Coupling of calcium to the interaction of troponin I with troponin C from cardiac muscle

Transcriptomic and proteomic analyses reveal the common and unique pathway(s) underlying different skin colors of leopard coral grouper (Plectropomus leopardus)

To gain a comprehensive and unbiased molecular understanding of the different skin colors of P. leopardus, we used Illumina HiSeq 2500 and TMT (Tandem Mass Tag) to compare transcription and protein levels between red and black skin of P. leopardus. We identified 797 upregulated and 314 downregulated genes (differentially expressed genes; DEGs) in red (RG) compared with black (BG) skin of P. leopardus. We also identified 377 differentially abundant proteins (DAPs), including 314 upregulated and 63 downregulated proteins. These DEGs and DAPs were significantly enriched in melanin synthesis (e.g., pyrimidine metabolism, Phenylalanine, tyrosine, and tryptophan biosynthesis, melanogenesis, phenylalanine metabolism, and tyrosine metabolism), oxidative phosphorylation (e.g., phosphonate and phosphinate metabolism, and oxidative phosphorylation), energy metabolism (e.g., HIF-1, glycolysis/gluconeogenesis, fatty acid biosynthesis, and fatty acid degradation), and signal transduction (e.g., Wnt, calcium, MAPK, and cGMP-PKG signaling pathways), etc. Further analysis of MAPKs showed that the activation levels of its main members JNK1 and ERK1/2 differed significantly between red and black skin colors. After RNAi was used to interfere with ERK1/2, it was found that the local skin of the tail of P. leopardus would turn black. Combined transcriptome and proteome analysis showed that most DEGs-DAPs in red skin were higher than in black skin (58 were upregulated, 1 was downregulated, and 4 were opposite). These DEGs-DAPs showed that the differences between red and black skin tissues of P. leopardus were related primarily to energy metabolism, signal transduction and cytoskeleton. These findings are not only conducive to understand the skin color regulation mechanism of P. leopardus and other coral reef fish, but also provide an important descriptive to the breeding of color strains. SIGNIFICANCE OF THE STUDY: The skin color of P. leopardus gradually darkens or blackens due to environmental factors such as changes in light intensity and human activities, and this directly affects its ornamental and economic value. In this study, RNAseq and TMT were used to conduct comparative quantitative transcriptomics and proteomics and analyze differences between red and black P. leopardus skin. The results showed that energy metabolism, signal transduction and cytoskeleton were the main metabolic pathways causing their skin color differences. These findings contribute to existing data describing fish skin color, and provide information about protein levels, which are of great significance to a deeper understanding of the skin color regulation mechanism in P. leopardus and other coral reef fishes.

Evaluation of a high-sensitivity cardiac troponin I assay compared to a first-generation cardiac troponin I assay in Doberman Pinschers with and without dilated cardiomyopathy

BACKGROUND

Echocardiography and 24-hour ECG are the gold standard tests to diagnose dilated cardiomyopathy (DCM) in Doberman Pinschers (DP), but myocardial damage might be detected earlier using a high-sensitivity cardiac troponin I (hs-cTnI) assay.

OBJECTIVE

To evaluate and compare an hs-cTnI assay (Advia Centaur TnI-Ultra assay) with a conventional cTnI assay in DP with different stages of DCM and in healthy DP.

ANIMALS

Three hundred forty-five examinations from 162 DP with and 179 DP without DCM.

METHODS

Prospective longitudinal study. Dogs were allocated into 6 groups based on echocardiographic and 24-hour ECG criteria: (1) healthy group (179 dogs), (2) last-normal group (29 dogs), which included dogs that were considered to be healthy at the time of their examination but were assigned to the last-normal group retrospectively when DCM was diagnosed at their next examination within 1.5 years, (3) only arrhythmias (45 dogs, 119 examinations), (4) only echocardiographic changes (24 dogs, 61 examinations), (5) echocardiographic changes with ventricular premature complexes (41 dogs, 100 examinations), and (6) decompensated (23 dogs, 36 examinations). Hs-cTnI and conventional cTnI concentration measurements were performed and compared.

RESULTS

A cutoff value of hs-cTnI concentration >0.113 ng/mL had a sensitivity of 81.2% and a specificity of 73.2% to identify the presence of DCM. The conventional cTnI assay showed a similar test performance, but the hs-cTnI assay identified more dogs (21/29 dogs, 72%) in the last-normal group compared to the conventional cTnI test (18/29 dogs, 62%).

CONCLUSIONS AND CLINICAL IMPORTANCE

The hs-cTnI is an additional test with good potential to identify early DCM.

Pathogenesis associated with a restrictive cardiomyopathy mutant in cardiac troponin T is due to reduced protein stability and greatly increased myofilament Ca2+ sensitivity

BACKGROUND

Dilated and hypertrophic cardiomyopathy mutations in troponin can blunt effects of protein kinase A (PKA) phosphorylation of cardiac troponin I (cTnI), decreasing myofilament Ca2+-sensitivity; however this effect has never been tested for restrictive cardiomyopathy (RCM) mutants. This study explores whether an RCM cardiac troponin T mutant (cTnT-ΔE96) interferes with convergent PKA regulation and if TnT instability contributes to greatly enhanced Ca2+-sensitivity in skinned fibers.

METHODS

Force of contraction in skinned cardiac porcine fiber and spectroscopic studies were performed.

RESULTS

A decrease of -0.26 and -0.25 pCa units in Ca2+-sensitivity of contraction after PKA incubation was observed for skinned fibers incorporated with WT or cTnT-ΔE96, respectively. To further assess whether cTnT-ΔE96 interferes solely with transmission of cTnI phosphorylation effects, skinned fibers were reconstituted with PKA pseudo-phosphorylated cTnI (cTnI-SS/DD.cTnC). Fibers displaced with cTnT-WT, reconstituted with cTnI-SS/DD.cTnC decreased Ca2+-sensitivity of force (pCa50=5.61) compared to control cTnI-WT.cTnC (pCa50=5.75), similarly affecting cTnT-ΔE96 (pCa50=6.03) compared to control \cTnI-WT.cTnC (pCa50=6.14). Fluorescence studies measuring cTnC(IAANS) Ca2+-affinity changes due to cTnT-ΔE96 indicated that higher complexity (thin filament) better recapitulates skinned fiber Ca2+ sensitive changes. Circular dichroism revealed reduced α-helicity and earlier thermal unfolding for cTnT-ΔE96 compared to WT.

CONCLUSIONS

Although ineffective in decreasing myofilament Ca2+-sensitivity to normal levels, cTnT-ΔE96 does not interfere with PKA cTnI phosphorylation mediated effects; 2) cTnT-ΔE96 requires actin to increase cTnC Ca2+-affinity; and 3) deletion of E96 reduces cTnT stability, likely disrupting crucial thin filament interactions.

GENERAL SIGNIFICANCE

The pathological effect of cTnT-ΔE96 is largely manifested by dramatic myofilament Ca2+-sensitization which still persists even after PKA phosphorylation mediated Ca2+-desensitization.

Troponin assays in the assessment of the equine myocardium

In 2000, troponin assays were adopted as the test of choice for detection of myocardial injury in man. This decision was made after extensive testing and followed a 60 year search for a biomarker of myocardial damage with sufficient analytical sensitivity and specificity. This has led to proliferation of assays for use in human medicine, each requiring extensive testing and validation before it could be made available on the open market for human use. The search for ever-more analytically sensitive assays and for a standard reference material continues. The adoption of troponin testing in veterinary medicine followed shortly after its development for use in man, providing a much-needed means of detecting and monitoring myocardial damage in horses. However, application of these tests in veterinary medicine has exclusively involved use of assays designed for and clinically validated in human patients. There is no mandated requirement for test validation in veterinary medicine and, while many of these assays have been shown to be capable of detecting equine troponin, the wide diversity of available tests, lack of validation, absence of protocols for their use and lack of standardisation make their application problematic. The objective of this review article is to address this issue, offering guidance where data are available and encouraging caution where there are none. Ultimately, the overall goal of this review is to examine critically the use of troponin assays in the horse and to promote the accurate and appropriate interpretation of valid results.

Expression and assembly of active human cardiac troponin in Escherichia coli

Cardiomyopathy-related mutations in human cardiac troponin subunits, including troponin C (hcTnC), troponin I (hcTnI), and troponin T (hcTnT), are well-documented. Recently, it has been recognised that human cardiac troponin (hcTn) is a sophisticated allosteric system. Therefore, the effect of drugs on this protein complex should be studied with assembled hcTn rather than a short fragment of a subunit or the subunit itself. Here, we describe the expression and assembly of active hcTn in Escherichia coli, a novel method that is rapid and simple, and produces large amounts of functional hcTn.

Long-range effects of familial hypertrophic cardiomyopathy mutations E180G and D175N on the properties of tropomyosin

Cardiac α-tropomyosin (Tm) single-site mutations D175N and E180G cause familial hypertrophic cardiomyopathy (FHC). Previous studies have shown that these mutations increase both Ca(2+) sensitivity and residual contractile activity at low Ca(2+) concentrations, which causes incomplete relaxation during diastole resulting in hypertrophy and sarcomeric disarray. However, the molecular basis for the cause and the difference in the severity of the manifested phenotypes of disease are not known. In this work we have (1) used ATPase studies using reconstituted thin filaments in solution to show that these FHC mutants result in an increase in Ca(2+) sensitivity and an increased residual level of ATPase, (2) shown that both FHC mutants increase the rate of cleavage at R133, ~45 residues N-terminal to the mutations, when free and bound to actin, (3) shown that for Tm-E180G, the increase in the rate of cleavage is greater than that for D175N, and (4) shown that for E180G, cleavage also occurs at a new site 53 residues C-terminal to E180G, in parallel with cleavage at R133. The long-range decreases in dynamic stability due to these two single-site mutations suggest increases in flexibility that may weaken the ability of Tm to inhibit activity at low Ca(2+) concentrations for D175N and to a greater degree for E180G, which may contribute to differences in the severity of FHC.

Alanine or aspartic acid substitutions at serine23/24 of cardiac troponin I decrease thin filament activation, with no effect on crossbridge detachment kinetics

Ala/Asp substitutions at Ser23/24 have been employed to investigate the functional impact of cardiac troponin I (cTnI) phosphorylation by protein kinase A (PKA). Some limitations of previous studies include the use of heterologous proteins and confounding effects arising from phosphorylation of cardiac myosin binding protein-C. Our goal was to probe the effects of cTnI phosphorylation using a homologous assay, so that altered function could be solely attributed to changes in cTnI. We reconstituted detergent-skinned rat cardiac papillary fibers with homologous rat cardiac troponin subunits to study the impact of Ala and Asp substitutions at Ser23/24 of rat cTnI (RcTnI S23A/24A and RcTnI S23D/24D). Both RcTnI S23A/24A and RcTnI S23D/24D showed a ~36% decrease in Ca(2+)-activated maximal tension. Both RcTnI S23A/24A and RcTnI S23D/24D showed a ~18% decrease in ATPase activity. Muscle fiber stiffness measurements suggested that the decrease in thin filament activation observed in RcTnI S23A/24A and RcTnI S23D/24D was due to a decrease in the number of strongly-bound crossbridges. Another major finding was that Ala and Asp substitutions in cTnI did not affect crossbridge detachment kinetics.

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

RATIONALE

Cardiac myosin-binding protein-C (cMyBP-C) phosphorylation at Ser-273, Ser-282, and Ser-302 regulates myocardial contractility. In vitro and in vivo experiments suggest the nonequivalence of these sites and the potential importance of Ser-282 phosphorylation in modulating the protein's overall phosphorylation and myocardial function.

OBJECTIVE

To determine whether complete cMyBP-C phosphorylation is dependent on Ser-282 phosphorylation and to define its role in myocardial function. We hypothesized that Ser-282 regulates Ser-302 phosphorylation and cardiac function during β-adrenergic stimulation.

METHODS AND RESULTS

Using recombinant human C1-M-C2 peptides in vitro, we determined that protein kinase A can phosphorylate Ser-273, Ser-282, and Ser-302. Protein kinase C can also phosphorylate Ser-273 and Ser-302. In contrast, Ca(2+)-calmodulin-activated kinase II targets Ser-302 but can also target Ser-282 at nonphysiological calcium concentrations. Strikingly, Ser-302 phosphorylation by Ca(2+)-calmodulin-activated kinase II was abolished by ablating the ability of Ser-282 to be phosphorylated via alanine substitution. To determine the functional roles of the sites in vivo, three transgenic lines, which expressed cMyBP-C containing either Ser-273-Ala-282-Ser-302 (cMyBP-C(SAS)), Ala-273-Asp-282-Ala-302 (cMyBP-C(ADA)), or Asp-273-Ala-282-Asp-302 (cMyBP-C(DAD)), were generated. Mutant protein was completely substituted for endogenous cMyBP-C by breeding each mouse line into a cMyBP-C null (t/t) background. Serine-to-alanine substitutions were used to ablate the abilities of the residues to be phosphorylated, whereas serine-to-aspartate substitutions were used to mimic the charged state conferred by phosphorylation. Compared to control nontransgenic mice, as well as transgenic mice expressing wild-type cMyBP-C, the transgenic cMyBP-C(SAS(t/t)), cMyBP-C(ADA(t/t)), and cMyBP-C(DAD(t/t)) mice showed no increases in morbidity and mortality and partially rescued the cMyBP-C((t/t)) phenotype. The loss of cMyBP-C phosphorylation at Ser-282 led to an altered β-adrenergic response. In vivo hemodynamic studies revealed that contractility was unaffected but that cMyBP-C(SAS(t/t)) hearts showed decreased diastolic function at baseline. However, the normal increases in cardiac function (increased contractility/relaxation) as a result of infusion of β-agonist was significantly decreased in all of the mutants, suggesting that competency for phosphorylation at multiple sites in cMyBP-C is a prerequisite for normal β-adrenergic responsiveness.

CONCLUSIONS

Ser-282 has a unique regulatory role in that its phosphorylation is critical for the subsequent phosphorylation of Ser-302. However, each residue plays a role in regulating the contractile response to β-agonist stimulation.

Structural studies of interactions between cardiac troponin I and actin in regulated thin filament using Förster resonance energy transfer

The Ca(2+)-induced interaction between cardiac troponin I (cTnI) and actin plays a key role in the regulation of cardiac muscle contraction and relaxation. In this report we have investigated changes of this interaction in response to strong cross-bridge formation between myosin S1 and actin and PKA phosphorylation of cTnI within reconstituted thin filament. The interaction was monitored by measuring Förster resonance energy transfer (FRET) between the fluorescent donor 5-(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid (AEDANS) attached to the residues 131, 151, 160 167, 188, and 210 of cTnI and the nonfluorescent acceptor 4-(dimethylamino)phenylazophenyl-4'-maleimide (DABM) attached to cysteine 374 of actin. The FRET distance measurements showed that bound Ca(2+) induced large increases in the distances from actin to the cTnI sites, indicating a Ca(2+)-triggered separation of cTnI from actin. Strongly bound myosin S1 induced additional increases in these distances in the presence of bound Ca(2+). The two ligand-induced increases were independent of each other. These two-step changes in distances provide a direct link of structural changes at the interface between cTnI and actin to the three-state model of thin filament regulation of muscle contraction and relaxation. When cTnC was inactivated through mutations of key residues within the 12-residue Ca(2+)-binding loop, strongly bound S1 alone induced increases in the distances in spite of the fact that the filaments no longer bound regulatory Ca(2+). These results suggest bound Ca(2+) or strongly bound S1 alone can partially activate thin filament, but full activation requires both bound Ca(2+) and strongly bound S1. The distributions of the FRET distances revealed different structural dynamics associated with different regions of cTnI in different biochemical states. The second actin-binding region appears more rigid than the inhibitory/regulatory region. In the Mg(2+) state, the regulatory region appears more flexible than the inhibitory region, and in the Ca(2+) state the inhibitory region becomes more flexible. PKA phosphorylation of cTnI at Ser23 and Ser24 distance from actin to cTnI residue 131 by 2.2-5.2 A in different biochemical states and narrowed the distributions of the distances from actin to the inhibitory and regulatory regions of cTnI. The observed phosphorylation effects are likely due to an intramolecular interaction of the phosphorylated N-terminal segment and the inhibitory region of cTnI.

The cardiac Ca2+-sensitive regulatory switch, a system in dynamic equilibrium

The Ca(2+)-sensitive regulatory switch of cardiac muscle is a paradigmatic example of protein assemblies that communicate ligand binding through allosteric change. The switch is a dimeric complex of troponin C (TnC), an allosteric sensor for Ca(2+), and troponin I (TnI), an allosteric reporter. Time-resolved equilibrium Förster resonance energy transfer (FRET) measurements suggest that the switch activates in two steps: a TnI-independent Ca(2+)-priming step followed by TnI-dependent opening. To resolve the mechanistic role of TnI in activation we performed stopped-flow FRET measurements of activation after rapid addition of a lacking component (Ca(2+) or TnI) and deactivation after rapid chelation of Ca(2+). Time-resolved measurements, stopped-flow measurements, and Ca(2+)-titration measurements were globally analyzed in terms of a new quantitative dynamic model of TnC-TnI allostery. The analysis provided a mesoscopic parameterization of distance changes, free energy changes, and transition rates among the accessible coarse-grained states of the system. The results reveal that 1), the Ca(2+)-induced priming step, which precedes opening, is the rate-limiting step in activation; 2), closing is the rate-limiting step in de-activation; 3), TnI induces opening; 4), there is an incompletely deactivated population when regulatory Ca(2+) is not bound, which generates an accessory pathway of activation; and 5), there is incomplete activation by Ca(2+)-when regulatory Ca(2+) is bound, a 3:2 mixture of dynamically interconverting open (active) and primed-closed (partially active) conformers is observed (15 degrees C). Temperature-dependent stopped-flow FRET experiments provide a near complete thermokinetic parameterization of opening: the enthalpy change (DeltaH = -33.4 kJ/mol), entropy change (DeltaS = -0.110 kJ/mol/K), heat capacity change (DeltaC(p) = -7.6 kJ/mol/K), the enthalpy of activation (delta(double dagger) = 10.6 kJ/mol) and the effective barrier crossing attempt frequency (nu(adj) = 1.8 x 10(4) s(-1)).

Structural kinetics of cardiac troponin C mutants linked to familial hypertrophic and dilated cardiomyopathy in troponin complexes

The key events in regulating cardiac muscle contraction involve Ca(2+) binding to and release from cTnC (troponin C) and structural changes in cTnC and other thin filament proteins triggered by Ca(2+) movement. Single mutations L29Q and G159D in human cTnC have been reported to associate with familial hypertrophic and dilated cardiomyopathy, respectively. We have examined the effects of these individual mutations on structural transitions in the regulatory N-domain of cTnC triggered by Ca(2+) binding and dissociation. This study was carried out with a double mutant or triple mutants of cTnC, reconstituted into troponin with tryptophanless cTnI and cTnT. The double mutant, cTnC(L12W/N51C) labeled with 1,5-IAEDANS at Cys-51, served as a control to monitor Ca(2+)-induced opening and closing of the N-domain by Förster resonance energy transfer (FRET). The triple mutants contained both L12W and N51C labeled with 1,5-IAEDANS, and either L29Q or G159D. Both mutations had minimal effects on the equilibrium distance between Trp-12 and Cys-51-AEDANS in the absence or presence of bound Ca(2+). L29Q had no effect on the closing rate of the N-domain triggered by release of Ca(2+), but reduced the Ca(2+)-induced opening rate. G159D reduced both the closing and opening rates. Previous results showed that the closing rate of cTnC N-domain triggered by Ca(2+) dissociation was substantially enhanced by PKA phosphorylation of cTnI. This rate enhancement was abolished by L29Q or G159D. These mutations alter the kinetics of structural transitions in the regulatory N-domain of cTnC that are involved in either activation (L29Q) or deactivation (G159D). Both mutations appear to be antagonistic toward phosphorylation signaling between cTnI and cTnC.

Effects of PKA phosphorylation of cardiac troponin I and strong crossbridge on conformational transitions of the N-domain of cardiac troponin C in regulated thin filaments

Regulation of cardiac muscle function is initiated by binding of Ca2+ to troponin C (cTnC) which induces a series of structural changes in cTnC and other thin filament proteins. These structural changes are further modulated by crossbridge formation and fine-tuned by phosphorylation of cTnI. The objective of the present study is to use a new Förster resonance energy transfer-based structural marker to distinguish structural and kinetic effects of Ca2+ binding, crossbridge interaction, and protein kinase A phosphorylation of cTnI on the conformational changes of the cTnC N-domain. The FRET-based structural marker was generated by attaching AEDANS to one cysteine of a double-cysteine mutant cTnC(13C/51C) as a FRET donor and attaching DDPM to the other cysteine as the acceptor. The doubly labeled cTnC mutant was reconstituted into the thin filament by adding cTnI, cTnT, tropomyosin, and actin. Changes in the distance between Cys13 and Cys51 induced by Ca2+ binding/dissociation were determined by FRET-sensed Ca2+ titration and stopped-flow studies, and time-resolved fluorescence measurements. The results showed that the presence of both Ca2+ and strong binding of myosin head to actin was required to achieve a fully open structure of the cTnC N-domain in regulated thin filaments. Equilibrium and stopped-flow studies suggested that strongly bound myosin head significantly increased the Ca2+ sensitivity and changed the kinetics of the structural transition of the cTnC N-domain. PKA phosphorylation of cTnI impacted the Ca2+ sensitivity and kinetics of the structural transition of the cTnC N-domain but showed no global structural effect on cTnC opening. These results provide an insight into the modulation mechanism of strong crossbridge and cTnI phosphorylation in cardiac thin filament activation/relaxation processes.

Development and cardiac contractility: cardiac troponin T isoforms and cytosolic calcium in rabbit

Cardiac contractility depends on calcium sensitivity of the myofilaments and cytosolic free calcium concentration ([Ca(2+)](i)) during activation. During development, the cardiac troponin T isoform cTnT(1) is replaced by shorter cTnT isoforms, including cTnT(4), and changes occur in other myofibrillar proteins and in calcium regulation. We expressed rabbit recombinant (r)cTnT(1) and rcTnT(4) in Spodoptera frugiperda cells and determined their effect on calcium binding to TnC in solution and on the calcium sensitivity of myofilaments in skinned rabbit ventricular fibers in vitro. We measured [Ca(2+)](i) and L-type calcium current (I(Ca)) in ventricular myocytes from 3-wk-old and adult rabbits. The dissociation constant (K(d)) of Ca-Tn(cTnT1) in solution was smaller than that of Ca-Tn(cTnT4) (mean +/- SE: 0.52 +/- 0.08 mumol/L versus 0.83 +/- 0.09 mumol/L). The Ca(2+) sensitivity of force development was greater in fibers reconstituted with rcTnT(1) (pCa(50) 6.07 +/- 0.04) than those reconstituted with rcTnT(4) (pCa(50) 5.75 +/- 0.07). Systolic [Ca](i) was lower in 3-wk-old than adult cells (443 +/- 35 nmol/L versus 882 +/- 88 nmol/L) as was I(Ca) (5.8 +/- 0.9 pA/pF versus 14.2 +/- 1.6 pA/pF). The higher calcium sensitivity of Tn-Ca binding and of force development conferred by rcTnT(1) suggest that higher neonatal cTnT(1) expression may partially compensate for the lower systolic [Ca(2+)](i).

Structural transition of the inhibitory region of troponin I within the regulated cardiac thin filament

Contraction and relaxation of cardiac muscle are regulated by the inhibitory and regulatory regions of troponin I (cTnI). Our previous FRET studies showed that the inhibitory region of cTnI in isolated troponin experiences a structural transition from a beta-turn/coil motif to an extended conformation upon Ca(2+) activation. During the relaxation process, the kinetics of the reversal of this conformation is coupled to the closing of the Ca(2+)-induced open conformation of the N-domain of troponin C (cTnC) and an interaction between cTnC and cTnI in their interface. We have since extended the structural kinetic study of the inhibitory region to fully regulated thin filament. Single-tryptophan and single-cysteine mutant cTnI(L129W/S151C) was labeled with 1,5-IAEDANS at Cys151, and the tryptophan-AEDANS pair served as a donor-acceptor pair. Labeled cTnI mutant was used to prepare regulated thin filaments. Ca(2+)-induced conformational changes in the segment of Trp129-Cys151 of cTnI were monitored by FRET sensitized acceptor (AEDANS) emission in Ca(2+) titration and stopped-flow measurements. Control experiments suggested energy transfer from endogenous tryptophan residues of actin and myosin S1 to AEDANS attached to Cys151 of cTnI was very small and Ca(2+) independent. The present results show that the rate of Ca(2+)-induced structural transition and Ca(2+) sensitivity of the inhibitory region of cTnI were modified by (1) thin filament formation, (2) the presence of strongly bound S1, and (3) PKA phosphorylation of the N-terminus of cTnI. Ca(2+) sensitivity was not significantly changed by the presence of cTm and actin. However, the cTn-cTm interaction decreased the cooperativity and kinetics of the structural transition within cTnI, while actin filaments elicited opposite effects. The strongly bound S1 significantly increased the Ca(2+) sensitivity and slowed down the kinetics of structural transition. In contrast, PKA phosphorylation of cTnI decreased the Ca(2+) sensitivity and accelerated the structural transition rate of the inhibitory region of cTnI on thin filaments. These results support the idea of a feedback mechanism by strong cross-bridge interaction with actin and provide insights on the molecular basis for the fine tuning of cardiac function by beta-adrenergic stimulation.

Ionic interventions that alter the association of troponin C C-domain with the thin filaments of vertebrate striated muscle

The regulatory complex of vertebrate skeletal muscle integrates information about cross-bridge binding, divalent cations and other intracellular ionic conditions to control activation of muscle contraction. Relatively little is known about the role of the troponin C (TnC) C-domain in the absence of Ca2+. Here, we use a standardized condition for measuring isometric tension in rabbit psoas skinned fibers to track TnC attachment and detachment in the absence of Ca2+ under different conditions of ionic strength, pH and MgATP. In the presence of MgATP and Mg2+, TnC detaches more readily and has a 1.5- to 2-fold lower affinity for the intact thin filament at pH 8 and 250 mM K+ than at pH 6 or in 30 mM K+; changes in affinity are fully reversible. The response to ionic strength is lost when Mg2+ and MgATP are absent, whereas the response to pH persists, suggesting that weaker electrostatic TnC-TnI-TnT interactions can be overridden by strongly bound cross-bridges. In solution, titration of a fluorescent C-domain mutant (F154W TnC) with Mg2+ reveals no significant changes in Mg2+ affinity with pH or ionic strength, suggesting that these parameters influence TnC binding by acting directly on electrostatic forces between TnC and TnI rather than by changing Mg2+ binding to C-domain sites III and IV.

Tissue expression of four troponin I genes and their molecular interactions with two troponin C isoforms in Caenorhabditis elegans

Gene duplication is a major genetic event that can produce multiple protein isoforms. Comparative sequence and functional analysis of related gene products can provide insights into protein family evolution. To characterize the Caenorhabditis elegans troponin I family, we analyzed gene structures, tissue expression patterns and RNAi phenotypes of four troponin I isoforms. Tissue expression patterns were determined using lacZ/gfp/rfp reporter gene assays. The tni-1, tni-2/unc-27 and tni-3 genes, each encoding a troponin I isoform, are uniquely expressed in body wall, vulval and anal muscles but at different levels; tni-4 was expressed solely in the pharynx. Expressing tni-1 and -2 gene RNAi caused motility defects similar to unc-27 (e155) mutant, a tni-2 null allele. The tni-3 RNAi expression produced egg laying defects while the tni-4 RNAi caused arrest at gastrulation. Overlay analyses were used to assay interactions between the troponin I and two troponin C isoforms. The three body wall troponin I isoforms interacted with body wall and pharyngeal troponin C isoforms; TNI-4 interacted only with pharyngeal troponin C. Our results suggest the body wall genes have evolved following duplication of the pharynx gene and provide important data about gene duplication and functional differentiation of nematode troponin I isoforms.

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.

Differential regulation of the actomyosin interaction by skeletal and cardiac troponin isoforms

There are significant isoform differences between the skeletal and cardiac troponin complexes. Studies of the regulatory properties of these proteins have previously shown only significant differences in the calcium dependence of their regulation. Using a sensitive myosin subfragment 1 (S1) binding assay we show that in the presence of calcium, thin filaments reconstituted with either skeletal or cardiac troponin produce virtually identical S1 binding curves. However in the absence of calcium the S1 binding curves differ considerably. Combined with kinetic measurements, curve fitting to the three-state thin filament regulatory model shows the main difference is that calcium produces a 4-fold change in K(T) (the closed-open equilibrium) for the skeletal system but little change in the cardiac system. The results show a significant difference in the range of regulatory effect between the cardiac and skeletal systems that we interpret as effects upon actin-troponin (Tn)I-TnC binding equilibria. As structural data show that the Ca(2+)-bound TnC structures differ, the additional counter-intuitive result here is that with respect to myosin binding the +Ca(2+) state of the two systems is similar whereas the -Ca(2+) state differs. This shows the regulatory tuning of the troponin complex produced by isoform variation is the net result of a complex series of interactions among all the troponin components.

Structural consequences of cardiac troponin I phosphorylation

beta-Adrenergic stimulation of the heart results in bisphosphorylation of the N-terminal extension of cardiac troponin I (TnI). Bisphosphorylation of TnI reduces the affinity of the regulatory site on troponin C (TnC) for Ca(2+) by increasing the rate of Ca(2+) dissociation. What remains unclear is how the phosphorylation signal is transmitted from one subunit of troponin to another. We have produced a series of mutations in the N-terminal extension of TnI designed to further our understanding of the mechanisms involved. The ability of phosphorylation of the mutant TnIs to affect Ca(2+) sensitivity has been assessed. We find that the Pro residues found in a conserved (Xaa-Pro)(4) motif N-terminal to the phosphorylation sites are not required for the effect of the N-terminal extension on Ca(2+) binding in the presence or absence of phosphorylation. Our experiments also reveal that the full effects of phosphorylation are seen even when residues 1-15 of TnI are deleted. If further residues are removed, not only does the effect of phosphorylation diminish but deletion of the N-terminal extension mimics phosphorylation. We propose that TnI residues 16-29 bind to TnC stabilizing the "open" Ca(2+)-bound state. Phosphorylation (or deletion) prevents this binding, accelerating Ca(2+) release.

Multicenter Evaluation of an Automated Assay for Troponin I

Background: Cardiac troponin I (cTnI) is a powerful tool to aid in the diagnosis of myocardial infarction and cardiac muscle damage. We describe an assay that overcomes problems of early assays that were often affected by cTnI degradation, assay interference, poor sensitivity, and imprecision.

Methods: The analytical performance of the Access® AccuTnITM assay (Beckman Coulter) was evaluated at five institutions. Controls, zero calibrator, and diluted patient samples were used to determine precision, detection limit, functional sensitivity, and linearity. The 97.5 and 99 percentiles of a reference population were determined. Common interferents and heterophilic patient samples were tested. Equimolarity was determined by assaying samples with various ratios of free and complexed cTnI. Matched samples drawn into serum, EDTA, lithium heparin, and sodium heparin sample tubes were compared.

Results: Total imprecision (CVs) was 4.0–8.8% between 0.40 and 31 μg/L cTnI. The detection limit was <0.01 μg/L. The 97.5 percentile upper reference limit (URL) was 0.03 μg/L (CV = 20%), and the 99 percentile URL was 0.04 μg/L (CV = 14%). Total CVs of 10% and 20% were seen at and above 0.06 and 0.03 μg/L, respectively. The assay was linear to >60 μg/L and not affected by common assay interferents. An equimolar response was observed with free, complexed, phosphorylated, and dephosphorylated forms of cTnI. Results were 4% lower in serum and 14% lower in EDTA plasma than in lithium heparin plasma (P <0.01), independent of cTnI concentration.

Conclusion: AccuTnI is a sensitive and precise assay for the measurement of cTnI.

The interaction of the bisphosphorylated N-terminal arm of cardiac troponin I-A 31P-NMR study

Cardiac troponin I, the inhibitory subunit of the heterotrimeric cardiac troponin (cTn) complex is phosphorylated by protein kinase A at two serine residues located in its heart-specific N-terminal extension. This flexible arm interacts at different sites within cTn dependent on its phosphorylation degree. Bisphosphorylation is known to induce conformational changes within cTnI which finally lead to a reduction of the calcium affinity of cTnC. However, as we show here, the bisphosphorylated cTnI arm does not interact with cTnC, but with cTnT and/or cTnI.

Factors contributing to troponin exchange in myofibrils and in solution

The troponin complex in a muscle fiber can be replaced with exogenous troponin by using a gentle exchange procedure in which the actin-tropomyosin complex is never devoid of a full complement of troponin (Brenner et al. (1999) Biophys J 77: 2677-2691). The mechanism of this exchange process and the factors that influence this exchange are poorly understood. In this study, the exchange process has now been examined in myofibrils and in solution. In myofibrils under rigor conditions, troponin exchange occurred preferentially in the region of overlap between actin and myosin when the free Ca2+ concentration was low. At higher concentrations of Ca2+, the exchange occurred uniformly along the actin. Ca2+ also accelerated troponin exchange in solution but the effect of S1 could not be confirmed in solution experiments. The rate of exchange in solution was insensitive to moderate changes in pH or ionic strength. Increasing the temperature resulted in a two-fold increase in rate with each 10 degrees C increase in temperature. A sequential two step model of troponin binding to actin-tropomyosin could simulate the observed association and dissociation transients. In the absence of Ca2+ or rigor S1, the following rate constants could describe the binding process: k1 = 7.12 microM(-1) s(-1), k(-1) = 0.65 s(-1), k2 = 0.07 s(-1), k(-2) = 0.0014 s(-1). The slow rate of detachment of troponin from actin (k(-2)) limits the rate of exchange in solution and most likely contributes to the slow rate of exchange in fibers.

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.

Additional PKA phosphorylation sites in human cardiac troponin I

We used mass spectrometry to monitor cAMP-dependent protein kinase catalysed phosphorylation of human cardiac troponin I in vitro. Phosphorylation of isolated troponin I by cAMP-dependent protein kinase resulted in the covalent incorporation of phosphate on at least five different sites on troponin I, and a S22/23A troponin I mutant incorporated phosphates on at least three sites. In addition to the established phosphorylation sites (S22 and S23) we found that S38 and S165 were the other two main sites of phosphorylation. These 'overphosphorylation' sites were not phosphorylated sufficiently slower than S22 and S23 that we could isolate pure S22/23 bisphosphorylated troponin I. Overphosphorylation of troponin I reduced its affinity for troponin C, as measured by isothermal titration microcalorimetry. Phosphorylation of S22/23A also decreased its affinity for troponin C indicating that phosphorylation of S38 and/or S165 impedes binding of troponin I to troponin C. Formation of a troponin I/troponin C complex prior to cAMP-dependent protein kinase treatment did not prevent overphosphorylation. When whole troponin was phosphorylated by cAMP-dependent protein kinase, however, [(32)P]phosphate was incorporated only into troponin I and only at S22 and S23. Mass spectrometry confirmed that overphosphorylation is abolished in the ternary complex. Troponin I bisphosphorylated exclusively at S22 and S23 troponin I showed reduced affinity for troponin C but the effect was diminished with respect to overphosphorylated troponin I. These results show that care should be exercised when interpreting data obtained with troponin I phosphorylated in vitro.

The importance of the carboxyl-terminal domain of cardiac troponin C in Ca2+-sensitive muscle regulation

The interactions between troponin I and troponin C are central to the Ca(2+)-regulated control of striated muscle. Using isothermal titration microcalorimetry we have studied the binding of human cardiac troponin C (cTnC) and its isolated domains to human cardiac troponin I (cTnI). We provide the first binding data for these proteins while they are free in solution and unmodified by reporter groups. Our data reveal that the C-terminal domain of cTnC is responsible for most of the free energy change upon cTnC.cTnI binding. Importantly, the interaction between cTnI and the C-terminal domain of cTnC is 8-fold stronger in the presence of Ca(2+) than in the presence of Mg(2+), suggesting that the C-terminal domain of cTnC may play a modulatory role in cardiac muscle regulation. Changes in the affinity of cTnI for cTnC and its isolated C-terminal domain in response to ionic strength support this finding, with both following similar trends. At physiological ionic strength the affinity of cTnC for cTnI changed very little in response to Ca(2+), although the thermodynamic data show a clear distinction between binding in the presence of Ca(2+) and in the presence of Mg(2+).

Lower Cardiac Troponin T and I Results in Heparin-Plasma Than in Serum

Background: The use of plasma rather than serum for determination of cardiac troponins can improve turnaround time and potentially avoid incomplete serum separation that may produce falsely increased results. We investigated the influence of incomplete serum separation and the effect of heparin-plasma on cardiac troponin concentrations.

Methods: Serum and heparin-plasma samples were drawn simultaneously from 100 patients (50 patients with acute coronary syndrome and 50 patients after open heart surgery) and measured on three different analytical systems, two for determination of cardiac troponin I (cTnI; Abbott AxSYM and Bayer ACS:Centaur) and one for cardiac troponin T (cTnT; Roche Elecsys cTnT STAT). Serum samples were reanalyzed after a second centrifugation to assess the influence of incomplete serum separation.

Results: Mean results (± 95% confidence interval) in heparin-plasma compared with serum were 101% ± 2% (AxSYM cTnI), 94% ± 3% (ACS:Centaur cTnI), and 99% ± 3% (Elecsys cTnT). Differences >20% were seen in 11% of results on the ACS:Centaur, 9% of results on Elecsys cTnT, and 2% of results on the AxSYM. For the Elecsys cTnT assay, the magnitude of the difference between serum and plasma was independent of the absolute concentration and confined to individual samples, and was reversed by treatment with heparinase. A second centrifugation had no effect on serum results by any of the assays.

Conclusion: The concentrations of troponins measured in heparin-plasma are markedly lower than in serum in some cases.

Systematic mapping of regions of human cardiac troponin I involved in binding to cardiac troponin C: N- and C-terminal low affinity contributing regions

The Spot method of multiple peptide synthesis was used to map in a systematic manner regions of the human cardiac troponin I sequence (hcTnI) involved in interactions with its physiological partner, troponin C (cTnC). Ninety-six 20-mer peptides describing the entire hcTnI sequence were chemically assembled; their reactivity with [125I]cTnC, in the presence of 3 mM Ca2+, enabled the assignment of six sites of interaction (residues 19-32, 45-54, 129-138, 145-164, 161-178 and 191-210). For several sites, a good correlation with literature data was obtained, thus validating this methodological approach. Synthetic peptides, each containing in their sequence an interaction site, were prepared. As assessed by BIACORE, all of them exhibited an affinity for cTnC in the range of 10(-6)-10(-7) M, except for hcTnI [39-58] which showed a nanomolar affinity. This peptide was also able to block the interaction between hcTnI and cTnC. We therefore postulate that despite the existence of multiple cTnC interaction sites on the hcTnI molecule, only that region of hcTnI allows a stabilization of the complex. Residues 19-32 from the N-terminal cardio-specific extension of hcTnI were also found to be involved in interaction with cTnC; residues 19-32 may correspond to the minimal sequence of the extension which could switch between the N- and C-terminal TnC domains, depending on its phosphorylation state. Finally, two Ca(2+)-dependent cTnC binding domains within the C-terminal part of hcTnI (residues 164-178 and 191-210) were also mapped. The latter site may be linked with the cardiac dysfunction observed in stunned myocardium.

Altered regulatory properties of human cardiac troponin I mutants that cause hypertrophic cardiomyopathy

Cardiac troponin I (cTnI) is the inhibitory component of the troponin complex and is involved in the calcium control of heart muscle contraction. Recently, specific missense mutations of the cTnI gene (TNNI3) have been shown to cause familial hypertrophic cardiomyopathy (HCM). We have analyzed the functional effects of two HCM mutations (R145G and R162W) using purified recombinant cTnI. Both mutations gave reduced inhibition of actin-tropomyosin-activated myosin ATPase, both in experiments using cTnI alone and in those using reconstituted human cardiac troponin under relaxing conditions. Both mutant troponin complexes also conferred increased calcium sensitivity of ATPase regulation. Studies on wild type/R145G mutant mixtures showed that the wild type phenotype was dominant in that the inhibition and the calcium sensitivity conferred by a 50:50 mixture was more similar to wild type than expected. Surface plasmon resonance-based assays showed that R162W mutant had an increased affinity for troponin C in the presence of calcium. This observation may contribute to the increased calcium sensitivity found with this mutant and also corroborates recent structural data. The observed decreased inhibition and increased calcium sensitivity suggest that these mutations may cause HCM via impaired relaxation rather than the impaired contraction seen with some other classes of HCM mutants.

Conformation of the regulatory domain of cardiac muscle troponin C in its complex with cardiac troponin I

Calcium activation of fast striated muscle results from an opening of the regulatory N-terminal domain of fast skeletal troponin C (fsTnC), and a substantial exposure of a hydrophobic patch, essential for Ca(2+)-dependent interaction with fast skeletal troponin I (fsTnI). This interaction is obligatory to relieve the inhibition of strong, force-generating actin-myosin interactions. We have determined intersite distances in the N-terminal domain of cardiac TnC (cTnC) by fluorescence resonance energy transfer measurements and found negligible increases in these distances when the single regulatory site is saturated with Ca(2+). However, in the presence of bound cardiac TnI (cTnI), activator Ca(2+) induces significant increases in the distances and a substantial opening of the N-domain. This open conformation within the cTnC.cTnI complex has properties favorable for the Ca(2+)-induced interaction with an additional segment of cTnI. Thus, the binding of cTnI to cTnC is a prerequisite to achieve a Ca(2+)-induced open N-domain similar to that previously observed in fsTnC with no bound fsTnI. This role of cardiac TnI has not been previously recognized. Our results also indicate that structural information derived from a single protein may not be sufficient for inference of a structure/function relationship.

National Academy of Clinical Biochemistry Standards of Laboratory Practice: Recommendations for the Use of Cardiac Markers in Coronary Artery Diseases

The Sixth Conference on the “Standards of Laboratory Practice Series”, sponsored by the National Academy of Clinical Biochemistry (NACB), was held on August 4–5, 1998, at the Annual Meeting of the American Association for Clinical Chemistry, in Chicago, IL. An expert committee was assembled to write recommendations on the use of cardiac markers in coronary artery diseases. The NACB Committee prepared a preliminary draft of the guidelines, made them available on the World Wide Web (www.nacb.org), and distributed them before the presentations. The recommendations were divided into four areas: the use of markers in the triage of patients with chest pain, acute coronary syndromes, clinical applications other than acute myocardial infarction and research, and assay platforms and markers of acute myocardial infarction. The recommendations were revised and subsequently re-presented in part at the “Biomarkers in Acute Cardiac Syndromes Conference”, sponsored by the Jewish Hospital Heart and Lung Institute, Louisville KY, on October 16–17, 1998. This report lists each recommendation, its scientific justification, and a summary of discussions from conference participants and reviewers.

Approximately 100 individuals responded to various versions of these recommendations via direct correspondences, telephone calls to Committee members, electronic mail correspondence to the Committee Chairman, or oral questions and comments raised during one of the two conference presentations. Some of the recommendations were changed to reflect the consensus opinion. In cases in which there was no consensus, the Committee included pertinent discussion without necessarily changing the original recommendations. At times, the Committee members felt that although a particular recommendation might not be the current standard of care today, they anticipate that it likely will be adopted in the near future.

NMR analysis of cardiac troponin C-troponin I complexes: effects of phosphorylation

Phosphorylation of the cardiac specific amino-terminus of troponin I has been demonstrated to reduce the Ca2+ affinity of the cardiac troponin C regulatory site. Recombinant N-terminal cardiac troponin I proteins, cardiac troponin I(33-80), cardiac troponin I(1-80), cardiac troponin I(1-80)DD and cardiac troponin I(1-80)pp, phosphorylated by protein kinase A, were used to form stable binary complexes with recombinant cardiac troponin C. Cardiac troponin I(1-80)DD, having phosphorylated Ser residues mutated to Asp, provided a stable mimetic of the phosphorylated state. In all complexes, the N-terminal domain of cardiac troponin I primarily makes contact with the C-terminal domain of cardiac troponin C. The nonphosphorylated cardiac specific amino-terminus, cardiac troponin I(1-80), was found to make additional interactions with the N-terminal domain of cardiac troponin C.

The N-terminal region of troponin T is essential for the maximal activation of rat cardiac myofilaments

Troponin T (TnT) is an essential protein in the transduction of the Ca2+-binding signal that triggers striated muscle contraction. Functional diversity among various TnT isoforms found in cardiac and skeletal muscles has been correlated with the sequence heterogeneity at the amino (N-) and the carboxyl (C-) terminal regions. The most striking difference between cardiac TnT (cTnT) and skeletal TnT (sTnT) is that cTnT has an extended N-terminus, which is rich in negatively charged amino acids. To investigate the role of this region in cTnT, we deleted the first 76 amino acids in rat cTnT (cTnT77-289) by site-directed mutagenesis. We exchanged the native troponin complex in rat cardiac myofibrillar preparations and detergent skinned cardiac fiber bundles by treatment with excess cTnT or cTnT77-289. After reconstituting the cTnT77-289 containing myofibrils with cardiac troponin I-cardiac troponin C (cTnI-cTnC), the MgATPase activity was 70% of the cTnT treated myofibrils in the relaxed state and 83% of the cTnT treated myofibrils in the maximal Ca2+-activated state. These observations were supported by force measurements in which cTnT and cTnT77-289 were exchanged into skinned fiber bundles. Prior to reconstitution with cTnI-cTnC, the Ca2+-independent maximal force developed by the cTnT77-289 containing fiber was 45% of the force developed by the cTnT containing fiber. After reconstituting with cTnI-cTnC, the Ca2+-activated maximal force of the cTnT77-289 containing fiber was 62% of the force developed by the cTnT containing +cTnI-cTnC reconstituted fiber. In both assays, no significant changes in the normalized Ca2+-activity relation or in co-operativity were observed. Fluorescence experiments using pyrene-labeled Tm demonstrated that the binding of cTnT77-289 to Tm was 3-4 fold stronger than that of cTnT. Our results suggest that strong interactions between cTnT77-289 and Tm stabilize cardiac myofilaments in a sub-maximally activated state. Our findings also indicate that the N-terminus of cTnT is essential for maximal activation of cardiac myofilaments.

Determination of Cardiac Troponin I Forms in the Blood of Patients with Acute Myocardial Infarction and Patients Receiving Crystalloid or Cold Blood Cardioplegia

To determine the forms of cardiac troponin I (cTnI) circulating in the bloodstream of patients with acute myocardial infarction (AMI) and patients receiving a cardioplegia during heart surgery, we developed three immunoenzymatic sandwich assays. The first assay involves the combination of two monoclonal antibodies (mAbs) specific for human cTnI. The second assay involves the combination of a mAb specific for troponin C (TnC) and an anti-cTnI mAb. The third assay was a combination of a mAb specific for human cardiac troponin T (cTnT) and an anti-cTnI mAb. Fifteen serum samples from patients with AMI, 10 serum samples from patients receiving crystalloid cardioplegia during heart surgery, and 10 serum samples from patients receiving cold blood cardioplegia during heart surgery were assayed by the three two-site immunoassays. We confirmed that cTnI circulates not only in free form but also complexed with the other troponin components (TnC and cTnT). We showed that the predominant form in blood is the cTnI-TnC binary complex (IC). Free cTnI, the cTnI-cTnT binary complex, and the cTnT-cTnI-TnC ternary complex were seldom present, and when present, were in small quantities compared with the binary complex IC. Similar results were obtained in both patient populations studied. These observations are essential for the development of new immunoassays with improved clinical sensitivity and for the selection of an appropriate cTnI primary calibrator.

The kinetic cycle of cardiac troponin C: calcium binding and dissociation at site II trigger slow conformational rearrangements

The goal of this study is to characterize the kinetic mechanism of Ca2+ activation and inactivation of cardiac troponin C (cTnC), the Ca2+ signaling protein which triggers heart muscle contraction. Previous studies have shown that IAANS covalently coupled to Cys84 of wild-type cTnC is sensitive to conformational change caused by Ca2+ binding to the regulatory site II; the present study also utilizes the C35S mutant, in which Cys84 is the lone cysteine, to ensure the specificity of IAANS labeling. Site II Ca2+ affinities for cTnC-wt, cTnC-C35S, cTnC-wt-IAANS2, and cTnC-C35S-IAANS were similar (KD = 2-5 microM at 25 degrees C; KD = 2-8 microM at 4 degrees C), indicating that neither the IAANS label nor the C35S mutation strongly perturbs site II Ca2+ affinity. To directly determine the rate of Ca2+ dissociation from site II, the Ca2+-loaded protein was rapidly mixed with a spectroscopically sensitive chelator in a stopped flow spectrometer. The resulting site II Ca2+ off-rates were k(off) = 700-800 s(-1) (4 degrees C) for both cTnC-wt and cTnC-C35S, yielding calculated macroscopic site II Ca2+ on-rates of k(on) = k(off)/KD = 2-4 x 10(8) M(-1) s(-1) (4 degrees C). As observed for Ca2+ affinities, neither the C35S mutation nor IAANS labeling significantly altered the Ca2+ on- and off-rates. Using IAANS fluorescence as a monitor of the protein conformational state, the intramolecular conformational changes (delta) induced by Ca2+ binding and release at site II were found to be significantly slower than the Ca2+ on- and off-rates. The conformational rate constants measured for cTnC-wt-IAANS2 and cTnC-C35S-IAANS were k(delta on) = 120-210 s(-1) and k(delta off) = 90-260 s(-1) (4 degrees C) . Both conformational events were slowed in cTnC-wt-IAANS2 relative to cTnC-C35S-IAANS, presumably due to the bulky IAANS probe coupled to Cys35. Together, the results provide a nearly complete kinetic description of the Ca2+ activation cycle of isolated cTnC, revealing rapid Ca2+ binding and release at site II accompanied by slow conformational steps that are likely to be retained by the full troponin complex during heart muscle contraction and relaxation.

Characterization of cardiac troponin subunit release into serum after acute myocardial infarction and comparison of assays for troponin T and I

We examined the release of cardiac troponin T (cTnT) and I (cTnI) into the blood of patients after acute myocardial infarction (AMI). Three postAMI serum samples were applied in separate analytical runs onto a calibrated gel filtration column (Sephacryl S-200), and the proteins were separated by molecular weight. Using commercial cTnT and cTnI assays measured on collected fractions, we found that troponin was released into blood as a ternary complex of cTnT-I-C, a binary complex of cTnI-C, and free cTnT, with no free cTnI within the limits of the analytical methodologies. The serum samples were also examined after incubation with EDTA and heparin. EDTA broke up troponin complexes into individual subunits, whereas heparin had no effect on the assays tested. We added free cTnC subunits to 24 AMI serum samples and found no marked increase in the total cTnI concentrations, using an immunoassay that gave higher values for the cTnI-C complex than free cTnI. To characterize the cross-reactivity of cTnT and cTnI assays, purified troponin standards in nine different forms were prepared, added to serum and plasma pools, and tested in nine quantitative commercial and pre-market assays for cTnI and one approved assay for cTnT. All nine cTnI assays recognized each of the troponin I forms (complexed and free). In five of these assays, the relative responses for cTnI were nearly equimolar. For the remainder, the response was substantially greater for complexed cTnI than for free cTnI. Moreover, there was a substantial difference in the absolute concentration of results between cTnI assays. The commercial cTnT assay recognized binary and ternary complexes of troponin on a near equimolar basis. We conclude that all assays are useful for detection of cardiac injury. However, there are differences in absolute cTnI results due to a lack of mass standardization and heterogeneity in the cross-reactivities of antibodies to various troponin I forms.

A kinetic model for the binding of Ca2+ to the regulatory site of troponin from cardiac muscle

The kinetics of the binding of Ca2+ to the single regulatory site of cardiac muscle troponin was investigated by using troponin reconstituted from the three subunits, using a monocysteine mutant of troponin C (cTnC) labeled with the fluorescent probe 2-[(4'-(iodoacetamido)anilino]naphthalene-6-sulfonic acid (IAANS) at Cys-35. The kinetic tracings of binding experiments for troponin determined at free [Ca2+] > 1 microM were resolved into two phases. The rate of the fast phase increased with increasing [Ca2+], reaching a maximum of about 35 s-1 at 4 degrees C, and the rate of the slow phase was approximately 5 s-1 and did not depend on [Ca2+]. Dissociation of bound Ca2+ occurred in two phases, with rates of about 23 and 4 s-1. The binding and dissociation results obtained with the binary complex formed between cardiac troponin I and the IAANS-labeled cTnC mutant were very similar to those obtained from reconstituted troponin. The kinetic data are consistent with a three-step sequential model similar to the previously reported mechanism for the binding of Ca2+ to a cTnC mutant labeled with the same probe at Cys-84 (Dong et al. (1996) J. Biol. Chem. 271, 688-694). In this model, the initial binding in the bimolecular step to form the Ca2+-troponin complex is assumed to be a rapid equilibrium, followed by two sequential first-order transitions. The apparent bimolecular rate constant is 5.1 x 10(7) M-1 s-1, a factor of 3 smaller than that for cTnC. The rates of the first-order transitions are an order of magnitude smaller for troponin than for cTnC. These kinetic differences form a basis for the enhanced Ca2+ affinity of troponin relative to the Ca2+ affinity of isolated cTnC. Phosphorylation of the monocysteine mutant of troponin I by protein kinase A resulted in a 3-fold decrease in the bimolecular rate constant but a 2-fold increase in the two observed Ca2+ dissociation rates. These changes in the kinetic parameters are responsible for a 5-fold reduction in Ca2+ affinity of phosphorylated troponin for the specific site.

Structure of cardiac muscle troponin C unexpectedly reveals a closed regulatory domain

The regulation of cardiac muscle contraction must differ from that of skeletal muscles to effect different physiological and contractile properties. Cardiac troponin C (TnC), the key regulator of cardiac muscle contraction, possesses different functional and Ca2+-binding properties compared with skeletal TnC and features a Ca2+-binding site I, which is naturally inactive. The structure of cardiac TnC in the Ca2+-saturated state has been determined by nuclear magnetic resonance spectroscopy. The regulatory domain exists in a "closed" conformation even in the Ca2+-bound (the "on") state, in contrast to all predicted models and differing significantly from the calcium-induced structure observed in skeletal TnC. This structure in the Ca2+-bound state, and its subsequent interaction with troponin I (TnI), are crucial in determining the specific regulatory mechanism for cardiac muscle contraction. Further, it will allow for an understanding of the action of calcium-sensitizing drugs, which bind to cardiac TnC and are known to enhance the ability of cardiac TnC to activate cardiac muscle contraction.

Disparate fluorescence properties of 2-[4'-(iodoacetamido)anilino]-naphthalene-6-sulfonic acid attached to Cys-84 and Cys-35 of troponin C in cardiac muscle troponin

Two monocysteine mutants of cardiac muscle troponin C, cTnC(C35S) and cTnC(C84S), were genetically generated and labeled with the fluorescent probe 2-[4'-(iodoacetamido)anilino]naphthalene-6-sulfonic acid (IAANS) at Cys-84 and Cys-35, respectively. Cys-84 is located on helix D in the regulatory N-domain, and Cys-35 is at the -y position of the inactive 12-residue loop of site I. These labeled mutants were studied by a variety of steady-state and time-resolved fluorescence methods. In the absence of divalent cation, the fluorescence of the attached IAANS indicated an exposed environment at Cys-35 and a relatively less-exposed environment at Cys-84. The binding of Ca2+ to the single regulatory site elicited a large enhancement of the emission of IAANS attached to Cys-84, but only marginal fluorescence changes of the probe at Cys-35. Upon reconstitution of the labeled cTnC mutants with troponin I and troponin T to form the three-subunit troponin, the fluorescence of IAANS-Cys-84 in apo-troponin was spectrally similar to that observed with the Ca(2+)-loaded uncomplexed cTnC mutant. Only very moderate changes in the fluorescence of IAANS-Cys-84 were observed when the regulatory site in reconstituted troponin was saturated. The exposed Cys-35 environment of the uncomplexed cTnC mutant became considerably less exposed and less polar when the mutant was incorporated into apo-troponin. In contrast to the Cys-84 site, saturation of the regulatory site II by Ca2+ in reconstituted troponin resulted in a reversal of the environment of the Cys-35 site toward a more exposed and more polar environment. These results indicated involvement of the inactive loop I in the Ca2+ trigger mechanism in cardiac muscle. The fluorescence of IAANS at both Cys-84 and Cys-35 was sensitive to phosphorylation of cTnl in reconstituted troponin, and the sensitivity was observed with both apo-troponin and Ca(2+)-loaded troponin.

Calcium-induced conformational change in cardiac troponin C studied by fluorescence probes attached to Cys-84

Residue Cys-84 of bovine cardiac troponin C (cTnC) located at the C-terminal end of helix D was selectively labeled in the presence of Ca2+ with two fluorescent probes: IAANS (2-(4-(iodoacetamido)anilino)naphthalene-6-sulfonic acid) and acrylodan (6-acrylol-2-(dimethylamino)naphthalene). The fluorescence of the attached probes was studied by the steady-state and time-resolved methods to gain an insight about the nature of Ca(2+)-induced conformational changes in the N-domain regulatory region of cTnC. Changes in the experimental emission spectra, quantum yields, and excited-state lifetimes suggested that bound Ca2+ at the single regulatory site induced a less polar microenvironment for both probes attached to Cys-84. However, a twofold increase in the bimolecular collisional quenching constant was observed for both probes in the presence of activator Ca2+, indicating an increased exposure of the probes to solvent. These data were interpreted with reference to the origins of the observed Stokes' shifts. In the apo and 2Mg states of cTnC, the attached probes were partially shielded by helices B and C, and their excited-states were highly quenched in the tertiary structure through strong interactions of a dipolar nature with neighboring amino-acid side chains. In the 3Ca state, these interactions were disrupted so that nonradiative decay processes were suppressed and radiative processes were enhanced, leading to the observed increases in quantum yields and lifetimes and blue-shifts of the emission spectra. As the disruption of internal quenching resulted from separation of helices B and C from helix D, the attached probes became more accessible to solvent and experienced increases in the rate of collisions with external molecules in the solvent. Although this increased exposure to solvent would lead to suppression of radiative decay processes, this effect apparently was overcompensated by the effect of elimination of internal quenching. The present results are consistent with a Ca(2+)-induced open conformation of the N-domain in cTnC.