PKC-betaII sensitizes cardiac myofilaments to Ca2+ by phosphorylating troponin I on threonine-144

Extracellular Histone-Induced Protein Kinase C Alpha Activation and Troponin Phosphorylation Is a Potential Mechanism of Cardiac Contractility Depression in Sepsis

Reduction in cardiac contractility is common in severe sepsis. However, the pathological mechanism is still not fully understood. Recently it has been found that circulating histones released after extensive immune cell death play important roles in multiple organ injury and disfunction, particularly in cardiomyocyte injury and contractility reduction. How extracellular histones cause cardiac contractility depression is still not fully clear. In this work, using cultured cardiomyocytes and a histone infusion mouse model, we demonstrate that clinically relevant histone concentrations cause significant increases in intracellular calcium concentrations with subsequent activation and enriched localization of calcium-dependent protein kinase C (PKC) α and βII into the myofilament fraction of cardiomyocytes in vitro and in vivo. Furthermore, histones induced dose-dependent phosphorylation of cardiac troponin I (cTnI) at the PKC-regulated phosphorylation residues (S43 and T144) in cultured cardiomyocytes, which was also confirmed in murine cardiomyocytes following intravenous histone injection. Specific inhibitors against PKCα and PKCβII revealed that histone-induced cTnI phosphorylation was mainly mediated by PKCα activation, but not PKCβII. Blocking PKCα also significantly abrogated histone-induced deterioration in peak shortening, duration and the velocity of shortening, and re-lengthening of cardiomyocyte contractility. These in vitro and in vivo findings collectively indicate a potential mechanism of histone-induced cardiomyocyte dysfunction driven by PKCα activation with subsequent enhanced phosphorylation of cTnI. These findings also indicate a potential mechanism of clinical cardiac dysfunction in sepsis and other critical illnesses with high levels of circulating histones, which holds the potential translational benefit to these patients by targeting circulating histones and downstream pathways.

Umbrella Sampling Simulations Measure Switch Peptide Binding and Hydrophobic Patch Opening Free Energies in Cardiac Troponin

The cardiac troponin (cTn) complex is an important regulatory protein in heart contraction. Upon binding of Ca2+, cTn undergoes a conformational shift that allows the troponin I switch peptide (cTnISP) to be released from the actin filament and bind to the troponin C hydrophobic patch (cTnCHP). Mutations and modifications to this complex can change its sensitivity to Ca2+ and alter the energetics of the transition from the Ca2+-unbound, cTnISP-unbound form to the Ca2+-bound, cTnISP-bound form. We utilized targeted molecular dynamics (TMD) to obtain a trajectory of this transition pathway, followed by umbrella sampling to estimate the free energy associated with the cTnISP-cTnCHP binding and the cTnCHP opening events for wild-type (WT) cTn. We were able to reproduce experimental values for the cTnISP-cTnCHP binding event and obtain cTnCHP opening free energies in agreement with previous computational measurements of smaller cTnC systems. This excellent agreement for WT cTn demonstrated the strength of computational methods in studying the dynamics and energetics of the cTn complex. We then introduced mutations to the cTn complex that cause cardiomyopathy or alter its Ca2+ sensitivity and observed a general decrease in the free energy of opening the cTnCHP. For these same mutations, we observed no general trend in the effect on the cTnISP-cTnCHP binding event. Our method sets the stage for future computational studies on this system that predict the consequences of yet uncharacterized mutations on cTn dynamics and energetics.

Alterations in ACE and ACE2 Activities and Cardiomyocyte Signaling Underlie Improved Myocardial Function in a Rat Model of Repeated Remote Ischemic Conditioning

Post-ischemic left ventricular (LV) remodeling and its hypothetical prevention by repeated remote ischemic conditioning (rRIC) in male Sprague–Dawley rats were studied. Myocardial infarction (MI) was evoked by permanent ligation of the left anterior descending coronary artery (LAD), and myocardial characteristics were tested in the infarcted anterior and non-infarcted inferior LV regions four and/or six weeks later. rRIC was induced by three cycles of five-minute-long unilateral hind limb ischemia and five minutes of reperfusion on a daily basis for a period of two weeks starting four weeks after LAD occlusion. Sham operated animals served as controls. Echocardiographic examinations and invasive hemodynamic measurements revealed distinct changes in LV systolic function between four and six weeks after MI induction in the absence of rRIC (i.e., LV ejection fraction (LVEF) decreased from 52.8 ± 2.1% to 50 ± 1.6%, mean ± SEM, p < 0.05) and in the presence of rRIC (i.e., LVEF increased from 48.2 ± 4.8% to 55.2 ± 4.1%, p < 0.05). Angiotensin-converting enzyme (ACE) activity was about five times higher in the anterior LV wall at six weeks than that in sham animals. Angiotensin-converting enzyme 2 (ACE2) activity roughly doubled in post-ischemic LVs. These increases in ACE and ACE2 activities were effectively mitigated by rRIC. Ca²⁺-sensitivities of force production (pCa₅₀) of LV permeabilized cardiomyocytes were increased at six weeks after MI induction together with hypophosphorylation of 1) cardiac troponin I (cTnI) in both LV regions, and 2) cardiac myosin-binding protein C (cMyBP-C) in the anterior wall. rRIC normalized pCa₅₀, cTnI and cMyBP-C phosphorylations. Taken together, post-ischemic LV remodeling involves region-specific alterations in ACE and ACE2 activities together with changes in cardiomyocyte myofilament protein phosphorylation and function. rRIC has the potential to prevent these alterations and to improve LV performance following MI.

Increased myofilament calcium sensitivity is associated with decreased cardiac troponin I phosphorylation in the diabetic rat heart

NEW FINDINGS

What is the central question of this study? In Zucker Diabetic Fatty rats, does cardiomyocyte myofilament function change through the time course of diabetes and what are the mechanisms behind alterations in calcium sensitivity? What is the main finding and its importance? Zucker Diabetic Fatty rats had increased myofilament calcium sensitivity and reduced phosphorylation at cardiac troponin I without differential O-GlcNAcylation.

ABSTRACT

The diabetic heart has impaired systolic and diastolic function independent of other comorbidities. The availability of calcium is altered, but does not fully explain the cardiac dysfunction seen in the diabetic heart. Thus, we explored if myofilament calcium regulation of contraction is altered while also categorizing the levels of phosphorylation and O-GlcNAcylation in the myofilaments. Calcium sensitivity (pCa₅₀ ) was measured in Zucker Diabetic Fatty (ZDF) rat hearts at the initial stage of diabetes (12 weeks old) and after 8 weeks of uncontrolled hyperglycaemia (20 weeks old) and in non-diabetic (nDM) littermates. Skinned cardiomyocytes were connected to a capacitance-gauge transducer and a torque motor to measure force as a function of pCa (-log[Ca²⁺ ]). Fluorescent gel stain (ProQ Diamond) was used to measure total protein phosphorylation. Specific phospho-sites on cardiac troponin I (cTnI) and total cTnI O-GlcNAcylation were quantified using immunoblot. pCa₅₀ was greater in both 12- and 20-week-old diabetic (DM) rats compared to nDM littermates (P = 0.0001). Total cTnI and cTnI serine 23/24 phosphorylation were lower in DM rats (P = 0.003 and P = 0.01, respectively), but cTnI O-GlcNAc protein expression was not different. pCa₅₀ is greater in DM rats and corresponds with an overall reduction in cTnI phosphorylation. These findings indicate that myofilament calcium sensitivity is increased and cTnI phosphorylation is reduced in ZDF DM rats and suggests an important role for cTnI phosphorylation in the DM heart.

Enhanced KCl-mediated contractility and Ca2+ sensitization in porcine collateral-dependent coronary arteries persist after exercise training

We have previously reported enhanced Ca²⁺ sensitivity of coronary arteries that is dependent upon collateral circulation for their blood supply. For the current study, we hypothesized that small collateral-dependent arteries would exhibit an enhanced KCl-mediated contractile response attributable to Ca²⁺ sensitization and increased Ca²⁺ channel current. Ameroid constrictors were surgically placed around the left circumflex (LCX) artery of female Yucatan miniature swine. Eight weeks postoperatively, pigs were randomized into sedentary or exercise-trained (treadmill run; 5 days/wk; 14 wk) groups. Small coronary arteries (150-300 μm luminal diameter) were isolated from myocardial regions distal to the collateral-dependent LCX and the nonoccluded left anterior descending arteries. Contractile tension and simultaneous measures of both tension and intracellular free Ca²⁺ levels (fura-2) were measured in response to increasing concentrations of KCl. In addition, whole cell Ca²⁺ currents were also obtained. Chronic occlusion enhanced contractile responses to KCl and increased Ca²⁺ sensitization in collateral-dependent compared with nonoccluded arteries of both sedentary and exercise-trained pigs. In contrast, smooth muscle cell Ca²⁺ channel current was not altered by occlusion or exercise training. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII; inhibited by KN-93, 0.3-1 μM) contributed to the enhanced contractile response in collateral-dependent arteries of sedentary pigs, whereas both CaMKII and Rho-kinase (inhibited by hydroxyfasudil, 30 μM or Y27632, 10 μM) contributed to increased contraction in exercise-trained animals. Taken together, these data suggest that chronic occlusion leads to enhanced contractile responses to KCl in collateral-dependent coronary arteries via increased Ca²⁺ sensitization, a response that is further augmented with exercise training.NEW & NOTEWORTHY Small coronary arteries distal to chronic occlusion displayed enhanced contractile responses, which were further augmented after exercise training and attributable to enhanced calcium sensitization without alterations in calcium channel current. The calcium sensitization mediators Rho-kinase and CaMKII significantly contributed to enhanced contraction in collateral-dependent arteries of exercise-trained, but not sedentary, pigs. Exercise-enhanced contractile responses may increase resting arterial tone, creating an enhanced coronary flow reserve that is accessible during periods of increased metabolic demand.

Cardiac changes during the peri-menopausal period in a VCD-induced murine model of ovarian failure

AIM

Cardiovascular disease (CVD) risk is lower in pre-menopausal females vs age matched males. After menopause risk equals or exceeds that of males. CVD protection of pre-menopausal females is ascribed to high circulating oestrogen levels. Despite experimental evidence that oestrogen are cardioprotective, oestrogen replacement therapy trials have not shown clear benefits. One hypothesis to explain the discrepancy proposed hearts remodel during peri-menopause. Peri-menopasual myocardial changes have never been investigated, nor has the ability of oestrogen to regulate heart function during peri-menopause.

METHODS

We injected female mice with 4-vinylcyclohexene diepoxide (VCD, 160 mg/kg/d IP) to cause gradual ovarian failure over 120d and act as a peri-menopausal model RESULTS: Left ventricular function assessed by Langendorff perfusion found no changes in VCD-injected mice at 60 or 120 days compared to intact mice. Cardiac myofilament activity was altered at 60 and 120 days indicating a molecular remodelling in peri-menopause. Myocardial TGF-β1 increased at 60 days post-VCD treatment along with reduced Akt phosphorylation. Acute activation of oestrogen receptor-α (ERα) or -β (ERβ) depressed left ventricular contractility in hearts from intact mice. ER-regulation of myocardial and myofilament function, and myofilament phosphorylation, were disrupted in the peri-menopausal model. Disruption occurred without alterations in total ERα or ERβ expression.

CONCLUSIONS

This is the first study to demonstrate remodelling of the heart in a model of peri-menopause, along with a disruption in ER-dependent regulation of the heart. These data indicate that oestrogen replacement therapy initiated after menopause affects a heart that is profoundly different from that found in reproductively intact animals.

Physiologic, Pathologic, and Therapeutic Paracrine Modulation of Cardiac Excitation-Contraction Coupling

Cardiac excitation-contraction coupling (ECC) is the orchestrated process of initial myocyte electrical excitation, which leads to calcium entry, intracellular trafficking, and subsequent sarcomere shortening and myofibrillar contraction. Neurohumoral β-adrenergic signaling is a well-established mediator of ECC; other signaling mechanisms, such as paracrine signaling, have also demonstrated significant impact on ECC but are less well understood. For example, resident heart endothelial cells are well-known physiological paracrine modulators of cardiac myocyte ECC mainly via NO and endothelin-1. Moreover, recent studies have demonstrated other resident noncardiomyocyte heart cells (eg, physiological fibroblasts and pathological myofibroblasts), and even experimental cardiotherapeutic cells (eg, mesenchymal stem cells) are also capable of altering cardiomyocyte ECC through paracrine mechanisms. In this review, we first focus on the paracrine-mediated effects of resident and therapeutic noncardiomyocytes on cardiomyocyte hypertrophy, electrophysiology, and calcium handling, each of which can modulate ECC, and then discuss the current knowledge about key paracrine factors and their underlying mechanisms of action. Next, we provide a case example demonstrating the promise of tissue-engineering approaches to study paracrine effects on tissue-level contractility. More specifically, we present new functional and molecular data on the effects of human adult cardiac fibroblast conditioned media on human engineered cardiac tissue contractility and ion channel gene expression that generally agrees with previous murine studies but also suggests possible species-specific differences. By contrast, paracrine secretions by human dermal fibroblasts had no discernible effect on human engineered cardiac tissue contractile function and gene expression. Finally, we discuss systems biology approaches to help identify key stem cell paracrine mediators of ECC and their associated mechanistic pathways. Such integration of tissue-engineering and systems biology methods shows promise to reveal novel insights into paracrine mediators of ECC and their underlying mechanisms of action, ultimately leading to improved cell-based therapies for patients with heart disease.

Myofilament Ca2+ sensitivity correlates with left ventricular contractility during the progression of pressure overload-induced left ventricular myocardial hypertrophy in rats

AIM

Here we aimed at investigating the relation between left ventricular (LV) contractility and myofilament function during the development and progression of pressure overload (PO)-induced LV myocardial hypertrophy (LVH).

METHODS

Abdominal aortic banding (AB) was performed to induce PO in rats for 6, 12 and 18 weeks. Sham operated animals served as controls. Structural and molecular alterations were investigated by serial echocardiography, histology, quantitative real-time PCR and western blot. LV function was assessed by pressure-volume analysis. Force measurement was carried out in permeabilized cardiomyocytes.

RESULTS

AB resulted in the development of pathological LVH as indicated by increased heart weight-to-tibial length ratio, LV mass index, cardiomyocyte diameter and fetal gene expression. These alterations were already present at early stage of LVH (AB-week6). Furthermore, at more advanced stages (AB-week12, AB-week18), myocardial fibrosis and chamber dilatation were also observed. From a hemodynamic point of view, the AB-wk6 group was associated with increased LV contractility, maintained ventriculo-arterial coupling (VAC) and preserved systolic function. In the same experimental group, increased myofilament Ca²⁺ sensitivity (pCa₅₀) and hyperphosphorylation of cardiac troponin-I (cTnI) at Threonine-144 was detected. In contrast, in the AB-wk12 and AB-wk18 groups, the initial augmentation of LV contractility, as well as the increased myofilament Ca²⁺ sensitivity and cTnI (Threonine-144) hyperphosphorylation diminished, leading to impaired VAC and reduced systolic performance. Strong correlation was found between LV contractility parameters and myofilament Ca²⁺-sensitivity among the study groups.

CONCLUSION

Changes in myofilament Ca²⁺ sensitivity might underlie the alterations in LV contractility during the development and progression of PO-induced LVH.

PKC and PKN in heart disease

The protein kinase C (PKC) and closely related protein kinase N (PKN) families of serine/threonine protein kinases play crucial cellular roles. Both kinases belong to the AGC subfamily of protein kinases that also include the cAMP dependent protein kinase (PKA), protein kinase B (PKB/AKT), protein kinase G (PKG) and the ribosomal protein S6 kinase (S6K). Involvement of PKC family members in heart disease has been well documented over the years, as their activity and levels are mis-regulated in several pathological heart conditions, such as ischemia, diabetic cardiomyopathy, as well as hypertrophic or dilated cardiomyopathy. This review focuses on the regulation of PKCs and PKNs in different pathological heart conditions and on the influences that PKC/PKN activation has on several physiological processes. In addition, we discuss mechanisms by which PKCs and the closely related PKNs are activated and turned-off in hearts, how they regulate cardiac specific downstream targets and pathways, and how their inhibition by small molecules is explored as new therapeutic target to treat cardiomyopathies and heart failure.

Heart Failure-Related Hyperphosphorylation in the Cardiac Troponin I C Terminus Has Divergent Effects on Cardiac Function In Vivo

BACKGROUND

In human heart failure, Ser199 (equivalent to Ser200 in mouse) of cTnI (cardiac troponin I) is significantly hyperphosphorylated, and in vitro studies suggest that it enhances myofilament calcium sensitivity and alters calpain-mediated cTnI proteolysis. However, how its hyperphosphorylation affects cardiac function in vivo remains unknown.

METHODS AND RESULTS

To address the question, 2 transgenic mouse models were generated: a phospho-mimetic cTnIS200D and a phospho-silenced cTnIS200A, each driven by the cardiomyocyte-specific α-myosin heavy chain promoter. Cardiac structure assessed by echocardiography and histology was normal in both transgenic models compared with littermate controls (n=5). Baseline in vivo hemodynamics and isolated muscle studies showed that cTnIS200D significantly prolonged relaxation and lowered left ventricular peak filling rate, whereas ejection fraction and force development were normal (n=5). However, with increased heart rate or β-adrenergic stimulation, cTnIS200D mice had less enhanced ejection fraction or force development versus controls, whereas relaxation improved similarly to controls (n=5). By contrast, cTnIS200A was functionally normal both at baseline and under the physiological stresses. To test whether either mutation impacted cardiac response to ischemic stress, isolated hearts were subjected to ischemia/reperfusion. cTnIS200D were protected, recovering 88±8% of contractile function versus 35±15% in littermate controls and 28±8% in cTnIS200A (n=5). This was associated with less cTnI proteolysis in cTnIS200D hearts.

CONCLUSIONS

Hyperphosphorylation of this serine in cTnI C terminus impacts heart function by depressing diastolic function at baseline and limiting systolic reserve under physiological stresses. However, paradoxically, it preserves heart function after ischemia/reperfusion injury, potentially by decreasing proteolysis of cTnI.

β-Arrestin mediates the Frank-Starling mechanism of cardiac contractility

The Frank-Starling law of the heart is a physiological phenomenon that describes an intrinsic property of heart muscle in which increased cardiac filling leads to enhanced cardiac contractility. Identified more than a century ago, the Frank-Starling relationship is currently known to involve length-dependent enhancement of cardiac myofilament Ca²⁺ sensitivity. However, the upstream molecular events that link cellular stretch to the length-dependent myofilament Ca²⁺ sensitivity are poorly understood. Because the angiotensin II type 1 receptor (AT1R) and the multifunctional transducer protein β-arrestin have been shown to mediate mechanosensitive cellular signaling, we tested the hypothesis that these two proteins are involved in the Frank-Starling mechanism of the heart. Using invasive hemodynamics, we found that mice lacking β-arrestin 1, β-arrestin 2, or AT1R were unable to generate a Frank-Starling force in response to changes in cardiac volume. Although wild-type mice pretreated with the conventional AT1R blocker losartan were unable to enhance cardiac contractility with volume loading, treatment with a β-arrestin-biased AT1R ligand to selectively activate β-arrestin signaling preserved the Frank-Starling relationship. Importantly, in skinned muscle fiber preparations, we found markedly impaired length-dependent myofilament Ca²⁺ sensitivity in β-arrestin 1, β-arrestin 2, and AT1R knockout mice. Our data reveal β-arrestin 1, β-arrestin 2, and AT1R as key regulatory molecules in the Frank-Starling mechanism, which potentially can be targeted therapeutically with β-arrestin-biased AT1R ligands.

Cardiac troponin structure-function and the influence of hypertrophic cardiomyopathy associated mutations on modulation of contractility

Cardiac troponin (cTn) acts as a pivotal regulator of muscle contraction and relaxation and is composed of three distinct subunits (cTnC: a highly conserved Ca(2+) binding subunit, cTnI: an actomyosin ATPase inhibitory subunit, and cTnT: a tropomyosin binding subunit). In this mini-review, we briefly summarize the structure-function relationship of cTn and its subunits, its modulation by PKA-mediated phosphorylation of cTnI, and what is known about how these properties are altered by hypertrophic cardiomyopathy (HCM) associated mutations of cTnI. This includes recent work using computational modeling approaches to understand the atomic-based structural level basis of disease-associated mutations. We propose a viewpoint that it is alteration of cTnC-cTnI interaction (rather than the Ca(2+) binding properties of cTn) per se that disrupt the ability of PKA-mediated phosphorylation at cTnI Ser-23/24 to alter contraction and relaxation in at least some HCM-associated mutations. The combination of state of the art biophysical approaches can provide new insight on the structure-function mechanisms of contractile dysfunction resulting cTnI mutations and exciting new avenues for the diagnosis, prevention, and even treatment of heart diseases.

IL-6 Activates PI3K and PKCζ Signaling and Determines Cardiac Differentiation in Rat Embryonic H9c2 Cells

INTRODUCTION

IL-6 influences several biological processes, including cardiac stem cell and cardiomyocyte physiology. Although JAK-STAT3 activation is the defining feature of IL-6 signaling, signaling molecules such as PI3K, PKCs, and ERK1/2 are also activated and elicit different responses. Moreover, most studies on the specific role of these signaling molecules focus on the adult heart, and few studies are available on the biological effects evoked by IL-6 in embryonic cardiomyocytes.

AIM

The aim of this study was to clarify the biological response of embryonic heart derived cells to IL-6 by analyzing the morphological modifications and the signaling cascades evoked by the cytokine in H9c2 cells.

RESULTS

IL-6 stimulation determined the terminal differentiation of H9c2 cells, as evidenced by the increased expression of cardiac transcription factors (NKX2.5 and GATA4), structural proteins (α-myosin heavy chain and cardiac Troponin T) and the gap junction protein Connexin 43. This process was mediated by the rapid modulation of PI3K, Akt, PTEN, and PKCζ phosphorylation levels. PI3K recruitment was an upstream event in the signaling cascade and when PI3K was inhibited, IL-6 failed to modify PKCζ, PTEN, and Akt phosphorylation. Blocking PKCζ activity affected only PTEN and Akt. Finally, the overexpression of a constitutively active form of PKCζ in H9c2 cells largely mimicked the morphological and molecular effects evoked by IL-6.

CONCLUSIONS

This study demonstrated that IL-6 induces the cardiac differentiation of H9c2 embryonic cells though a signaling cascade that involves PI3K, PTEN, and PKCζ activities.

TNNI1, TNNI2 and TNNI3: Evolution, regulation, and protein structure-function relationships

Troponin I (TnI) is the inhibitory subunit of the troponin complex in the sarcomeric thin filament of striated muscle and plays a central role in the calcium regulation of contraction and relaxation. Vertebrate TnI has evolved into three isoforms encoded by three homologous genes: TNNI1 for slow skeletal muscle TnI, TNNI2 for fast skeletal muscle TnI and TNNI3 for cardiac TnI, which are expressed under muscle type-specific and developmental regulations. To summarize the current knowledge on the TnI isoform genes and products, this review focuses on the evolution, gene regulation, posttranslational modifications, and structure-function relationship of TnI isoform proteins. Their physiological and medical significances are also discussed.

A novel phosphorylation site, Serine 199, in the C-terminus of cardiac troponin I regulates calcium sensitivity and susceptibility to calpain-induced proteolysis

Phosphorylation of cardiac troponin I (cTnI) by protein kinase C (PKC) is implicated in cardiac dysfunction. Recently, Serine 199 (Ser199) was identified as a target for PKC phosphorylation and increased Ser199 phosphorylation occurs in end-stage failing compared with non-failing human myocardium. The functional consequences of cTnI-Ser199 phosphorylation in the heart are unknown. Therefore, we investigated the impact of phosphorylation of cTnI-Ser199 on myofilament function in human cardiac tissue and the susceptibility of cTnI to proteolysis. cTnI-Ser199 was replaced by aspartic acid (199D) or alanine (199A) to mimic phosphorylation and dephosphorylation, respectively, with recombinant wild-type (Wt) cTn as a negative control. Force development was measured at various [Ca(2+)] and at sarcomere lengths of 1.8 and 2.2 μm in demembranated cardiomyocytes in which endogenous cTn complex was exchanged with the recombinant human cTn complexes. In idiopathic dilated cardiomyopathy samples, myofilament Ca(2+)-sensitivity (pCa50) at 2.2 μm was significantly higher in 199D (pCa50 = 5.79 ± 0.01) compared to 199A (pCa50 = 5.65 ± 0.01) and Wt (pCa50 = 5.66 ± 0.02) at ~63% cTn exchange. Myofilament Ca(2+)-sensitivity was significantly higher even with only 5.9 ± 2.5% 199D exchange compared to 199A, and saturated at 12.3 ± 2.6% 199D exchange. Ser199 pseudo-phosphorylation decreased cTnI binding to both actin and actin-tropomyosin. Moreover, altered susceptibility of cTnI to proteolysis by calpain I was found when Ser199 was pseudo-phosphorylated. Our data demonstrate that low levels of cTnI-Ser199 pseudo-phosphorylation (~6%) increase myofilament Ca(2+)-sensitivity in human cardiomyocytes, most likely by decreasing the binding affinity of cTnI for actin-tropomyosin. In addition, cTnI-Ser199 pseudo-phosphorylation or mutation regulates calpain I mediated proteolysis of cTnI.

Troponin I phosphorylation in human myocardium in health and disease

Cardiac troponin I (cTnI) is well known as a biomarker for the diagnosis of myocardial damage. However, because of its central role in the regulation of contraction and relaxation in heart muscle, cTnI may also be a potential target for the treatment of heart failure. Studies in rodent models of cardiac disease and human heart samples showed altered phosphorylation at various sites on cTnI (i.e. site-specific phosphorylation). This is caused by altered expression and/or activity of kinases and phosphatases during heart failure development. It is not known whether these (transient) alterations in cTnI phosphorylation are beneficial or detrimental. Knowledge of the effects of site-specific cTnI phosphorylation on cardiomyocyte contractility is therefore of utmost importance for the development of new therapeutic strategies in patients with heart failure. In this review we focus on the role of cTnI phosphorylation in the healthy heart upon activation of the beta-adrenergic receptor pathway (as occurs during increased stress and exercise) and as a modulator of the Frank-Starling mechanism. Moreover, we provide an overview of recent studies which aimed to reveal the functional consequences of changes in cTnI phosphorylation in cardiac disease.

Top-down mass spectrometry of cardiac myofilament proteins in health and disease

Myofilaments are composed of thin and thick filaments that coordinate with each other to regulate muscle contraction and relaxation. PTMs together with genetic variations and alternative splicing of the myofilament proteins play essential roles in regulating cardiac contractility in health and disease. Therefore, a comprehensive characterization of the myofilament proteins in physiological and pathological conditions is essential for better understanding the molecular basis of cardiac function and dysfunction. Due to the vast complexity and dynamic nature of proteins, it is challenging to obtain a holistic view of myofilament protein modifications. In recent years, top-down MS has emerged as a powerful approach to study isoform composition and PTMs of proteins owing to its advantage of complete sequence coverage and its ability to identify PTMs and sequence variants without a priori knowledge. In this review, we will discuss the application of top-down MS to the study of cardiac myofilaments and highlight the insights it provides into the understanding of molecular mechanisms in contractile dysfunction of heart failure. Particularly, recent results of cardiac troponin and tropomyosin modifications will be elaborated. The limitations and perspectives on the use of top-down MS for myofilament protein characterization will also be briefly discussed.

Gene regulation, alternative splicing, and posttranslational modification of troponin subunits in cardiac development and adaptation: a focused review

Troponin plays a central role in regulating the contraction and relaxation of vertebrate striated muscles. This review focuses on the isoform gene regulation, alternative RNA splicing, and posttranslational modifications of troponin subunits in cardiac development and adaptation. Transcriptional and posttranscriptional regulations such as phosphorylation and proteolysis modifications, and structure-function relationships of troponin subunit proteins are summarized. The physiological and pathophysiological significances are discussed for impacts on cardiac muscle contractility, heart function, and adaptations in health and diseases.

Reactive oxygen species and excitation-contraction coupling in the context of cardiac pathology

Reactive oxygen species (ROS) are highly reactive oxygen-derived chemical compounds that are by-products of aerobic cellular metabolism as well as crucial second messengers in numerous signaling pathways. In excitation-contraction-coupling (ECC), which links electrical signaling and coordinated cardiac contraction, ROS have a severe impact on several key ion handling proteins such as ion channels and transporters, but also on regulating proteins such as protein kinases (e.g. CaMKII, PKA or PKC), thereby pivotally influencing the delicate balance of this finely tuned system. While essential as second messengers, ROS may be deleterious when excessively produced due to a disturbed balance in Na(+) and Ca(2+) handling, resulting in Na(+) and Ca(2+) overload, SR Ca(2+) loss and contractile dysfunction. This may, in the end, result in systolic and diastolic dysfunction and arrhythmias. This review aims to provide an overview of the single targets of ROS in ECC and to outline the role of ROS in major cardiac pathologies, such as heart failure and arrhythmogenesis. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System"

Cardiac resynchronization sensitizes the sarcomere to calcium by reactivating GSK-3β

Cardiac resynchronization therapy (CRT), the application of biventricular stimulation to correct discoordinate contraction, is the only heart failure treatment that enhances acute and chronic systolic function, increases cardiac work, and reduces mortality. Resting myocyte function also increases after CRT despite only modest improvement in calcium transients, suggesting that CRT may enhance myofilament calcium responsiveness. To test this hypothesis, we examined adult dogs subjected to tachypacing-induced heart failure for 6 weeks, concurrent with ventricular dyssynchrony (HF(dys)) or CRT. Myofilament force-calcium relationships were measured in skinned trabeculae and/or myocytes. Compared with control, maximal calcium-activated force and calcium sensitivity declined globally in HF(dys); however, CRT restored both. Phosphatase PP1 induced calcium desensitization in control and CRT-treated cells, while HF(dys) cells were unaffected, implying that CRT enhances myofilament phosphorylation. Proteomics revealed phosphorylation sites on Z-disk and M-band proteins, which were predicted to be targets of glycogen synthase kinase-3β (GSK-3β). We found that GSK-3β was deactivated in HF(dys) and reactivated by CRT. Mass spectrometry of myofilament proteins from HF(dys) animals incubated with GSK-3β confirmed GSK-3β–dependent phosphorylation at many of the same sites observed with CRT. GSK-3β restored calcium sensitivity in HF(dys), but did not affect control or CRT cells. These data indicate that CRT improves calcium responsiveness of myofilaments following HF(dys) through GSK-3β reactivation, identifying a therapeutic approach to enhancing contractile function

Length-dependent activation is modulated by cardiac troponin I bisphosphorylation at Ser23 and Ser24 but not by Thr143 phosphorylation

Frank-Starling's law reflects the ability of the heart to adjust the force of its contraction to changes in ventricular filling, a property based on length-dependent myofilament activation (LDA). The threonine at amino acid 143 of cardiac troponin I (cTnI) is prerequisite for the length-dependent increase in Ca(2+) sensitivity. Thr143 is a known target of protein kinase C (PKC) whose activity is increased in cardiac disease. Thr143 phosphorylation may modulate length-dependent myofilament activation in failing hearts. Therefore, we investigated if pseudo-phosphorylation at Thr143 modulates length dependence of force using troponin exchange experiments in human cardiomyocytes. In addition, we studied effects of protein kinase A (PKA)-mediated cTnI phosphorylation at Ser23/24, which has been reported to modulate LDA. Isometric force was measured at various Ca(2+) concentrations in membrane-permeabilized cardiomyocytes exchanged with recombinant wild-type (WT) troponin or troponin mutated at the PKC site Thr143 or Ser23/24 into aspartic acid (D) or alanine (A) to mimic phosphorylation and dephosphorylation, respectively. In troponin-exchanged donor cardiomyocytes experiments were repeated after incubation with exogenous PKA. Pseudo-phosphorylation of Thr143 increased myofilament Ca(2+) sensitivity compared with WT without affecting LDA in failing and donor cardiomyocytes. Subsequent PKA treatment enhanced the length-dependent shift in Ca(2+) sensitivity after WT and 143D exchange. Exchange with Ser23/24 variants demonstrated that pseudo-phosphorylation of both Ser23 and Ser24 is needed to enhance the length-dependent increase in Ca(2+) sensitivity. cTnI pseudo-phosphorylation did not alter length-dependent changes in maximal force. Thus phosphorylation at Thr143 enhances myofilament Ca(2+) sensitivity without affecting LDA, while Ser23/24 bisphosphorylation is needed to enhance the length-dependent increase in myofilament Ca(2+) sensitivity.

Protein kinase C phosphomimetics alter thin filament Ca2+ binding properties

Adrenergic stimulation modulates cardiac function by altering the phosphorylation status of several cardiac proteins. The Troponin complex, which is the Ca²⁺ sensor for cardiac contraction, is a hot spot for adrenergic phosphorylation. While the effect of β-adrenergic related PKA phosphorylation of troponin I at Ser23/24 is well established, the effects of α-adrenergic induced PKC phosphorylation on multiple sites of TnI (Ser43/45, Thr144) and TnT (Thr194, Ser198, Thr203 and Thr284) are much less clear. By utilizing an IAANS labeled fluorescent troponin C, , we systematically examined the site specific effects of PKC phosphomimetic mutants of TnI and TnT on TnC’s Ca²⁺ binding properties in the Tn complex and reconstituted thin filament. The majority of the phosphomemetics had little effect on the Ca²⁺ binding properties of the isolated Tn complex. However, when incorporated into the thin filament, the phosphomimetics typically altered thin filament Ca²⁺ sensitivity in a way consistent with their respective effects on Ca²⁺ sensitivity of skinned muscle preparations. The altered Ca²⁺ sensitivity could be generally explained by a change in Ca²⁺ dissociation rates. Within TnI, phosphomimetic Asp and Glu did not always behave similar, nor were Ala mutations (used to mimic non-phosphorylatable states) benign to Ca²⁺ binding. Our results suggest that Troponin may act as a hub on the thin filament, sensing physiological stimuli to modulate the contractile performance of the heart.

Cardiac myosin binding protein-C as a central target of cardiac sarcomere signaling: a special mini review series

Cardiac myosin binding protein-C (cMyBP-C) is a cardiac-specific thick filament assembly, accessory, and regulatory protein. Physiologically, it is a key regulator of cardiac contractility. With more than 200 mutations in the cMyBP-C gene directly linked to the development of cardiomyopathy and heart failure, cMyBP-C clearly plays a critical role in heart function. At baseline, cMyBP-C is highly phosphorylated, a condition required for normal cardiac function. However, the level of cMyBP-C phosphorylation is significantly decreased during heart failure, indicating that the level of cMyBP-C phosphorylation is directly linked to signaling and cardiac function. Early studies indicated that cMyBP-C interacts with myosin and titin, whereas studies now show that it also interacts with thin filament proteins. However, the exact role(s) of cMyBP-C in the heart remain(s) to be elucidated. As such, we invited experts in the field of cardiac muscle to identify and address key issues related to cMyBP-C by contributing a mini review on such topics as structure, function, regulation, cardiomyopathy, proteolysis, and gene therapy. Starting from this issue, Pflügers Archiv European Journal of Physiology will publish two invited mini review articles each month to discuss the most recent advances in the study of cMyBP-C.

Post-translational control of cardiac hemodynamics through myosin binding protein C

Cardiac myosin binding protein C (cMyBP-C) is an integral sarcomeric protein that associates with the thick, thin, and titin filament systems in the contractile apparatus. Three different isoforms of MyBP-C exist in mammalian muscle: slow skeletal (MyBPC1), fast skeletal (MyBP-C2, with several variants), and cardiac (cMyBP-C). Genetic screening studies show that mutations in MYBPC3 occur frequently and are responsible for as many as 30-35 % of identified cases of familial hypertrophic cardiomyopathy. The function of cMyBP-C is stringently regulated by its post-translational modification. In particular, the addition of phosphate groups occurs with high frequency on certain serine residues that are located in the cardiac-specific regulatory M domain. Phosphorylation of this domain has been extensively studied in vitro and in vivo. Phosphorylation of the M domain can regulate the manner in which actin and myosin interact, affecting the cross bridge cycle and ultimately, cardiac hemodynamics.

Agonist activated PKCβII translocation and modulation of cardiac myocyte contractile function

Elevated protein kinase C β_II (PKCβ_II) 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 PKCβ_II 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 PKCβ_II-expressing myocytes. PHE also caused PKCβ_II 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 PKCβ_II-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 PKCβ_II-expressing myocytes. These patterns are consistent with bifurcated activation of downstream signaling activity by PKCβ_II.

PKCα-specific phosphorylation of the troponin complex in human myocardium: a functional and proteomics analysis

Aims

Protein kinase Cα (PKCα) is one of the predominant PKC isoforms that phosphorylate cardiac troponin. PKCα is implicated in heart failure and serves as a potential therapeutic target, however, the exact consequences for contractile function in human myocardium are unclear. This study aimed to investigate the effects of PKCα phosphorylation of cardiac troponin (cTn) on myofilament function in human failing cardiomyocytes and to resolve the potential targets involved.

Methods and Results

Endogenous cTn from permeabilized cardiomyocytes from patients with end-stage idiopathic dilated cardiomyopathy was exchanged (∼69%) with PKCα-treated recombinant human cTn (cTn (DD+PKCα)). This complex has Ser23/24 on cTnI mutated into aspartic acids (D) to rule out in vitro cross-phosphorylation of the PKA sites by PKCα. Isometric force was measured at various [Ca²⁺] after exchange. The maximal force (F_max) in the cTn (DD+PKCα) group (17.1±1.9 kN/m²) was significantly reduced compared to the cTn (DD) group (26.1±1.9 kN/m²). Exchange of endogenous cTn with cTn (DD+PKCα) increased Ca²⁺-sensitivity of force (pCa₅₀ = 5.59±0.02) compared to cTn (DD) (pCa₅₀ = 5.51±0.02). In contrast, subsequent PKCα treatment of the cells exchanged with cTn (DD+PKCα) reduced pCa₅₀ to 5.45±0.02. Two PKCα-phosphorylated residues were identified with mass spectrometry: Ser198 on cTnI and Ser179 on cTnT, although phosphorylation of Ser198 is very low. Using mass spectrometry based-multiple reaction monitoring, the extent of phosphorylation of the cTnI sites was quantified before and after treatment with PKCα and showed the highest phosphorylation increase on Thr143.

Conclusion

PKCα-mediated phosphorylation of the cTn complex decreases F_max and increases myofilament Ca²⁺-sensitivity, while subsequent treatment with PKCα in situ decreased myofilament Ca²⁺-sensitivity. The known PKC sites as well as two sites which have not been previously linked to PKCα are phosphorylated in human cTn complex treated with PKCα with a high degree of specificity for Thr143.

Role of oxidants on calcium and sodium movement in healthy and diseased cardiac myocytes

In this review article we give an overview of current knowledge with respect to redox-sensitive alterations in Na(+) and Ca(2+) handling in the heart. In particular, we focus on redox-activated protein kinases including cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), and Ca/calmodulin-dependent protein kinase II (CaMKII), as well as on redox-regulated downstream targets such as Na(+) and Ca(2+) transporters and channels. We highlight the pathological and physiological relevance of reactive oxygen species and some of its sources (such as NADPH oxidases, NOXes) for excitation-contraction coupling (ECC). A short outlook with respect to the clinical relevance of redox-dependent Na(+) and Ca(2+) imbalance will be given.

Post-translational modifications of myofilament proteins involved in length-dependent prolongation of relaxation in rabbit right ventricular myocardium

The phosphorylation state of several cardiac myofilament proteins changes with the level of stretch in intact, twitch-contracting cardiac muscles. It remains unclear which kinases are involved in the length-dependent phosphorylation of these proteins. We set out to investigate which kinases are involved after a step-wise change in cardiac muscle length. We hypothesize that myofilament protein phosphorylation by PKCβII and PKA alters contractile kinetics during length-dependent activation. Right ventricular intact trabeculae were isolated from New Zealand White rabbit hearts and stimulated to contract at 1Hz. Twitch force recordings where taken at taut and optimal muscle lengths before and after administration of kinase inhibitors at 37°C. PKCβII inhibition significantly decreased time from stimulation to peak force (TTP), time from peak force to 50% relaxation (RT50), and 90% relaxation (RT90) at optimal muscle length. This led to a loss in the length-dependent increase of RT50 and RT90 in the presence of the PKCβII inhibitor, whereas the length-dependent increase in RT50 and RT90 was seen in the controls. PKA inhibition using H-89 significantly decreased TTP at both taut and optimal muscle lengths. Detection of Ser/Thr phosphorylation with ProQ-diamond staining indicates a role for PKCβII in the phosphorylation of tropomyosin and myosin light chain-2 (MLC2) and PKA for tropomyosin, troponin-I, MLC2, myosin binding protein-C, troponin-T (TnT) 3 and TnT4. Our data provide evidence for two signaling kinases acting upon myofilament proteins during length-dependent activation, and provide further insight for length-dependent myofilament function.

The role of the paracrine/autocrine mediator endothelin-1 in regulation of cardiac contractility and growth

Endothelin-1 (ET-1) is a critical autocrine and paracrine regulator of cardiac physiology and pathology. Produced locally within the myocardium in response to diverse mechanical and neurohormonal stimuli, ET-1 acutely modulates cardiac contractility. During pathological cardiovascular conditions such as ischaemia, left ventricular hypertrophy and heart failure, myocyte expression and activity of the entire ET-1 system is enhanced, allowing the peptide to both initiate and maintain maladaptive cellular responses. Both the acute and chronic effects of ET-1 are dependent on the activation of intracellular signalling pathways, regulated by the inositol-trisphosphate and diacylglycerol produced upon activation of the ET(A) receptor. Subsequent stimulation of protein kinases C and D, calmodulin-dependent kinase II, calcineurin and MAPKs modifies the systolic calcium transient, myofibril function and the activity of transcription factors that coordinate cellular remodelling. The precise nature of the cellular response to ET-1 is governed by the timing, localization and context of such signals, allowing the peptide to regulate both cardiomyocyte physiology and instigate disease.

LINKED ARTICLES

This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Redox regulation of sodium and calcium handling

SIGNIFICANCE

In heart failure (HF), contractile dysfunction and arrhythmias result from disturbed intracellular Ca handling. Activated stress kinases like cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), and Ca/calmodulin-dependent protein kinase II (CaMKII), which are known to influence many Ca-regulatory proteins, are mechanistically involved.

RECENT ADVANCES

Beside classical activation pathways, it is becoming increasingly evident that reactive oxygen species (ROS) can directly oxidize these kinases, leading to alternative activation. Since HF is associated with increased ROS generation, ROS-activated serine/threonine kinases may play a crucial role in the disturbance of cellular Ca homeostasis. Many of the previously described ROS effects on ion channels and transporters are possibly mediated by these stress kinases. For instance, ROS have been shown to oxidize and activate CaMKII, thereby increasing Na influx through voltage-gated Na channels, which can lead to intracellular Na accumulation and action potential prolongation. Consequently, Ca entry via activated NCX is favored, which together with ROS-induced dysfunction of the sarcoplasmic reticulum can lead to dramatic intracellular Ca accumulation, diminished contractility, and arrhythmias.

CRITICAL ISSUES

While low amounts of ROS may regulate kinase activity, excessive uncontrolled ROS production may lead to direct redox modification of Ca handling proteins. Therefore, depending on the source and amount of ROS generated, ROS could have very different effects on Ca-handling proteins.

FUTURE DIRECTIONS

The discrimination between fine-tuned ROS signaling and unspecific ROS damage may be crucial for the understanding of heart failure development and important for the investigation of targeted treatment strategies.

Oxidative stress and sarcomeric proteins

Oxidative stress accompanies a wide spectrum of clinically important cardiac disorders, including ischemia/reperfusion, diabetes mellitus, and hypertensive heart disease. Although reactive oxygen species (ROS) can activate signaling pathways that contribute to ischemic preconditioning and cardioprotection, high levels of ROS induce structural modifications of the sarcomere that impact on pump function and the pathogenesis of heart failure. However, the precise nature of the redox-dependent change in contractility is determined by the source/identity of the oxidant species, the level of oxidative stress, and the chemistry/position of oxidant-induced posttranslational modifications on individual proteins within the sarcomere. This review focuses on various ROS-induced posttranslational modifications of myofilament proteins (including direct oxidative modifications of myofilament proteins, myofilament protein phosphorylation by ROS-activated signaling enzymes, and myofilament protein cleavage by ROS-activated proteases) that have been implicated in the control of cardiac contractility.

Myofilament proteins: From cardiac disorders to proteomic changes

Myofilament proteins of the cardiac sarcomere house the molecular machinery responsible for generating tension and pressure. Release of intracellular Ca(2+) triggers myofilament tension generation and shortening, but the response to Ca(2+) is modulated by changes in key regulatory proteins. We review how these proteomic changes are essential to adaptive physiological regulation of cardiac output and become maladaptive in cardiac disorders. We also review the essentials of proteomic techniques used to study myofilament protein changes, including degradation, isoform expression, phosphorylation and oxidation. Selected proteomic studies illustrate the applications of these approaches.

Cardiac myofilaments: from proteome to pathophysiology

This review addresses the functional consequences of altered post-translational modifications of cardiac myofilament proteins in cardiac diseases such as heart failure and ischemia. The modifications of thick and thin filament proteins as well as titin are addressed. Understanding the functional consequences of altered protein modifications is an essential step in the development of targeted therapies for common cardiac diseases.

Multiple reaction monitoring to identify site-specific troponin I phosphorylated residues in the failing human heart

BACKGROUND

Human cardiac troponin I is known to be phosphorylated at multiple amino acid residues by several kinases. Advances in mass spectrometry allow sensitive detection of known and novel phosphorylation sites and measurement of the level of phosphorylation simultaneously at each site in myocardial samples.

METHODS AND RESULTS

On the basis of in silico prediction and liquid chromatography/mass spectrometry data, 14 phosphorylation sites on cardiac troponin I, including 6 novel residues (S4, S5, Y25, T50, T180, S198), were assessed in explanted hearts from end-stage heart failure transplantation patients with ischemic heart disease or idiopathic dilated cardiomyopathy and compared with samples obtained from nonfailing donor hearts (n=10 per group). Thirty mass spectrometry-based multiple reaction monitoring quantitative tryptic peptide assays were developed for each phosphorylatable and corresponding nonphosphorylated site. The results show that in heart failure there is a decrease in the extent of phosphorylation of the known protein kinase A sites (S22, S23) and other newly discovered phosphorylation sites located in the N-terminal extension of cardiac troponin I (S4, S5, Y25), an increase in phosphorylation of the protein kinase C sites (S41, S43, T142), and an increase in phosphorylation of the IT-arm domain residues (S76, T77) and C-terminal domain novel phosphorylation sites of cardiac troponin I (S165, T180, S198). In a canine dyssynchronous heart failure model, enhanced phosphorylation at 3 novel sites was found to decline toward control after resynchronization therapy.

CONCLUSIONS

Selective, functionally significant phosphorylation alterations occurred on individual residues of cardiac troponin I in heart failure, likely reflecting an imbalance in kinase/phosphatase activity. Such changes can be reversed by cardiac resynchronization.

Role of endothelin in the induction of cardiac hypertrophy in vitro

Endothelin (ET-1) is a peptide hormone mediating a wide variety of biological processes and is associated with development of cardiac dysfunction. Generally, ET-1 is regarded as a molecular marker released only in correlation with the observation of a hypertrophic response or in conjunction with other hypertrophic stress. Although the cardiac hypertrophic effect of ET-1 is demonstrated, inotropic properties of cardiac muscle during chronic ET-1-induced hypertrophy remain largely unclear. Through the use of a novel in vitro multicellular culture system, changes in contractile force and kinetics of rabbit cardiac trabeculae in response to 1 nM ET-1 for 24 hours can be observed. Compared to the initial force at t = 0 hours, ET-1 treated muscles showed a ~2.5 fold increase in developed force after 24 hours without any effect on time to peak contraction or time to 90% relaxation. ET-1 increased muscle diameter by 12.5 ± 3.2% from the initial size, due to increased cell width compared to non-ET-1 treated muscles. Using specific signaling antagonists, inhibition of NCX, CaMKII, MAPKK, and IP3 could attenuate the effect of ET-1 on increased developed force. However, among these inhibitions only IP3 receptor blocker could not prevent the increase muscle size by ET-1. Interestingly, though calcineurin-NFAT inhibition could not suppress the effect of ET-1 on force development, it did prevent muscle hypertrophy. These findings suggest that ET-1 provokes both inotropic and hypertrophic activations on myocardium in which both activations share the same signaling pathway through MAPK and CaMKII in associated with NCX activity.

PKCβII modulation of myocyte contractile performance

Significant up-regulation of the protein kinase Cβ(II) (PKCβ(II)) develops during heart failure and yet divergent functional outcomes are reported in animal models. The goal here is to investigate PKCβ(II) modulation of contractile function and gain insights into downstream targets in adult cardiac myocytes. Increased PKCβ(II) protein expression and phosphorylation developed after gene transfer into adult myocytes while expression remained undetectable in controls. The PKCβ(II) was distributed in a peri-nuclear pattern and this expression resulted in diminished rates and amplitude of shortening and re-lengthening compared to controls and myocytes expressing dominant negative PKCβ(II) (PKCβDN). Similar decreases were observed in the Ca(2+) transient and the Ca(2+) decay rate slowed in response to caffeine in PKCβ(II)-expressing myocytes. Parallel phosphorylation studies indicated PKCβ(II) targets phosphatase activity to reduce phospholamban (PLB) phosphorylation at residue Thr17 (pThr17-PLB). The PKCβ inhibitor, LY379196 (LY) restored pThr17-PLB to control levels. In contrast, myofilament protein phosphorylation was enhanced by PKCβ(II) expression, and individually, LY and the phosphatase inhibitor, calyculin A each failed to block this response. Further work showed PKCβ(II) increased Ca(2+)-activated, calmodulin-dependent kinase IIδ (CaMKIIδ) expression and enhanced both CaMKIIδ and protein kinase D (PKD) phosphorylation. Phosphorylation of both signaling targets also was resistant to acute inhibition by LY. These later results provide evidence PKCβ(II) modulates contractile function via intermediate downstream pathway(s) in cardiac myocytes.

Staurosporine inhibits frequency-dependent myofilament desensitization in intact rabbit cardiac trabeculae

Myofilament calcium sensitivity decreases with frequency in intact healthy rabbit trabeculae and associates with Troponin I and Myosin light chain-2 phosphorylation. We here tested whether serine-threonine kinase activity is primarily responsible for this frequency-dependent modulations of myofilament calcium sensitivity. Right ventricular trabeculae were isolated from New Zealand White rabbit hearts and iontophoretically loaded with bis-fura-2. Twitch force-calcium relationships and steady state force-calcium relationships were measured at frequencies of 1 and 4 Hz at 37 °C. Staurosporine (100 nM), a nonspecific serine-threonine kinase inhibitor, or vehicle (DMSO) was included in the superfusion solution before and during the contractures. Staurosporine had no frequency-dependent effect on force development, kinetics, calcium transient amplitude, or rate of calcium transient decline. The shift in the pCa₅₀ of the force-calcium relationship was significant from 6.05 ± 0.04 at 1 Hz versus 5.88 ± 0.06 at 4 Hz under control conditions (vehicle, P < 0.001) but not in presence of staurosporine (5.89 ± 0.08 at 1 Hz versus 5.94 ± 0.07 at 4 Hz, P = NS). Phosphoprotein analysis (Pro-Q Diamond stain) confirmed that staurosporine significantly blunted the frequency-dependent phosphorylation at Troponin I and Myosin light chain-2. We conclude that frequency-dependent modulation of calcium sensitivity is mediated through a kinase-specific effect involving phosphorylation of myofilament proteins.

Regulation of cardiac excitability by protein kinase C isozymes

Cardiac excitability and electrical activity are determined by the sum of individual ion channels, gap junctions and exchanger activities. Electrophysiological remodeling during heart disease involves changes in membrane properties of cardiomyocytes and is related to higher prevalence of arrhythmia-associated morbidity and mortality. Pharmacological and genetic manipulation of cardiac cells as well as animal models of cardiovascular diseases are used to identity changes in electrophysiological properties and the molecular mechanisms associated with the disease. Protein kinase C (PKC) and several other kinases play a pivotal role in cardiac electrophysiological remodeling. Therefore, identifying specific therapies that regulate these kinases is the main focus of current research. PKC, a family of serine/threonine kinases, has been implicated as potential signaling nodes associated with biochemical and biophysical stress in cardiovascular diseases. In this review, we describe the role of PKC isozymes that are involved in cardiac excitability and discuss both genetic and pharmacological tools that were used, their attributes and limitations. Selective and effective pharmacological interventions to normalize cardiac electrical activities and correct cardiac arrhythmias will be of great clinical benefit.

Augmented phosphorylation of cardiac troponin I in hypertensive heart failure

An altered cardiac myofilament response to activating Ca(2+) is a hallmark of human heart failure. Phosphorylation of cardiac troponin I (cTnI) is critical in modulating contractility and Ca(2+) sensitivity of cardiac muscle. cTnI can be phosphorylated by protein kinase A (PKA) at Ser(22/23) and protein kinase C (PKC) at Ser(22/23), Ser(42/44), and Thr(143). Whereas the functional significance of Ser(22/23) phosphorylation is well understood, the role of other cTnI phosphorylation sites in the regulation of cardiac contractility remains a topic of intense debate, in part, due to the lack of evidence of in vivo phosphorylation. In this study, we utilized top-down high resolution mass spectrometry (MS) combined with immunoaffinity chromatography to determine quantitatively the cTnI phosphorylation changes in spontaneously hypertensive rat (SHR) model of hypertensive heart disease and failure. Our data indicate that cTnI is hyperphosphorylated in the failing SHR myocardium compared with age-matched normotensive Wistar-Kyoto rats. The top-down electron capture dissociation MS unambiguously localized augmented phosphorylation sites to Ser(22/23) and Ser(42/44) in SHR. Enhanced Ser(22/23) phosphorylation was verified by immunoblotting with phospho-specific antibodies. Immunoblot analysis also revealed up-regulation of PKC-α and -δ, decreased PKCε, but no changes in PKA or PKC-β levels in the SHR myocardium. This provides direct evidence of in vivo phosphorylation of cTnI-Ser(42/44) (PKC-specific) sites in an animal model of hypertensive heart failure, supporting the hypothesis that PKC phosphorylation of cTnI may be maladaptive and potentially associated with cardiac dysfunction.

Protein kinase Cα as a heart failure therapeutic target

Heart failure afflicts ~5 million people and causes ~300,000 deaths a year in the United States alone. Heart failure is defined as a deficiency in the ability of the heart to pump sufficient blood in response to systemic demands, which results in fatigue, dyspnea, and/or edema. Identifying new therapeutic targets is a major focus of current research in the field. We and others have identified critical roles for protein kinase C (PKC) family members in programming aspects of heart failure pathogenesis. More specifically, mechanistic data have emerged over the past 6-7 years that directly implicate PKCα, a conventional PKC family member, as a nodal regulator of heart failure propensity. Indeed, deletion of the PKCα gene in mice, or its inhibition in rodents with drugs or a dominant negative mutant and/or inhibitory peptide, has shown dramatic protective effects that antagonize the development of heart failure. This review will weigh all the evidence implicating PKCα as a novel therapeutic target to consider for the treatment of heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

G protein-dependent and G protein-independent signaling pathways and their impact on cardiac function

G protein-coupled receptors signal through a variety of mechanisms that impact cardiac function, including contractility and hypertrophy. G protein-dependent and G protein-independent pathways each have the capacity to initiate numerous intracellular signaling cascades to mediate these effects. G protein-dependent signaling has been studied for decades and great strides continue to be made in defining the intricate pathways and effectors regulated by G proteins and their impact on cardiac function. G protein-independent signaling is a relatively newer concept that is being explored more frequently in the cardiovascular system. Recent studies have begun to reveal how cardiac function may be regulated via G protein-independent signaling, especially with respect to the ever-expanding cohort of β-arrestin-mediated processes. This review primarily focuses on the impact of both G protein-dependent and β-arrestin-dependent signaling pathways on cardiac function, highlighting the most recent data that illustrate the comprehensive nature of these mechanisms of G protein-coupled receptor signaling.

Pharmacological inhibition of βIIPKC is cardioprotective in late-stage hypertrophy

We previously found that in the hearts of hypertensive Dahl salt-sensitive rats, βIIPKC levels increase during the transition from compensated cardiac hypertrophy to cardiac dysfunction. Here we showed that a six-week treatment of these hypertensive rats with a βIIPKC-specific inhibitor, βIIV5-3, prolonged their survival by at least 6weeks, suppressed myocardial fibrosis and inflammation, and delayed the transition from compensated hypertrophy to cardiac dysfunction. In addition, changes in the levels of the Ca(2+)-handling proteins, SERCA2 and the Na(+)/Ca(2+) exchanger, as well as troponin I phosphorylation, seen in the control-treated hypertensive rats were not observed in the βΙΙPKC-treated rats, suggesting that βΙΙPKC contributes to the regulation of calcium levels in the myocardium. In contrast, treatment with the selective inhibitor of βIPKC, an alternative spliced form of βIIPKC, had no beneficial effects in these rats. We also found that βIIV5-3, but not βIV5-3, improved calcium handling in isolated rat cardiomyocytes and enhanced contractility in isolated rat hearts. In conclusion, our data using an in vivo model of cardiac dysfunction (late-phase hypertrophy), suggest that βIIPKC contributes to the pathology associated with heart failure and thus an inhibitor of βIIPKC may be a potential treatment for this disease.

The role of protein kinase C-mediated phosphorylation of sarcomeric proteins in the heart-detrimental or beneficial?

Protein kinase C (PKC) is a family of serine/threonine protein kinases, and alterations have been found in PKC isoform expression and localization in the failing heart. These alterations in PKC activation levels influence the PKC-mediated phosphorylation status of cellular target proteins involved in Ca²⁺-handling and sarcomeric contraction. The differences observed in the effects due to PKC-mediated phosphorylation may underlie part of the contractile dysfunction observed in the failing heart. It is therefore important to establish the beneficial and detrimental effects of this kinase in the healthy and failing heart. The function of PKC has been studied intensively; however, the complexity of the regulation of this kinase makes the interpretation of the different effects difficult. The main focus of this review is the (patho)physiological impact of phosphorylation of sarcomeric proteins, myosin light chain-2, troponin I and T, desmin, myosin binding protein-C, and titin by PKC.

A preferred AMPK phosphorylation site adjacent to the inhibitory loop of cardiac and skeletal troponin I

5'-AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that is activated when cellular AMP to ATP ratios rise, potentially serving as a key regulator of cellular energetics. Among the known targets of AMPK are catabolic and anabolic enzymes, but little is known about the ability of this kinase to phosphorylate myofilament proteins and thereby regulating the contractile apparatus of striated muscles. Here, we demonstrate that troponin I isoforms of cardiac (cTnI) and fast skeletal (fsTnI) muscles are readily phosphorylated by AMPK. For cTnI, two highly conserved serine residues were identified as AMPK sites using a combination of high-resolution top-down electron capture dissociation mass spectrometry, (32) P-incorporation, synthetic peptides, phospho-specific antibodies, and site-directed mutagenesis. These AMPK sites in cTnI were Ser149 adjacent to the inhibitory loop and Ser22 in the cardiac-specific N-terminal extension, at the level of cTnI peptides, the intact cTnI subunit, whole cardiac troponin complexes and skinned cardiomyocytes. Phosphorylation time-course experiments revealed that Ser149 was the preferred site, because it was phosphorylated 12-16-fold faster than Ser22 in cTnI. Ser117 in fsTnI, analogous to Ser149 in cTnI, was phosphorylated with similar kinetics as cTnI Ser149. Hence, the master energy-sensing protein AMPK emerges as a possibly important regulator of cardiac and skeletal contractility via phosphorylation of a preferred site adjacent to the inhibitory loop of the thin filament protein TnI.

Exercise training does not improve cardiac function in compensated or decompensated left ventricular hypertrophy induced by aortic stenosis

There is ample evidence that regular exercise exerts beneficial effects on left ventricular (LV) hypertrophy, remodeling and dysfunction produced by ischemic heart disease or systemic hypertension. In contrast, the effects of exercise on pathological LV hypertrophy and dysfunction produced by LV outflow obstruction have not been studied to date. Consequently, we evaluated the effects of 8 weeks of voluntary wheel running in mice (which mitigates post-infarct LV dysfunction) on LV hypertrophy and dysfunction produced by mild (mTAC) and severe (sTAC) transverse aortic constriction. mTAC produced ~40% LV hypertrophy and increased myocardial expression of hypertrophy marker genes but did not affect LV function, SERCA2a protein levels, apoptosis or capillary density. Exercise had no effect on global LV hypertrophy and function in mTAC but increased interstitial collagen, and ANP expression. sTAC produced ~80% LV hypertrophy and further increased ANP expression and interstitial fibrosis and, in contrast with mTAC, also produced LV dilation, systolic as well as diastolic dysfunction, pulmonary congestion, apoptosis and capillary rarefaction and decreased SERCA2a and ryanodine receptor (RyR) protein levels. LV diastolic dysfunction was likely aggravated by elevated passive isometric force and Ca(2+)-sensitivity of myofilaments. Exercise training failed to mitigate the sTAC-induced LV hypertrophy and capillary rarefaction or the decreases in SERCA2a and RyR. Exercise attenuated the sTAC-induced increase in passive isometric force but did not affect myofilament Ca(2+)-sensitivity and tended to aggravate interstitial fibrosis. In conclusion, exercise had no effect on LV function in compensated and decompensated cardiac hypertrophy produced by LV outflow obstruction, suggesting that the effect of exercise on pathologic LV hypertrophy and dysfunction depends critically on the underlying cause.

Redox signaling and cardiac sarcomeres

Oxidative stress is common in many clinically important cardiac disorders, including ischemia/reperfusion, diabetes, and hypertensive heart disease. Oxidative stress leads to derangements in pump function due to changes in the expression or function of proteins that regulate intracellular Ca(2+) homeostasis. There is growing evidence that the cardiodepressant actions of reactive oxygen species (ROS) also are attributable to ROS-dependent signaling events in the sarcomere. This minireview focuses on myofilament protein post-translational modifications induced by ROS or ROS-activated signaling enzymes that regulate cardiac contractility.

H2O2 alters rat cardiac sarcomere function and protein phosphorylation through redox signaling

ROS, such as H(2)O(2), are a component of pathological conditions in many organ systems and have been reported to be elevated in cardiac pathophysiology. The experiments presented here test the hypothesis that H(2)O(2) induces alterations in cardiac myofilament function by the posttranslational modification of sarcomeric proteins indirectly through PKC signaling. In vitro assessment of actomyosin Mg(2+)-ATPase activity of myofibrillar fractions showed blunted relative ATP consumption in the relaxed state (pCa 8.0) in response to treatment with 0.5 mM H(2)O(2) before myofilament isolation. The effect was attributable to downstream "redox signaling," inasmuch as the direct application of H(2)O(2) to isolated myofibrils did not alter Mg(2+)-ATPase activity. Ca(2+)-ATPase activity, which was used as a measure of myofibrillar myosin function, was unaffected by H(2)O(2). Functional experiments using rat cardiac trabeculae treated with 0.5 or 5 mM H(2)O(2) followed by detergent extraction of membranes demonstrated increased Ca(2+) sensitivity of force production, a faster rate of force redevelopment, and (for 5 mM) decreased maximum tension. Biochemical analysis of myocardial samples treated with 0.5 mM H(2)O(2) demonstrated increased phosphorylation of two sarcomeric proteins: cardiac troponin I and myosin-binding protein-C. These changes were eliminated by a general PKC inhibitor. However, H(2)O(2) and the general PKC activator PMA induced different phosphorylation patterns in cardiomyocytes in which PKC-delta was elevated by viral infection. These data provide evidence that PKC-dependent redox signaling affects the function of cardiac myofilaments and indicate modification of specific proteins through this signaling mechanism.

Deciphering modifications in swine cardiac troponin I by top-down high-resolution tandem mass spectrometry

Cardiac troponin I (cTnI) is an important regulatory protein in cardiac muscle, and its modification represents a key mechanism in the regulation of cardiac muscle contraction and relaxation. cTnI is often referred to as the "gold-standard" serum biomarker for diagnosing patients with acute cardiac injury since it is unique to the heart and released into the circulation following necrotic death of cardiac tissue. The swine (Sus scrofa) heart model is extremely valuable for cardiovascular research since the heart anatomy and coronary artery distribution of swine are almost identical to those of humans. Herein, we report a comprehensive characterization of the modifications in swine cTnI using top-down high-resolution tandem mass spectrometry in conjugation with immunoaffinity chromatography purification. High-resolution high accuracy mass spectrometry revealed that swine cTnI affinity purified from domestic pig hearts was N-terminally acetylated and phosphorylated. Electron capture disassociation is uniquely suited for localization of labile phosphorylations, which unambiguously identified Ser22/Ser23 as the only basally phosphorylated sites that are well-known to be regulated by protein kinase A and protein kinase C. Moreover, a combination of tandem mass spectrometry with sequence homology alignment effectively localized a single amino acid polymorphism, V116A, representing a novel genetic variant of swine cTnI. Overall, our studies demonstrated the unique power of top-down high-resolution tandem mass spectrometry in the characterization of protein modifications, including labile phosphorylation and unexpected sequence variants.