Sites phosphorylated in bovine cardiac troponin T and I. Characterization by 31P-NMR spectroscopy and phosphorylation by protein kinases

Abnormal phosphorylation / dephosphorylation and Ca2+ dysfunction in heart failure

Heart failure (HF) can be caused by a variety of causes characterized by abnormal myocardial systole and diastole. Ca2+ current through the L-type calcium channel (LTCC) on the membrane is the initial trigger signal for a cardiac cycle. Declined systole and diastole in HF are associated with dysfunction of myocardial Ca2+ function. This disorder can be correlated with unbalanced levels of phosphorylation / dephosphorylation of LTCC, endoplasmic reticulum (ER), and myofilament. Kinase and phosphatase activity changes along with HF progress, resulting in phased changes in the degree of phosphorylation / dephosphorylation. It is important to realize the phosphorylation / dephosphorylation differences between a normal and a failing heart. This review focuses on phosphorylation / dephosphorylation changes in the progression of HF and summarizes the effects of phosphorylation / dephosphorylation of LTCC, ER function, and myofilament function in normal conditions and HF based on previous experiments and clinical research. Also, we summarize current therapeutic methods based on abnormal phosphorylation / dephosphorylation and clarify potential therapeutic directions.

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.

Multisite phosphorylation of the cardiac ryanodine receptor: a random or coordinated event?

Many proteins are phosphorylated at more than one phosphorylation site to achieve precise tuning of protein function and/or integrate a multitude of signals into the activity of one protein. Increasing the number of phosphorylation sites significantly broadens the complexity of molecular mechanisms involved in processing multiple phosphorylation sites by one or more distinct kinases. The cardiac ryanodine receptor (RYR2) is a well-established multiple phospho-target of kinases activated in response to β-adrenergic stimulation because this Ca²⁺ channel is a critical component of Ca²⁺ handling machinery which is responsible for β-adrenergic enhancement of cardiac contractility. Our review presents a selective overview of the extensive, often conflicting, literature which focuses on identifying reliable lines of evidence to establish if multiple RYR2 phosphorylation is achieved randomly or in a specific sequence, and whether phosphorylation at individual sites is functionally specific and additive or similar and can therefore be substituted.

ADP-stimulated contraction: A predictor of thin-filament activation in cardiac disease

Diastolic dysfunction is general to all idiopathic dilated (IDCM) and hypertrophic cardiomyopathy (HCM) patients. Relaxation deficits may result from increased actin-myosin formation during diastole due to altered tropomyosin position, which blocks myosin binding to actin in the absence of Ca(2+). We investigated whether ADP-stimulated force development (without Ca(2+)) can be used to reveal changes in actin-myosin blockade in human cardiomyopathy cardiomyocytes. Cardiac samples from HCM patients, harboring thick-filament (MYH7mut, MYBPC3mut) and thin-filament (TNNT2mut, TNNI3mut) mutations, and IDCM were compared with sarcomere mutation-negative HCM (HCMsmn) and nonfailing donors. Myofilament ADP sensitivity was higher in IDCM and HCM compared with donors, whereas it was lower for MYBPC3. Increased ADP sensitivity in IDCM, HCMsmn, and MYH7mut was caused by low phosphorylation of myofilament proteins, as it was normalized to donors by protein kinase A (PKA) treatment. Troponin exchange experiments in a TNNT2mut sample corrected the abnormal actin-myosin blockade. In MYBPC3trunc samples, ADP sensitivity highly correlated with cardiac myosin-binding protein-C (cMyBP-C) protein level. Incubation of cardiomyocytes with cMyBP-C antibody against the actin-binding N-terminal region reduced ADP sensitivity, indicative of cMyBP-C's role in actin-myosin regulation. In the presence of Ca(2+), ADP increased myofilament force development and sarcomere stiffness. Enhanced sarcomere stiffness in sarcomere mutation-positive HCM samples was irrespective of the phosphorylation background. In conclusion, ADP-stimulated contraction can be used as a tool to study how protein phosphorylation and mutant proteins alter accessibility of myosin binding on actin. In the presence of Ca(2+), pathologic [ADP] and low PKA-phosphorylation, high actin-myosin formation could contribute to the impaired myocardial relaxation observed in cardiomyopathies.

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.

Impact of site-specific phosphorylation of protein kinase A sites Ser23 and Ser24 of cardiac troponin I in human cardiomyocytes

PKA-mediated phosphorylation of contractile proteins upon β-adrenergic stimulation plays an important role in the regulation of cardiac performance. Phosphorylation of the PKA sites (Ser(23)/Ser(24)) of cardiac troponin (cTn)I results in a decrease in myofilament Ca(2+) sensitivity and an increase in the rate of relaxation. However, the relation between the level of phosphorylation of the sites and the functional effects in the human myocardium is unknown. Therefore, site-directed mutagenesis was used to study the effects of phosphorylation at Ser(23) and Ser(24) of cTnI on myofilament function in human cardiac tissue. Serines were replaced by aspartic acid (D) or alanine (A) to mimic phosphorylation and dephosphorylation, respectively. cTnI-DD mimics both sites phosphorylated, cTnI-AD mimics Ser(23) unphosphorylated and Ser(24) phosphorylated, cTnI-DA mimics Ser(23) phosphorylated and Ser(24) unphosphorylated, and cTnI-AA mimics both sites unphosphorylated. Force development was measured at various Ca(2+) concentrations in permeabilized cardiomyocytes in which the endogenous troponin complex was exchanged with these recombinant human troponin complexes. In donor cardiomyocytes, myofilament Ca(2+) sensitivity (pCa(50)) was significantly lower in cTnI-DD (pCa(50): 5.39 ± 0.01) compared with cTnI-AA (pCa(50): 5.50 ± 0.01), cTnI-AD (pCa(50): 5.48 ± 0.01), and cTnI-DA (pCa(50): 5.51 ± 0.01) at ~70% cTn exchange. No effects were observed on the rate of tension redevelopment. In cardiomyocytes from idiopathic dilated cardiomyopathic tissue, a linear decline in pCa(50) with cTnI-DD content was observed, saturating at ~55% bisphosphorylation. Our data suggest that in the human myocardium, phosphorylation of both PKA sites on cTnI is required to reduce myofilament Ca(2+) sensitivity, which is maximal at ~55% bisphosphorylated cTnI. The implications for in vivo cardiac function in health and disease are detailed in the DISCUSSION in this article.

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.

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.

Protein kinase C alpha and epsilon phosphorylation of troponin and myosin binding protein C reduce Ca2+ sensitivity in human myocardium

Previous studies indicated that the increase in protein kinase C (PKC)-mediated myofilament protein phosphorylation observed in failing myocardium might be detrimental for contractile function. This study was designed to reveal and compare the effects of PKCα- and PKCε-mediated phosphorylation on myofilament function in human myocardium. Isometric force was measured at different [Ca²⁺] in single permeabilized cardiomyocytes from failing human left ventricular tissue. Activated PKCα and PKCε equally reduced Ca²⁺ sensitivity in failing cardiomyocytes (ΔpCa₅₀ = 0.08 ± 0.01). Both PKC isoforms increased phosphorylation of troponin I- (cTnI) and myosin binding protein C (cMyBP-C) in failing cardiomyocytes. Subsequent incubation of failing cardiomyocytes with the catalytic subunit of protein kinase A (PKA) resulted in a further reduction in Ca²⁺ sensitivity, indicating that the effects of both PKC isoforms were not caused by cross-phosphorylation of PKA sites. Both isozymes showed no effects on maximal force and only PKCα resulted in a modest significant reduction in passive force. Effects of PKCα were only minor in donor cardiomyocytes, presumably because of already saturated cTnI and cMyBP-C phosphorylation levels. Donor tissue could therefore be used as a tool to reveal the functional effects of troponin T (cTnT) phosphorylation by PKCα. Massive dephosphorylation of cTnT with alkaline phosphatase increased Ca²⁺ sensitivity. Subsequently, PKCα treatment of donor cardiomyocytes reduced Ca²⁺ sensitivity (ΔpCa₅₀ = 0.08 ± 0.02) and solely increased phosphorylation of cTnT, but did not affect maximal and passive force. PKCα- and PKCε-mediated phosphorylation of cMyBP-C and cTnI as well as cTnT decrease myofilament Ca²⁺ sensitivity and may thereby reduce contractility and enhance relaxation of human myocardium.

Single amino acid sequence polymorphisms in rat cardiac troponin revealed by top-down tandem mass spectrometry

Heterotrimeric cardiac troponin (cTn) is a critical component of the thin filament regulatory complex in cardiac muscle. Two of the three subunits, cTnI and cTnT, are subject to post-translational modifications such as proteolysis and phosphorylation, but linking modification patterns to function remains a major challenge. To obtain a global view of the biochemical state of cTn in native tissue, we performed high resolution top-down mass spectrometry of cTn heterotrimers from healthy adult rat hearts. cTn heterotrimers were affinity purified, desalted and then directly subjected to mass spectrometry using a 7 Tesla Thermo LTQ-FT-ICR instrument equipped with an ESI source. Molecular ions for N-terminally processed and acetylated cTnI and cTnT were readily detected as were other post-translationally modified forms of these proteins. cTnI was phosphorylated with a distribution of un-, mono- and bisphosphorylated forms of 41 +/- 3%, 46 +/- 1%, 13 +/- 3%, respectively. cTnT was predominantly monophosphorylated and partially proteolyzed at the Glu(29)-Pro(30) peptide bond. Also observed in high resolution spectra were 'shadow' peaks of similar intensity to 'parent' peaks exhibiting masses of cTnI+16 Da and cTnT+128 Da, subsequently shown by tandem mass spectrometry (MS/MS) to be single amino acid polymorphisms. Intact and protease-digested cTn subunits were fragmented by electron capture dissociation or collision activated dissociation to localize an Ala/Ser polymorphism at residue 7 of cTnI. Similar analysis of cTnT localized an additional Gln within a three residue alternative splice site beginning at residue 192. Besides being able to provide unique insights into the global state of post-translational modification of cTn subunits, high resolution top-down mass spectrometry readily revealed naturally occurring single amino acid sequence variants including a genetic polymorphism at residue 7 in cTnI, and an alternative splice isoform that affects a putative hinge region around residue 192 of cTnT, all of which co-exist within a single rat heart.

Present and Future Biochemical Markers for Detection of Acute Coronary Syndrome

The use of biochemical markers in the diagnosis and management of patients with acute coronary syndrome has increased continually in recent decades. The development of highly sensitive and cardiac-specific troponin assays has changed the view on diagnosis of myocardial infarction and also extended the role of biochemical markers of necrosis into risk stratification and guidance for treatment. The consensus definition of myocardial infarction places increased emphasis on cardiac marker testing, with cardiac troponin replacing creatine kinase MB as the "gold standard" for diagnosis of myocardial infarction. Along with advances in the use of more cardiac-specific markers of myocardial necrosis, biochemical markers that are involved in the progression of atherosclerotic plaques to the vulnerable state or that signal the presence of vulnerable plaques have recently been identified. These markers have variable abilities to predict the risk of an individual for acute coronary syndrome. The aim of this review is to provide an overview of the well-established markers of myocardial necrosis, with a special focus on cardiac troponin I, together with a summary of some of the potential future markers of inflammation, plaque instability, and ischemia.

Multiple molecular forms of circulating cardiac troponin: analytical and clinical significance

Cardiac troponin T (cTnT) and I (cTnI) are highly specific and sensitive biomarkers of myocardial cell damage and are now accepted as the ‘gold standard’ diagnostic test for acute coronary syndrome and supersede the classical muscle enzyme biomarkers. While the understanding of the development and structure of the troponins has advanced, detailed biochemistry of the troponin molecules is complex and poorly understood. Many post-translational molecular forms of troponin are known to exist. The diversity of these circulating forms may have a clinical impact and the notion of a disease-specific troponin protein signature has been suggested. However, the effects of these multiple forms on commercial assay performance and their impact clinically are currently unknown and should be the focus of future research and assay design.

Frequency-dependent myofilament Ca2+ desensitization in failing rat myocardium

The positive force-frequency relation, one of the key factors modulating performance of healthy myocardium, has been attributed to an increased Ca(2+) influx per unit of time. In failing hearts, a blunted, flat or negative force-frequency relation has been found. In healthy and failing hearts frequency-dependent alterations in Ca(2+) sensitivity of the myofilaments, related to different phosphorylation levels of contractile proteins, could contribute to this process. Therefore, the frequency dependency of force, intracellular free Ca(2+) ([Ca(2+)](i)), Ca(2+) sensitivity and contractile protein phosphorylation were determined in control and monocrotaline-treated, failing rat hearts. An increase in frequency from 0.5 to 6 Hz resulted in an increase in force in control (14.3 +/- 3.0 mN mm(-2)) and a decrease in force in failing trabeculae (9.4 +/- 3.2 mN mm(-2)), whereas in both groups the amplitude of [Ca(2+)](i) transient increased. In permeabilized cardiomyocytes, isolated from control hearts paced at 0 and 9 Hz, Ca(2+) sensitivity remained constant with frequency (pCa(50): 5.55 +/- 0.02 and 5.58 +/- 0.01, respectively, P>0.05), whereas in cardiomyocytes from failing hearts Ca(2+) sensitivity decreased with frequency (pCa(50): 5.62 +/- 0.01 and 5.57 +/- 0.01, respectively, P<0.05). After incubation of the cardiomyocytes with protein kinase A (PKA) this frequency dependency of Ca(2+) sensitivity was abolished. Troponin I (TnI) and myosin light chain 2 (MLC2) phosphorylation remained constant in control hearts but both increased with frequency in failing hearts. In conclusion, in heart failure frequency-dependent myofilament Ca(2+) desensitization, through increased TnI phosphorylation, contributes to the negative force-frequency relation and is counteracted by a frequency-dependent MLC2 phosphorylation. We propose a novel role for PKC-mediated TnI phosphorylation in modulating the force-frequency relation.

pkaPS: prediction of protein kinase A phosphorylation sites with the simplified kinase-substrate binding model

Background

Protein kinase A (cAMP-dependent kinase, PKA) is a serine/threonine kinase, for which ca. 150 substrate proteins are known. Based on a refinement of the recognition motif using the available experimental data, we wished to apply the simplified substrate protein binding model for accurate prediction of PKA phosphorylation sites, an approach that was previously successful for the prediction of lipid posttranslational modifications and of the PTS1 peroxisomal translocation signal.

Results

Approximately 20 sequence positions flanking the phosphorylated residue on both sides have been found to be restricted in their sequence variability (region -18...+23 with the site at position 0). The conserved physical pattern can be rationalized in terms of a qualitative binding model with the catalytic cleft of the protein kinase A. Positions -6...+4 surrounding the phosphorylation site are influenced by direct interaction with the kinase in a varying degree. This sequence stretch is embedded in an intrinsically disordered region composed preferentially of hydrophilic residues with flexible backbone and small side chain. This knowledge has been incorporated into a simplified analytical model of productive binding of substrate proteins with PKA.

Conclusion

The scoring function of the pkaPS predictor can confidently discriminate PKA phosphorylation sites from serines/threonines with non-permissive sequence environments (sensitivity of ~96% at a specificity of ~94%). The tool "pkaPS" has been applied on the whole human proteome. Among new predicted PKA targets, there are entirely uncharacterized protein groups as well as apparently well-known families such as those of the ribosomal proteins L21e, L22 and L6.

Availability

The supplementary data as well as the prediction tool as WWW server are available at .

Reviewers

Erik van Nimwegen (Biozentrum, University of Basel, Switzerland), Sandor Pongor (International Centre for Genetic Engineering and Biotechnology, Trieste, Italy), Igor Zhulin (University of Tennessee, Oak Ridge National Laboratory, USA).

Troponin phosphorylation and regulatory function in human heart muscle: dephosphorylation of Ser23/24 on troponin I could account for the contractile defect in end-stage heart failure

We made quantitative measurements of phosphorylation in troponin isolated from 6 non-failing donor hearts and 6 explanted hearts with end-stage heart failure in SDS-PAGE gels using Pro-Q Diamond phosphoprotein stain. The troponin T phosphorylation level was the same in troponin from failing and non-failing heart (3.1 mol Pi/mol). However, troponin I phosphorylation was significantly lower in failing (0.37+/-0.18 mol Pi/mol) compared with non-failing heart troponin (2.25+/-0.36 mol Pi/mol). Levels of troponin I PKA-dependent phosphorylation, measured with a phosphoserine 23/24-specific antibody, were also significantly lower in failing heart troponin (0.19+/-0.06 mol Pi/mol) compared to non-failing troponin (1.14+/-0.09 mol Pi/mol). We calculate that there is phosphorylation in addition to serine 23/24 of 1.11+/-0.34 mol Pi/mol in non-failing reduced to 0.18+/-0.17 mol Pi/mol in failing heart troponin, attributed to phosphorylation on the PKC sites. To test for the functional role of troponin I phosphorylation, the native troponin I from either non-failing or failing heart troponin was exchanged for a recombinant (unphosphorylated) human cardiac troponin I. Thin filament Ca(2+)-regulatory function was studied with the quantitative in vitro motility assay: thin filaments containing the replaced troponin I resulted in a failing phenotype of a 17-26% reduced sliding speed and an increased Ca(2+)-sensitivity relative to non-failing troponin (EC(50) TnI-exchanged/non-failing=0.57, p<0.001). When exchanged with troponin I phosphorylated with PKA motility parameters reverted to a pattern indistinguishable from non-failing troponin (p=0.35-0.75). We suggest that changes in troponin function can account for the contractile abnormality in failing heart muscle and that the functional changes in troponin are due to reduced phosphorylation of troponin I at the PKA sites.

Myocardial regulatory proteins and heart failure

Cardiac troponin T (cTnT) and cardiac troponin I (cTnI) are considered to be the most specific and sensitive biochemical markers of myocardial damage. Troponins have been studied in a wide range of clinical settings, including heart failure; however, there are few data on the role of regulatory proteins in the pathogenesis of heart failure, although a few interesting hypotheses have been proposed. A considerable body of evidence favours the view that alteration of the myocardial thin filament is the primary event leading to defective contractility of the failing myocardium, while the changes in Ca(2+) handling are a compensatory response. A better understanding of the role of regulatory proteins under different physiological and pathological conditions could lead to new therapeutic approaches in heart failure. Recently, calcium sensitisation has been proposed as a novel method by which cardiac performance may be enhanced via an increase in the affinity of troponin C for calcium but without affecting intracellular calcium concentration. To date, the only calcium sensitizer used in clinical practice is levosimendan.

In Vivo and in Vitro Analysis of Cardiac Troponin I Phosphorylation

Adrenergic stimulation induces positive changes in cardiac contractility and relaxation. Cardiac troponin I is phosphorylated at different sites by protein kinase A and protein kinase C, but the effects of these post-translational modifications on the rate and extent of contractility and relaxation during beta-adrenergic stimulation in the intact animal remain obscure. To investigate the effect(s) of complete and chronic cTnI phosphorylation on cardiac function, we generated transgenic animals in which the five possible phosphorylation sites were replaced with aspartic acid, mimicking a constant state of complete phosphorylation (cTnI-AllP). We hypothesized that chronic and complete phosphorylation of cTnI might result in increased morbidity or mortality, but complete replacement with the transgenic protein was benign with no detectable pathology. To differentiate the effects of the different phosphorylation sites, we generated another mouse model, cTnI-PP, in which only the protein kinase A phosphorylation sites (Ser(23)/Ser(24)) were mutated to aspartic acid. In contrast to the cTnIAllP, the cTnI-PP mice showed enhanced diastolic function under basal conditions. The cTnI-PP animals also showed augmented relaxation and contraction at higher heart rates compared with the nontransgenic controls. Nuclear magnetic resonance amide proton/nitrogen chemical shift analysis of cardiac troponin C showed that, in the presence of cTnI-AllP and cTnI-PP, the N terminus exhibits a more closed conformation, respectively. The data show that protein kinase C phosphorylation of cTnI plays a dominant role in depressing contractility and exerts an antithetic role on the ability of protein kinase A to increase relaxation.

Covalent and noncovalent modification of thin filament action: the essential role of troponin in cardiac muscle regulation

Troponin is essential for the regulation of cardiac contraction. Troponin is a sarcomeric molecular switch, directly regulating the contractile event in concert with intracellular calcium signals. Troponin isoform switching, missense mutations, proteolytic cleavage, and posttranslational modifications are known to directly affect sarcomeric regulation. This review focuses on physiologically relevant covalent and noncovalent modifications in troponin as part of a thematic series on cardiac thin filament function in health and disease.

Protein kinase C and A sites on troponin I regulate myofilament Ca2+ sensitivity and ATPase activity in the mouse myocardium

Cardiac troponin I (cTnI) is a phosphoprotein subunit of the troponin-tropomyosin complex that is thought to inhibit cardiac muscle contraction during diastole. To investigate the contributions of cTnI phosphorylation to cardiac regulation, transgenic mice were created with the phosphorylation sites of cTnI mutated to alanine. Activation of protein kinase C (PKC) by perfusion of hearts with phorbol-12-myristate-13-acetate (PMA) or endothelin-1 (ET-1) inhibited the maximum ATPase rate by up to 25 % and increased the Ca2+ sensitivity of ATPase activity and of isometric tension by up to 0.15 pCa units. PKC activation no longer altered cTnI phosphorylation, depressed ATPase rates or enhanced myofilament Ca2+ sensitivity in transgenic mice expressing cTnI that could not be phosphorylated on serines43/45 and threonine144 (PKC sites). Modest changes in myosin regulatory light chain phosphorylation occurred in all mouse lines, but increases in myofilament Ca2+ sensitivity required the presence of phosphorylatable cTnI. For comparison, the beta-adrenergic agonist isoproterenol caused a 38 % increase in maximum ATPase rate and a 0.12 pCa unit decrease in myofilament Ca2+ sensitivity. These beta-adrenergic effects were absent in transgenic mice expressing cTnI that could not be phosphorylated on serines23/24 (protein kinase A, PKA, sites). Overall, the results indicate that PKC and PKA exert opposing effects on actomyosin function by phosphorylating cTnI on distinct sites. A primary role of PKC phosphorylation of cTnI may be to reduce the requirements of the contractile apparatus for both Ca2+ and ATP, thereby promoting efficient ATP utilisation during contraction.

Phosphorylation and flexibility of cyclic-AMP-dependent protein kinase (PKA) using (31)P NMR spectroscopy

Cell signaling pathways rely on phosphotransfer reactions that are catalyzed by protein kinases. The protein kinases themselves are typically regulated by phosphorylation and concurrent structural rearrangements, both near the catalytic site and elsewhere. Thus, physiological function requires posttranslational modification and deformable structures. A prototypical example is provided by cyclic AMP-dependent protein kinase (PKA). It is activated by phosphorylation, is inhomogeneously phosphorylated when expressed in bacteria, and exhibits a wide range of dynamic properties. Here we use (31)P nuclear magnetic resonance (NMR) spectroscopy to characterize the phosphorylation states and to estimate the flexibility of the phosphorylation sites of 2-, 3-, and 4-fold phosphorylated PKA. The phosphorylation sites Ser10, Ser139, Thr197, and Ser338 are assigned to individual NMR resonances, assisted by complexation with AMP-PNP and dephosphorylation with alkaline phosphatase. Rotational diffusion correlation times estimated from resonance line widths show progressively increasing flexibilities for phosphothreonine 197, phosphoserines 139 and 338, and disorder at phosphoserine 10, consistent with crystal structures of PKA. However, because the apparent rotational diffusion correlation time fitted for phosphothreonine 197 of the activation loop is longer than the overall PKA rotational diffusion time, microsecond to millisecond time scale conformational exchange effects involving motions of phosphothreonine 197 are probable. These may represent "open"-"closed" transitions of the uncomplexed protein in solution. These data represent direct measurements of flexibilities also associated with functional properties, such as ATP binding and membrane association, and illustrate the applicability of (31)P NMR for functional and dynamic characterization of protein kinase phosphorylation sites.

Detection technologies in proteome analysis

Common strategies employed for general protein detection include organic dye, silver stain, radiolabeling, reverse stain, fluorescent stain, chemiluminescent stain and mass spectrometry-based approaches. Fluorescence-based protein detection methods have recently surpassed conventional technologies such as colloidal Coomassie blue and silver staining in terms of quantitative accuracy, detection sensitivity, and compatibility with modern downstream protein identification and characterization procedures, such as mass spectrometry. Additionally, specific detection methods suitable for revealing protein post-translational modifications have been devised over the years. These include methods for the detection of glycoproteins, phosphoproteins, proteolytic modifications, S-nitrosylation, arginine methylation and ADP-ribosylation. Methods for the detection of a range of reporter enzymes and epitope tags are now available as well, including those for visualizing beta-glucuronidase, beta-galactosidase, oligohistidine tags and green fluorescent protein. Fluorescence-based and mass spectrometry-based methodologies are just beginning to offer unparalleled new capabilities in the field of proteomics through the performance of multiplexed quantitative analysis. The primary objective of differential display proteomics is to increase the information content and throughput of proteomics studies through multiplexed analysis. Currently, three principal approaches to differential display proteomics are being actively pursued, difference gel electrophoresis (DIGE), multiplexed proteomics (MP) and isotope-coded affinity tagging (ICAT). New multiplexing capabilities should greatly enhance the applicability of the two-dimensional gel electrophoresis technique with respect to addressing fundamental questions related to proteome-wide changes in protein expression and post-translational modification.

Troponin I chimera analysis of the cardiac myofilament tension response to protein kinase A

Viral-mediated gene transfer of troponin I (TnI) isoforms and chimeras into adult rat cardiac myocytes was used to investigate the role TnI domains play in the myofilament tension response to protein kinase A (PKA). In myocytes expressing endogenous cardiac TnI (cTnI), PKA phosphorylated TnI and myosin-binding protein C and decreased the Ca2+ sensitivity of myofilament tension. In marked contrast, PKA did not influence Ca2+-activated tension in myocytes expressing the slow skeletal isoform of TnI or a chimera (N-slow/card-C TnI), which lack the unique phosphorylatable amino terminal extension found in cTnI. PKA-mediated phosphorylation of a second TnI chimera, N-card/slow-C TnI, which has the amino terminal region of cTnI, caused a decrease in the Ca2+ sensitivity of tension comparable in magnitude to control myocytes. Based on these results, we propose the amino terminal region shared by cTnI and N-card/slow-C TnI plays a central role in determining the magnitude of the PKA-mediated shift in myofilament Ca2+ sensitivity, independent of the isoform-specific functional domains previously defined within the carboxyl terminal backbone of TnI. Interestingly, exposure of permeabilized myocytes to acidic pH after PKA-mediated phosphorylation of cTnI resulted in an additive decrease in myofilament Ca2+ sensitivity. The isoform-specific, pH-sensitive region within TnI lies in the carboxyl terminus of TnI, and the additive response provides further evidence for the presence of a separate domain that directly transduces the PKA phosphorylation signal.

Overexpression of human cardiac troponin in Escherichia coli: its purification and characterization

All three subunits of the human cardiac troponin complex (cTn), namely the major isoform of the tropomyosin binding subunit (hcTnT3), the inhibitory subunit (cTnI), and the calcium binding subunit (cTnC), have been coexpressed in Escherichia coli. The cDNAs of each subunit have been cloned into the pSBET vector and transformed into E. coli. The coexpressed subunits assembled within the bacterial cells to form the hcTn complex (hcTnT3.hcTnI.hcTnC). The complex was isolated and purified by three chromatographic steps. Per 6-L cell culture about 10 mg of a highly purified troponin complex showing the expected 1:1:1 molar ratio of hcTnT3:cTnI:cTnC was obtained. Upon phosphorylation by protein kinase A at Ser22 and Ser23 in cTnI, this recombinant troponin complex shows a nearly identical (31)P NMR spectrum to the native one isolated from bovine heart. By measuring the rate of myosin S1 binding to reconstituted thin filaments it was shown that the dependence of the regulation of S1 binding upon calcium concentration and bisphosphorylation was comparable to the native complex.

Stepwise subunit interaction changes by mono- and bisphosphorylation of cardiac troponin I

Four phosphorylation degrees of cardiac troponin I (cTnI) have been characterized, namely, a dephospho, a bisphospho, and two monophospho states. Here we describe for the first time a role of the monophosphorylated forms. We have investigated the interaction between the cardiac troponin subunits dependent on the phosphorylation state of cTnI by surface plasmon resonance (SPR) spectroscopy. The monophosphorylated forms were generated by mutating each of the two serine residues, located in human cTnI at positions 22 and 23, to alanine. Association and dissociation rate constants of binary (cTnI-cTnT and cTnI-cTnC) and ternary (cTnI/cTnC complex-cTnT) complexes were determined. Mono- and consecutive bisphosphorylation of cTnI gradually reduces the affinity to cTnC and cTnT by lowering the association rate constants; the dissociation rate constants remain unchanged. Phosphorylation also affects formation of the ternary complexes; however, in this instance, association rate constants are constant, and dissociation rate constants are enhanced. A model of cardiac troponin is presented describing an induction of distinct conformational changes by mono- and bisphosphorylation of cTnI.

Troponin T: genetics, properties and function

Troponin T (TnT) is present in striated muscle of vertebrates and invertebrates as a group of homologous proteins with molecular weights usually in the 31-36 kDa range. It occupies a unique role in the regulatory protein system in that it interacts with TnC and TnI of the troponin complex and the proteins of the myofibrillar thin filament, tropomyosin and actin. In the myofibril the molecule is about 18 nm long and for much its length interacts with tropomyosin. The ability of TnT to form a complex with tropomyosin is responsible for locating the troponin complex with a periodicity of 38.5 nm along the thin filament of the myofibril. In addition to it structural role, TnT has the important function of transforming the TnI-TnC complex into a system, the inhibitory activity of which, on the tropomyosin-actomyosin MgATPase of the myofibril, becomes sensitive to calcium ions. Different genes control the expression of TnT in fast skeletal, slow skeletal and cardiac muscles. In all muscles, and particularly in fast skeletal, alternative splicing of mRNA produces a series of isoforms in a developmentally regulated manner. In consequence TnT exists in many more isoforms than any of the other thin filament proteins, the TnT superfamily. Despite the general homology of TnT isoforms, this alternative splicing leads to variable regions close to the N- and C-termini. As the isoforms have slightly different effects on the calcium sensitivity of the actomyosin MgATPase, modulation of the contractile response to calcium can occur during development and in different muscle types. TnT has recently aroused clinical interest in its potential for detecting myocardial damage and the association of mutations in the cardiac isoform with hypertrophic cardiomyopathy.

Microanalysis and distribution of cardiac troponin I phospho species in heart areas

Sequential phosphorylation and dephosphorylation of cTnI by the cAMP dependent protein kinase and by protein phosphatase 2A, respectively, produce the non-, mono- and bisphosphorylated species (Jaquet et al., 1995, Eur. J. Biochem. 231, 486-490). The aim of this study was to determine these forms even in small tissue samples, e.g. in biopsy probes of approximately 30 mg which would allow to define the phosphorylation state of cTnI in heart areas. In order to do so a micro isolation procedure for cTnI had to be established. cTnI is extracted from small bovine, rabbit and human heart tissue samples (30-100 mg) under special conditions avoiding dephosphorylation and is isolated by affinity chromatography on cTnC Sepharose. All three species, the bis-, mono- and dephospho cTnI, are precipitated quantitatively by acetone, then they are separated by non-equilibrium isoelectric focusing and quantified by scanning densitometry. The method presented here allows to quantify the three cTnI species reproducibly. No other phosphorylated species are detected. Truncated cTnI forms of each phospho species are found in human biopsy samples due to removal of a approximately 36 amino acid peptide from the C-terminus. In bovine, human and rabbit heart the pattern of the three cTnI phospho species is characteristic for left and right atrium, left and right ventricle and septum.

The ordered phosphorylation of cardiac troponin I by the cAMP-dependent protein kinase--structural consequences and functional implications

The pattern of phosphorylation of adjacent serine residues in several peptides based on the N-terminal region of human cardiac troponin I has been analysed by PAGE and 1H NMR spectroscopy to identify the products. With cAMP-dependent protein kinase, Ser24 is rapidly phosphorylated, and subsequent much slower phosphorylation of Ser23 occurs only after phosphorylation of Ser24 is almost complete. Monophosphorylation of the peptide at Ser23 was not detected at any time. On replacement of Arg22 with Ala or Met the sole phosphorylation target was Ser23, phosphorylation being considerably slower than for Ser24 in the wild-type peptide, while diphosphorylation could not be detected after prolonged incubation. The results emphasise the importance of the N-terminal sequence RRRSS for the function of cardiac troponin I and imply that in human cardiac muscle unstimulated by adrenaline, troponin I is phosphorylated on Ser24. Comparative two-dimensional NOESY data indicate that in the diphosphorylated form at physiological pH values, specific structural constraints are imposed on the N-terminal peptide region. These constraints result in the effective screening of the two phosphate groups from each other by the arginine residues N-terminal to the serine pair and stabilisation of the structure in the region of residues 25-29, which is adjacent to a site of interaction between troponin I and troponin C. These conformational changes presumably underlie the decrease in calcium sensitivity of the myofibrillar ATPase that occurs after adrenaline intervention.

Phosphorylation specificities of protein kinase C isozymes for bovine cardiac troponin I and troponin T and sites within these proteins and regulation of myofilament properties

Protein kinase C (PKC) isozymes alpha, delta, epsilon, and zeta, shown to be expressed in adult rat cardiomyocytes, displayed distinct substrate specificities in phosphorylating troponin I and troponin T subunits in the bovine cardiac troponin complex. Thus, because they have different substrate affinities, PKC-alpha, -delta, and -epsilon phosphorylated troponin I more than troponin T, but PKC-zeta conversely phosphorylated the latter more than the former. Furthermore, PKC isozymes exhibited discrete specificities in phosphorylating distinct sites in these proteins as free subunits or in the troponin complex. Unlike other isozymes, PKC-delta was uniquely able to phosphorylate Ser-23/Ser-24 in troponin I, the bona fide phosphorylation sites for protein kinase A (PKA); and consequently, like PKA, it reduced Ca2+ sensitivity of Ca2+-stimulated MgATPase of reconstituted actomyosin S-1. In addition, PKC-delta, like PKC-alpha, readily phosphorylated Ser-43/Ser-45 (sites common for all PKC isozymes) and reduced maximal activity of MgATPase. In this respect, PKC-delta functioned as a hybrid of PKC-alpha and PKA. In contrast to PKC-alpha, -delta, and -epsilon, PKC-zeta exclusively phosphorylated two previously unknown sites in troponin T. Phosphorylation of troponin T by PKC-alpha resulted in decreases in both Ca2+ sensitivity and maximal activity, whereas phosphorylation by PKC-zeta resulted in a slight increase of the Ca2+ sensitivity without affecting the maximal activity of MgATPase. Most of the in vitro phosphorylation sites in troponin I and troponin T were confirmed in situ in adult rat cardiomyocytes. The present study has demonstrated for the first time distinct specificities of PKC isozymes for phosphorylation of two physiological substrates in the myocardium, with functional consequences.

Bisphosphorylation of cardiac troponin I modulates the Ca(2+)-dependent binding of myosin subfragment S1 to reconstituted thin filaments

We have reconstituted thin filaments comprising pyrene-labelled actin (pyr-actin), tropomyosin (Tm) and cardiac troponin (cTn). cTn was isolated in two defined phosphorylation states; completely dephosphorylated on all subunits and with only the cTnI subunit bisphosphorylated. The thin filament was saturated with cTn at a pyr-actin/Tm/cTn ratio of 7:1:1. The calcium-dependent binding of S1 to thin filaments was measured in a stopped-flow spectrophotometer and the dependence of the observed rate constant on [Ca2+] fitted to the Hill equation. The only significant difference between the two phosphorylation states of the filaments was a 0.36 decrease in the pCa50 on bisphosphorylation.

Phosphorylation of both serine residues in cardiac troponin I is required to decrease the Ca2+ affinity of cardiac troponin C

The phosphorylation of cardiac muscle troponin I (CTnI) at two adjacent N-terminal serine residues by cAMP-dependent protein kinase (PKA) has been implicated in the inotropic response of the heart to beta-agonists. Phosphorylation of these residues has been shown to reduce the Ca2+ affinity of the single Ca(2+)-specific regulatory site of cardiac troponin C (CTnC) and to increase the rate of Ca2+ dissociation from this site (Robertson, S. P., Johnson, J. D., Holroyde, M. J., Kranias, E. G., Potter, J. D., and Solaro, R. J. (1982) J. Biol. Chem. 257, 260-263). Recent studies (Zhang, R., Zhao, J., and Potter, J. D. (1995) Circ. Res. 76, 1028-1035) have correlated this increase in Ca2+ dissociation with a reduced Ca2+ sensitivity of force development and a faster rate of cardiac muscle relaxation in a PKA phosphorylated skinned cardiac muscle preparation. To further determine the role of the two PKA phosphorylation sites in mouse CTnI (serine 22 and 23), serine 22 or 23, or both were mutated to alanine. The wild type and the mutated CTnIs were expressed in Escherichia coli and purified. Using these mutants, it was found that serine 23 was phosphorylated more rapidly than serine 22 and that both serines are required to be phosphorylated in order to observe the characteristic reduction in the Ca2+ sensitivity of force development seen in a skinned cardiac muscle preparation. The latter result confirms that PKA phosphorylation of CTnI, and not other proteins, is responsible for this change in Ca2+ sensitivity. The results also suggest that one of the serines (23) may be constitutively phosphorylated and that serine 22 may be functionally more important.

Cardiac troponin I mutants. Phosphorylation by protein kinases C and A and regulation of Ca(2+)-stimulated MgATPase of reconstituted actomyosin S-1

The significance of site-specific phosphorylation of cardiac troponin I (TnI) by protein kinase C and protein kinase A in the regulation of Ca(2+)-stimulated MgATPase of reconstituted actomyosin S-1 was investigated. The TnI mutants used were T144A, S43A/S45A, and S43A/S45A/T144A (in which the identified protein kinase C phosphorylation sites, Thr-144 and Ser-43/ Ser-45, were, respectively, substituted by Ala) and S23A/S24A and N32 (in which the protein kinase A phosphorylation sites Ser-23/Ser-24 were either substituted by Ala or deleted). The mutations caused subtle changes in the kinetics of phosphorylation by protein kinase C, and all mutants were maximally phosphorylated to various extents (1.3-2.7 mol of phosphate/mol of protein). Protein kinase C could cross-phosphorylate protein kinase A sites but the reverse essentially could not occur. Compared to wild-type TnI and T144A, un-phosphorylated S43A/S45A, S43A/S45A/T144, S23A/ S24A, and N32 caused a decreased Ca2+ sensitivity of Ca(2+)-stimulated MgATPase of reconstituted actomyosin. S-1. Phosphorylation by protein kinase C of wild-type and all mutants except S43A/S45A and S43A/S45A/T144A caused marked reductions in both the maximal activity of Ca(2+)-stimulated MgATPase and apparent affinity of myosin S-1 for reconstitued (regulated) actin. It was further noted that protein kinase C acted in an additive manner with protein kinase A by phosphorylating Ser-23/Ser-24 to bring about a decreased Ca2+ sensitivity of the myofilament. It is suggested that Ser-43/Ser-45 and Ser-23/Ser-24 in cardiac TnI are important for normal Ca2+ sensitivity of the myofilament, and that phosphorylation of Ser-43/Ser-45 and Ser-23/Ser-24 is primarily involved in the protein kinase C regulation of the activity and Ca2+ sensitivity, respectively, of actomyosin S-1 MgATPase.

Pattern formation on cardiac troponin I by consecutive phosphorylation and dephosphorylation

Two serine residues located adjacently in the heart-specific N-terminus of cardiac troponin I can be phosphorylated in vivo. Both residues are sequentially phosphorylated and dephosphorylated by cAMP-dependent protein kinase (PKA) and protein phosphatase 2A (PP2A). The concentration changes of the different troponin I species have been determined separately for the phosphorylation and dephosphorylation reaction and approximated by time courses predicted by a reaction model. Dependent on the concentration ratio of active protein kinase/protein phosphatase, four different troponin I species can be generated; one nonphosphorylated, two monophosphorylated and one bisphosphorylated. This pattern generation will be observed in proteins phosphorylated and dephosphorylated by a single protein kinase and phosphatase on more than one site and is a new principle inherent in signal cascades.

A site phosphorylated in bovine cardiac troponin T by cardiac CaM kinase II

Ca2+/Calmodulin-dependent protein kinase II isolated from heart phosphorylates bovine cardiac troponin T present in the holotroponin complex. Thr-190 has been determined as the main phosphorylation site.

Characterization of the cardiac troponin I phosphorylation domain by 31P nuclear magnetic resonance spectroscopy

Cardiac holotroponin can be phosphorylated at serine 23 and/or 24 in the heart-specific region of bovine troponin I. When isolated freshly it is composed of a mixture of non-, two mono-, and bisphosphorylated species. At neutral pH the monophosphorylated form carrying phosphate at serine 24 yields a resonance signal at 4.6 ppm and that carrying phosphate at serine 23 at 4.4 ppm; the two phosphate groups of the bisphosphorylated form yield only one 31P-NMR signal at 4.2 ppm. From the chemical shift dependence on pH, pKa values have been estimated to be 5.3 and 5.6 for the phosphate groups at serine 24 and serine 23, respectively. Both phosphates of the bisphosphorylated form exhibit very similar pKa values of approximately 5.8. Separation of bisphosphotroponin I from the complex results in a downfield shift and the appearance of two 31P-NMR signals at positions comparable to those of the two monophospho forms. Complex formation of cardiac troponin I with C or T does not alter the spectrum obtained with isolated troponin I; however, the original troponin spectrum is restored by reconstitution of the holocomplex from all three components T, I, and C. Two signals are also observed with a bisphosphorylated synthetic peptide [PVRRRS(P)S(P)ANYR] representing the phosphorylation domain. pKa values of about 5.3 and 5.6 have been determined for serine 7 (corresponding to serine 24 of troponin I) and serine 6 of the peptide (corresponding to serine 23 of troponin I).

Protein kinase C phosphorylation of cardiac troponin T decreases Ca(2+)-dependent actomyosin MgATPase activity and troponin T binding to tropomyosin-F-actin complex

Effects of phosphorylation of bovine cardiac troponin T (TnT) by protein kinase C on the Ca(2+)-stimulated MgATPase activity of reconstituted actomyosin complex and the binding of TnT to tropomyosin(Tm)-F-actin were investigated. The Ca(2+)-stimulated MgATPase of actomyosin containing phosphorylated TnT (1.8 mol of P/mol), compared with that containing unphosphorylated TnT, was decreased by up to 48%. Phosphorylation of TnT also decreased (up to 48%) its maximum binding to Tm-F-actin, which was accompanied by a decrease (up to 3.5-fold) in its apparent binding affinity. The findings indicate that the effects of phosphorylated TnT in decreasing actomyosin MgATPase might be secondary to its decreased interactions with the other components of the thin filament, representing a new mechanism underlying the negative inotropic responses of various cardiac preparations to protein kinase C-activating phorbol esters.

Ordered phosphorylation of a duplicated minimal recognition motif for cAMP-dependent protein kinase present in cardiac troponin I

Cardiac troponin I contains two adjacent serines in sequence after three arginine residues thus making up a minimally duplicated recognition motif for cAMP-dependent protein kinase. In a synthetic peptide, PVRRRSSANY, the two serine residues are phosphorylated sequentially with the intermediate formation of a monophosphorylated species according to the following reaction sequence: Peptide k1----Peptide-P k2----Peptide-P2. The calculated rat constants are: k1 = 0.435.min-1 and k2 = 0.034.min-1. Sequence analyses of the monophosphopeptide and its tryptic fragments show that the predominant monophosphoform carries phosphate at the second serine.