Cardioprotection through a PKC-dependent decrease in myofilament ATPase

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.

PRKCE gene encoding protein kinase C-epsilon-Dual roles at sarcomeres and mitochondria in cardiomyocytes

Protein kinase C-epsilon (PKCε) is an isoform of a large PKC family of enzymes that has a variety of functions in different cell types. Here we discuss two major roles of PKCε in cardiac muscle cells; specifically, its role in regulating cardiac muscle contraction via targeting the sarcomeric proteins, as well as modulating cardiac cell energy production and metabolism by targeting cardiac mitochondria. The importance of PKCε action is described within the context of intracellular localization, as substrate selectivity and specificity is achieved through spatiotemporal targeting of PKCε. Accordingly, the role of PKCε in regulating myocardial function in physiological and pathological states has been documented in both cardioprotection and cardiac hypertrophy.

Circulating Pneumolysin Is a Potent Inducer of Cardiac Injury during Pneumococcal Infection

Streptococcus pneumoniae accounts for more deaths worldwide than any other single pathogen through diverse disease manifestations including pneumonia, sepsis and meningitis. Life-threatening acute cardiac complications are more common in pneumococcal infection compared to other bacterial infections. Distinctively, these arise despite effective antibiotic therapy. Here, we describe a novel mechanism of myocardial injury, which is triggered and sustained by circulating pneumolysin (PLY). Using a mouse model of invasive pneumococcal disease (IPD), we demonstrate that wild type PLY-expressing pneumococci but not PLY-deficient mutants induced elevation of circulating cardiac troponins (cTns), well-recognized biomarkers of cardiac injury. Furthermore, elevated cTn levels linearly correlated with pneumococcal blood counts (r=0.688, p=0.001) and levels were significantly higher in non-surviving than in surviving mice. These cTn levels were significantly reduced by administration of PLY-sequestering liposomes. Intravenous injection of purified PLY, but not a non-pore forming mutant (PdB), induced substantial increase in cardiac troponins to suggest that the pore-forming activity of circulating PLY is essential for myocardial injury in vivo. Purified PLY and PLY-expressing pneumococci also caused myocardial inflammatory changes but apoptosis was not detected. Exposure of cultured cardiomyocytes to PLY-expressing pneumococci caused dose-dependent cardiomyocyte contractile dysfunction and death, which was exacerbated by further PLY release following antibiotic treatment. We found that high PLY doses induced extensive cardiomyocyte lysis, but more interestingly, sub-lytic PLY concentrations triggered profound calcium influx and overload with subsequent membrane depolarization and progressive reduction in intracellular calcium transient amplitude, a key determinant of contractile force. This was coupled to activation of signalling pathways commonly associated with cardiac dysfunction in clinical and experimental sepsis and ultimately resulted in depressed cardiomyocyte contractile performance along with rhythm disturbance. Our study proposes a detailed molecular mechanism of pneumococcal toxin-induced cardiac injury and highlights the major translational potential of targeting circulating PLY to protect against cardiac complications during pneumococcal infections.

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.

Low molecular weight fibroblast growth factor-2 signals via protein kinase C and myofibrillar proteins to protect against postischemic cardiac dysfunction

Among its many biological roles, fibroblast growth factor-2 (FGF2) acutely protects the heart from dysfunction associated with ischemia/reperfusion (I/R) injury. Our laboratory has demonstrated that this is due to the activity of the low molecular weight (LMW) isoform of FGF2 and that FGF2-mediated cardioprotection relies on the activity of protein kinase C (PKC); however, which PKC isoforms are responsible for LMW FGF2-mediated cardioprotection, and their downstream targets, remain to be elucidated. To identify the PKC pathway(s) that contributes to postischemic cardiac recovery by LMW FGF2, mouse hearts expressing only LMW FGF2 (HMWKO) were bred to mouse hearts not expressing PKCα (PKCαKO) or subjected to a selective PKCε inhibitor (εV(1-2)) before and during I/R. Hearts only expressing LMW FGF2 showed significantly improved postischemic recovery of cardiac function following I/R (P < 0.05), which was significantly abrogated in the absence of PKCα (P < 0.05) or presence of PKCε inhibition (P < 0.05). Hearts only expressing LMW FGF2 demonstrated differences in actomyosin ATPase activity as well as increases in the phosphorylation of troponin I and T during I/R compared with wild-type hearts; several of these effects were dependent on PKCα activity. This evidence indicates that both PKCα and PKCε play a role in LMW FGF2-mediated protection from cardiac dysfunction and that PKCα signaling to the contractile apparatus is a key step in the mechanism of LMW FGF2-mediated protection against myocardial dysfunction.

Rapid changes in cardiac myofilament function following the acute activation of estrogen receptor-alpha

Estrogens have well-recognized and complex cardiovascular effects, including altering myocardial contractility through changes in myofilament function. The presence of multiple estrogen receptor (ER) isoforms in the heart may explain some discrepant findings about the cardiac effects of estrogens. Most studies examining the impact of estrogens on the heart have focused on chronic changes in estrogen levels, and have not investigated rapid, non-genomic pathways. The first objective of this study was to determine how acute activation of ERα impacts cardiac myofilaments. Nongenomic myocardial estrogen signaling is associated with the activation of a variety of signaling pathways. p38 MAPK has been implicated in acute ER signaling in the heart, and is known to affect myofilament function. Thus, the second objective of this study was to determine if acute ERα activation mediates its myofilament effects through p38 MAPK recruitment. Hearts from female C57Bl/6 mice were perfused with the ERα agonist PPT and myofilaments isolated. Activation of ERα depressed actomyosin MgATPase activity and decreased myofilament calcium sensitivity. Inhibition of p38 MAPK attenuated the myofilament effects of ERα activation. ERα stimulation did not affect global myofilament protein phosphorylation, but troponin I phosphorylation at the putative PKA phosphorylation sites was decreased. Changes in myofilament activation did not translate into alterations in whole heart function. The present study provides evidence supporting rapid, non-genomic changes in cardiac myofilament function following acute ERα stimulation mediated by the p38 MAPK pathway.

Cardiac myosin binding protein C phosphorylation in cardiac disease

Perturbations in sarcomeric function may in part underlie systolic and diastolic dysfunction of the failing heart. Sarcomeric dysfunction has been ascribed to changes in phosphorylation status of sarcomeric proteins caused by an altered balance between intracellular kinases and phosphatases during the development of cardiac disease. In the present review we discuss changes in phosphorylation of the thick filament protein myosin binding protein C (cMyBP-C) reported in failing myocardium, with emphasis on phosphorylation changes observed in familial hypertrophic cardiomyopathy caused by mutations in MYBPC3. Moreover, we will discuss assays which allow to distinguish between functional consequences of mutant sarcomeric proteins and (mal)adaptive changes in sarcomeric protein phosphorylation.

cMyBP-C as a promiscuous substrate: phosphorylation by non-PKA kinases and its potential significance

It is now generally accepted that phosphorylation of cMyBP-C is critically important in maintaining normal cardiac function. Although much of the work to date on phospho-regulation of cMyBP-C has focused on the role of protein kinase A (PKA, also known as cAMP-dependent protein kinase), recent evidence suggests that a number of non-PKA serine/threonine kinases, such as Ca(2+)/calmodulin-dependent protein kinase II, protein kinase C, protein kinase D and the 90-kDa ribosomal S6 kinase are also capable of targeting this key regulatory sarcomeric protein. This article reviews such evidence and proposes a hypothetical role for some of the pertinent signalling pathways in phospho-regulation of cMyBP-C in the setting of heart failure.

A critical function for Ser-282 in cardiac Myosin binding protein-C phosphorylation and cardiac function

RATIONALE

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.

OBJECTIVE

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 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 C can also phosphorylate Ser-273 and Ser-302. In contrast, Ca(2+)-calmodulin-activated kinase II targets Ser-302 but can also target Ser-282 at nonphysiological calcium concentrations. Strikingly, Ser-302 phosphorylation by Ca(2+)-calmodulin-activated kinase II was abolished by ablating the ability of Ser-282 to be phosphorylated via alanine substitution. To determine the functional roles of the sites in vivo, three transgenic lines, which expressed cMyBP-C containing either Ser-273-Ala-282-Ser-302 (cMyBP-C(SAS)), Ala-273-Asp-282-Ala-302 (cMyBP-C(ADA)), or Asp-273-Ala-282-Asp-302 (cMyBP-C(DAD)), 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 abilities of the residues to be phosphorylated, whereas serine-to-aspartate substitutions were used to mimic the charged state conferred by phosphorylation. Compared to control nontransgenic mice, as well as transgenic mice expressing wild-type cMyBP-C, the transgenic cMyBP-C(SAS(t/t)), cMyBP-C(ADA(t/t)), and cMyBP-C(DAD(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-C(SAS(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.

CONCLUSIONS

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.

Lysine deacetylation in ischaemic preconditioning: the role of SIRT1

AIMS

Acute ischaemic preconditioning (IPC) induces protection against cardiac ischaemia-reperfusion (IR) via post-translational modification of key proteins. Lysine (Lys) acetylation is an important regulator of protein function, but this type of modification has not been studied in the context of IPC. We investigated Lys acetylation in IPC and its upstream regulation by SIRT1.

METHODS AND RESULTS

Hearts from C57BL/6 mice were Langendorff-perfused and subjected to IPC and IR injury. Mice were exposed to IPC by in vivo coronary artery occlusion. An isolated cardiomyocyte model of IPC was also developed. Lys acetylation was measured by western blotting, and pharmacological modulators of Lys acetylation were tested. More Lys deacetylation was observed in IPC, in the Langendorff, in vivo, and cellular IPC models; this was concurrent with an increase in SIRT1 activity measured by p53 Lys₃₇₉ deacetylation. IPC was not accompanied by changes in SIRT1 protein level, but evidence was obtained for SIRT1 modification by Small Ubiquitin-like Modifier (SUMOylation) in IPC. Furthermore, the specific SIRT1 inhibitor splitomicin reversed both IPC-mediated Lys deacetylation and IPC-induced cardioprotection. Inhibition of nicotinamide phosphoribosyltransferase (Nampt, an important enzyme which regulates SIRT1 activity by maintaining availability of the substrate NAD(+)) also blocked both IPC-induced deacetylation and cardioprotection.

CONCLUSION

Lys deacetylation occurs during IPC and an elevation in SIRT1 activity plays a role in this phenomenon. Inhibition of SIRT1, either directly or by restricting the availability of its substrate NAD(+), inhibits IPC. Together these data suggest a role for SIRT1-mediated Lys deacetylation in the mechanism of acute IPC.

Phosphorylation and function of cardiac myosin binding protein-C in health and disease

During the past 5 years there has been an increasing body of literature describing the roles cardiac myosin binding protein C (cMyBP-C) phosphorylation play in regulating cardiac function and heart failure. cMyBP-C is a sarcomeric thick filament protein that interacts with titin, myosin and actin to regulate sarcomeric assembly, structure and function. Elucidating the function of cMyBP-C is clinically important because mutations in this protein have been linked to cardiomyopathy in more than sixty million people worldwide. One function of cMyBP-C is to regulate cross-bridge formation through dynamic phosphorylation by protein kinase A, protein kinase C and Ca(2+)-calmodulin-activated kinase II, suggesting that cMyBP-C phosphorylation serves as a highly coordinated point of contractile regulation. Moreover, dephosphorylation of cMyBP-C, which accelerates its degradation, has been shown to associate with the development of heart failure in mouse models and in humans. Strikingly, cMyBP-C phosphorylation presents a potential target for therapeutic development as protection against ischemic-reperfusion injury, which has been demonstrated in mouse hearts. Also, emerging evidence suggests that cMyBP-C has the potential to be used as a biomarker for diagnosing myocardial infarction. Although many aspects of cMyBP-C phosphorylation and function remain poorly understood, cMyBP-C and its phosphorylation states have significant promise as a target for therapy and for providing a better understanding of the mechanics of heart function during health and disease. In this review we discuss the most recent findings with respect to cMyBP-C phosphorylation and function and determine potential future directions to better understand the functional role of cMyBP-C and phosphorylation in sarcomeric structure, myocardial contractility and cardioprotection.

Relaxin alters cardiac myofilament function through a PKC-dependent pathway

The pregnancy hormone relaxin (RLX) is a powerful cardiostimulatory peptide. Despite its well-characterized effects on the heart, the intracellular mechanisms responsible for RLX's positive inotropic effects are unknown. Cardiac myofilaments are the central contractile elements of the heart, and changes in the phosphorylation status of myofilament proteins are known to mediate changes in function. The first objective of this study was to determine whether RLX stimulates myofilament activation and alters the phosphorylation of one or more myofilament proteins. RLX works through a variety of intracellular signaling cascades in different tissue types. Protein kinases A (PKA) and C (PKC) are two common molecules implicated in RLX signaling and are known to affect myofilament function. Thus the second objective of this study was to determine whether RLX mediates its myocardial effects through PKA or PKC activation. Murine myocardium was treated with recombinant H2-RLX, and cardiac myofilaments were isolated. RLX increased cardiac myofilament force development at physiological levels of intracellular Ca2+ without altering myofilament ATP consumption. Myosin binding protein C, troponin T, and troponin I phosphorylation levels were increased with RLX treatment. Immunoblot analysis revealed an increase in myofilament-associated PKC-δ, decreases in PKC-α and -βII, but no effect on PKC-ε. Inhibition of PKC with chelerythrine chloride or PKC-δ with rottlerin prevented the RLX-dependent changes in myofilament function and protein phosphorylation. PKA antagonism with H-89 had no effect on the myofilament effects of RLX. This study is the first to show that RLX-dependent changes in myofilament-associated PKC alters myofilament activation in a manner consistent with its cardiostimulatory effects.

Modest actomyosin energy conservation increases myocardial postischemic function

We have proposed that pharmacological preconditioning, leading to PKC-epsilon activation, in hearts improves postischemic functional recovery through a decrease in actomyosin ATPase activity and subsequent ATP conservation. The purpose of the present study was to determine whether moderate PKC-independent decreases in actomyosin ATPase are sufficient to improve myocardial postischemic function. Rats were given propylthiouracil (PTU) for 8 days to induce a 25% increase in beta-myosin heavy chain with a 28% reduction in actomyosin ATPase activity. Recovery of postischemic left ventricular developed pressure (LVDP) was significantly higher in PTU-treated rat hearts subjected to 30 min of global ischemia than in control hearts: 57.9 +/- 6.2 vs. 32.6 +/- 5.1% of preischemic values. In addition, PTU-treated hearts exhibited a delayed onset of rigor contracture during ischemia and a higher global ATP content after ischemia. In the second part of our study, we demonstrated a lower maximal actomyosin ATPase and a higher global ATP content after ischemia in human troponin T (TnT) transgenic mouse hearts. In mouse hearts with and without a point mutation at F110I of human TnT, recovery of postischemic LVDP was 55.4 +/- 5.5 and 62.5 +/- 14.5% compared with 20.0 +/- 2.9% in nontransgenic mouse hearts after 35 min of global ischemia. These results are consistent with the hypothesis that moderate decreases in actomyosin ATPase activity result in net ATP conservation that is sufficient to improve postischemic contractile function.