Following activation by G-protein-coupled receptor agonists, protein kinase C (PKC) modulates cardiac myocyte function by phosphorylation of intracellular targets including myofilament proteins cardiac troponin I (cTnI) and cardiac myosin binding protein C (cMyBP-C). Since PKC phosphorylation has been shown to decrease myofibril ATPase activity, we hypothesized that PKC phosphorylation of cTnI and cMyBP-C will lower myocyte power output and, in addition, attenuate the elevation in power in response to protein kinase A (PKA)-mediated phosphorylation. We compared isometric force and power generating capacity of rat skinned cardiac myocytes before and after treatment with the catalytic subunit of PKC. PKC increased phosphorylation levels of cMyBP-C and cTnI and decreased both maximal Ca2+ activated force and Ca2+ sensitivity of force. Moreover, during submaximal Ca2+ activations PKC decreased power output by 62 %, which arose from both the fall in force and slower loaded shortening velocities since depressed power persisted even when force levels were matched before and after PKC. In addition, PKC blunted the phosphorylation of cTnI by PKA, reduced PKA-induced spontaneous oscillatory contractions, and diminished PKA-mediated elevations in myocyte power. To test whether altered thin filament function plays an essential role in these contractile changes we investigated the effects of chronic cTnI pseudo-phosphorylation on myofilament function using myocyte preparations from transgenic animals in which either only PKA phosphorylation sites (Ser-23/Ser-24) (PP) or both PKA and PKC phosphorylation sites (Ser-23/Ser-24/Ser-43/Ser-45/T-144) (All-P) were replaced with aspartic acid. Cardiac myocytes from All-P transgenic mice exhibited reductions in maximal force, Ca2+ sensitivity of force, and power. Similarly diminished power generating capacity was observed in hearts from All-P mice as determined by in situ pressure–volume measurements. These results imply that PKC-mediated phosphorylation of cTnI plays a dominant role in depressing contractility, and, thus, increased PKC isozyme activity may contribute to maladaptive behavior exhibited during the progression to heart failure.
PKC; PKA; Cardiac myocyte; Cardiac troponin I; Power output
Gravin, an A-kinase anchoring protein, targets protein kinase A (PKA), protein kinase C (PKC), calcineurin and other signaling molecules to the beta2-adrenergic receptor (β2-AR). Gravin mediates desensitization/resensitization of the receptor by facilitating its phosphorylation by PKA and PKC. The role of gravin in β-AR mediated regulation of cardiac function is unclear. The purpose of this study was to determine the effect of acute β-AR stimulation on cardiac contractility in mice lacking functional gravin. Using echocardiographic analysis, we observed that contractility parameters such as left ventricular fractional shortening and ejection fraction were increased in gravin mutant (gravin-t/t) animals lacking functional protein compared to wild-type (WT) animals both at baseline and following acute isoproterenol (ISO) administration. In isolated gravin-t/t cardiomyocytes, we observed increased cell shortening fraction and decreased intracellular Ca2+ in response to 1 µmol/L ISO stimulation. These physiological responses occurred in the presence of decreased β2-AR phosphorylation in gravin-t/t hearts, where PKA-dependent β2-AR phosphorylation has been shown to lead to receptor desensitization. cAMP production, PKA activity and phosphorylation of phospholamban and troponin I was comparable in WT and gravin-t/t hearts both with and without ISO stimulation. However, cardiac myosin binding protein C (cMyBPC) phosphorylation site at position 273 was significantly increased in gravin-t/t versus WT hearts, in the absence of ISO. Additionally, the cardioprotective heat shock protein 20 (Hsp20) was significantly more phosphorylated in gravin-t/t versus WT hearts, in response to ISO. Our results suggest that disruption of gravin’s scaffold mediated signaling is able to increase baseline cardiac function as well as to augment contractility in response to acute β-AR stimulation by decreasing β2-AR phosphorylation and thus attenuating receptor desensitization and perhaps by altering PKA localization to increase the phosphorylation of cMyBPC and the nonclassical PKA substrate Hsp20.
Myosin binding protein-C (MyBP-C) exists in three major isoforms: slow skeletal, fast skeletal, and cardiac. While cardiac MyBP-C (cMyBP-C) expression is restricted to the heart in the adult, it is transiently expressed in neonatal stages of some skeletal muscles. However, it is unclear whether this expression is necessary for the proper development and function of skeletal muscle. Our aim was to determine whether the absence of cMyBP-C alters the structure, function, or MyBP-C isoform expression in adult skeletal muscle using a cMyBP-C null mouse model (cMyBP-C(t/t)). Slow MyBP-C was expressed in both slow and fast skeletal muscles, whereas fast MyBP-C was mostly restricted to fast skeletal muscles. Expression of these isoforms was unaffected in skeletal muscle from cMyBP-C(t/t) mice. Slow and fast skeletal muscles in cMyBP-C(t/t) mice showed no histological or ultrastructural changes in comparison to the wild-type control. In addition, slow muscle twitch, tetanus tension, and susceptibility to injury were all similar to the wild-type controls. Interestingly, fMyBP-C expression was significantly increased in the cMyBP-C(t/t) hearts undergoing severe dilated cardiomyopathy, though this does not seem to prevent dysfunction. Additionally, expression of both slow and fast isoforms was increased in myopathic skeletal muscles. Our data demonstrate that i) MyBP-C isoforms are differentially regulated in both cardiac and skeletal muscles, ii) cMyBP-C is dispensable for the development of skeletal muscle with no functional or structural consequences in the adult myocyte, and iii) skeletal isoforms can transcomplement in the heart in the absence of cMyBP-C.
Inflammation plays a critical role in adverse cardiac remodeling and heart failure. Therefore, approaches geared towards inhibiting inflammation may provide therapeutic benefits. We tested the hypothesis that genetic deletion of interleukin-10 (IL10), a potent anti-inflammatory cytokine, exacerbates pressure-overload induced adverse cardiac remodeling and hypertrophy and that IL10 therapy inhibits this pathology.
Methods and Results
Cardiac hypertrophy was induced in Wild-type (WT) and IL10-knockout (KO) mice by isoproterenol (ISO) infusion. ISO-induced left ventricular (LV) dysfunction and hypertrophic remodeling, including fibrosis and fetal gene expression, were further exaggerated in KO mice compared to WT. Systemic recombinant mouse IL10 administration markedly improved LV function and not only inhibited but also reversed ISO-induced cardiac remodeling. Intriguingly, very similar cardio-protective response of IL10 was found in transverse aortic constriction (TAC)-induced hypertrophy and heart failure model. In neonatal rat ventricular myocytes (NRCM) and H9c2 myoblasts, ISO activated NFκB while it inhibited STAT3 phosphorylation. Interestingly, IL10 suppressed ISO-induced NFκB activation and attenuated STAT3 inhibition. Moreover, pharmacological and genetic inhibition of STAT3 reversed the protective effects of IL10 while ectopic expression of constitutively active STAT3 mimicked the IL10 responses on the ISO effects, confirming that IL10 mediated inhibition of NFκB is STAT3 dependent.
Taken together our studies suggest IL10 treatment as a potential therapeutic approach to limit the progression of pressure overload-induced adverse cardiac remodeling.
heart failure; hypertrophy; interleukins; myocardium; signal transduction
Elevated protein kinase C βII (PKCβII) expression develops during heart failure and yet the role of this isoform in modulating contractile function remains controversial. The present study examines the impact of agonist-induced PKCβII activation on contractile function in adult cardiac myocytes. Diminished contractile function develops in response to low dose phenylephrine (PHE, 100 nM) in controls, while function is preserved in response to PHE in PKCβII-expressing myocytes. PHE also caused PKCβII translocation and a punctate distribution pattern in myocytes expressing this isoform. The preserved contractile function and translocation responses to PHE are blocked by the inhibitor, LY379196 (30 nM) in PKCβII-expressing myocytes. Further analysis showed downstream protein kinase D (PKD) phosphorylation and phosphatase activation are associated with the LY379196-sensitive contractile response. PHE also triggered a complex pattern of end-target phosphorylation in PKCβII-expressing myocytes. These patterns are consistent with bifurcated activation of downstream signaling activity by PKCβII.
Earlier studies have shown that cardiac myosin binding protein-C (cMyBP-C) is easily releasable into the circulation following myocardial infarction (MI) in animal models and patients. However, since its release kinetics has not been clearly demonstrated, no parameters are available to judge its efficacy as a bona fide biomarker of MI in patients with MI. To make this assessment, plasma levels of cMyBP-C and six known biomarkers of MI were determined by sandwich enzyme-linked immunosorbent assay in patients with MI who had before and after Percutaneous Transcoronary Angioplasty (PTCA), as well as healthy controls. Compared to healthy controls (22.3 ± 2.4 ng/mL (n=54)), plasma levels of cMyBP-C were significantly increased in patients with MI (105.1 ± 8.8 ng/mL (n=65), P<0.001). Out of 65 patients, 24 had very high levels of plasma cMyBP-C (116.5 ± 13.3 ng/mL), indicating high probability of MI. Importantly, cMyBP-C levels were significantly decreased in patients (n=40) at 12 hours post-PTCA (41.2 ± 9.3 ng/mL, P<0.001), compared to the patients with MI. Receiver operating characteristic analysis revealed that a plasma cMyBP-C reading of 68.1 ng/mL provided a sensitivity of 66.2% and a specificity of 100%. Also, myoglobin, carbonic anhydrase and creatine kinase-MB levels were significantly increased in MI patients who also had higher cMyBP-C levels. In contrast, levels of cardiac troponin I, glycogen phosphorylase and heart-type fatty acid binding protein were not significantly changed in the samples, indicating the importance of evaluating the differences in release kinetics of these biomarkers in the context of accurate diagnosis. Our findings suggest that circulating cMyBP-C is a sensitive and cardiac-specific biomarker with potential utility for the accurate diagnosis of MI.
Acute coronary syndrome; cardiac biomarker; cardiac myosin binding protein-C; contractile protein; cMyBP-C; myocardial infarction
Mutations of cardiac myosin binding protein-C (cMyBP-C) are inherited by an estimated 60 million people worldwide, and the protein is the target of several kinases. Recent evidence further suggests that cMyBP-C mutations alter Ca2+ transients, leading to electrophysiological dysfunction. Thus, while the importance of studying this cardiac sarcomere protein is clear, preliminary data in the literature have raised many questions. Therefore, in this article, we propose to review the structure and function of cMyBP-C with particular respect to the role(s) in cardiac contractility and whether its release into the circulatory system is a potential biomarker of myocardial infarction. We also discuss future directions and experimental designs that may lead to expanding the role(s) of cMyBP-C in the heart. In conclusion, we suggest that cMyBP-C is a regulatory protein that could offer a broad clinical utility in maintaining normal cardiac function.
Sarcomeric proteins; Cardiomyopathy; Phosphorylation; Cardiac biomarker; Heart failure; X-ray diffraction
Cardiac myosin binding protein-C (cMyBP-C) plays a role in sarcomeric structure and stability, as well as modulating heart muscle contraction. The 150 kDa full-length (FL) cMyBP-C has been shown to undergo proteolytic cleavage during ischemia–reperfusion injury, producing an N-terminal 40 kDa fragment (mass 29 kDa) that is predominantly associated with post-ischemic contractile dysfunction. Thus far, the pathogenic properties of such truncated cMyBP-C proteins have not been elucidated. In the present study, we hypothesized that the presence of these 40 kDa fragments is toxic to cardiomyocytes, compared to the 110 kDa C-terminal fragment and FL cMyBP-C. To test this hypothesis, we infected neonatal rat ventricular cardiomyocytes and adult rabbit ventricular cardiomyocytes with adenoviruses expressing the FL, 110 and 40 kDa fragments of cMyBP-C, and measured cytotoxicity, Ca2+ transients, contractility, and protein–protein interactions. Here we show that expression of 40 kDa fragments in neonatal rat ventricular cardiomyocytes significantly increases LDH release and caspase 3 activity, significantly reduces cell viability, and impairs Ca2+ handling. Adult cardiomyocytes expressing 40 kDa fragments exhibited similar impairment of Ca2+ handling along with a significant reduction of sarcomere length shortening, relaxation velocity, and contraction velocity. Pull-down assays using recombinant proteins showed that the 40 kDa fragment binds significantly to sarcomeric actin, comparable to C0–C2 domains. In addition, we discovered several acetylation sites within the 40 kDa fragment that could potentially affect actomyosin function. Altogether, our data demonstrate that the 40 kDa cleavage fragments of cMyBP-C are toxic to cardiomyocytes and significantly impair contractility and Ca2+ handling via inhibition of actomyosin function. By elucidating the deleterious effects of endogenously expressed cMyBP-C N-terminal fragments on sarcomere function, these data contribute to the understanding of contractile dysfunction following myocardial injury.
Proteolysis; Pathogenesis; Muscle contractility; Actin; Acetylation; Ca2+; Transients
Cardiac myosin binding protein-C (cMyBP-C) has 11 immunoglobulin or fibronectin-like domains, C0 through C10, which bind sarcomeric proteins, including titin, myosin and actin. Using bacterial expressed mouse N-terminal fragments (C0 through C3) in an in vitro motility assay of myosin-generated actin movement and the laser trap assay to assess single molecule actin-binding capacity, we determined that the first N-terminal 17 amino acids of the cMyBP-C motif (the linker between C1 and C2) contain a strong, stereospecific actin-binding site that depends on positive charge due to a cluster of arginines. Phosphorylation of 4 serines within the motif decreases the fragments’ actin-binding capacity and actomyosin inhibition. Using the laser trap assay, we observed individual cMyBP-C fragments transiently binding to a single actin filament with both short (~20ms) and long (~300ms) attached lifetimes, similar to that of a known actin-binding protein, α-actinin. These experiments suggest that cMyBP-C N-terminal domains containing the cMyBP-C motif tether actin filaments and provide one mechanism by which cMyBP-C modulates actomyosin motion generation, i.e. by imposing an effective viscous load within the sarcomere.
single molecule biophysics; laser trap; PKA phosphorylation; contractile proteins; contractility; heart
Cardiac myosin binding protein-C (cMyBP-C) is a thick filament assembly protein that stabilizes sarcomeric structure and regulates cardiac function; however, the profile of cMyBP-C degradation after myocardial infarction (MI) is unknown. We hypothesized that cMyBP-C is sensitive to proteolysis and is specifically increased in the bloodstream post-MI in rats and humans. Under these circumstances, elevated levels of degraded cMyBP-C could be used as a diagnostic tool to confirm MI. To test this hypothesis, we first established that cMyBP-C dephosphorylation is directly associated with increased degradation of this myofilament protein, leading to its release in vitro. Using neonatal rat ventricular cardiomyocytes in vitro, we were able to correlate the induction of hypoxic stress with increased cMyBP-C dephosphorylation, degradation, and the specific release of N′-fragments. Next, to define the proteolytic pattern of cMyBP-C post-MI, the left anterior descending coronary artery was ligated in adult male rats. Degradation of cMyBP-C was confirmed by a reduction in total cMyBP-C and the presence of degradation products in the infarct tissue. Phosphorylation levels of cMyBP-C were greatly reduced in ischemic areas of the MI heart compared to non-ischemic regions and sham control hearts. Post-MI plasma samples from these rats, as well as humans, were assayed for cMyBP-C and its fragments by sandwich ELISA and immunoprecipitation analyses. Results showed significantly elevated levels of cMyBP-C in the plasma of all post-MI samples. Overall, this study suggests that cMyBP-C is an easily releasable myofilament protein that is dephosphorylated, degraded and released into the circulation post-MI. The presence of elevated levels of cMyBP-C in the blood provides a promising novel biomarker able to accurately rule in MI, thus aiding in the further assessment of ischemic heart disease.
Myosin binding protein-C; Phosphorylation; Cardiac troponin I; Cardiac biomarker
Cardiac myosin binding protein-C (cMyBP-C) phosphorylation at Ser-273, Ser-282 and Ser-302 regulates myocardial contractility. In vitro and in vivo experiments suggest the nonequivalence of these sites and the potential importance of Ser-282 phosphorylation in modulating the protein's overall phosphorylation and myocardial function.
To determine whether complete cMyBP-C phosphorylation is dependent on Ser-282 phosphorylation and to define its role in myocardial function. We hypothesized that Ser-282 regulates Ser-302 phosphorylation and cardiac function during β-adrenergic (β-AR) stimulation.
Methods and Results
Using recombinant human C1-M-C2 peptides in vitro, we determined that protein kinase A can phosphorylate Ser-273, Ser-282 and Ser-302. Protein kinase Cε can also phosphorylate Ser-273 and Ser-302. In contrast, Ca2+-calmodulin-activated kinase II (CaMKII) targets Ser-302 but can also target Ser-282 at non-physiological calcium concentrations. Strikingly, Ser-302 phosphorylation by CaMKII was abolished by ablating Ser-282's ability to be phosphorylated via alanine substitution. To determine the sites’ functional roles in vivo, three transgenic lines, which expressed cMyBP-C containing either Ser-273-Ala-282-Ser-302 (cMyBP-CSAS), Ala-273-Asp-282-Ala-302 (cMyBP-CADA) or Asp-273-Ala-282-Asp-302 (cMyBP-CDAD), were generated. Mutant protein was completely substituted for endogenous cMyBP-C by breeding each mouse line into a cMyBP-C null (t/t) background. Serine to alanine substitutions were used to ablate the residues’ abilities to be phosphorylated while serine to aspartate substitutions were used to mimic the charged state conferred by phosphorylation. Compared to control non-transgenic mice, as well as transgenic mice expressing wild-type cMyBP-C, the transgenic cMyBP-CSAS(t/t), cMyBP-CADA(t/t) and cMyBP-CDAD(t/t) mice showed no increases in morbidity and mortality and partially rescued the cMyBP-C(t/t) phenotype. The loss of cMyBP-C phosphorylation at Ser-282 led to an altered β-adrenergic response. In vivo hemodynamic studies revealed that contractility was unaffected but that cMyBP-CSAS(t/t) hearts showed decreased diastolic function at baseline. However, the normal increases in cardiac function (increased contractility/relaxation) as a result of infusion of β-agonist was significantly decreased in all of the mutants, suggesting that competency for phosphorylation at multiple sites in cMyBP-C is a prerequisite for normal β-adrenergic responsiveness.
Ser-282 has a unique regulatory role in that its phosphorylation is critical for the subsequent phosphorylation of Ser-302. However, each residue plays a role in regulating the contractile response to β-agonist stimulation.
Myofilament Phosphorylation; Contractile Function; Myosin Binding Protein-C
cardiac biomarker; cardiac myosin binding protein-C; cardiac troponin-I; Meso Scale Discovery; myocardial infarction; selective reaction monitoring
Only a small fraction of large genomes such as that of the human contains the functional regions such as the exons, promoters, and polyA sites. A platform technique for selective enrichment of functional genomic regions will enable several next-generation sequencing applications that include the discovery of causal mutations for disease and drug response. Here, we describe a powerful platform technique, termed “functional genomic fingerprinting” (FGF), for the multiplexed genomewide isolation and analysis of targeted regions such as the exome, promoterome, or exon splice enhancers. The technique employs a fixed part of a uniquely designed Fixed-Randomized primer, while the randomized part contains all the possible sequence permutations. The Fixed-Randomized primers bind with full sequence complementarity at multiple sites where the fixed sequence (such as the splice signals) occurs within the genome, and multiplex amplify many regions bounded by the fixed sequences (e.g., exons). Notably, validation of this technique using cardiac myosin binding protein-C (MYBPC3) gene as an example strongly supports the application and efficacy of this method. Further, assisted by genomewide computational analyses of such sequences, the FGF technique may provide a unique platform for high-throughput sample production and analysis of targeted genomic regions by the next-generation sequencing techniques, with powerful applications in discovering disease and drug response genes.
Ischemic heart disease, which remains the leading cause of morbidity and mortality in the Western world, is invariably characterized by impaired cardiac function and disturbed Ca2+ homeostasis. Because enhanced inhibitor-1 (I-1) activity has been suggested to preserve Ca2+ cycling, we sought to define whether increases in I-1 activity in the adult heart may ameliorate contractile dysfunction and cellular injury in the face of an ischemic insult. To this end, we generated an inducible transgenic mouse model that enabled temporally controlled expression of active I-1 (T35D). Active I-1 expression in the adult heart elicited significant enhancement of contractile function, associated with preferential phospholamban phosphorylation and enhanced sarcoplasmic reticulum Ca2+ -transport. Further phosphoproteomic analysis revealed alterations in proteins associated with energy production and protein synthesis, possibly to support the increased metabolic demands of the hyperdynamic hearts. Importantly, on ischemia/reperfusion-induced injury, active I-1 expression augmented contractile function and recovery. Further examination revealed that the infarct region and apoptotic as well as necrotic injuries were significantly attenuated by enhanced I-1 activity. These cardioprotective effects were associated with suppression of the endoplasmic reticulum stress response. The present findings indicate that increased I-1 activity in the adult heart enhances Ca2+ cycling and improves mechanical recovery, as well as cell survival after an ischemic insult, suggesting that active I-1 may represent a potential therapeutic strategy in myocardial infarction.
ischemia; reperfusion; protein phosphatase-1 inhibitor-1; phospholamban; ER stress
Cardiac myosin binding protein-C (cMyBP-C) phosphorylation modulates cardiac contractility. When expressed in cMyBP-C null (cMyBP-C(t/t)) hearts, a cMyBP-C phosphomimetic (cMyBP-CAllP+), rescued cardiac dysfunction and protected the hearts from ischemic-reperfusion (I/R) injury. However, cMyBP-C function may be dependent upon the myosin isoform type. Since these replacements were carried out in the mouse heart, which contains predominantly α-myosin heavy chain (α-MyHC), the applicability of the data to the human, whose cardiomyocytes contain predominantly β-MyHC, is unclear. We determined the effect(s) of cMyBP-C phosphorylation in a “humanized” mouse heart in which >80% of the α-MyHC was replaced by β-MyHC, which is the predominant myosin isoform in human cardiac muscle.
Methods and Results
To determine the effects of cMyBP-C phosphorylation in a β-MyHC background, transgenic mice expressing normal cMyBP-C (cMyBP-CWT), nonphosphorylatable cMyBP-C (cMyBP-CAllP-), or cMyBP-CAllP+ were bred into the β-MyHC background (β). These mice were then crossed into the cMyBP-C(t/t) background to ensure the absence of endogenous cMyBP-C. cMyBP-C(t/t)/β and cMyBP-CAllP-:(t/t)/β mice died prematurely due to heart failure, confirming that cMyBP-C phosphorylation is essential in the β-MyHC background. cMyBP-CAllP+:(t/t)/β and cMyBP-CWT:(t/t)/β hearts showed no morbidity and mortality and cMyBP-CAllP+:(t/t)/β hearts were significantly cardioprotected from I/R injury.
cMyBP-C phosphorylation is necessary for basal myocardial function in the β-MyHC background and can preserve function after I/R injury. Our studies justify exploration of cMyBP-C phosphorylation as a therapeutic target in the human heart.
heart failure; myosin heavy chain; cardioprotection; myosin binding protein-C
Heart failure is a leading cause of mortality in South Asians. However, its genetic etiology remains largely unknown1. Cardiomyopathies due to sarcomeric mutations are a major monogenic cause for heart failure (MIM600958). Here, we describe a deletion of 25 bp in the gene encoding cardiac myosin binding protein C (MYBPC3) that is associated with heritable cardiomyopathies and an increased risk of heart failure in Indian populations (initial study OR = 5.3 (95% CI = 2.3–13), P = 2 × 10−6; replication study OR = 8.59 (3.19–25.05), P = 3 × 10−8; combined OR = 6.99 (3.68–13.57), P = 4 × 10−11) and that disrupts cardiomyocyte structure in vitro. Its prevalence was found to be high (~4%) in populations of Indian subcontinental ancestry. The finding of a common risk factor implicated in South Asian subjects with cardiomyopathy will help in identifying and counseling individuals predisposed to cardiac diseases in this region.
Cardiac troponin I (cTnI) phosphorylation helps regulate myocardial contractility and relaxation during β-adrenergic stimulation. cTnI differs from the skeletal isoform in that it has a cardiac specific N′-extension of 32 residues (N′-extension). The role of the acidic-N′-region in modulating cardiac contractility has not been fully defined. To test the hypothesis that the acidic-N′-region of cTnI helps regulate myocardial function, we generated cardiac-specific transgenic mice in which residues 2–11 (cTnIΔ2–11) were deleted. The hearts displayed significantly decreased contraction and relaxation under basal and β-adrenergic stress compared to non-transgenic hearts, with a reduction in maximal Ca2+ dependent force and maximal Ca2+-activated Mg2+-ATPase activity. However, Ca2+ sensitivity of force development and cTnI-Ser23/24 phosphorylation were not affected. NMR amide proton/nitrogen chemical shift analysis shows that phosphorylation at Ser23/24 in cTnI and cTnIΔ2–11 decrease interactions with the N-lobe of cardiac troponin C. We hypothesized that phosphorylation at Ser23/24 induces a large conformational change positioning the conserved acidic-N-region to compete with actin for the inhibitory region of cTnI. Consistent with this hypothesis, deletion of the conserved acidic-N′-region results in a decrease in myocardial contractility in the cTnIΔ2–11 mice demonstrating the importance of acidic-N′-region in regulating myocardial contractility and mediating the heart’s response to β-AR stimulation.
muscle; transgenic mouse model; contractility; heart
The role of cardiac myosin binding protein-C (cMyBP-C) phosphorylation in cardiac physiology or pathophysiology is unclear. To investigate the status of cMyBP-C phosphorylation in vivo, we determined its phosphorylation state in stressed and unstressed mouse hearts. cMyBP-C phosphorylation is significantly decreased during the development of heart failure or pathologic hypertrophy. We then generated transgenic (TG) mice in which cMyBP-C’s phosphorylation sites were changed to nonphosphorylatable alanines (MyBP-CAllP−). A TG line showing ~40% replacement with MyBP-CAllP− showed no changes in morbidity or mortality but displayed depressed cardiac contractility, altered sarcomeric structure and upregulation of transcripts associated with a hypertrophic response. To explore the effect of complete replacement of endogenous cMyBP-C with MyBP-CAllP−, the mice were bred into the MyBP-C(t/t) background, in which less than 10% of normal levels of a truncated MyBP-C are present. Although MyBP-CAllP− was incorporated into the sarcomere and expressed at normal levels, the mutant protein could not rescue the MyBP-C(t/t) phenotype. The mice developed significant cardiac hypertrophy with myofibrillar disarray and fibrosis, similar to what was observed in the MyBP-C(t/t) animals. In contrast, when the MyBP-C(t/t) mice were bred to a TG line expressing normal MyBP-C (MyBP-CWT), the MyBP-C(t/t) phenotype was rescued. These data suggest that cMyBP-C phosphorylation is essential for normal cardiac function.