Published online before print November 5, 2007, doi: 10.1161/CIRCULATIONAHA.107.706523
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(Circulation. 2007;116:2399-2408.)
© 2007 American Heart Association, Inc.
Molecular Cardiology |
Control of In Vivo Contraction/Relaxation Kinetics by Myosin Binding Protein C
Protein Kinase A Phosphorylation–Dependent and –Independent Regulation
From the Division of Cardiology, Department of Medicine (T.N., E.T., J.O.M., D.A.K.), Johns Hopkins Medical Institutions, Baltimore, Md; the Department of Pediatrics, Division of Molecular Cardiovascular Biology (S.S., J.R.), Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio; and the Department of Genetics (J.G.S.), Harvard Medical School, Boston, Mass.
Correspondence to Dr David A. Kass, Division of Cardiology, Johns Hopkins Medical Institutions, Ross Research Building, Room 835, 720 Rutland Ave, Baltimore, MD 21205. E-mail dkass{at}jhmi.edu
Received March 29, 2007; accepted September 6, 2007.
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Abstract |
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Background— Cardiac myosin binding protein-C (cMyBP-C) is a thick-filament protein whose presence and phosphorylation by protein kinase A (PKA) regulates cross-bridge formation and kinetics in isolated myocardium. We tested the influence of cMyBP-C and its PKA-phosphorylation on contraction/relaxation kinetics in intact hearts and revealed its essential role in several classic properties of cardiac function.
Methods and Results— Comprehensive in situ cardiac pressure–volume analysis was performed in mice harboring a truncation mutation of cMyBP-C (cMyBP-C(t/t)) that resulted in nondetectable protein versus hearts re-expressing solely wild-type (cMyBP-CWT:(t/t)) or mutated protein in which known PKA-phosphorylation sites were constitutively suppressed (cMyBP-CAllP-:(t/t)). Hearts lacking cMyBP-C had faster early systolic activation, which then terminated prematurely, limiting ejection. Systole remained short at faster heart rates; thus, cMyBP-C(t/t) hearts displayed minimal rate-dependent decline in diastolic time and cardiac preload. Furthermore, prolongation of pressure relaxation by afterload was markedly blunted in cMyBP-C(t/t) hearts. All 3 properties were similarly restored to normal in cMyBP-CWT:(t/t) and cMyBP-CAllP-:(t/t) hearts, which supports independence of PKA-phosphorylation. However, the dependence of peak rate of pressure rise on preload was specifically depressed in cMyBP-CAllP-:(t/t) hearts, whereas cMyBP-C(t/t) and cMyBP-CAllP-:(t/t) hearts had similar blunted adrenergic and rate-dependent contractile reserve, which supports linkage of these behaviors to PKA-cMyBP-C modification.
Conclusions— cMyBP-C is essential for major properties of cardiac function, including sustaining systole during ejection, the heart-rate dependence of the diastolic time period, and relaxation delay from increased arterial afterload. These are independent of its phosphorylation by PKA, which more specifically modulates early pressure rise rate and adrenergic/heart rate reserve.
Key Words: diastole • heart • myocardial contraction • sarcomeres
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Introduction |
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The ability of the intact mammalian heart to rapidly develop systolic pressure, eject blood, and perform external work are all potently influenced by net muscle stretch at end-diastole (ie, preload volume). Two key factors that influence preload are heart rate, which modulates the time available for diastole, and arterial load, which can delay relaxation before filling commences. Furthermore, in order to effectively eject, the heart must sustain systole well after the aortic valve opens despite falling sarcomere lengths coupled to muscle shortening.1 Although such fundamental properties of cardiac contraction have been recognized for over a century, the molecular mechanisms that underlie them have remained largely unknown.
Clinical Perspective p 2408
One sarcomeric protein that has recently taken center stage as a potential key regulator of contraction kinetics is cardiac myosin binding protein C (cMyBP-C).2 cMyBP-C is a large thick-filament accessory protein localized in the A-band of the sarcomere and binds actin, myosin, and titin.2–6 cMyBP-C mutations are a major and common cause of genetic hypertrophic cardiomyopathy,3,7–10 and changes in its phosphorylation by protein kinase A (PKA) may influence contractility in heart failure and postischemic syndromes.11,12 Isolated myocyte and muscle studies suggest cMyBP-C normally functions to constrain the kinetics and extent of cross-bridge formation.13–16 Myocytes lacking cMyBP-C display faster shortening and force redevelopment when muscle is suddenly stretched.1,14,17,18 cMyBP-C also has unique phosphorylation sites6,19,20 that alter its function. In particular, PKA phosphorylates Ser-273, Ser-282, and Ser-302,21 releasing cMyBP-C from its binding to the S2 site on myosin that lies proximal to the lever arm and links the motor head to the thick filament backbone,3,6,15,22 and enhancing actin binding.23 This results in acceleration and enhancement of crossbridge-dependent force much like that observed in muscle lacking cMyBP-C.24
Despite its potential importance, very little is known about how cMyBP-C and/or its phosphorylation by PKA modulate intact heart contraction kinetics. Genetically modified mice lacking cMyBP-C develop dilated cardiomyopathy with a reduced ejection fraction and end-systolic elastance.25–28 Intriguingly, however, the peak rate of pressure rise (dP/dtmax), a property that almost always decreases in failing hearts, is preserved in those lacking cMyBP-C.25–28 One mechanism appears related to an unusual abbreviation of systole in such hearts,26 which limits ejection. The impact of this behavior on other key features of heart contraction remains unexplored. Furthermore, as hearts re-expressing PKA-phosphorylation–deficient cMyBP-C also dilate and develop functional depression,12 posttranslational regulation of cMyBP-C might also influence activation kinetics.2,24 Determining its role in vivo is important for understanding basic cardiac physiology and the potential changes that could accompany cMyBP-C mutations.
To test the role of cMyBP-C and/or its PKA-phosphorylation in fundamental properties of intact heart contraction/relaxation kinetics, comprehensive pressure–volume (PV) analyses were performed in mice lacking cMyBP-C and in those re-expressing wild-type (WT) protein or PKA-phosphorylation–deficient (AllP–) protein. We show that cMyBP-C underlies shortening of diastole at faster heart rates, a property that limits cardiac filling with tachycardia. We confirm its central role in sustaining cardiac activation during ejection, and we reveal a novel potent influence of cMyBP-C on prolongation of pressure relaxation at higher afterloads. These regulations are independent of PKA phosphorylation, whereas the latter influences the preload dependence of early pressure rise rates as well as adrenergic and rate-dependent contractile reserve.
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Methods |
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Mouse Models
Four groups of mice (either sex, age 20 to 24 weeks) were studied: nontransgenic (NTG) animals, mice harboring a truncation mutation (cMyBP-C(t/t)) that results in no detectable expressed protein, and the same mutant strain in which WT (cMyBP-CWT:(t/t)) or AllP– (cMyBP-CAllP-:(t/t)) protein was re-expressed at 40% of the levels for normal native protein. The generation of these latter hybrid animals and assessment of their cardiac morphology and resting global function have been previously described.12 All of the data presented in the present study were derived de novo, and the protocols were approved by the Animal Care and Use Committee of Johns Hopkins University.
In Vivo Left Ventricular Function Studies
Left ventricular (LV) function was determined in in situ hearts with the use of a miniature PV catheter (SPR-839; Millar Instruments Inc, Houston, Tex).29 Mice were anesthetized (urethane 0.8 to 1 g/kg; etomidate 20 to 25 mg/kg; morphine 1 to 2 mg/kg; ip administration) and ventilated via tracheostomy (10 µL/g tidal volume, 130 breaths/min). The PV catheter was inserted via the LV apex and advanced until its distal tip lay in the aortic root. The volume signal was calibrated to independently measured flow (Transonics Systems Inc, Ithica, NY) and the offset determined by hypertonic saline method.29 An external jugular vein was cannulated to achieve volume expansion (12.5% human albumin) and infusion of isoproterenol. PV relations were measured during transient obstruction of the inferior vena cava. These data yielded the end-systolic PV relationship from which a slope (ie, end-systolic elastance (Ees)) and volume intercept (Vo) were determined.29 Chamber activation and relaxation kinetics were assessed by the time-varying elastance (stiffening): E(t)=P(t)/(V(t)–Vo), which was further normalized to its peak value at end-systole (En (t)=E(t)/Ees). Resting chamber function was determined as described29 and included heart rate, end-systolic and end-diastolic pressures and volumes, peak rates of pressure rise and decline (dP/dtmax and dP/dtmin), maximal LV power index (PWRmax/end-diastolic volume), and relaxation time constant (logistic model30). The dependence of stroke volume, stroke work, and dP/dtmax on cardiac preload was also determined. All abbreviations are shown in Table 1.
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To test the influence of heart rate on En (t), resting rate was lowered without altering LV function by the If blocker (ULFS-49, 15 to 20 mg/kg ip),29 and then hearts atrially paced via an esophageal lead. Steady-state pacing at 400 to 700 minutes–1 was performed. En (t) relations among all groups were compared at 500 minutes–1. The influence of afterload on relaxation was determined by measuring PV data during graded proximal aortic occlusion. Modulation by β-adrenergic stimulation (PKA activation) was tested with isoproterenol (20 to 60 ng/kg per min) at a fixed pacing rate of 500 minutes–1.
Statistical Analysis
Data are presented as mean±SEM. Between-group comparisons were performed by one-way ANOVA with a Tukey multiple comparisons test. The dependence of En (t) kinetics on heart rate and β-adrenergic stimulation was assessed by analysis of covariance, with the genotype serving as the grouping factor. Adrenergic reserve was assessed at the maximal isoproterenol dose (60 ng · kg–1 · min–1) by paired t test. P<0.05 were considered significant.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
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Results |
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Altered Activation Kinetics in cMyBP-C(t/t) but Not cMyBP-CAllP-:(t/t) Hearts
Figure 1A depicts how chamber activation kinetics was assessed from amplitude-normalized time-varying elastance relations (En(t)). For each curve, the peak rate of initial stiffening (dEn/dtmax), systolic time period (time to end-systole (tes)), and ratio of elastance at the onset of ejection relative to its peak value at end-systole (Eoe/Ees), were determined. Consistent with our prior report,26 hearts lacking cMyBP-C had a rapid initial rise in En(t), but this peaked shortly after ejection started and then decreased (Figure 1B). Thus, nearly 75% of total stiffening occurred before ejection, with little further change after the aortic valve opened, which led to a reduced tes (Figure 1C). As shown here for the first time, these properties were fully normalized by re-expressing either WT or AllP– protein (even at 40% levels). Figure 1D provides absolute Ees, which was lower in cMyBP-C(t/t) hearts, as previously reported.25–27 This was restored to normal not only by re-expression of WT protein, but intriguingly also by AllP– protein.
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To test whether PKA phosphorylation influenced the kinetics of ventricular elastance, En(t) curves were examined before and after isoproterenol stimulation, with heart rate maintained constant at 500 minutes–1 (Figure 2). Because the amplitudes of these relations are normalized, absolute contractility change is not depicted, but the time-course of activation is. Isoproterenol did not alter the primary kinetics of En(t) in any of the models, (although the En(t) wave-shape changed slightly in NTG mice, timing kinetics remained unaltered; Figure 2, right). Thus, the differences in En(t) kinetics between cMyBP-C(t/t) and the other genotypes were unrelated to PKA phosphorylation. These results further indicate that even if alternative PKA sites exist on cMyBP-C, they do not appear to regulate these properties.
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cMyBP-C Controls Rate-Dependent Shortening of the Diastolic Time Period
Systolic abbreviation in hearts lacking cMyBP-C could influence the heart rate dependence of diastolic filling time. This is because, at faster rates, systole is relatively prolonged and thus diastole is shortened, which limits filling time and thereby reduces end-diastolic volume (EDV). This was observed in NTG hearts, with diastolic period shortening by 14% and EDV decreasing by 25% (Figure 3A to 3C). However, in cMyBP-C(t/t) hearts, relative durations of systole and diastole were little affected by heart rate, and EDV was less reduced. Re-expression with either WT or AllP– protein normalized rate-dependent shortening of diastole and preload reduction with tachycardia, consistent with the restoration of En (t) kinetics. Figure 3D shows the relation between change in EDV and normalized diastolic time period at varying heart rates. Data from all groups fell along a single relation, with cMyBP-C mice having little change in either parameter.
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Another feature highlighted by the data shown in Figure 3A is a rate-dependent change in end-systolic volume (ESV), which reflects contractile reserve rather than diastolic filling.29 In both NTG and cMyBP-C(t/t) hearts, ESV remained low with faster rates, but it increased significantly in both cMyBP-C(t/t) and cMyBP-CAllP-:(t/t)hearts (P<0.01 by ANCOVA), which reflected a blunted systolic force-frequency response. This was further confirmed by load-independent, LV power index analysis, which displayed a blunted rate response in both (t/t) and AllP– hearts compared with controls (P<0.01; data not shown). Thus, lack of PKA-phosphorylation of cMyBP-C depressed the rate-dependent contractile reserve in a manner similar to hearts that lack the protein entirely.
cMyBP-C Regulates Afterload Dependence of Relaxation
Increasing late-systolic afterload prolongs the time course of subsequent pressure relaxation.31 Given the early decline of systolic activation of cMyBP-C(t/t) hearts, we postulated that afterload-relaxation dependence may also be diminished. Cardiac afterload was indexed by effective arterial elastance (Ea = end-systolic pressure/stroke volume) measured beat-to-beat during acute aortic occlusion (Figure 4A). Higher afterload led to delayed relaxation in NTG hearts, but this was quite blunted in cMyBP-C(t/t) hearts (Figure 4B). Re-expression of either WT or AllP– protein restored the afterload-relaxation dependence to normal.
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) and Ea at baseline. The dependence for mice lacking cMyBP-C was much lower but very similar for the other 3 models. *P<0.05 versus all other groups by ANOVA. C, Reduction in afterload-relaxation dependence with stimulation by ISO. The relations appear to be rotated clockwise and all display a reduction in slope. Mice lacking cMyBP-C or with the AllP– protein had even less dependence than the other groups (
P<0.05 versus other 2 groups by ANOVA).Prior studies have shown that afterload-relaxation dependence is also sensitive to PKA phosphorylation through an interaction with troponin I (TnI), decreasing with stimulation,32 and increasing if PKA-phosphorylation of TnI is prevented.33 This may relate to the impact of TnI phosphorylation on reducing myofilament calcium sensitivity. Muscle lacking cMyBP-C reportedly shows little change in calcium-sensitivity when stimulated by PKA, which suggests its own role or interaction with TnI modulation.34 Given these data, we performed afterload-relaxation analysis in hearts administered isoproterenol (Figure 4C). In all groups, the load-relaxation relations rotated clockwise (reduced sensitivity), which supports an independent effect of PKA stimulation. Data for cMyBP-C(t/t) hearts rotated similarly as for NTG and cMyBP-CWT:(t/t) hearts, whereas cMyBP-CAllP-:(t/t) hearts exhibited an even greater shift (P<0.01 by ANCOVA).
cMyBP-C PKA-Phosphorylation Modulates Early Pressure Rise and Adrenergic Reserve
Although En (t) kinetics and behaviors related to it were similarly restored to normal by re-expression of WT or AllP– protein, other features in cMyBP-CAllP-:(t/t) remained abnormal. Figure 5A shows sample PV loops generated over a range of preloads, and Figure 5B summarizes analysis of these data. Additional parameters are provided in Table 2. cMyBP-CAllP-:(t/t) hearts were not normal, but had an ejection fraction, ESV, and dP/dtmin that were intermediate between controls and cMyBP-C(t/t), and cardiac mass that was similarly increased as cMyBP-C(t/t). However, as with Ees (Figure 1D), re-expression of AllP– protein restored systolic function indexed by relations between end-ejection parameters and preload (ie, Frank-Starling, and stroke work-EDV), whereas these were depressed in cMyBP-C(t/t) hearts. In contrast, the preload dependence of dP/dtmax declined only in cMyBP-CAllP-:(t/t) hearts (Figure 5B), which suggests that PKA-phosphorylation of cMyBP-C played an important role in isovolumic-phase contractile behavior (early systole).
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As previously noted, rate-dependent contractile reserve was depressed in both cMyBP-C(t/t) and cMyBP-CAllP-:(t/t) hearts, and this was also true of β-adrenergic reserve. Figure 6 shows results for 3 load-independent indexes of contractile function at baseline and after isoproterenol stimulation. Little response was observed in cMyBP-C(t/t) and cMyBP-CAllP-:(t/t), whereas the 2 control groups showed similar positive increases. The key difference between this analysis and the En(t) waveforms shown in Figure 2 (which appear unaltered by isoproterenol) is that the latter were amplitude-normalized to facilitate analysis of wave-shape, whereas Figure 6 shows absolute contractility. It is notable that cMyBP-CWT:(t/t) mice showed recovery of adrenergic (and heart rate) contractile reserve despite expression levels of 40% normal. Thus, lack of full expression cannot explain the abnormalities observed in cMyBP-CAllP-:(t/t) mice.
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Discussion |
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The present study reveals several roles of cMyBP-C in the intact heart and clarifies the importance of PKA-phosphorylation to this regulation. First, cMyBP-C slows early contraction but then allows systole to be sustained and reach higher levels of stiffening so the heart can effectively eject. The present data both confirms our prior findings regarding this behavior26 and extends them by showing that abnormal chamber elastance kinetics in hearts lacking cMyBP-C are unaltered by PKA-phosphorylation or stimulation frequency. This latter finding leads to a second novel role of cMyBP-C, its contribution to relative shortening of diastole at faster heart rates, and thus to the well known decline in cardiac filling with tachycardia. Last, we show that cMyBP-C is important to coupling higher ventricular late-systolic afterload with delay in relaxation. These properties all appear to be independent of PKA-phosphorylation of cMyBP-C. In contrast, the latter influences the dependence of early systolic pressure rise on chamber preload, and its absence leads to marked suppression of adrenergic and rate-dependent contractile reserve.
Comparisons of In Vitro and In Vivo Influences of cMyBP-C
In isolated systems, loaded shortening and force development occur more quickly if cMyBP-C is absent or is modified/inhibited to prevent thick-filament binding.6,14,15,26 This cross-bridge constraint effect is proposed to stem from binding of regulatory domains C0 to C4 with actin and myosin S2,6,15,23,35 and possibly to the formation of a trimeric collar-like structure around the myosin rod, with C5 to C10 domains interacting in a staggered manner.10 The former is modulated by PKA-phosphorylation, which reduces S2 binding and may enhance actin binding.6,21,23 Tension26 and myocyte calcium transient decay27 are also prolonged in the absence of cMyBP-C, which may be a direct effect or the result of concomitant declines in phospholamban and SERCA2a expression observed in cMyBP-C knockout models.12 In addition, PKA-dependent reduction of myofilament Ca2+ sensitivity is reportedly blunted in muscle lacking cMyBP-C,34 whereas modest basal effects on Ca2+-sensitivity have varied among studies.28,34
In the intact heart, the behavior most consistent with faster muscle kinetics is the rapid initial rise of En (t), which likely underlies preservation of dP/dtmax despite global systolic dysfunction. Consistent with isolated tissues, basal relaxation decay is prolonged. Some other findings, however, are less easily linked between isolated tissue and intact heart data, which suggests that physiological conditions and loads may be central to the role of cMyBP-C. For example, basal and isoproterenol-stimulated shortening in isolated myocytes from WT and cMyBP-C–/- are similar,27 whereas intact heart contractility and adrenergic reserve are markedly blunted. The afterload-relaxation dependence was blunted by PKA stimulation in both cMyBP-C(t/t) and cMyBP-CAllP-:(t/t) as in normal muscle, so its relation to altered PKA-Ca sensitivity modulation is unclear.
Another difference lies in the PKA-cMyBP-C modulation of in vitro versus in vivo contraction kinetics. PKA enhances and accelerates force redevelopment of skinned myocardium subjected to acute length stretch,13,17,24 but this does not occur in muscle lacking cMyBP-C.13 This has been suggested to underlie the early rapid and abbreviated time-varying elastance in vivo,2,13,16,24,26 yet the present data showing that these are normalized in cMyBP-CAllP-(t/t) makes the link between these behaviors less clear. We propose stretch-activation behavior may rather correspond to the preload dependence of early systolic kinetics (ie, dP/dtmax),36 as this was depressed in cMyBP-CAllP-:(t/t). Cardiac inotropes can differentially impact early (eg, dP/dtmax) versus late (eg, Ees) ejection-phase contractile parameters, with those working via PKA stimulation enhancing both, and those independent of PKA often having greater effects on the latter.37 Our results support a greater role of PKA-phosphorylation of cMyBP-C in releasing its constraint over early contraction kinetics. This would also explain why dP/dtmax-EDV relations were reduced more in cMyBP-CAllP-:(t/t) hearts than those lacking cMyBP-C entirely. The former reflects the presence of a protein fixed in a more constraining mode, whereas the latter lacks this inhibitory effect (regardless of PKA stimulation) because the protein is absent.
cMyBP-C and Intact Heart Contraction Kinetics
Recognition of a dependence of diastolic time intervals and chamber filling on heart rate dates back over a century to the seminal work of Porter,38 Henderson,39 and Katz,40 who showed that systolic time was little altered whereas diastole shortened at faster rates, and this was accompanied by a decline in preload. cMyBP-C appears to play a key role in this behavior. At faster heart rates, systole is relatively prolonged so ejection can better occur, but this comes at a cost to diastolic filling time. In normal hearts, this results in a flat response in net cardiac output at higher heart rates.29,41 It remains unknown whether any of the reported human mutations in cMyBP-C alter this dependence, but it would be intriguing to investigate this.
Increased afterload particularly in late systole delays pressure relaxation42,43 and may contribute to ventricular diastolic dysfunction.31,44 As previously noted, this dependence is blunted by PKA-phosphorylation of TnI.32,33 Here, we revealed a non-PKA–dependent influence of cMyBP-C on this behavior, which may relate to the role of protein in constraining cross-bridge kinetics and sustaining systole. cMyBP-C(t/t) mice had prolonged basal relaxation, and one could speculate this limited any further delay at higher afterloads. However, load-dependent relaxation is often enhanced in heart failure despite marked basal delay,31 and cMyBP-C–/- hearts exposed to sustained pressure-overload have even longer delays,27 which suggests this to be unlikely.
In Vivo Role of cMyBP-C Phosphorylation
Beyond its influence on early systolic kinetics, PKA-phosphorylation of cMyBP-C appears important to contractile reserve. Studies in failing and postischemic hearts reveal a decline in cMyBP-C phosphorylation,11,12 whereas cMyBP-CAllP-:(t/t) hearts are enlarged and display abnormal fetal gene expression much like those that lack cMyBP-C. Whether these changes purely reflect the loss of cMyBP-C phosphorylation at the targeted sites remains unknown, as such mutations might trigger other unknown changes as well. As shown here, one result is a decline in both β-adrenergic and rate-dependent systolic reserve, the former consistent with earlier reports.12 In studies of myocardial stunning, 3 novel phosphorylation sites at S290, S313, and S331 were recently identified,19 though how they are regulated and by what kinase remains unknown. Though these sites were not mutated in cMyBP-CAllP-:(t/t) hearts, the present finding that En (t) curves were unaltered by PKA stimulation regardless of genotype indicates they are unlikely to be involved in the behavior.
Sadayappan et al12 first reported hemodynamic data for all 4 models used in the present study, though this was limited to echocardiographic volumes, ejection fraction, and chamber pressures. On the basis of this analysis, both cMyBP-CAllP-:(t/t) and cMyBP-C(t/t) appeared to have similar cardiac dysfunction. The present analysis shows that the underlying contractile behavior is more complex, and that in fact substantial differences between the models exist. The disparities in chamber volumes and ejection fraction we found between the models are also suggested in the earlier report,12 though they had not reached statistical significance for perhaps methodological reasons.
Conclusion
In conclusion, we have revealed a novel fundamental role of cMyBP-C in regulating basal and rate-dependent systolic kinetics and afterload-dependent diastolic relaxation rate in the intact heart. These properties appear to be independent of cMyBP-C phosphorylation by PKA, but rather suggest that binding interactions between cMyBP-C and other sarcomere proteins likely underlies its constraint on contraction/relaxation. PKA regulation, however, plays a central role in both early contraction kinetics and in adrenergic and rate-dependent contractile reserve. Future studies in animals with cMyBP-C mutations at sites influencing such non-PKA–dependent binding are needed to clarify the physiology further.
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Acknowledgments |
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Sources of Funding
Dr Kass is supported by National Institute of Health grant PO1 HL-59408, the Abraham and Virginia Weiss Professorship, the Belfer Research Laboratory; Dr Nagayama is supported by a grant from Daiichi-Sankyo Inc; and Drs Kass and Mudd are supported by the grant T32-HL-07227-31.
Disclosures
None.
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[Abstract/Free Full Text]
CLINICAL PERSPECTIVE
Cardiac myosin binding protein C (cMyBP-C) is a regulatory sarcomeric protein that links with actin and myosin and is commonly mutated in genetic hypertrophic cardiomyopathy. Growing evidence suggests it plays a key role in modulating heart contraction. cMyBP-C constrains rates of cross-bridge formation, slowing things down but also allowing force to be sustained longer during systole. Hearts that lack cMyBP-C start contraction normally, but then systole is abbreviated and ejection is reduced. cMyBP-C is also modified by phosphorylation, notably by protein kinase A (stimulated by adrenergic tone), and this is proposed to enhance contractility. Reduced phosphorylation occurs in ischemic heart disease and cardiac failure. Using mouse mutation models that either lacked cMyBP-C, or expressed protein that could or could not be phosphorylated by protein kinase A, we assessed its regulation of intact heart function. cMyBP-C was central to relative shortening of the diastolic time period at faster heart rates and thus rate-dependence of ventricular filling. cMyBP-C also contributed to delayed cardiac relaxation at higher afterloads and sustaining systole, and none of these properties depended on cMyBP-C phosphorylation. Phosphorylation of cMyBP-C was important for heart rate, adrenergic contractile reserve, and the dependence of early rapid pressure rise on cardiac filling. These data provide a molecular mechanism for several classic properties of cardiac function and suggest how mutations in cMyBP-C might impact contraction. Changes that modify structural/protein interactions or cMyBP-C phosphorylation will likely result in different phenotypes, and the present findings may help in the diagnostic interpretation of such syndromes.
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