Acceleration of crossbridge kinetics by protein kinase A phosphorylation of cardiac myosin binding protein C modulates cardiac function

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Simultaneous imaging of local calcium and single sarcomere length in rat neonatal cardiomyocytes using yellow Cameleon-Nano140

In cardiac muscle, contraction is triggered by sarcolemmal depolarization, resulting in an intracellular Ca(2+) transient, binding of Ca(2+) to troponin, and subsequent cross-bridge formation (excitation-contraction [EC] coupling). Here, we develop a novel experimental system for simultaneous nano-imaging of intracellular Ca(2+) dynamics and single sarcomere length (SL) in rat neonatal cardiomyocytes. We achieve this by expressing a fluorescence resonance energy transfer (FRET)-based Ca(2+) sensor yellow Cameleon-Nano (YC-Nano) fused to α-actinin in order to localize to the Z disks. We find that, among four different YC-Nanos, α-actinin-YC-Nano140 is best suited for high-precision analysis of EC coupling and α-actinin-YC-Nano140 enables quantitative analyses of intracellular calcium transients and sarcomere dynamics at low and high temperatures, during spontaneous beating and with electrical stimulation. We use this tool to show that calcium transients are synchronized along the length of a myofibril. However, the averaging of SL along myofibrils causes a marked underestimate (∼50%) of the magnitude of displacement because of the different timing of individual SL changes, regardless of the absence or presence of positive inotropy (via β-adrenergic stimulation or enhanced actomyosin interaction). Finally, we find that β-adrenergic stimulation with 50 nM isoproterenol accelerated Ca(2+) dynamics, in association with an approximately twofold increase in sarcomere lengthening velocity. We conclude that our experimental system has a broad range of potential applications for the unveiling molecular mechanisms of EC coupling in cardiomyocytes at the single sarcomere level.

Dose-Dependent Effects of the Myosin Activator Omecamtiv Mecarbil on Cross-Bridge Behavior and Force Generation in Failing Human Myocardium

BACKGROUND

Omecamtiv mecarbil (OM) enhances systolic function in vivo by directly binding the myosin cross-bridges (XBs) in the sarcomere. However, the mechanistic details governing OM-induced modulation of XB behavior in failing human myocardium are unclear.

METHODS AND RESULTS

The effects of OM on steady state and dynamic XB behavior were measured in chemically skinned myocardial preparations isolated from human donor and heart failure (HF) left ventricle. HF myocardium exhibited impaired contractile function as evidenced by reduced maximal force, magnitude of XB recruitment (P_df), and a slowed rate of XB detachment (k_rel) at submaximal Ca²⁺ activations. Ca²⁺ sensitivity of force generation (pCa₅₀) was higher in HF myocardium when compared with donor myocardium, both prior to and after OM incubations. OM incubation (0.5 and 1.0 μmol/L) enhanced force generation at submaximal Ca²⁺ activations in a dose-dependent manner. Notably, OM induced a slowing in k_rel with 1.0 μmol/L OM but not with 0.5 μmol/L OM in HF myocardium. Additionally, OM exerted other differential effects on XB behavior in HF myocardium as evidenced by a greater enhancement in P_df and slowing in the time course of cooperative XB recruitment (T_rec), which collectively prolonged achievement of peak force development (T_pk), compared with donor myocardium.

CONCLUSIONS

Our findings demonstrate that OM augments force generation but also prolongs the time course of XB transitions to force-bearing states in remodeled HF myocardium, which may extend the systolic ejection time in vivo. Optimal OM dosing is critical for eliciting enhanced systolic function without excessive prolongation of systolic ejection time, which may compromise diastolic filling.

Increased Postnatal Cardiac Hyperplasia Precedes Cardiomyocyte Hypertrophy in a Model of Hypertrophic Cardiomyopathy

Rationale: Hypertrophic cardiomyopathy (HCM) occurs in ~0.5% of the population and is a leading cause of sudden cardiac death (SCD) in young adults. Cardiomyocyte hypertrophy has been the accepted mechanism for cardiac enlargement in HCM, but the early signaling responsible for initiating hypertrophy is poorly understood. Mutations in cardiac myosin binding protein C (MYBPC3) are among the most common HCM-causing mutations. Ablation of Mybpc3 in an HCM mouse model (cMyBP-C^−/−) rapidly leads to cardiomegaly by postnatal day (PND) 9, though hearts are indistinguishable from wild-type (WT) at birth. This model provides a unique opportunity to explore early processes involved in the dramatic postnatal transition to hypertrophy.

Methods and Results: We performed microarray analysis, echocardiography, qPCR, immunohistochemistry (IHC), and isolated cardiomyocyte measurements to characterize the perinatal cMyBP-C^−/− phenotype before and after overt hypertrophy. cMyBP-C^−/− hearts showed elevated cell cycling at PND1 that transitioned to hypertrophy by PND9. An expanded time course revealed that increased cardiomyocyte cycling was associated with elevated heart weight to body weight ratios prior to cellular hypertrophy, suggesting that cell cycling resulted in cardiomyocyte proliferation. Animals heterozygous for the cMyBP-C deletion trended in the direction of the homozygous null, yet did not show increased heart size by PND9.

Conclusions: Results indicate that altered regulation of the cell cycling pathway and elevated proliferation precedes hypertrophy in the cMyBP-C^−/− HCM model, and suggests that increased cardiomyocyte number contributes to increased heart size in cMyBP-C^−/− mice. This pre-hypertrophic period may reflect a unique time during which the commitment to HCM is determined and disease severity is influenced.

Functional communication between PKC-targeted cardiac troponin I phosphorylation sites

Increased protein kinase C (PKC) activity is associated with heart failure, and can target multiple cardiac troponin I (cTnI) residues in myocytes, including S23/24, S43/45 and T144. In earlier studies, cTnI-S43D and/or -S45D augmented S23/24 and T144 phosphorylation, which suggested there is communication between clusters. This communication is now explored by evaluating the impact of phospho-mimetic cTnI S43/45D combined with S23/24D (cTnIS4D) or T144D (cTnISDTD). Gene transfer of epitope-tagged cTnIS4D and cTnISDTD into adult cardiac myocytes progressively replaced endogenous cTnI. Partial replacement with cTnISDTD or cTnIS4D accelerated the time to peak (TTP) shortening and time to 50% re-lengthening (TTR_50%) on day 2, but peak shortening was only diminished by cTnIS4D. Extensive cTnIS4D replacement continued to accelerate TTP, and decrease shortening amplitude, while TTR_50% returned to baseline levels on day 4. In contrast, cTnISDTD modestly reduced shortening amplitude and continued to accelerate myocyte TTP and TTR_50%. These results indicate cTnIS43/45 communicates with S23/24 and T144, with S23/24 exacerbating and T144 attenuating the S43/45D-dependent functional deficit. In addition, more severe functional alterations in cTnIS4D myocytes were accompanied by higher levels of secondary phosphorylation compared to cTnISDTD. These results suggest that secondary phosphorylation helps to maintain steady-state contractile function during chronic cTnI phosphorylation at PKC sites.

Force development and intracellular Ca2+ in intact cardiac muscles from gravin mutant mice

Gravin (AKAP12) is an A-kinase-anchoring-protein that scaffolds protein kinase A (PKA), β₂-adrenergic receptor (β₂-AR), protein phosphatase 2B and protein kinase C. Gravin facilitates β₂-AR-dependent signal transduction through PKA to modulate cardiac excitation-contraction coupling and its removal positively affects cardiac contraction. Trabeculae from the right ventricles of gravin mutant (gravin-t/t) mice were employed for force determination. Simultaneously, corresponding intracellular Ca²⁺ transient ([Ca²⁺]_i) were measured. Twitch force (T_f)-interval relationship, [Ca²⁺]_i-interval relationship, and the rate of decay of post-extrasysolic potentiation (R_f) were also obtained. Western blot analysis were performed to correlate sarcomeric protein expression with alterations in calcium cycling between the WT and gravin-t/t hearts. Gravin-t/t muscles had similar developed force compared to WT muscles despite having lower [Ca²⁺]_i at any given external Ca²⁺ concentration ([Ca²⁺]_o). The time to peak force and peak [Ca²⁺]_i were slower and the time to 75% relaxation was significantly prolonged in gravin-t/t muscles. Both T_f-interval and [Ca²⁺]_i-interval relations were depressed in gravin-t/t muscles. R_f, however, did not change. Furthermore, Western blot analysis revealed decreased ryanodine receptor (RyR2) phosphorylation in gravin-t/t hearts. Gravin-t/t cardiac muscle exhibits increased force development in responsiveness to Ca²⁺. The Ca²⁺ cycling across the SR appears to be unaltered in gravin-t/t muscle. Our study suggests that gravin is an important component of cardiac contraction regulation via increasing myofilament sensitivity to calcium. Further elucidation of the mechanism can provide insights to role of gravin if any in the pathophysiology of impaired contractility.

Cardiac myosin binding protein-C Ser302 phosphorylation regulates cardiac β-adrenergic reserve

Phosphorylation of cardiac myosin binding protein-C (MyBP-C) modulates cardiac contractile function; however, the specific roles of individual serines (Ser) within the M-domain that are targets for β-adrenergic signaling are not known. Recently, we demonstrated that significant accelerations in in vivo pressure development following β-agonist infusion can occur in transgenic (TG) mouse hearts expressing phospho-ablated Ser²⁸² (that is, TG^S282A) but not in hearts expressing phospho-ablation of all three serines [that is, Ser²⁷³, Ser²⁸², and Ser³⁰² (TG^3SA)], suggesting an important modulatory role for other Ser residues. In this regard, there is evidence that Ser³⁰² phosphorylation may be a key contributor to the β-agonist-induced positive inotropic responses in the myocardium, but its precise functional role has not been established. Thus, to determine the in vivo and in vitro functional roles of Ser³⁰² phosphorylation, we generated TG mice expressing nonphosphorylatable Ser³⁰² (that is, TG^S302A). Left ventricular pressure-volume measurements revealed that TG^S302A mice displayed no accelerations in the rate of systolic pressure rise and an inability to maintain systolic pressure following dobutamine infusion similar to TG^3SA mice, implicating Ser³⁰² phosphorylation as a critical regulator of enhanced systolic performance during β-adrenergic stress. Dynamic strain-induced cross-bridge (XB) measurements in skinned myocardium isolated from TG^S302A hearts showed that the molecular basis for impaired β-adrenergic-mediated enhancements in systolic function is due to the absence of protein kinase A-mediated accelerations in the rate of cooperative XB recruitment. These results demonstrate that Ser³⁰² phosphorylation regulates cardiac contractile reserve by enhancing contractile responses during β-adrenergic stress.

Phosphorylation of cardiac myosin binding protein C releases myosin heads from the surface of cardiac thick filaments

Cardiac myosin binding protein C (cMyBP-C) has a key regulatory role in cardiac contraction, but the mechanism by which changes in phosphorylation of cMyBP-C accelerate cross-bridge kinetics remains unknown. In this study, we isolated thick filaments from the hearts of mice in which the three serine residues (Ser273, Ser282, and Ser302) that are phosphorylated by protein kinase A in the m-domain of cMyBP-C were replaced by either alanine or aspartic acid, mimicking the fully nonphosphorylated and the fully phosphorylated state of cMyBP-C, respectively. We found that thick filaments from the cMyBP-C phospho-deficient hearts had highly ordered cross-bridge arrays, whereas the filaments from the cMyBP-C phospho-mimetic hearts showed a strong tendency toward disorder. Our results support the hypothesis that dephosphorylation of cMyBP-C promotes or stabilizes the relaxed/superrelaxed quasi-helical ordering of the myosin heads on the filament surface, whereas phosphorylation weakens this stabilization and binding of the heads to the backbone. Such structural changes would modulate the probability of myosin binding to actin and could help explain the acceleration of cross-bridge interactions with actin when cMyBP-C is phosphorylated because of, for example, activation of β1-adrenergic receptors in myocardium.

Sarcomeric protein modification during adrenergic stress enhances cross-bridge kinetics and cardiac output

Molecular adaptations to chronic neurohormonal stress, including sarcomeric protein cleavage and phosphorylation, provide a mechanism to increase ventricular contractility and enhance cardiac output, yet the link between sarcomeric protein modifications and changes in myocardial function remains unclear. To examine the effects of neurohormonal stress on posttranslational modifications of sarcomeric proteins, mice were administered combined α- and β-adrenergic receptor agonists (isoproterenol and phenylephrine, IPE) for 14 days using implantable osmotic pumps. In addition to significant cardiac hypertrophy and increased maximal ventricular pressure, IPE treatment accelerated pressure development and relaxation (74% increase in dP/dt_max and 14% decrease in τ), resulting in a 52% increase in cardiac output compared with saline (SAL)-treated mice. Accelerated pressure development was maintained when accounting for changes in heart rate and preload, suggesting that myocardial adaptations contribute to enhanced ventricular contractility. Ventricular myocardium isolated from IPE-treated mice displayed a significant reduction in troponin I (TnI) and myosin-binding protein C (MyBP-C) expression and a concomitant increase in the phosphorylation levels of the remaining TnI and MyBP-C protein compared with myocardium isolated from saline-treated control mice. Skinned myocardium isolated from IPE-treated mice displayed a significant acceleration in the rate of cross-bridge (XB) detachment (46% increase) and an enhanced magnitude of XB recruitment (43% increase) at submaximal Ca²⁺ activation compared with SAL-treated mice but unaltered myofilament Ca²⁺ sensitivity of force generation. These findings demonstrate that sarcomeric protein modifications during neurohormonal stress are molecular adaptations that enhance in vivo ventricular contractility through accelerated XB kinetics to increase cardiac output.NEW & NOTEWORTHY Posttranslational modifications to sarcomeric regulatory proteins provide a mechanism to modulate cardiac function in response to stress. In this study, we demonstrate that neurohormonal stress produces modifications to myosin-binding protein C and troponin I, including a reduction in protein expression within the sarcomere and increased phosphorylation of the remaining protein, which serve to enhance cross-bridge kinetics and increase cardiac output. These findings highlight the importance of sarcomeric regulatory protein modifications in modulating ventricular function during cardiac stress.

The contributions of cardiac myosin binding protein C and troponin I phosphorylation to β-adrenergic enhancement of in vivo cardiac function

KEY POINTS

β-adrenergic stimulation increases cardiac myosin binding protein C (MyBP-C) and troponin I phosphorylation to accelerate pressure development and relaxation in vivo, although their relative contributions remain unknown. Using a novel mouse model lacking protein kinase A-phosphorylatable troponin I (TnI) and MyBP-C, we examined in vivo haemodynamic function before and after infusion of the β-agonist dobutamine. Mice expressing phospho-ablated MyBP-C displayed cardiac hypertrophy and prevented full acceleration of pressure development and relaxation in response to dobutamine, whereas expression of phosphor-ablated TnI alone had little effect on the acceleration of contractile function in response to dobutamine. Our data demonstrate that MyBP-C phosphorylation is the principal mediator of the contractile response to increased β-agonist stimulation in vivo. These results help us understand why MyBP-C dephosphorylation in the failing heart contributes to contractile dysfunction and decreased adrenergic reserve in response to acute stress. β-adrenergic stimulation plays a critical role in accelerating ventricular contraction and speeding relaxation to match cardiac output to changing circulatory demands. Two key myofilaments proteins, troponin I (TnI) and myosin binding protein-C (MyBP-C), are phosphorylated following β-adrenergic stimulation; however, their relative contributions to the enhancement of in vivo cardiac contractility are unknown. To examine the roles of TnI and MyBP-C phosphorylation in β-adrenergic-mediated enhancement of cardiac function, transgenic (TG) mice expressing non-phosphorylatable TnI protein kinase A (PKA) residues (i.e. serine to alanine substitution at Ser23/24; TnI(PKA-)) were bred with mice expressing non-phosphorylatable MyBP-C PKA residues (i.e. serine to alanine substitution at Ser273, Ser282 and Ser302; MyBPC(PKA-)) to generate a novel mouse model expressing non-phosphorylatable PKA residues in TnI and MyBP-C (DBL(PKA-)). MyBP-C dephosphorylation produced cardiac hypertrophy and increased wall thickness in MyBPC(PKA-) and DBL(PKA-) mice, and in vivo echocardiography and pressure-volume catheterization studies revealed impaired systolic function and prolonged diastolic relaxation compared to wild-type and TnI(PKA-) mice. Infusion of the β-agonist dobutamine resulted in accelerated rates of pressure development and relaxation in all mice; however, MyBPC(PKA-) and DBL(PKA-) mice displayed a blunted contractile response compared to wild-type and TnI(PKA-) mice. Furthermore, unanaesthesized MyBPC(PKA-) and DBL(PKA-) mice displayed depressed maximum systolic pressure in response to dobutamine as measured using implantable telemetry devices. Taken together, our data show that MyBP-C phosphorylation is a critical modulator of the in vivo acceleration of pressure development and relaxation as a result of enhanced β-adrenergic stimulation, and reduced MyBP-C phosphorylation may underlie depressed adrenergic reserve in heart failure.

The genetic basis of hypertrophic cardiomyopathy in cats and humans

Mutations in genes that encode for muscle sarcomeric proteins have been identified in humans and two breeds of domestic cats with hypertrophic cardiomyopathy (HCM). This article reviews the history, genetics, and pathogenesis of HCM in the two species in order to give veterinarians a perspective on the genetics of HCM. Hypertrophic cardiomyopathy in people is a genetic disease that has been called a disease of the sarcomere because the preponderance of mutations identified that cause HCM are in genes that encode for sarcomeric proteins (Maron and Maron, 2013). Sarcomeres are the basic contractile units of muscle and thus sarcomeric proteins are responsible for the strength, speed, and extent of muscle contraction. In people with HCM, the two most common genes affected by HCM mutations are the myosin heavy chain gene (MYH7), the gene that encodes for the motor protein β-myosin heavy chain (the sarcomeric protein that splits ATP to generate force), and the cardiac myosin binding protein-C gene (MYBPC3), a gene that encodes for the closely related structural and regulatory protein, cardiac myosin binding protein-C (cMyBP-C). To date, the two mutations linked to HCM in domestic cats (one each in Maine Coon and Ragdoll breeds) also occur in MYBPC3 (Meurs et al., 2005, 2007). This is a review of the genetics of HCM in both humans and domestic cats that focuses on the aspects of human genetics that are germane to veterinarians and on all aspects of feline HCM genetics.

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.

Loss of myocardial retinoic acid receptor α induces diastolic dysfunction by promoting intracellular oxidative stress and calcium mishandling in adult mice

Retinoic acid receptor (RAR) has been implicated in pathological stimuli-induced cardiac remodeling. To determine whether the impairment of RARα signaling directly contributes to the development of heart dysfunction and the involved mechanisms, tamoxifen-induced myocardial specific RARα deletion (RARαKO) mice were utilized. Echocardiographic and cardiac catheterization studies showed significant diastolic dysfunction after 16wks of gene deletion. However, no significant differences were observed in left ventricular ejection fraction (LVEF), between RARαKO and wild type (WT) control mice. DHE staining showed increased intracellular reactive oxygen species (ROS) generation in the hearts of RARαKO mice. Significantly increased NOX2 (NADPH oxidase 2) and NOX4 levels and decreased SOD1 and SOD2 levels were observed in RARαKO mouse hearts, which were rescued by overexpression of RARα in cardiomyocytes. Decreased SERCA2a expression and phosphorylation of phospholamban (PLB), along with decreased phosphorylation of Akt and Ca²⁺/calmodulin-dependent protein kinase II δ (CaMKII δ) was observed in RARαKO mouse hearts. Ca²⁺ reuptake and cardiomyocyte relaxation were delayed by RARα deletion. Overexpression of RARα or inhibition of ROS generation or NOX activation prevented RARα deletion-induced decrease in SERCA2a expression/activation and delayed Ca²⁺ reuptake. Moreover, the gene and protein expression of RARα was significantly decreased in aged or metabolic stressed mouse hearts. RARα deletion accelerated the development of diastolic dysfunction in streptozotocin (STZ)-induced type 1 diabetic mice or in high fat diet fed mice. In conclusion, myocardial RARα deletion promoted diastolic dysfunction, with a relative preserved LVEF. Increased oxidative stress have an important role in the decreased expression/activation of SERCA2a and Ca²⁺ mishandling in RARαKO mice, which are major contributing factors in the development of diastolic dysfunction. These data suggest that impairment of cardiac RARα signaling may be a novel mechanism that is directly linked to pathological stimuli-induced diastolic dysfunction.

Comparison of the effects of a truncating and a missense MYBPC3 mutation on contractile parameters of engineered heart tissue

Hypertrophic cardiomyopathy (HCM) is a cardiac genetic disease characterized by left ventricular hypertrophy, diastolic dysfunction and myocardial disarray. The most frequently mutated gene is MYBPC3, encoding cardiac myosin-binding protein-C (cMyBP-C). We compared the pathomechanisms of a truncating mutation (c.2373_2374insG) and a missense mutation (c.1591G>C) in MYBPC3 in engineered heart tissue (EHT). EHTs enable to study the direct effects of mutants without interference of secondary disease-related changes. EHTs were generated from Mybpc3-targeted knock-out (KO) and wild-type (WT) mouse cardiac cells. MYBPC3 WT and mutants were expressed in KO EHTs via adeno-associated virus. KO EHTs displayed higher maximal force and sensitivity to external [Ca(2+)] than WT EHTs. Expression of WT-Mybpc3 at MOI-100 resulted in ~73% cMyBP-C level but did not prevent the KO phenotype, whereas MOI-300 resulted in ≥95% cMyBP-C level and prevented the KO phenotype. Expression of the truncating or missense mutation (MOI-300) or their combination with WT (MOI-150 each), mimicking the homozygous or heterozygous disease state, respectively, failed to restore force to WT level. Immunofluorescence analysis revealed correct incorporation of WT and missense, but not of truncated cMyBP-C in the sarcomere. In conclusion, this study provides evidence in KO EHTs that i) haploinsufficiency affects EHT contractile function if WT cMyBP-C protein levels are ≤73%, ii) missense or truncating mutations, but not WT do not fully restore the disease phenotype and have different pathogenic mechanisms, e.g. sarcomere poisoning for the missense mutation, iii) the direct impact of (newly identified) MYBPC3 gene variants can be evaluated.

Myocardial reverse remodeling: how far can we rewind?

Heart failure (HF) is a systemic disease that can be divided into HF with reduced ejection fraction (HFrEF) and with preserved ejection fraction (HFpEF). HFpEF accounts for over 50% of all HF patients and is typically associated with high prevalence of several comorbidities, including hypertension, diabetes mellitus, pulmonary hypertension, obesity, and atrial fibrillation. Myocardial remodeling occurs both in HFrEF and HFpEF and it involves changes in cardiac structure, myocardial composition, and myocyte deformation and multiple biochemical and molecular alterations that impact heart function and its reserve capacity. Understanding the features of myocardial remodeling has become a major objective for limiting or reversing its progression, the latter known as reverse remodeling (RR). Research on HFrEF RR process is broader and has delivered effective therapeutic strategies, which have been employed for some decades. However, the RR process in HFpEF is less clear partly due to the lack of information on HFpEF pathophysiology and to the long list of failed standard HF therapeutics strategies in these patient's outcomes. Nevertheless, new proteins, protein-protein interactions, and signaling pathways are being explored as potential new targets for HFpEF remodeling and RR. Here, we review recent translational and clinical research in HFpEF myocardial remodeling to provide an overview on the most important features of RR, comparing HFpEF with HFrEF conditions.

Alterations in Multi-Scale Cardiac Architecture in Association With Phosphorylation of Myosin Binding Protein-C

Background

The geometric organization of myocytes in the ventricular wall comprises the structural underpinnings of cardiac mechanical function. Cardiac myosin binding protein‐C (MYBPC3) is a sarcomeric protein, for which phosphorylation modulates myofilament binding, sarcomere morphology, and myocyte alignment in the ventricular wall. To elucidate the mechanisms by which MYBPC3 phospho‐regulation affects cardiac tissue organization, we studied ventricular myoarchitecture using generalized Q‐space imaging (GQI). GQI assessed geometric phenotype in excised hearts that had undergone transgenic (TG) modification of phospho‐regulatory serine sites to nonphosphorylatable alanines (MYBPC3^{AllP−/(t/t)}) or phospho‐mimetic aspartic acids (MYBPC3^AllP+/(t/t)).

Methods and Results

Myoarchitecture in the wild‐type (MYBPC3^WT) left‐ventricle (LV) varied with transmural position, with helix angles ranging from −90/+90 degrees and contiguous circular orientation from the LV mid‐myocardium to the right ventricle (RV). Whereas MYBPC3^AllP+/(t/t) hearts were not architecturally distinct from MYBPC3^WT, MYBPC3^{AllP−/(t/t)} hearts demonstrated a significant reduction in LV transmural helicity. Null MYBPC3^(t/t) hearts, as constituted by a truncated MYBPC3 protein, demonstrated global architectural disarray and loss in helicity. Electron microscopy was performed to correlate the observed macroscopic architectural changes with sarcomere ultrastructure and demonstrated that impaired phosphorylation of MYBPC3 resulted in modifications of the sarcomere aspect ratio and shear angle. The mechanical effect of helicity loss was assessed through a geometric model relating cardiac work to ejection fraction, confirming the mechanical impairments observed with echocardiography.

Conclusions

We conclude that phosphorylation of MYBPC3 contributes to the genesis of ventricular wall geometry, linking myofilament biology with multiscale cardiac mechanics and myoarchitecture.

Cardiac Myosin Binding Protein-C Phosphorylation Modulates Myofilament Length-Dependent Activation

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is an important regulator of contractile function, however, its contributions to length-dependent changes in cross-bridge (XB) kinetics is unknown. Therefore, we performed mechanical experiments to quantify contractile function in detergent-skinned ventricular preparations isolated from wild-type (WT) hearts, and hearts expressing non-phosphorylatable cMyBP-C [Ser to Ala substitutions at residues Ser273, Ser282, and Ser302 (i.e., 3SA)], at sarcomere length (SL) 1.9 μm or 2.1μm, prior and following protein kinase A (PKA) treatment. Steady-state force generation measurements revealed a blunting in the length-dependent increase in myofilament Ca(2+)-sensitivity of force generation (pCa50) following an increase in SL in 3SA skinned myocardium compared to WT skinned myocardium. Dynamic XB behavior was assessed at submaximal Ca(2+)-activations by imposing an acute rapid stretch of 2% of initial muscle length, and measuring both the magnitudes and rates of resultant phases of force decay due to strain-induced XB detachment and delayed force rise due to recruitment of additional XBs with increased SL (i.e., stretch activation). The magnitude (P2) and rate of XB detachment (k rel) following stretch was significantly reduced in 3SA skinned myocardium compared to WT skinned myocardium at short and long SL, and prior to and following PKA treatment. Furthermore, the length-dependent acceleration of k rel due to decreased SL that was observed in WT skinned myocardium was abolished in 3SA skinned myocardium. PKA treatment accelerated the rate of XB recruitment (k df) following stretch at both SL's in WT but not in 3SA skinned myocardium. The amplitude of the enhancement in force generation above initial pre-stretch steady-state levels (P3) was not different between WT and 3SA skinned myocardium at any condition measured. However, the magnitude of the entire delayed force phase which can dip below initial pre-stretch steady-state levels (Pdf) was significantly lower in 3SA skinned myocardium under all conditions, in part due to a reduced magnitude of XB detachment (P2) in 3SA skinned myocardium compared to WT skinned myocardium. These findings demonstrate that cMyBP-C phospho-ablation regulates SL- and PKA-mediated effects on XB kinetics in the myocardium, which would be expected to contribute to the regulation of the Frank-Starling mechanism.

S-glutathiolation impairs phosphoregulation and function of cardiac myosin-binding protein C in human heart failure

Cardiac myosin-binding protein C (cMyBP-C) regulates actin-myosin interaction and thereby cardiac myocyte contraction and relaxation. This physiologic function is regulated by cMyBP-C phosphorylation. In our study, reduced site-specific cMyBP-C phosphorylation coincided with increased S-glutathiolation in ventricular tissue from patients with dilated or ischemic cardiomyopathy compared to nonfailing donors. We used redox proteomics, to identify constitutive and disease-specific S-glutathiolation sites in cMyBP-C in donor and patient samples, respectively. Among those, a cysteine cluster in the vicinity of the regulatory phosphorylation sites within the myosin S2 interaction domain C1-M-C2 was identified and showed enhanced S-glutathiolation in patients. In vitro S-glutathiolation of recombinant cMyBP-C C1-M-C2 occurred predominantly at Cys(249), which attenuated phosphorylation by protein kinases. Exposure to glutathione disulfide induced cMyBP-C S-glutathiolation, which functionally decelerated the kinetics of Ca(2+)-activated force development in ventricular myocytes from wild-type, but not those from Mybpc3-targeted knockout mice. These oxidation events abrogate protein kinase-mediated phosphorylation of cMyBP-C and therefore potentially contribute to the reduction of its phosphorylation and the contractile dysfunction observed in human heart failure.-Stathopoulou, K., Wittig, I., Heidler, J., Piasecki, A., Richter, F., Diering, S., van der Velden, J., Buck, F., Donzelli, S., Schröder, E., Wijnker, P. J. M., Voigt, N., Dobrev, D., Sadayappan, S., Eschenhagen, T., Carrier, L., Eaton, P., Cuello, F. S-glutathiolation impairs phosphoregulation and function of cardiac myosin-binding protein C in human heart failure.

Slowing of contractile kinetics by myosin-binding protein C can be explained by its cooperative binding to the thin filament

Cardiac myosin binding protein C (cMyBP-C) is a thick filament-associated protein that participates in the regulation of muscle contraction. Simplified in vitro systems show that cMyBP-C binds not only to myosin, but also to the actin filament. The physiological significance of these separate binding interactions remains unclear, as does the question of whether either interaction is capable of explaining the behavior of intact muscle from which cMyBP-C has been removed. We have used a computational model to explore the characteristic effects of myosin-binding versus actin-binding by cMyBP-C. Simulations suggest that myosin-cMyBP-C interactions reduce peak force and Ca2 + sensitivity of the myofilaments, but have no appreciable effect on the rate of force redevelopment (ktr). In contrast, cMyBP-C binding to actin increases myofilament Ca2 + sensitivity and slows ktrat sub-maximal Ca2 + values. This slowing is due to cooperation between cMyBP-C ‘crossbridges’ and traditional myosin crossbridges as they bind to and activate the actin thin filament. We further observed that an overall recapitulation of skinned myocardial data from wild type and cMyBP-C knockout mice requires the interaction of cMyBP-C with of both of its binding targets in our model. The assumption of significant interactions with both partners was also sufficient to explain published effects of cMyBP-C ablation on twitch kinetics. These modeling results strongly support the view that both binding interactions play critical roles in the physiology of intact muscle. Furthermore, they suggest that the widely observed phenomenon of slowed force development in the presence of cMyBP-C may actually be a manifestation of cooperative binding of this protein to the thin filament.

Contractile apparatus dysfunction early in the pathophysiology of diabetic cardiomyopathy

Diabetes mellitus significantly increases the risk of cardiovascular disease and heart failure in patients. Independent of hypertension and coronary artery disease, diabetes is associated with a specific cardiomyopathy, known as diabetic cardiomyopathy (DCM). Four decades of research in experimental animal models and advances in clinical imaging techniques suggest that DCM is a progressive disease, beginning early after the onset of type 1 and type 2 diabetes, ahead of left ventricular remodeling and overt diastolic dysfunction. Although the molecular pathogenesis of early DCM still remains largely unclear, activation of protein kinase C appears to be central in driving the oxidative stress dependent and independent pathways in the development of contractile dysfunction. Multiple subcellular alterations to the cardiomyocyte are now being highlighted as critical events in the early changes to the rate of force development, relaxation and stability under pathophysiological stresses. These changes include perturbed calcium handling, suppressed activity of aerobic energy producing enzymes, altered transcriptional and posttranslational modification of membrane and sarcomeric cytoskeletal proteins, reduced actin-myosin cross-bridge cycling and dynamics, and changed myofilament calcium sensitivity. In this review, we will present and discuss novel aspects of the molecular pathogenesis of early DCM, with a special focus on the sarcomeric contractile apparatus.

A change of heart: oxidative stress in governing muscle function?

Redox/cysteine modification of proteins that regulate calcium cycling can affect contraction in striated muscles. Understanding the nature of these modifications would present the possibility of enhancing cardiac function through reversible cysteine modification of proteins, with potential therapeutic value in heart failure with diastolic dysfunction. Both heart failure and muscular dystrophy are characterized by abnormal redox balance and nitrosative stress. Recent evidence supports the synergistic role of oxidative stress and inflammation in the progression of heart failure with preserved ejection fraction, in concert with endothelial dysfunction and impaired nitric oxide-cyclic guanosine monophosphate-protein kinase G signalling via modification of the giant protein titin. Although antioxidant therapeutics in heart failure with diastolic dysfunction have no marked beneficial effects on the outcome of patients, it, however, remains critical to the understanding of the complex interactions of oxidative/nitrosative stress with pro-inflammatory mechanisms, metabolic dysfunction, and the redox modification of proteins characteristic of heart failure. These may highlight novel approaches to therapeutic strategies for heart failure with diastolic dysfunction. In this review, we provide an overview of oxidative stress and its effects on pathophysiological pathways. We describe the molecular mechanisms driving oxidative modification of proteins and subsequent effects on contractile function, and, finally, we discuss potential therapeutic opportunities for heart failure with diastolic dysfunction.

Molecular effects of the myosin activator omecamtiv mecarbil on contractile properties of skinned myocardium lacking cardiac myosin binding protein-C

Decreased expression of cardiac myosin binding protein-C (cMyBP-C) in the myocardium is thought to be a contributing factor to hypertrophic cardiomyopathy in humans, and the initial molecular defect is likely abnormal cross-bridge (XB) function which leads to impaired force generation, decreased contractile performance, and hypertrophy in vivo. The myosin activator omecamtiv mecarbil (OM) is a pharmacological drug that specifically targets the myosin XB and recent evidence suggests that OM induces a significant decrease in the in vivo motility velocity and an increase in the XB duty cycle. Thus, the molecular effects of OM maybe beneficial in improving contractile function in skinned myocardium lacking cMyBP-C because absence of cMyBP-C in the sarcomere accelerates XB kinetics and enhances XB turnover rate, which presumably reduces contractile efficiency. Therefore, parameters of XB function were measured in skinned myocardium lacking cMyBP-C prior to and following OM incubation. We measured ktr, the rate of force redevelopment as an index of XB transition from both the weakly- to strongly-bound state and from the strongly- to weakly-bound states and performed stretch activation experiments to measure the rates of XB detachment (krel) and XB recruitment (kdf) in detergent-skinned ventricular preparations isolated from hearts of wild-type (WT) and cMyBP-C knockout (KO) mice. Samples from donor human hearts were also used to assess the effects of OM in cardiac muscle expressing a slow β-myosin heavy chain (β-MHC). Incubation of skinned myocardium with OM produced large enhancements in steady-state force generation which were most pronounced at low levels of [Ca(2+)] activations, suggesting that OM cooperatively recruits additional XB's into force generating states. Despite a large increase in steady-state force generation following OM incubation, parallel accelerations in XB kinetics as measured by ktr were not observed, and there was a significant OM-induced decrease in krel which was more pronounced in the KO skinned myocardium compared to WT skinned myocardium (58% in WT vs. 76% in KO at pCa 6.1), such that baseline differences in krel between KO and WT skinned myocardium were no longer apparent following OM-incubation. A significant decrease in the kdf was also observed following OM incubation in all groups, which may be related to the increase in the number of cooperatively recruited XB's at low Ca(2+)-activations which slows the overall rate of force generation. Our results indicate that OM may be a useful pharmacological approach to normalize hypercontractile XB kinetics in myocardium with decreased cMyBP-C expression due to its molecular effects on XB behavior.

Cardiac MyBP-C regulates the rate and force of contraction in mammalian myocardium

Cardiac myosin-binding protein-C (cMyBP-C) is a thick filament-associated protein that seems to contribute to the regulation of cardiac contraction through interactions with either myosin or actin or both. Several studies over the past several years have suggested that the interactions of cardiac myosin-binding protein-C with its binding partners vary with its phosphorylation state, binding predominantly to myosin when dephosphorylated and to actin when it is phosphorylated by protein kinase A or other kinases. Here, we summarize evidence suggesting that phosphorylation of cardiac myosin binding protein-C is a key regulator of the kinetics and amplitude of cardiac contraction during β-adrenergic stimulation and increased stimulus frequency. We propose a model for these effects via a phosphorylation-dependent regulation of the kinetics and extent of cooperative recruitment of cross bridges to the thin filament: phosphorylation of cardiac myosin binding protein-C accelerates cross bridge binding to actin, thereby accelerating recruitment and increasing the amplitude of the cardiac twitch. In contrast, enhanced lusitropy as a result of phosphorylation seems to be caused by a direct effect of phosphorylation to accelerate cross-bridge detachment rate. Depression or elimination of one or both of these processes in a disease, such as end-stage heart failure, seems to contribute to the systolic and diastolic dysfunction that characterizes the disease.

Phosphoregulation of Cardiac Inotropy via Myosin Binding Protein-C During Increased Pacing Frequency or β1-Adrenergic Stimulation

BACKGROUND

Mammalian hearts exhibit positive inotropic responses to β-adrenergic stimulation as a consequence of protein kinase A-mediated phosphorylation or as a result of increased beat frequency (the Bowditch effect). Several membrane and myofibrillar proteins are phosphorylated under these conditions, but the relative contributions of these to increased contractility are not known. Phosphorylation of cardiac myosin-binding protein-C (cMyBP-C) by protein kinase A accelerates the kinetics of force development in permeabilized heart muscle, but its role in vivo is unknown. Such understanding is important because adrenergic responsiveness of the heart and the Bowditch effect are both depressed in heart failure.

METHODS AND RESULTS

The roles of cMyBP-C phosphorylation were studied using mice in which either WT or nonphosphorylatable forms of cMyBP-C [ser273ala, ser282ala, ser302ala: cMyBP-C(t3SA)] were expressed at similar levels on a cMyBP-C null background. Force and [Ca(2+)]in measurements in isolated papillary muscles showed that the increased force and twitch kinetics because increased pacing or β1-adrenergic stimulation were nearly absent in cMyBP-C(t3SA) myocardium, even though [Ca(2+)]in transients under each condition were similar to WT. Biochemical measurements confirmed that protein kinase A phosphorylated ser273, ser282, and ser302 in WT cMyBP-C. In contrast, CaMKIIδ, which is activated by increased pacing, phosphorylated ser302 principally, ser282 to a lesser degree, and ser273 not at all.

CONCLUSIONS

Phosphorylation of cMyBP-C increases the force and kinetics of twitches in living cardiac muscle. Further, cMyBP-C is a principal mediator of increased contractility observed with β-adrenergic stimulation or increased pacing because of protein kinase A and CaMKIIδ phosphorylations of cMyB-C.

Phosphorylation of cardiac Myosin-binding protein-C is a critical mediator of diastolic function

BACKGROUND

Heart failure (HF) with preserved ejection fraction (HFpEF) accounts for ≈50% of all cases of HF and currently has no effective treatment. Diastolic dysfunction underlies HFpEF; therefore, elucidation of the mechanisms that mediate relaxation can provide new potential targets for treatment. Cardiac myosin-binding protein-C (cMyBP-C) is a thick filament protein that modulates cross-bridge cycling rates via alterations in its phosphorylation status. Thus, we hypothesize that phosphorylated cMyBP-C accelerates the rate of cross-bridge detachment, thereby enhancing relaxation to mediate diastolic function.

METHODS AND RESULTS

We compared mouse models expressing phosphorylation-deficient cMyBP-C(S273A/S282A/S302A)-cMyBP-C(t3SA), phosphomimetic cMyBP-C(S273D/S282D/S302D)-cMyBP-C(t3SD), and wild-type-control cMyBP-C(tWT) to elucidate the functional effects of cMyBP-C phosphorylation. Decreased voluntary running distances, increased lung/body weight ratios, and increased brain natriuretic peptide levels in cMyBP-C(t3SA) mice demonstrate that phosphorylation deficiency is associated with signs of HF. Echocardiography (ejection fraction and myocardial relaxation velocity) and pressure/volume measurements (-dP/dtmin, pressure decay time constant τ-Glantz, and passive filling stiffness) show that cMyBP-C phosphorylation enhances myocardial relaxation in cMyBP-C(t3SD) mice, whereas deficient cMyBP-C phosphorylation causes diastolic dysfunction with HFpEF in cMyBP-C(t3SA) mice. Simultaneous force and [Ca(2+)]i measurements on intact papillary muscles show that enhancement of relaxation in cMyBP-C(t3SD) mice and impairment of relaxation in cMyBP-C(t3SA) mice are not because of altered [Ca(2+)]i handling, implicating that altered cross-bridge detachment rates mediate these changes in relaxation rates.

CONCLUSIONS

cMyBP-C phosphorylation enhances relaxation, whereas deficient phosphorylation causes diastolic dysfunction and phenotypes resembling HFpEF. Thus, cMyBP-C is a potential target for treatment of HFpEF.

β-adrenergic effects on cardiac myofilaments and contraction in an integrated rabbit ventricular myocyte model

A five-state model of myofilament contraction was integrated into a well-established rabbit ventricular myocyte model of ion channels, Ca(2+) transporters and kinase signaling to analyze the relative contribution of different phosphorylation targets to the overall mechanical response driven by β-adrenergic stimulation (β-AS). β-AS effect on sarcoplasmic reticulum Ca(2+) handling, Ca(2+), K(+) and Cl(-) currents, and Na(+)/K(+)-ATPase properties was included based on experimental data. The inotropic effect on the myofilaments was represented as reduced myofilament Ca(2+) sensitivity (XBCa) and titin stiffness, and increased cross-bridge (XB) cycling rate (XBcy). Assuming independent roles of XBCa and XBcy, the model reproduced experimental β-AS responses on action potentials and Ca(2+) transient amplitude and kinetics. It also replicated the behavior of force-Ca(2+), release-restretch, length-step, stiffness-frequency and force-velocity relationships, and increased force and shortening in isometric and isotonic twitch contractions. The β-AS effect was then switched off from individual targets to analyze their relative impact on contractility. Preventing β-AS effects on L-type Ca(2+) channels or phospholamban limited Ca(2+) transients and contractile responses in parallel, while blocking phospholemman and K(+) channel (IKs) effects enhanced Ca(2+) and inotropy. Removal of β-AS effects from XBCa enhanced contractile force while decreasing peak Ca(2+) (due to greater Ca(2+) buffering), but had less effect on shortening. Conversely, preventing β-AS effects on XBcy preserved Ca(2+) transient effects, but blunted inotropy (both isometric force and especially shortening). Removal of titin effects had little impact on contraction. Finally, exclusion of β-AS from XBCa and XBcy while preserving effects on other targets resulted in preserved peak isometric force response (with slower kinetics) but nearly abolished enhanced shortening. β-AS effects on XBCa and XBcy have greater impact on isometric and isotonic contraction, respectively.

Independent modulation of contractile performance by cardiac troponin I Ser43 and Ser45 in the dynamic sarcomere

Protein kinase C (PKC) targets cardiac troponin I (cTnI) S43/45 for phosphorylation in addition to other residues. During heart failure, cTnI S43/45 phosphorylation is elevated, and yet there is ongoing debate about its functional role due, in part, to the emergence of complex phenotypes in animal models. The individual functional influences of phosphorylated S43 and S45 also are not yet known. The present study utilizes viral gene transfer of cTnI with phosphomimetic S43D and/or S45D substitutions to evaluate their individual and combined influences on function in intact adult cardiac myocytes. Partial replacement (≤40%) with either cTnIS43D or cTnIS45D reduced the amplitude of contraction, and cTnIS45D slowed contraction and relaxation rates, while there were no significant changes in function with cTnIS43/45D. More extensive replacement (≥70%) with cTnIS43D, cTnIS45D, and cTnIS43/45D each reduced the amplitude of contraction. Additional experiments also showed cTnIS45D reduced myofilament Ca(2+) sensitivity of tension. At the same time, shortening rates returned toward control values with cTnIS45D and the later stages of relaxation also became accelerated in myocytes expressing cTnIS43D and/or S45D. Further studies demonstrated this behavior coincided with adaptive changes in myofilament protein phosphorylation. Taken together, the results observed in myocytes expressing cTnIS43D and/or S45D suggest these 2 residues reduce function via independent mechanism(s). The changes in function associated with the onset of adaptive myofilament signaling suggest the sarcomere is capable of fine tuning PKC-mediated cTnIS43/45 phosphorylation and contractile performance. This modulatory behavior also provides insight into divergent phenotypes reported in animal models with cTnI S43/45 phosphomimetic substitutions.

Haploinsufficiency of MYBPC3 exacerbates the development of hypertrophic cardiomyopathy in heterozygous mice

Mutations in MYBPC3, the gene encoding cardiac myosin binding protein-C (cMyBP-C), account for ~40% of hypertrophic cardiomyopathy (HCM) cases. Most pathological MYBPC3 mutations encode truncated protein products not found in tissue. Reduced protein levels occur in symptomatic heterozygous human HCM carriers, suggesting haploinsufficiency as an underlying mechanism of disease. However, we do not know if reduced cMyBP-C content results from, or initiates the development of HCM. In previous studies, heterozygous (HET) mice with a MYBPC3 C'-terminal truncation mutation and normal cMyBP-C levels show altered contractile function prior to any overt hypertrophy. Therefore, this study aimed to test whether haploinsufficiency occurs, with decreased cMyBP-C content, following cardiac stress and whether the functional impairment in HET MYBPC3 hearts leads to worsened disease progression. To address these questions, transverse aortic constriction (TAC) was performed on three-month-old wild-type (WT) and HET MYBPC3-truncation mutant mice and then characterized at 4 and 12weeks post-surgery. HET-TAC mice showed increased hypertrophy and reduced ejection fraction compared to WT-TAC mice. At 4weeks post-surgery, HET myofilaments showed significantly reduced cMyBP-C content. Functionally, HET-TAC cardiomyocytes showed impaired force generation, higher Ca(2+) sensitivity, and blunted length-dependent increase in force generation. RNA sequencing revealed several differentially regulated genes between HET and WT groups, including regulators of remodeling and hypertrophic response. Collectively, these results demonstrate that haploinsufficiency occurs in HET MYBPC3 mutant carriers following stress, causing, in turn, reduced cMyBP-C content and exacerbating the development of dysfunction at myofilament and whole-heart levels.

A hypertrophic cardiomyopathy-associated MYBPC3 mutation common in populations of South Asian descent causes contractile dysfunction

Hypertrophic cardiomyopathy (HCM) results from mutations in genes encoding sarcomeric proteins, most often MYBPC3, which encodes cardiac myosin binding protein-C (cMyBP-C). A recently discovered HCM-associated 25-base pair deletion in MYBPC3 is inherited in millions worldwide. Although this mutation causes changes in the C10 domain of cMyBP-C (cMyBP-C(C10mut)), which binds to the light meromyosin (LMM) region of the myosin heavy chain, the underlying molecular mechanism causing HCM is unknown. In this study, adenoviral expression of cMyBP-C(C10mut) in cultured adult rat cardiomyocytes was used to investigate protein localization and evaluate contractile function and Ca(2+) transients, compared with wild-type cMyBP-C expression (cMyBP-C(WT)) and controls. Forty-eight hours after infection, 44% of cMyBP-C(WT) and 36% of cMyBP-C(C10mut) protein levels were determined in total lysates, confirming equal expression. Immunofluorescence experiments showed little or no localization of cMyBP-C(C10mut) to the C-zone, whereas cMyBP-C(WT) mostly showed C-zone staining, suggesting that cMyBP-C(C10mut) could not properly integrate in the C-zone of the sarcomere. Subcellular fractionation confirmed that most cMyBP-C(C10mut) resided in the soluble fraction, with reduced presence in the myofilament fraction. Also, cMyBP-C(C10mut) displayed significantly reduced fractional shortening, sarcomere shortening, and relaxation velocities, apparently caused by defects in sarcomere function, because Ca(2+) transients were unaffected. Co-sedimentation and protein cross-linking assays confirmed that C10(mut) causes the loss of C10 domain interaction with myosin LMM. Protein homology modeling studies showed significant structural perturbation in cMyBP-C(C10mut), providing a potential structural basis for the alteration in its mode of interaction with myosin LMM. Therefore, expression of cMyBP-C(C10mut) protein is sufficient to cause contractile dysfunction in vitro.

Myosin-binding protein C corrects an intrinsic inhomogeneity in cardiac excitation-contraction coupling

The beating heart exhibits remarkable contractile fidelity over a lifetime, which reflects the tight coupling of electrical, chemical, and mechanical elements within the sarcomere, the elementary contractile unit. On a beat-to-beat basis, calcium is released from the ends of the sarcomere and must diffuse toward the sarcomere center to fully activate the myosin- and actin-based contractile proteins. The resultant spatial and temporal gradient in free calcium across the sarcomere should lead to nonuniform and inefficient activation of contraction. We show that myosin-binding protein C (MyBP-C), through its positioning on the myosin thick filaments, corrects this nonuniformity in calcium activation by exquisitely sensitizing the contractile apparatus to calcium in a manner that precisely counterbalances the calcium gradient. Thus, the presence and correct localization of MyBP-C within the sarcomere is critically important for normal cardiac function, and any disturbance of MyBP-C localization or function will contribute to the consequent cardiac pathologies.

Tri-modal regulation of cardiac muscle relaxation; intracellular calcium decline, thin filament deactivation, and cross-bridge cycling kinetics

Cardiac muscle relaxation is an essential step in the cardiac cycle. Even when the contraction of the heart is normal and forceful, a relaxation phase that is too slow will limit proper filling of the ventricles. Relaxation is too often thought of as a mere passive process that follows contraction. However, many decades of advancements in our understanding of cardiac muscle relaxation have shown it is a highly complex and well-regulated process. In this review, we will discuss three distinct events that can limit the rate of cardiac muscle relaxation: the rate of intracellular calcium decline, the rate of thin-filament de-activation, and the rate of cross-bridge cycling. Each of these processes are directly impacted by a plethora of molecular events. In addition, these three processes interact with each other, further complicating our understanding of relaxation. Each of these processes is continuously modulated by the need to couple bodily oxygen demand to cardiac output by the major cardiac physiological regulators. Length-dependent activation, frequency-dependent activation, and beta-adrenergic regulation all directly and indirectly modulate calcium decline, thin-filament deactivation, and cross-bridge kinetics. We hope to convey our conclusion that cardiac muscle relaxation is a process of intricate checks and balances, and should not be thought of as a single rate-limiting step that is regulated at a single protein level. Cardiac muscle relaxation is a system level property that requires fundamental integration of three governing systems: intracellular calcium decline, thin filament deactivation, and cross-bridge cycling kinetics.

Ascribing novel functions to the sarcomeric protein, myosin binding protein H (MyBPH) in cardiac sarcomere contraction

Myosin binding protein H (MyBPH) is a protein of unknown function, which shares sequence and structural similarities with myosin binding protein C (cMyBPC), a protein frequently implicated in hypertrophic cardiomyopathy (HCM). Given the similarity between cMyBPC and MyBPH, we proposed that MyBPH, like cMyBPC, could be involved in HCM pathogenesis and we therefore sought to determine its function. We identified MyBPH-interacting proteins by using yeast two-hybrid (Y2H) analysis. The role of MyBPH and cMyBPC in cardiac cell contractility was analysed by measuring the planar cell surface area of differentiated H9c2 rat cardiomyocytes in response to β-adrenergic stress after siRNA knockdown of MyBPH and cMyBPC. Individual knockdown of either protein had no effect on cardiac contractility, while concurrent knockdowns reduced cardiac contractility. These proteins therefore functionally compensate for one another and are critical for cardiac contractility. We further show that both proteins co-localise with the autophagosomal membrane protein LC3, suggesting that both proteins are involved in autophagosomal membrane maturation processes. The results of this study ascribe novel functions to MyBPH, which may contribute to our understanding of its role in the sarcomere. This study provides evidence for a potential role of MyBPH in HCM, which warrants further investigation.

Pathomechanisms in heart failure: the contractile connection

Heart failure is a multi-factorial progressive disease in which eventually the contractile performance of the heart is insufficient to meet the demands of the body, even at rest. A distinction can be made on the basis of the cause of the disease in genetic and acquired heart failure and at the functional level between systolic and diastolic heart failure. Here the basic determinants of contractile function of myocardial cells will be reviewed and an attempt will be made to elucidate their role in the development of heart failure. The following topics are addressed: the tension generating capacity, passive tension, the rate of tension development, the rate of ATP utilisation, calcium sensitivity of tension development, phosphorylation of contractile proteins, length dependent activation and stretch activation. The reduction in contractile performance during systole can be attributed predominantly to a loss of cardiomyocytes (necrosis), myocyte disarray and a decrease in myofibrillar density all resulting in a reduction in the tension generating capacity and likely also to a mismatch between energy supply and demand of the myocardium. This leads to a decline in the ejection fraction of the heart. Diastolic dysfunction can be attributed to fibrosis and an increase in titin stiffness which result in an increase in stiffness of the ventricular wall and hampers the filling of the heart with blood during diastole. A large number of post translation modifications of regulatory sarcomeric proteins influence myocardial function by altering calcium sensitivity of tension development. It is still unclear whether in concert these influences are adaptive or maladaptive during the disease process.

Orientation of myosin binding protein C in the cardiac muscle sarcomere determined by domain-specific immuno-EM

Myosin binding protein C is a thick filament protein of vertebrate striated muscle. The cardiac isoform [cardiac myosin binding protein C (cMyBP-C)] is essential for normal cardiac function, and mutations in cMyBP-C cause cardiac muscle disease. The rod-shaped molecule is composed primarily of 11 immunoglobulin- or fibronectin-like domains and is located at nine sites, 43nm apart, in each half of the A-band. To understand how cMyBP-C functions, it is important to know its structural organization in the sarcomere, as this will affect its ability to interact with other sarcomeric proteins. Several models, in which cMyBP-C wraps around, extends radially from, or runs axially along the thick filament, have been proposed. Our goal was to define cMyBP-C orientation by determining the relative axial positions of different cMyBP-C domains. Immuno-electron microscopy was performed using mouse cardiac myofibrils labeled with antibodies specific to the N- and C-terminal domains and to the middle of cMyBP-C. Antibodies to all regions of the molecule, except the C-terminus, labeled at the same nine axial positions in each half A-band, consistent with a circumferential and/or radial rather than an axial orientation of the bulk of the molecule. The C-terminal antibody stripes were slightly displaced axially, demonstrating an axial orientation of the C-terminal three domains, with the C-terminus closer to the M-line. These results, combined with previous studies, suggest that the C-terminal domains of cMyBP-C run along the thick filament surface, while the N-terminus extends toward neighboring thin filaments. This organization provides a structural framework for understanding cMyBP-C's modulation of cardiac muscle contraction.

Phosphorylation of cMyBP-C affects contractile mechanisms in a site-specific manner

Cardiac myosin binding protein-C (cMyBP-C) is a cardiac-specific, thick-filament regulatory protein that is differentially phosphorylated at Ser(273), Ser(282), and Ser(302) by various kinases and modulates contraction. In this study, phosphorylation-site-specific effects of cMyBP-C on myocardial contractility and cross-bridge kinetics were studied by sinusoidal analysis in papillary and trabecular muscle fibers isolated from t/t (cMyBP-C-null) mice and in their counterparts in which cMyBP-C contains the ADA (Ala(273)-Asp(282)-Ala(302)), DAD (Asp(273)-Ala(282)-Asp(302)), and SAS (Ser(273)-Ala(282)-Ser(302)) mutations; the results were compared to those from mice expressing the wild-type (WT) transgene on the t/t background. Under standard activating conditions, DAD fibers showed significant decreases in tension (~50%), stiffness, the fast apparent rate constant 2πc, and its magnitude C, as well as its magnitude H, but an increase in the medium rate constant 2πb, with respect to WT. The t/t fibers showed a smaller drop in stiffness and a significant decrease in 2πc that can be explained by isoform shift of myosin heavy chain. In the pCa-tension study using the 8 mM phosphate (Pi) solution, there was hardly any difference in Ca(2+) sensitivity (pCa50) and cooperativity (nH) between the mutant and WT samples. However, in the solutions without Pi, DAD showed increased nH and slightly decreased pCa50. We infer from these observations that the nonphosphorylatable residue 282 combined with phosphomimetic residues Asp(273) and/or Asp(302) (in DAD) is detrimental to cardiomyocytes by lowering isometric tension and altering cross-bridge kinetics with decreased 2πc and increased 2πb. In contrast, a single change of residue 282 to nonphosphorylatable Ala (SAS), or to phosphomimetic Asps together with the changes of residues 273 and 302 to nonphosphorylatable Ala (ADA) causes minute changes in fiber mechanics.

Ceramide-mediated depression in cardiomyocyte contractility through PKC activation and modulation of myofilament protein phosphorylation

Although ceramide accumulation in the heart is considered a major factor in promoting apoptosis and cardiac disorders, including heart failure, lipotoxicity and ischemia-reperfusion injury, little is known about ceramide's role in mediating changes in contractility. In the present study, we measured the functional consequences of acute exposure of isolated field-stimulated adult rat cardiomyocytes to C6-ceramide. Exogenous ceramide treatment depressed the peak amplitude and the maximal velocity of shortening without altering intracellular calcium levels or kinetics. The inactive ceramide analog C6-dihydroceramide had no effect on myocyte shortening or [Ca(2+)]i transients. Experiments testing a potential role for C6-ceramide-mediated effects on activation of protein kinase C (PKC) demonstrated evidence for signaling through the calcium-independent isoform, PKCε. We employed 2-dimensional electrophoresis and anti-phospho-peptide antibodies to test whether treatment of the cardiomyocytes with C6-ceramide altered myocyte shortening via PKC-dependent phosphorylation of myofilament proteins. Compared to controls, myocytes treated with ceramide exhibited increased phosphorylation of myosin binding protein-C (cMyBP-C), specifically at Ser273 and Ser302, and troponin I (cTnI) at sites apart from Ser23/24, which could be attenuated with PKC inhibition. We conclude that the altered myofilament response to calcium resulting from multiple sites of PKC-dependent phosphorylation contributes to contractile dysfunction that is associated with cardiac diseases in which elevations in ceramides are present.

Single acute stress-induced progesterone and ovariectomy alter cardiomyocyte contractile function in female rats

Aim

To assess how ovarian-derived sex hormones (in particular progesterone) modify the effects of single acute stress on the mechanical and biochemical properties of left ventricular cardiomyocytes in the rat.

Methods

Non-ovariectomized (control, n = 8) and ovariectomized (OVX, n = 8) female rats were kept under normal conditions or were exposed to stress (control-S, n = 8 and OVX-S, n = 8). Serum progesterone levels were measured using a chemiluminescent immunoassay. Left ventricular myocardial samples were used for isometric force measurements and protein analysis. Ca²⁺-dependent active force (F_active), Ca²⁺-independent passive force (F_passive), and Ca²⁺-sensitivity of force production were determined in single, mechanically isolated, permeabilized cardiomyocytes. Stress- and ovariectomy-induced alterations in myofilament proteins (myosin-binding protein C [MyBP-C], troponin I [TnI], and titin) were analyzed by sodium dodecyl sulfate gel electrophoresis using protein and phosphoprotein stainings.

Results

Serum progesterone levels were significantly increased in stressed rats (control-S, 35.6 ± 4.8 ng/mL and OVX-S, 21.9 ± 4.0 ng/mL) compared to control (10 ± 2.9 ng/mL) and OVX (2.8 ± 0.5 ng/mL) groups. F_active was higher in the OVX groups (OVX, 25.9 ± 3.4 kN/m² and OVX-S, 26.3 ± 3.0 kN/m²) than in control groups (control, 16.4 ± 1.2 kN/m² and control-S, 14.4 ± 0.9 kN/m²). Regarding the potential molecular mechanisms, F_active correlated with MyBP-C phosphorylation, while myofilament Ca²⁺-sensitivity inversely correlated with serum progesterone levels when the mean values were plotted for all animal groups. F_passive was unaffected by any treatment.

Conclusion Stress increases ovary-independent synthesis and release of progesterone, which may regulate Ca²⁺-sensitivity of force production in left ventricular cardiomyocytes. Stress and female hormones differently alter Ca²⁺-dependent cardiomyocyte contractile force production, which may have pathophysiological importance during stress conditions affecting postmenopausal women.

Surviving the infarct: A profile of cardiac myosin binding protein-C pathogenicity, diagnostic utility, and proteomics in the ischemic myocardium

Cardiac myosin binding protein-C (cMyBP-C) is a regulatory protein of the contractile apparatus within the cardiac sarcomere. Ischemic injury to the heart during myocardial infarction (MI) results in the cleavage of cMyBP-C in a phosphorylation-dependent manner and release of an N-terminal fragment (C0C1f) into the circulation. C0C1f has been shown to be pathogenic within cardiac tissue, leading to the development of heart failure. Based on its high levels and early release into the circulation post-MI, C0C1f may serve as a novel biomarker for diagnosing MI more effectively than current clinically used biomarkers. Over time, circulating C0C1f could trigger an autoimmune response leading to myocarditis and progressive cardiac dysfunction. Given the importance of cMyBP-C phosphorylation state in the context of proteolytic cleavage and release into the circulation post-MI, understanding the posttranslational modifications (PTMs) of cMyBP-C would help in further elucidating the role of this protein in health and disease. Accordingly, recent studies have implemented the latest proteomics approaches to define the PTMs of cMyBP-C. The use of such proteomics assays may provide accurate quantitation of the levels of cMyBP-C in the circulation following MI, which could, in turn, demonstrate the efficacy of using plasma cMyBP-C as a cardiac-specific early biomarker of MI. In this review, we define the pathogenic and potential immunogenic effects of C0C1f on cardiac function in the post-MI heart. We also discuss the most advanced proteomics approaches now used to determine cMyBP-C PTMs with the aim of validating C0C1f as an early biomarker of MI.

The contribution of cardiac myosin binding protein-c Ser282 phosphorylation to the rate of force generation and in vivo cardiac contractility

Cardiac myosin binding protein-C phosphorylation plays an important role in modulating cardiac muscle function and accelerating contraction. It has been proposed that Ser282 phosphorylation may serve as a critical molecular switch that regulates the phosphorylation of neighbouring Ser273 and Ser302 residues, and thereby govern myofilament contractile acceleration in response to protein kinase A (PKA). Therefore, to determine the regulatory roles of Ser282 we generated a transgenic (TG) mouse model expressing cardiac myosin binding protein-C with a non-phosphorylatable Ser282 (i.e. serine to alanine substitution, TG(S282A)). Myofibrils isolated from TG(S282A) hearts displayed robust PKA-mediated phosphorylation of Ser273 and Ser302, and the increase in phosphorylation was identical to TG wild-type (TG(WT)) controls. No signs of pathological cardiac hypertrophy were detected in TG(S282A) hearts by either histological examination of cardiac sections or echocardiography. Baseline fractional shortening, ejection fraction, isovolumic relaxation time, rate of pressure development and rate of relaxation (τ) were unaltered in TG(S282A) mice. However, the increase in cardiac contractility as well as the acceleration of pressure development observed in response to β-adrenergic stimulation was attenuated in TG(S282A) mice. In agreement with our in vivo data, in vitro force measurements revealed that PKA-mediated acceleration of cross-bridge kinetics in TG(S282A) myocardium was significantly attenuated compared to TG(WT) myocardium. Taken together, our data suggest that while Ser282 phosphorylation does not regulate the phosphorylation of neighbouring Ser residues and basal cardiac function, full acceleration of cross-bridge kinetics and left ventricular pressure development cannot be achieved in its absence.

Titin-mediated control of cardiac myofibrillar function

According to the Frank-Starling relationship, ventricular pressure or stroke volume increases with end-diastolic volume. This is regulated, in large part, by the sarcomere length (SL) dependent changes in cardiac myofibrillar force, loaded shortening, and power. Consistent with this, both cardiac myofibrillar force and absolute power fall at shorter SL. However, when Ca(2+) activated force levels are matched between short and long SL (by increasing the activator [Ca(2+)]), short SL actually yields faster loaded shortening and greater peak normalized power output (PNPO). A potential mechanism for faster loaded shortening at short SL is that, at short SL, titin becomes less taut, which increases the flexibility of the cross-bridges, a process that may be mediated by titin's interactions with thick filament proteins. We propose a more slackened titin yields greater myosin head radial and azimuthal mobility and these flexible cross-bridges are more likely to maintain thin filament activation, which would allow more force-generating cross-bridges to work against a fixed load resulting in faster loaded shortening. We tested this idea by measuring SL-dependence of power at matched forces in rat skinned cardiac myocytes containing either N2B titin or a longer, more compliant N2BA titin. We predicted that, in N2BA titin containing cardiac myocytes, power-load curves would not be shifted upward at short SL compared to long SL (when force is matched). Consistent with this, peak normalized power was actually less at short SL versus long SL (at matched force) in N2BA-containing myocytes (N2BA titin: ΔPNPO (Short SL peak power minus long SL peak power)=-0.057±0.049 (n=5) versus N2B titin: ΔPNPO=+0.012±0.012 (n=5). These findings support a model whereby SL per se controls mechanical properties of cross-bridges and this process is mediated by titin. This myofibrillar mechanism may help sustain ventricular power during periods of low preloads, and perhaps a breakdown of this mechanism is involved in impaired function of failing hearts.

Contractile dysfunction in a mouse model expressing a heterozygous MYBPC3 mutation associated with hypertrophic cardiomyopathy

The etiology of hypertrophic cardiomyopathy (HCM) has been ascribed to mutations in genes encoding sarcomere proteins. In particular, mutations in MYBPC3, a gene which encodes cardiac myosin binding protein-C (cMyBP-C), have been implicated in over one third of HCM cases. Of these mutations, 70% are predicted to result in C'-truncated protein products, which are undetectable in tissue samples. Heterozygous carriers of these truncation mutations exhibit varying penetrance of HCM, with symptoms often occurring later in life. We hypothesize that heterozygous carriers of MYBPC3 mutations, while seemingly asymptomatic, have subtle functional impairments that precede the development of overt HCM. This study compared heterozygous (+/t) knock-in MYBPC3 truncation mutation mice with wild-type (+/+) littermates to determine whether functional alterations occur at the whole-heart or single-cell level before the onset of hypertrophy. The +/t mice show ∼40% reduction in MYBPC3 transcription, but no changes in cMyBP-C level, phosphorylation status, or cardiac morphology. Nonetheless, +/t mice show significantly decreased maximal force development at sarcomere lengths of 1.9 μm (+/t 68.5 ± 4.1 mN/mm(2) vs. +/+ 82.2 ± 3.2) and 2.3 μm (+/t 79.2 ± 3.1 mN/mm(2) vs. +/+ 95.5 ± 2.4). In addition, heterozygous mice show significant reductions in vivo in the early/after (E/A) (+/t 1.74 ± 0.12 vs. +/+ 2.58 ± 0.43) and E'/A' (+/t 1.18 ± 0.05 vs. +/+ 1.52 ± 0.15) ratios, indicating diastolic dysfunction. These results suggest that seemingly asymptomatic heterozygous MYBPC3 carriers do suffer impairments that may presage the onset of HCM.

Myosin-binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism

Myosin-binding protein C (MyBP-C) is an accessory protein of striated muscle thick filaments and a modulator of cardiac muscle contraction. Defects in the cardiac isoform, cMyBP-C, cause heart disease. cMyBP-C includes 11 Ig- and fibronectin-like domains and a cMyBP-C-specific motif. In vitro studies show that in addition to binding to the thick filament via its C-terminal region, cMyBP-C can also interact with actin via its N-terminal domains, modulating thin filament motility. Structural observations of F-actin decorated with N-terminal fragments of cMyBP-C suggest that cMyBP-C binds to actin close to the low Ca(2+) binding site of tropomyosin. This suggests that cMyBP-C might modulate thin filament activity by interfering with tropomyosin regulatory movements on actin. To determine directly whether cMyBP-C binding affects tropomyosin position, we have used electron microscopy and in vitro motility assays to study the structural and functional effects of N-terminal fragments binding to thin filaments. 3D reconstructions suggest that under low Ca(2+) conditions, cMyBP-C displaces tropomyosin toward its high Ca(2+) position, and that this movement corresponds to thin filament activation in the motility assay. At high Ca(2+), cMyBP-C had little effect on tropomyosin position and caused slowing of thin filament sliding. Unexpectedly, a shorter N-terminal fragment did not displace tropomyosin or activate the thin filament at low Ca(2+) but slowed thin filament sliding as much as the larger fragments. These results suggest that cMyBP-C may both modulate thin filament activity, by physically displacing tropomyosin from its low Ca(2+) position on actin, and govern contractile speed by an independent molecular mechanism.

Cardiac myosin-binding protein-C is a critical mediator of diastolic function

Diastolic dysfunction prominently contributes to heart failure with preserved ejection fraction (HFpEF). Owing partly to inadequate understanding, HFpEF does not have any effective treatments. Cardiac myosin-binding protein-C (cMyBP-C), a component of the thick filament of heart muscle that can modulate cross-bridge attachment/detachment cycling process by its phosphorylation status, appears to be involved in the diastolic dysfunction associated with HFpEF. In patients, cMyBP-C mutations are associated with diastolic dysfunction even in the absence of hypertrophy. cMyBP-C deletion mouse models recapitulate diastolic dysfunction despite in vitro evidence of uninhibited cross-bridge cycling. Reduced phosphorylation of cMyBP-C is also associated with diastolic dysfunction in patients. Mouse models of reduced cMyBP-C phosphorylation exhibit diastolic dysfunction while cMyBP-C phosphorylation mimetic mouse models show enhanced diastolic function. Thus, cMyBP-C phosphorylation mediates diastolic function. Experimental results of both cMyBP-C deletion and reduced cMyBP-C phosphorylation causing diastolic dysfunction suggest that cMyBP-C phosphorylation level modulates cross-bridge detachment rate in relation to ongoing attachment rate to mediate relaxation. Consequently, alteration in cMyBP-C regulation of cross-bridge detachment is a key mechanism that causes diastolic dysfunction. Regardless of the exact molecular mechanism, ample clinical and experimental data show that cMyBP-C is a critical mediator of diastolic function. Furthermore, targeting cMyBP-C phosphorylation holds potential as a future treatment for diastolic dysfunction.

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.

Novel control of cardiac myofilament response to calcium by S-glutathionylation at specific sites of myosin binding protein C

Our previous studies demonstrated a relation between glutathionylation of cardiac myosin binding protein C (cMyBP-C) and diastolic dysfunction in a hypertensive mouse model stressed by treatment with salt, deoxycorticosterone acetate, and unilateral nephrectomy. Although these results strongly indicated an important role for S-glutathionylation of myosin binding protein C as a modifier of myofilament function, indirect effects of other post-translational modifications may have occurred. Moreover, we did not determine the sites of thiol modification by glutathionylation. To address these issues, we developed an in vitro method to mimic the in situ S-glutathionylation of myofilament proteins and determined direct functional effects and sites of oxidative modification employing Western blotting and mass spectrometry. We induced glutathionylation in vitro by treatment of isolated myofibrils and detergent extracted fiber bundles (skinned fibers) with oxidized glutathione (GSSG). Immuno-blotting results revealed increased glutathionylation with GSSG treatment of a protein band around 140 kDa. Using tandem mass spectrometry, we identified the 140 kDa band as cMyBP-C and determined the sites of glutathionylation to be at cysteines 655, 479, and 627. Determination of the relation between Ca²⁺-activation of myofibrillar acto-myosin ATPase rate demonstrated an increased Ca²⁺-sensitivity induced by the S-glutathionylation. Force generating skinned fiber bundles also showed an increase in Ca-sensitivity when treated with oxidized glutathione, which was reversed with the reducing agent, dithiothreitol (DTT). Our data demonstrate that a specific and direct effect of S-glutathionylation of myosin binding protein C is a significant increase in myofilament Ca²⁺-sensitivity. Our data also provide new insights into the functional significance of oxidative modification of myosin binding protein C and the potential role of domains not previously considered to be functionally significant as controllers of myofilament Ca²⁺-responsiveness and dynamics.

Cardiac myosin-binding protein C: hypertrophic cardiomyopathy mutations and structure-function relationships

Cardiac myosin-binding protein C (cMyBP-C) research has been characterized by two waves. Initial interest was piqued by its discovery in 1973 as a contaminant of myosin preparations from skeletal muscle. The second wave started in 1995 by the discovery that mutations in the gene encoding cMyBP-C cause hypertrophic cardiomyopathy (HCM). In this review, we will address what is known of cMyBP-C's role as a regulator of contraction as well as its role in HCM.