Interactions at the NH2-terminal interface of cardiac troponin I modulate myofilament activation

Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca(2+) depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca(2+)-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (∼0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (∼1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca(2+) was mostly (61%), but not completely eliminated with cTnI1-163. TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin.

The functional significance of the last 5 residues of the C-terminus of cardiac troponin I

The C-terminal region of cardiac troponin I (cTnI) is known to be important in cardiac function, as removal of the last 17 C-terminal residues of human cTnI has been associated with myocardial stunning. To investigate the C-terminal region of cTnI, three C-terminal deletion mutations in human cTnI were generated: Δ1 (deletion of residue 210), Δ3 (deletion of residues 208-210), and Δ5 (deletion of residues 206-210). Mammalian two-hybrid studies showed that the interactions between cTnI mutants and cardiac troponin C (cTnC) or cardiac troponin T (cTnT) were impaired in Δ3 and Δ5 mutants when compared to wild-type cTnI. Troponin complexes containing 2-[4'-(iodoacetamido) anilino] naphthalene-6-sulfonic acid (IAANS) labeled cTnC showed that the troponin complex containing cTnI Δ5 had a small increase in Ca(2+) affinity (P < 0.05); while the cTnI Δ1- and Δ3 troponin complexes showed no difference in Ca(2+) affinity when compared to wild-type troponin. In vitro motility assays showed that all truncation mutants had increased Ca(2+) dependent motility relative to wild-type cTnI. These results suggest that the last 5 C-terminal residues of cTnI influence the binding of cTnI with cTnC and cTnT and affect the Ca(2+) dependence of filament sliding, and demonstrate the importance of this region of cTnI.

Effect of cold acclimation on troponin I isoform expression in striated muscle of rainbow trout

In vertebrates each of the three striated muscle types (fast skeletal, slow skeletal, and cardiac) contain distinct isoforms of a number of different contractile proteins including troponin I (TnI). The functional characteristics of these proteins have a significant influence on muscle function and contractility. The purpose of this study was to characterize which TnI gene and protein isoforms are expressed in the different muscle types of rainbow trout ( Oncorhynchus mykiss) and to determine whether isoform expression changes in response to cold acclimation (4°C). Semiquantitative real-time PCR was used to characterize the expression of seven different TnI genes. The sequence of these genes, cloned from Atlantic salmon ( Salmo salar) and rainbow trout, were obtained from the National Center for Biotechnology Information databases. One-dimensional gel electrophoresis and tandem mass spectrometry were used to identify the TnI protein isoforms expressed in each muscle type. Interestingly, the results indicate that each muscle type expresses the gene transcripts of up to seven TnI isoforms. There are significant differences, however, in the expression pattern of these genes between muscle types. In addition, cold acclimation was found to increase the expression of specific gene transcripts in each muscle type. The proteomics analysis demonstrates that fast skeletal and cardiac muscle contain three TnI isoforms, whereas slow skeletal muscle contains four. No other vertebrate muscle to date has been found to express as many TnI protein isoforms. Overall this study underscores the complex molecular composition of teleost striated muscle and suggests there is an adaptive value to the unique TnI profiles of each muscle type.

The influence of trout cardiac troponin I and PKA phosphorylation on the Ca2+ affinity of the cardiac troponin complex

The trout heart is 10-fold more sensitive to Ca(2+) than the mammalian heart. This difference is due, in part, to cardiac troponin C (cTnC) from trout having a greater Ca(2+) affinity than human cTnC. To determine what other proteins are involved, we cloned cardiac troponin I (cTnI) from the trout heart and determined how it alters the Ca(2+) affinity of a cTn complex containing all mammalian components (mammalian cTn). Ca(2+) activation of the complex was characterized using a human cTnC mutant that contains anilinonapthalenesulfote iodoacetamide attached to Cys53. When the cTn complex containing labeled human cTnC was titrated with Ca(2+), its fluorescence changed, reaching an asymptote upon saturation. Our results reveal that trout cTnI lacks the N-terminal extension found in cTnI from all other vertebrate groups. This protein domain contains two targets (Ser23 and Ser24) for protein kinase A (PKA) and protein kinase C. When these are phosphorylated, the rate of cardiomyocyte relaxation increases. When rat cTnI in the mammalian cTn complex was replaced with trout cTnI, the Ca(2+) affinity was increased ∼1.8-fold. This suggests that trout cTnI contributes to the high Ca(2+) sensitivity of the trout heart. Treatment of the two cTn complexes with PKA decreased the Ca(2+) affinity of both complexes. However, the change for the complex containing rat cTnI was 2.2-fold that of the complex containing trout cTnI. This suggests that the phosphorylation of trout cTnI does not play as significant a role in regulating cTn function in trout.

The influence of PKA treatment on the Ca2+ activation of force generation by trout cardiac muscle

β-Adrenergic stimulation of the mammalian heart increases heart rate, the strength of contraction as well as the kinetics of force generation and relaxation. These effects are due to the phosphorylation of select membrane and thin filament proteins by cAMP-activated protein kinase (PKA). At the level of the sarcomere, it is typically the phosphorylation of cardiac myosin binding protein C (cMyBP-C) and cardiac troponin I (cTnI) that is responsible for the change in the kinetics of contraction and relaxation. Trout cTnI (ScTnI) lacks two critical PKA targets within the N-terminus of the protein that, when phosphorylated in mammalian cTnI, cause a reduction in myofilament Ca2+ affinity. To determine what role the contractile element plays in the response of the trout heart to β-adrenergic stimulation, we characterized the influence of PKA treatment on the Ca2+ activation of skinned preparations dissected from ventricular trabeculae. In these experiments, isometric force generation and the rate of force development were measured over a range of Ca2+ concentrations. The results demonstrate that PKA treatment does not influence the Ca2+ sensitivity of force generation but it decreases maximum force generation by 25% and the rate of force re-development at maximal activation by 46%. Analysis of the trabeculae preparations for phosphoproteins revealed that PKA treatment phosphorylated myosin light chain 2 but not cTnI or cMyBP-C. These results indicate that the function of the trout cardiac contractile element is altered by PKA phosphorylation but in a manner different from that in mammalian heart.

Invited review: probing the structures of muscle regulatory proteins using small-angle solution scattering

Small-angle X-ray and neutron scattering with contrast variation have made important contributions in advancing our understanding of muscle regulatory protein structures in the context of the dynamic molecular processes governing muscle action. The contributions of the scattering investigations have depended upon the results of key crystallographic, NMR, and electron microscopy experiments that have provided detailed structural information that has aided in the interpretation of the scattering data. This review will cover the advances made using small-angle scattering techniques, in combination with the results from these complementary techniques, in probing the structures of troponin and myosin binding protein C. A focus of the troponin work has been to understand the isoform differences between the skeletal and cardiac isoforms of this major calcium receptor in muscle. In the case of myosin binding protein C, significant data are accumulating, indicating that this protein may act to modulate the primary calcium signals from troponin, and interest in its biological role has grown because of linkages between gene mutations in the cardiac isoform and serious heart disease.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Calcium, thin filaments, and the integrative biology of cardiac contractility

Although well known as the location of the mechanism by which the cardiac sarcomere is activated by Ca2+ to generate force and shortening, the thin filament is now also recognized as a vital component determining the dynamics of contraction and relaxation. Molecular signaling in the thin filament involves steric, allosteric, and cooperative mechanisms that are modified by protein phosphorylation, sarcomere length and load, the chemical environment, and isoform composition. Approaches employing transgenesis and mutagenesis now permit investigation of these processes at the level of the systems biology of the heart. These studies reveal that the thin filaments are not merely slaves to the levels of Ca2+ determined by membrane channels, transporters and exchangers, but are actively involved in beat to beat control of cardiac function by neural and hormonal factors and by the Frank-Starling mechanism.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

Adenosine attenuates C-terminal but not N-terminal proteolysis of cTnI during cardioplegic arrest

BACKGROUND

Specific site proteolysis and loss of troponin I (TnI) during myocardial ischemic events can contribute to myocardial dysfunction. Adenosine supplementation of cardioplegic solutions results in improved functional preservation of the heart. We investigated the effect of adenosine on N-terminal and C-terminal proteolysis of TnI in the heart.

MATERIALS AND METHODS

Hearts from male Sprague-Dawley rats were isolated and perfused at a constant pressure. Cardioplegic arrest (St. Thomas #2 +/- 100 microm adenosine) was induced and hearts frozen at various times during the arrest. Antibodies directed against specific regions of TnI were used to visualize TnI in whole heart homogentates, as well as from cellular fractions, using western blot analysis.

RESULTS

Cardioplegic arrest alone resulted in early N-terminal proteolysis of TnI, followed by later loss of sequences from the C-terminal end of the molecule. In addition, secondary protein bands that were immunoreactive to amino acid sequences centrally located on the TnI molecule were observed. There was also evidence of dissociation of TnI from the other myofibrillar proteins. The supplementation of cardioplegic solution with adenosine significantly attenuated the late C-terminal proteolytic degradation of TnI and its apparent dissociation from myofibrils proteins but had no effect on the early N-terminal proteolysis associated with cardioplegic arrest.

CONCLUSIONS

These data may provide an explanation for partial protection against postarrest myocardial dysfunction provided by adenosine.

Novel mutation in cardiac troponin I in recessive idiopathic dilated cardiomyopathy

Idiopathic dilated cardiomyopathy is a common cause of heart failure. Half of cases are believed to be hereditary, and mutations in cardiac sarcomeric contractile protein genes have been reported with autosomal dominant inheritance. We used mutation analysis suitable for identification of both dominant and recessive mutations to investigate the sarcomeric gene for cardiac troponin I (TNNI3) in 235 patients with dilated cardiomyopathy. We identified a novel TNNI3 mutation in a family with recessive disease. Functional studies showed impairment of troponin interactions that could lead to diminished myocardial contractility. TNNI3 is the first recessive gene identified for this condition, and we suggest that other such genes could be pinpointed by mutation analyses designed to identify homozygous mutations.

Calcium-dependent protein-protein interactions induce changes in proximity relationships of Cys48 and Cys64 in chicken skeletal troponin I

The goal of this study was to relate conformational changes in the N-terminal domain of chicken troponin I (TnI) to Ca2+ activation of the actin-myosin interaction. The two cysteine residues in this region (Cys48 and Cys64) were labeled with two sulfhydryl-reactive pyrene-containing fluorophores [N-(1-pyrene)maleimide, and N-(1-pyrene)iodoacetamide]. The labeled TnI showed a typical fluorescence spectrum: two sharp peaks of monomer fluorescence and a broad peak of excimer fluorescence arising from the formation of an excited dimer (excimer). Results obtained show that forming a binary complex of labeled TnI with skeletal TnC (sTnC) in the absence of Ca2+ decreases the excimer fluorescence, indicating a separation of the two residues. This reduction in excimer fluorescence does not occur when labeled TnI is complexed with cardiac TnC (cTnC). The latter causes only partial activation of the Ca2+-dependent myofibrillar ATPase. The binding of Ca2+ to the two N-terminal sites of sTnC causes a significant decrease in excimer fluorescence and an increase in monomer fluorescence in complexes of labeled TnI with skeletal TnC or TnC/TnT, while Ca2+ binding to site II of cTnC only causes an increase in monomer fluorescence but no change in excimer fluorescence. Thus a conformational change in the N-terminal region of TnI may be necessary for full activation of muscle contraction.

Phosphorylation or glutamic acid substitution at protein kinase C sites on cardiac troponin I differentially depress myofilament tension and shortening velocity

There is evidence that multi-site phosphorylation of cardiac troponin I (cTnI) by protein kinase C is important in both long- and short-term regulation of cardiac function. To determine the specific functional effects of these phosphorylation sites (Ser-43, Ser-45, and Thr-144), we measured tension and sliding speed of thin filaments in reconstituted preparations in which endogenous cTnI was replaced with cTnI phosphorylated by protein kinase C-epsilon or mutated to cTnI-S43E/S45E/T144E, cTnI-S43E/S45E, or cTnI-T144E. We used detergent-skinned mouse cardiac fiber bundles to measure changes in Ca(2+)-dependence of force. Compared with controls, fibers reconstituted with phosphorylated cTnI, cTnI-S43E/S45E/T144E, or cTnI-S43E/S45E were desensitized to Ca(2+), and maximum tension was as much as 27% lower, whereas fibers reconstituted with cTnI-T144E showed no change. In the in vitro motility assay actin filaments regulated by troponin complexes containing phosphorylated cTnI or cTnI-S43E/S45E/T144E showed both a decrease in Ca(2+) sensitivity and maximum sliding speed compared with controls, whereas filaments regulated by cTnI-S43E/S45E showed only decreased maximum sliding speed and filaments regulated by cTnI-T144E demonstrated only desensitization to Ca(2+). Our results demonstrate novel site specificity of effects of PKC phosphorylation on cTnI function and emphasize the complexity of modulation of the actin-myosin interaction by specific changes in the thin filament.

Cell biology of cardiac development

Building a vertebrate heart is a complex task and involves several tissues, including the myocardium, endocardium, neural crest, and epicardium. Interactions between these tissues result in the changes in function and morphology (and also in the extracellular matrix, which serves as a substrate for morphological change) that are requisite for development of the heart. Some of the signaling pathways that mediate these changes have now been identified and several investigators are now filling in the missing pieces in these pathways in hopes of ultimately understanding the molecular mechanisms that govern healthy heart development. In addition, transcription factors that regulate various aspects of heart development have been identified. Transcription factors of the GATA and Nkx2 families are of particular importance for early specification of the heart field and for regulating expression of genes that encode proteins of the contractile apparatus. This chapter highlights some of the most significant discoveries made in the rapidly expanding field of heart development.

Transgenic incorporation of skeletal TnT into cardiac myofilaments blunts PKC-mediated depression of force

Protein kinase C (PKC)-mediated phosphorylation of cardiac troponin I (cTnI) and troponin T (cTnT) has been shown to diminish maximum activation of myofilaments. The functional role of cTnI phosphorylation has been investigated. However, the impact of cTnT phosphorylation on myofilament force is not well studied. We tested the effect of endogenous PKC activation on steady-state tension development and Ca(2+) sensitivity in skinned fiber bundles from transgenic (TG) mouse hearts expressing fast skeletal TnT (fsTnT), which naturally lacks the PKC sites present in cTnT. The 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment induced a 29% (46.1 +/- 2.5 vs. 33.4 +/- 2.6 mN/mm(2)) reduction in maximum tension in the nontransgenic (NTG) preparations (n = 7) and was inhibited with chelerythrine. However, TPA did not induce a change in the maximum tension in the TG preparations (n = 11). TPA induced a small but significant (P < 0.02) increase in Ca(2+) sensitivity (untreated pCa(50) = 5.63 +/- 0.01 vs. treated pCa(50) = 5.72 +/- 0.01) only in TG preparations. In TG preparations, (32)P incorporation was not evident in TnT and was also significantly diminished in cTnI, compared with NTG. Our data indicate that incorporation of fsTnT into the cardiac myofilament lattice blunts PKC-mediated depression of maximum tension. These data also suggest that cTnT may play an important role in amplifying the myofilament depression induced by PKC-mediated phosphorylation of cTnI.

Chimera analysis of troponin I domains that influence Ca(2+)-activated myofilament tension in adult cardiac myocytes

The goal of this study was to investigate isoform-specific functional domains of the inhibitory troponin subunit, troponin I (TnI), as it functions within the intact myofilaments of adult cardiac myocytes. Adenovirus-mediated gene transfer was used to deliver and express a TnI chimera composed of the amino terminus of cardiac TnI (cTnI) and the carboxy terminus of slow skeletal TnI (ssTnI) in adult rat cardiac myocytes. The TnI chimera, designated N-card/slow-C TnI, was expressed and incorporated into myofilaments after gene transfer, without detectable changes in contractile protein stoichiometry or sarcomere architecture. Interestingly, force at submaximal Ca(2+) levels was markedly elevated in single permeabilized myocytes expressing the N-card/slow-C TnI chimera relative to force generated in adult myocytes expressing ssTnI or cTnI. Based on these results, a hierarchy of myofilament Ca(2+) sensitivity is emerging by use of TnI chimera analysis, with the order of sensitivity being N-card/slow-C TnI>>ssTnI>>cTnI. These results also strongly suggest that independent isoform-specific domains in both the amino and carboxy portions of TnI influence myofilament Ca(2+) sensitivity. In additional studies carried out under pathophysiological ionic conditions (pH 6.2), the dramatic acidosis-induced decrease in myofilament Ca(2+) sensitivity observed in myocytes expressing cTnI was blunted in myocytes expressing N-card/slow-C TnI in a manner similar to that in ssTnI-expressing myocytes. These results demonstrate that there is a pH-sensitive domain residing in the carboxy-terminal portion of TnI. The dissection of isoform-specific functional domains under physiological and acidic pH conditions demonstrates the utility of TnI chimeras for analysis of TnI function and provides important insights into the overall function of TnI within the intact myofilament of adult cardiac myocytes.