing Compounds.

We have investigated the mechanism and reactivity of covalent bond formation between cysteine-84 of the regulatory domain of cardiac troponin C and compounds containing a nitrile moiety similar to the calcium sensitizer levosimendan. The results of modifications to the levosimendan framework ranged from a large increase in covalent bond formation to complete inactivity. We present the biological activity of one of the most potent compounds. Limitations, including compound solubility and degradation at acidic pH, have prevented thorough investigation of the potential of these compounds. Our studies reveal the efficacious nature of the malononitrile moiety in targeting cNTnC and its potential in future cardiotonic drug design.