Fluorescent Immunoassays for Detection and Quantification of Cardiac Troponin I: A Short Review

Cardiovascular diseases are considered one of the major causes of human death globally. Myocardial infarction (MI), characterized by a diminished flow of blood to the heart, presents the highest rate of morbidity and mortality among all other cardiovascular diseases. These fatal effects have triggered the need for early diagnosis of appropriate biomarkers so that countermeasures can be taken. Cardiac troponin, the central key element of muscle regulation and contraction, is the most specific biomarker for cardiac injury and is considered the “gold standard”. Due to its high specificity, the measurement of cardiac troponin levels has become the predominant indicator of MI. Various forms of diagnostic methods have been developed so far, including chemiluminescence, fluorescence immunoassay, enzyme-linked immunosorbent assay, surface plasmon resonance, electrical detection, and colorimetric protein assays. However, fluorescence-based immunoassays are considered fast, accurate and most sensitive of all in the determination of cardiac troponins post-MI. This review represents the strategies, methods and levels of detection involved in the reported fluorescence-based immunoassays for the detection of cardiac troponin I.

Discrepancy between Cardiac Troponin Assays Due to Endogenous Antibodies

BACKGROUND

Despite well-described analytical effects of autoantibodies against cardiac troponin (cTn) I on experimental assays, no study has systematically examined their impact on cTn assays in clinical use. We determined the effects of endogenous antibodies on 5 different cTnI assays and a cTnT assay.

METHODS

cTn was measured by 6 methods: Siemens hs-cTnI Centaur, Siemens hs-cTnI Vista, Abbott hs-cTnI Architect, Beckman hs-cTnI Access, Beckman cTnI Access, and Roche hs-cTnT Elecsys. Measurements were repeated on 5 assays (all except Siemens hs-cTnI Vista) following immunoglobulin depletion by incubation with protein A. Low recovery of cTnI (<40%) following immunoglobulin depletion was considered positive for macro-cTnI. Protein A findings were validated by gel filtration chromatography and polyethylene glycol precipitation.

RESULTS

In a sample of 223 specimens selected from a community laboratory that uses the Siemens hs-cTnI Centaur assay and from which cTn was requested, 76% of samples demonstrated increased cTnI (median, 88 ng/L; interquartile range, 62-204 ng/L). Macro-cTnI was observed in 123 (55%) of the 223 specimens. Comparisons of cTnI assays markedly improved once patients with macro-cTnI were removed. Passing-Bablok regression analysis between hs-cTnI assays demonstrated different slopes for patients with and without macro-cTnI. In patients with macro-cTnI, 89 (72%) showed no effect on the recovery of cTnT, whereas 34 (28%) had reduced recovery of cTnT. The proportion of results above the manufacturers' 99th percentile varied with the cTn assay and macro-cTnI status.

CONCLUSION

We suggest that the observed discrepancy between hs-cTnI assays may be attributed in part to the presence of macro-cTnI.

Automated, Universal, and Mass-Producible Paper-Based Lateral Flow Biosensing Platform for High-Performance Point-of-Care Testing

Paper-based lateral flow assays (LFAs) are among the most widely used biosensing platforms for point-of-care testing (POCT). However, the conventional colloidal gold label of LFAs show low sensitivity and limited quantitative capacity. Alternatively, the use of enzyme/chemical reaction-based signal amplification with structural modifications has enhanced analytical capacity but requires multiple user interventions as a trade-off, increasing complexity, test imprecision, and time. These platforms are also difficult to manufacture, limiting their practical applications. In this study, within the current LFA production framework, we developed a highly sensitive, automated, universal, and manufacturable LFA biosensing platform by (i) incorporating gold nanoparticles into a polymer-networked peroxidase with an antibody as a new scheme for enhanced enzyme conjugation and (ii) integrating a mass-producible and time-programmable amplification part based on a water-swellable polymer for automating the sequential reactions in the immunoassay and signal amplification, without compromising performance, simplicity, and production feasibility. We applied this platform to evaluate cardiac troponin I (cTnI), a gold-standard biomarker for myocardial infarction diagnosis. Quantitative analysis of cTnI in clinical setting remains limited to the laboratory-based high-end and costly standard equipment. Coupled with an enzyme-catalyzed chemiluminescence method, this platform enables automated, cost-effective (0.66 USD per test), and high-performance testing of human cTnI in serum samples within 20 min with a detection range of 6 orders of magnitude, detection limit of 0.84 pg mL^-1 (595-fold higher than conventional cTnI-LFA), and a coefficient of variation of 2.9-8.5%, which are comparable to the standard equipment and acceptable for clinical use. Moreover, cTnI analysis results using clinical serum/plasma samples revealed a strong correlation (R² = 0.991) with contemporary standard equipment, demonstrating the practical application of this platform for high-performance POCT.

Universal amplification-free molecular diagnostics by billion-fold hierarchical nanofluidic concentration

Rapid and reliable detection of ultralow-abundance nucleic acids and proteins in complex biological media may greatly advance clinical diagnostics and biotechnology development. Currently, nucleic acid tests rely on enzymatic processes for target amplification (e.g., PCR), which have many inherent issues restricting their implementation in diagnostics. On the other hand, there exist no protein amplification techniques, greatly limiting the development of protein-based diagnosis. We report a universal biomolecule enrichment technique termed hierarchical nanofluidic molecular enrichment system (HOLMES) for amplification-free molecular diagnostics using massively paralleled and hierarchically cascaded nanofluidic concentrators. HOLMES achieves billion-fold enrichment of both nucleic acids and proteins within 30 min, which not only overcomes many inherent issues of nucleic acid amplification but also provides unprecedented enrichment performance for protein analysis. HOLMES features the ability to selectively enrich target biomolecules and simultaneously deplete nontargets directly in complex crude samples, thereby enormously enhancing the signal-to-noise ratio of detection. We demonstrate the direct detection of attomolar nucleic acids in urine and serum within 35 min and HIV p24 protein in serum within 60 min. The performance of HOLMES is comparable to that of nucleic acid amplification tests and near million-fold improvement over standard enzyme-linked immunosorbent assay (ELISA) for protein detection, being much simpler and faster in both applications. We additionally measured human cardiac troponin I protein in 9 human plasma samples, and showed excellent agreement with ELISA and detection below the limit of ELISA. HOLMES is in an unparalleled position to unleash the potential of protein-based diagnosis.

A Multi-Region Magnetoimpedance-Based Bio-Analytical System for Ultrasensitive Simultaneous Determination of Cardiac Biomarkers Myoglobin and C-Reactive Protein

Cardiac biomarkers (CBs) are substances that appear in the blood when the heart is damaged or stressed. Measurements of the level of CBs can be used in course of diagnostics or monitoring the state of the health of group risk persons. A multi-region bio-analytical system (MRBAS) based on magnetoimpedance (MI) changes was proposed for ultrasensitive simultaneous detection of CBs myoglobin (Mb) and C-reactive protein (CRP). The microfluidic device was designed and developed using standard microfabrication techniques for their usage in different regions, which were pre-modified with specific antibody for specified detection. Mb and CRP antigens labels attached to commercial Dynabeads with selected concentrations were trapped in different detection regions. The MI response of the triple sensitive element was carefully evaluated in initial state and in the presence of biomarkers. The results showed that the MI-based bio-sensing system had high selectivity and sensitivity for detection of CBs. Compared with the control region, ultrasensitive detections of CRP and Mb were accomplished with the detection limits of 1.0 pg/mL and 0.1 pg/mL, respectively. The linear detection range contained low concentration detection area and high concentration detection area, which were 1 pg/mL⁻10 ng/mL, 10⁻100 ng/mL for CRP, and 0.1 pg/mL⁻1 ng/mL, 1 n/mL⁻80 ng/mL for Mb. The measurement technique presented here provides a new methodology for multi-target biomolecules rapid testing.

Fully Automated Centrifugal Microfluidic Device for Ultrasensitive Protein Detection from Whole Blood

Enzyme-linked immunosorbent assay (ELISA) is a promising method to detect small amount of proteins in biological samples. The devices providing a platform for reduced sample volume and assay time as well as full automation are required for potential use in point-of-care-diagnostics. Recently, we have demonstrated ultrasensitive detection of serum proteins, C-reactive protein (CRP) and cardiac troponin I (cTnI), utilizing a lab-on-a-disc composed of TiO2 nanofibrous (NF) mats. It showed a large dynamic range with femto molar (fM) detection sensitivity, from a small volume of whole blood in 30 min. The device consists of several components for blood separation, metering, mixing, and washing that are automated for improved sensitivity from low sample volumes. Here, in the video demonstration, we show the experimental protocols and know-how for the fabrication of NFs as well as the disc, their integration and the operation in the following order: processes for preparing TiO2 NF mat; transfer-printing of TiO2 NF mat onto the disc; surface modification for immune-reactions, disc assembly and operation; on-disc detection and representative results for immunoassay. Use of this device enables multiplexed analysis with minimal consumption of samples and reagents. Given the advantages, the device should find use in a wide variety of applications, and prove beneficial in facilitating the analysis of low abundant proteins.

Cardiac troponins--Translational biomarkers in cardiology: Theory and practice of cardiac troponin high-sensitivity assays

Tn is a unique translational biomarker in cardiology whose potential has not been diminished in the new era of high sensitive assays. cTns can be valuable markers in cardiac diseases as well as in infectious diseases and respiratory diseases. Furthermore, the role of cTns is growing in the routine evaluation of cardioxicity and in determining the efficacy/safety ratio of novel cardioprotective strategies in clinical settings. cTns can detect myocardial injury not only in a wide spectrum of laboratory animals in experimental studies in vivo, but also in isolated heart models or cardiomyocytes in vitro. The crucial issue regarding the cross-species usage of cardiac troponin investigation remains the choice of cardiac troponin testing. This review summarizes the recent proteomic data on aminoacid sequences of cTnT and cTnI in various species, as well as selected analytical characteristics of human cardiac troponin high-sensitivity assays. Due to the highly phylogenetically conserved structure of troponins, the same bioindicator can be investigated using the same method in both clinical and experimental cardiology, thus contributing to a better understanding of the pathogenesis of cardiac diseases as well as to increased effectiveness of troponin use in clinical practice. Measuring cardiac troponins using commercially available human high-sensitivity cardiac troponin tests with convenient antibodies selected on the basis of adequate proteomic knowledge can solve many issues which would otherwise be difficult to address in clinical settings for various ethical and practical reasons. Our survey could help elaborate the practical guidelines for optimizing the choice of cTns assay in cardiology.

Structure of the Ca(2+)-saturated C-terminal domain of scallop troponin C in complex with a troponin I fragment

Troponin C (TnC) is the Ca(2+)-sensing subunit of troponin that triggers the contraction of striated muscles. In scallops, the striated muscles consume little ATP energy in sustaining strong contractile forces. The N-terminal domain of TnC works as the Ca(2+) sensor in vertebrates, whereas scallop TnC uses the C-terminal domain as the Ca(2+) sensor, suggesting that there are differences in the mechanism of the Ca(2+)-dependent regulation of muscles between invertebrates and vertebrates. Here, we report the crystal structure of Akazara scallop (Chlamys nipponensis akazara) adductor muscle TnC C-terminal domain (asTnCC) complexed with a short troponin I fragment (asTnIS) and Ca(2+). The electron density of a Ca(2+) ion is observed in only one of the two EF-hands. The EF-hands of asTnCC can only be in the fully open conformation with the assistance of asTnIS. The number of hydrogen bonds between asTnCC and asTnIS is markedly lower than the number in the vertebrate counterparts. The Ca(2+) modulation on the binding between asTnCC and asTnIS is weaker, but structural change of the complex depending on Ca(2+) concentration was observed. Together, these findings provide a detailed description of the distinct molecular mechanism of contractile regulation in the scallop adductor muscle from that of vertebrates.

An inhibitor of the δPKC interaction with the d subunit of F1Fo ATP synthase reduces cardiac troponin I release from ischemic rat hearts: utility of a novel ammonium sulfate precipitation technique

We have previously reported protection against hypoxic injury by a cell-permeable, mitochondrially-targeted δPKC-d subunit of F1Fo ATPase (dF1Fo) interaction inhibitor [NH2-YGRKKRRQRRRMLA TRALSLIGKRAISTSVCAGRKLALKTIDWVSFDYKDDDDK-COOH] in neonatal cardiac myo-cytes. In the present work we demonstrate the partitioning of this peptide to the inner membrane and matrix of mitochondria when it is perfused into isolated rat hearts. We also used ammonium sulfate ((NH4)2SO4) and chloroform/methanol precipitation of heart effluents to demonstrate reduced card-iac troponin I (cTnI) release from ischemic rat hearts perfused with this inhibitor. 50% (NH4)2SO4 saturation of perfusates collected from Langendorff rat heart preparations optimally precipitated cTnI, allowing its detection in Western blots. In hearts receiving 20 min of ischemia followed by 30, or 60 min of reperfusion, the Mean±S.E. (n=5) percentage of maximal cTnI release was 30 ± 7 and 60 ± 17, respectively, with additional cTnI release occurring after 150 min of reperfusion. Perfusion of hearts with the δPKC-dF1Fo interaction inhibitor, prior to 20 min of ischemia and 60-150 min of reperfusion, reduced cTnI release by 80%. Additionally, we found that when soybean trypsin inhibitor (SBTI), was added to rat heart effluents, it could also be precipitated using (NH4)2SO4 and detected in western blots. This provided a convenient method for normalizing protein recoveries between groups. Our results support the further development of the δPKC-dF1Fo inhibitor as a potential therapeutic for combating cardiac ischemic injury. In addition, we have developed an improved method for the detection of cTnI release from perfused rat hearts.

Preconcentration and detection of the phosphorylated forms of cardiac troponin I in a cascade microchip by cationic isotachophoresis

This paper describes the detection of a cardiac biomarker, cardiac troponin I (cTnI), spiked into depleted human serum using cationic isotachophoresis (ITP) in a 3.9 cm long poly(methyl methacrylate) (PMMA) microfluidic channel. The microfluidic chip incorporates a 100× cross-sectional area reduction, including a 10× depth reduction and a 10× width reduction, to increase sensitivity during ITP. The cross-sectional area reductions in combination with ITP allowed visualization of lower concentrations of fluorescently labeled cTnI. ITP was performed in both "peak mode" and "plateau mode" and the final concentrations obtained were linear with initial cTnI concentration. We were able to detect and quantify cTnI at initial concentrations as low as 46 ng mL(-1) in the presence of human serum proteins and obtain cTnI concentrations factors as high as ~ 9000. In addition, preliminary ITP experiments including both labeled cTnI and labeled protein kinase A (PKA) phosphorylated cTnI were performed to visualize ITP migration of different phosphorylated forms of cTnI. The different phosphorylated states of cTnI formed distinct ITP zones between the leading and terminating electrolytes. To our knowledge, this is the first attempt at using ITP in a cascade microchip to quantify cTnI in human serum and detect different phosphorylated forms.

10,000-fold concentration increase of the biomarker cardiac troponin I in a reducing union microfluidic chip using cationic isotachophoresis

This paper describes the preconcentration of the biomarker cardiac troponin I (cTnI) and a fluorescent protein (R-phycoerythrin) using cationic isotachophoresis (ITP) in a 3.9 cm long poly(methyl methacrylate) (PMMA) microfluidic chip. The microfluidic chip includes a channel with a 5× reduction in depth and a 10× reduction in width. Thus, the overall cross-sectional area decreases by 50× from inlet (anode) to outlet (cathode). The concentration is inversely proportional to the cross-sectional area so that as proteins migrate through the reductions, the concentrations increase proportionally. In addition, the proteins gain additional concentration by ITP. We observe that by performing ITP in a cross-sectional area reducing microfluidic chip we can attain concentration factors greater than 10,000. The starting concentration of cTnI was 2.3 μg mL⁻¹ and the final concentration after ITP concentration in the microfluidic chip was 25.52 ± 1.25 mg mL⁻¹. To the author's knowledge this is the first attempt at concentrating the cardiac biomarker cTnI by ITP. This experimental approach could be coupled to an immunoassay based technique and has the potential to lower limits of detection, increase sensitivity, and quantify different isolated cTnI phosphorylation states.

pT7MT, a metallothionein 2A-tagged novel prokaryotic fusion expression vector

In the present article, a novel fusion expression vector for Escherichia coli was developed based on the pTORG plasmid, a derivative of pET32a. This vector, named pT7MT (GenBank Accession No DQ504436), carries a T7 promoter and it drives the downstream gene encoding Metallothionein 2A (MT2A). There are in-framed multiple cloning sites (MCS) downstream of the MT2A gene. A target gene can be cloned into the MCS and fused to the C-terminal of the MT2A gene in a compatible open reading frame (ORF) to achieve fusion expression. The metal-binding capability of MT2A allows the purification of fusion proteins by metal chelating affinity chromatogralhy, known as Ni2+-affinity chromatography. Using this expression vector, we successfully got the stable and high-yield expression of MT2A-GST and MT2A-Troponin I fusion proteins. These two proteins were easily purified from the supernatant of cell lysates by one-step Ni2+ -affinity chromatography. The final yields of MT2A-GST and MT2A-Troponin I were 30 mg/l and 28 mg/l in LB culture, respectively. Taken together, our data suggest that pT7MT can be applied as a useful expression vector for stable and high-yield production of fusion proteins.

Isoelectric point determination of cardiac troponin I forms present in plasma from patients with myocardial infarction

BACKGROUND

Cardiac troponin I (cTnI) is a specific marker of myocardial injury. In blood of patients with cardiovascular diseases, cTnI is released as a mixture of free, complexed and post-translationally modified forms.

METHODS

The cTnI forms present in the plasma from 8 patients with acute myocardial infarction (AMI) have been analysed by two-dimensional gel electrophoresis (2-DE) and Western Blot using anti-cTnI mAb 19C7 and anti-phosphorylated cTnI (Serines 22-23) mAb 5E6.

RESULTS

After immunoextraction of cTnI in plasma samples by 19C7 and 2-DE separation, 4 different forms were detected by 19C7 in 7 out the 8 AMI plasma samples. Two 29 kDa spots corresponding to intact free cTnI forms were detected at pIs 5.2 and 5.4. However, spot with pI 5.4 was also recognized by mAb 5E6, and should be bis-phosphorylated cTnI. Two 55 kDa spots with pIs 6.6 and 6.7 could be IC complexes. CTnI forms with pIs lower than the theoretical pI were also found in free cTnI and phosphorylated cTnI purified materials.

CONCLUSIONS

2-DE analysis of AMI plasma showed the presence of acidic cTnI forms, one of them being phosphorylated. The clinical significance of these forms has to be further investigated.

Toward Standardization of Cardiac Troponin I Measurements Part II: Assessing Commutability of Candidate Reference Materials and Harmonization of Cardiac Troponin I Assays

BACKGROUND

Cardiac tropoin I (cTnI) measurements show an approximately 20- to 40-fold difference between assays, and better standardization and harmonization are needed. Toward this goal, the AACC cTnI Standardization Committee collaborated with the National Institute of Standards and Technology (NIST) in an earlier study to select 2 candidate reference materials (cRMs).

METHODS

Two troponin cRMs, a troponin C-troponin I-troponin T (CIT) complex from human heart tissue and a CIT complex from recombinant technology, were supplied to NIST for assessment of composition and purity, and cTnI value assignment. These cRMs and 6 cTnI-positive human serum pools were shipped to manufacturers of 15 cTnI assays. Commutability of the materials was examined by determining the numerical relationship for the cRM preparations between each manufacturer-specified field method and each of the other 14 field methods. These relationships were then compared with the corresponding numerical relationships for the human serum pools. Harmonization of methods was accomplished by determining regression parameters relative to the analytical system yielding values closest to the median for each serum pool. These regression parameters were used to recalculate pool values to harmonize the assays. Interassay CVs before and after harmonization were determined.

RESULTS

Characterization of the CIT and CI cRMs showed that these materials were of specified composition. The proportion of cTnI methods that demonstrated commutability for the CIT cRM was 45%; for the CI cRM, 39% of methods demonstrated commutability. Interassay cTnI variability for the field methods ranged from 82% to 97%, median 88%. After harmonization, variability of the serum pools for the cTnI methods was decreased to between 9.0% and 23%, median 15.5%.

CONCLUSIONS

The proportion of methods demonstrating commutability was too low for use as a common calibrator for the cTnI field methods. However a simple strategy using serum pools can improve harmonization of field cTnI methods by more than 5-fold. The CIT cRM was selected by the AACC cTnI standardization committee, and a new lot has been classified as the cTnI certified reference material Standard Reference Material 2921 by NIST.

Phosphorylation of human cardiac troponin I G203S and K206Q linked to familial hypertrophic cardiomyopathy affects actomyosin interaction in different ways

cAMP-dependent protein kinase (PKA)-dependent phosphorylation of the two serine residues in the amino terminal region unique to cardiac troponin I (cTnI) is known to cause two effects: (i) decrease of the maximum Ca2+-controlled thin filament-activated myosin S1-ATPase (actoS1-ATPase) activity and mean sliding velocity of reconstituted thin filaments; (ii) rightward shift of the Ca2+ activation curves of actoS1-ATPase activity, filament sliding velocity, and force generation. We have studied the influence of phosphorylation of human wild-type cTnI and of two mutant cTnI (G203S and K206Q) causing familial hypertrophic cardiomyopathy (fHCM) on the secondary structure by circular dichroism spectroscopy and on the Ca2+ regulation of actin-myosin interaction using actoS1-ATPase activity and in vitro motility assays. Both mutations slightly influence the backbone structure of cTnI but only the secondary structure of cTnI-G203S is also affected by bis-phosphorylation of cTnI. In functional studies, cTnI-G203S behaves similarly to wild-type cTnI, i.e. the mutation itself has no measurable effect and bis-phosphorylation alters the actoS1-ATPase activity and the in vitro thin filament motility in the same way as does bis-phosphorylation of wild-type cTnI. In contrast, the mutation K206Q leads to a considerable increase in the maximum actoS1-ATPase activity as well as filament motility compared to wild-type cTnI. Bis-phosphorylation of this mutant cTnI still suppresses the maximum actoS1-ATPase activity and filament sliding velocity but does no longer affect the Ca2+ sensitivity of these processes. Thus, these two fHCM-linked cTnI mutations, although reflecting similar pathological situations, exert different effects on the actomyosin system per se and in response to bis-phosphorylation of cTnI.

Strategy for analysis of cardiac troponins in biological samples with a combination of affinity chromatography and mass spectrometry

BACKGROUND

Cardiac troponins are modified during ischemic injury and are found as a heterogeneous mixture in blood of patients with cardiovascular diseases. We present a strategy to isolate cardiac troponins from human biological material, by use of affinity chromatography, and to provide samples ready for direct analysis by mass spectrometry.

METHODS

Cardiac troponins were isolated from human left ventricular tissue by affinity chromatography. Isolated troponins were either eluted and analyzed by Western blot or enzymatically digested while bound to affinity beads. The resulting peptide mixture was subjected to mass spectrometry for protein identification and characterization. The same method was used to analyze serum from patients with acute myocardial infarction (AMI).

RESULTS

Affinity chromatography with antibodies specific for one cardiac troponin subunit facilitated the isolation of the entire cardiac troponin complex from myocardial tissue. The three different proteases used for enzymatic digestion increased the total protein amino acid sequence coverage by mass spectrometry for the three cardiac troponin subunits. Combined amino acid sequence coverage for cardiac troponin I, T, and C (cTnI, cTnT, cTnC) was 54%, 48%, and 40%, respectively. To simulate matrix effects on the affinity chromatography-mass spectrometry approach, we diluted tissue homogenate in cardiac troponin-free serum. Sequence coverage in this case was 44%, 41%, and 19%, respectively. Finally, affinity chromatography-mass spectrometry analysis of AMI serum revealed the presence of cardiac troponins in a wide variety of its free and/or complexed subunits, including the binary cTnI-cTnC and cTnI-cTnC-cTnT complexes.

CONCLUSIONS

Affinity chromatography-mass spectrometry allows the extraction and analysis of cardiac troponins from biological samples in their natural forms. We were, for the first time, able to directly confirm the presence of cardiac troponin complexes in human serum after AMI. This approach could assist in more personalized risk stratification as well as the search for reference materials for cardiac troponin diagnostics.

An improved method for exchanging troponin subunits in detergent skinned rat cardiac fiber bundles

We describe a method for the removal of endogenous troponin (Tn) complex from bundles of detergent-treated cardiac fibers. After 70 min treatment with cTnT-cTnI most of the endogenous Tn complex was removed from fiber bundles. Complete reconstitution of the Tn complex was achieved by reconstituting with cardiac troponin C (cTnC) in fully relaxing conditions. Ca(2+)-dependent maximum force of the fibers treated with cTnT-cTnI or cTnT-cTnI(33-211), which was used to aid in the visualization of the troponin exchange, decreased to 85-90% of the force developed by fibers before the treatment. SDS-PAGE analysis of the cTnT-cTnI(33-211) and the cTnT(77-289)-cTnI(33-211) treated fiber bundles demonstrated that 70-80% of the endogenous Tn subunits were removed. After reconstitution with cTnC, approximately 80-85% of the Ca(2+)-regulated force was restored in cTnT-cTnI/cTnI(33-211) treated fibers. Our results demonstrate that by minimizing the prolonged exposure of skinned cardiac fiber bundles to rigor conditions, successful exchange of all three subunits of the Tn complex can be accomplished with minimal loss of function.

Characterization of the cardiac holotroponin complex reconstituted from native cardiac troponin T and recombinant I and C

Cardiac troponin I (cTnI), the inhibitory subunit of cardiac troponin (cTn), is phosphorylated by the cAMP-dependent protein kinase A at two adjacently located serine residues within the heart-specific N-terminal elongation. Four different phosphorylation states can be formed. To investigate each monophosphorylated form cTnI mutants, in which each of the two serine residues is replaced by an alanine, were generated. These mutants, as well as the wild-type cardiac troponin I (cTnI-WT) have been expressed in Escherichia coli, purified and characterized by isoelectric focusing, MS and CD-spectroscopy. Monophosphorylation induces conformational changes within cTnI that are different from those induced by bisphosphorylation. Functionality was assessed by measuring the calcium dependence of myosin S1 binding to thin filaments containing reconstituted native, wild-type and mutant cTn complexes. In all cases a functional holotroponin complex was obtained. Upon bisphosphorylation of cTnI-WT the pCa curve was shifted to the right to the same extent as that observed with bisphosphosphorylated native cTnI. However, the absolute values for the midpoints were higher when recombinant cTn subunits were used for reconstitution. Reconstitution itself changed the calcium affinity of cTnC: pCa50-values were higher than those obtained with the native cardiac holotroponin complex. Apparently only bisphosphorylation of cTnI influences the calcium sensitivity of the thin filament, thus monophosphorylation has a function different from that of bisphosphorylation; this function has not yet been identified.

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

Cardiac troponin I (cTnI) is an essential element in activation of myofilaments by Ca2+ binding to cardiac troponin C (cTnC). Yet, its role in transduction of the Ca2+ binding signal to cardiac troponin T (cTnT) and tropomyosin-actin remain poorly understood. We have recently discovered that regions of cTnI C-terminal to a previously defined inhibitory peptide are essential for full inhibitory activity and Ca(2+)-sensitivity of cardiac myofilaments (Rarick et al., 1997). However, apart from its role in structural binding to cTnC, there is little knowledge concerning the role of the N-terminus of cTnI in the activation and regulation of cardiac myofilaments. To address this question, we generated wild-type mouse cardiac TnI (WT-cTnI; 211 residues) and two N-terminal deletion mutants of mouse cTnI, cTnI54-211 (missing 53 residues), and cTnI80-211 (missing 79 residues). The cTnI54-211 mutant retained the ability to bind to cTnT, but lost the ability to bind to cTnC, whereas the cTnI80-211 mutant lost the ability to bind to cTnT, but bound weakly to cTnC. Both mutants bound to F-actin. In the absence of Ca2+, cTnI54-211 was able to inhibit the unregulated MgATPase activity of myofibrils lacking endogenous cTnI-cTnC to the same extent as WT-cTnI, whereas cTnI80-211 had some impairment of its inhibitory capability. Reconstitution with cTnI54-211/cTnC complex did not restore Ca(2+)-activation of myofibrillar MgATPase activity at all, however, the cTnI80-211/cTnC complex restored Ca(2+)-activation to nearly 50% of that obtained with WT-cTnI/cTnC. These data provide the first evidence of a significant function of a cTnT-binding domain on cTnI. They also indicate that the structural cTnC binding site on cTnI is required for Ca(2+)-dependent activation of cardiac myofilaments, and that cTnT binding to the N-terminus of cTnI is a negative regulator of activation.

Troponin I degradation and covalent complex formation accompanies myocardial ischemia/reperfusion injury

Selective troponin I (TnI) modification has been demonstrated to be in part responsible for the contractile dysfunction observed with myocardial ischemia/reperfusion injury. We have isolated and characterized modified TnI products in isolated rat hearts after 0, 15, or 60 minutes of ischemia followed by 45 minutes of reperfusion using affinity chromatography with cardiac troponin C (TnC) and an anti-TnI antibody, immunological mapping, reversed-phase high-performance liquid chromatography, and mass spectrometry. Rat cardiac TnI becomes progressively degraded from 210 amino acid residues to residues 1-193, 63-193, and 73-193 with increased severity of injury. Degradation is accompanied by formation of covalent complexes between TnI 1-193 and, respectively, TnC residues 1-94 and troponin T (TnT) residues 191-298. The covalent complexes are likely a result of isopeptide bond formation between lysine 193 of TnI and glutamine 191 of TnT by the cross-linking enzyme transglutaminase. With severe ischemia, cellular necrosis results in specific release of TnI 1-193 into the reperfusion effluent and TnT degradation in the myocardium (25-, 27-, and 33-kDa products). Two-dimensional electrophoresis demonstrated that phosphorylation of TnI prevents ischemia-induced degradation. This study characterized the modified TnI products in isolated rat hearts reperfused after a brief or severe period of ischemia, revealing the progressive nature of TnI degradation, changes in phosphorylation, and covalent complexes with ischemia/reperfusion injury. Finally, we propose a model for ischemia/reperfusion injury in which the extent of proteolytic and transglutaminase activities ultimately determines whether apoptosis or necrosis is achieved.

Conformational modulation of troponin T by configuration of the NH2-terminal variable region and functional effects

Troponin T (TnT) is an essential element in the thin filament-based regulatory system of striated muscle. Alternative mRNA splicing generates multiple TnT isoforms with primary structural differences in the NH2-terminal region. The functional significance of this hypervariable NH2-terminal domain and the developmental or muscle type-specific TnT isoforms is not fully understood. We have analyzed chicken breast muscle TnT containing a metal-binding cluster [H(E/A)EAH]4-7 (Tx) in the NH2-terminal region to demonstrate potential effects of the NH2-terminal structure on the conformation of TnT [Ogut, O., and Jin, J.-P. (1996) Biochemistry 35, 16581-16590]. Using specific antibody epitope analysis on this metal-binding TnT model, this study revealed that the binding of Zn2+ to the NH2-terminal region of chicken breast muscle TnT induces extensive conformational changes in the whole protein as demonstrated by a significant decrease in binding avidity of a polyclonal anti-TnT serum which recognizes multiple epitopes on the TnT molecule. This NH2-terminal configuration-based effect is not restricted to the metal ion interaction, whereas the binding of anti-NH2 terminus monoclonal antibodies to TnT induced similar changes. Protein-binding assays have shown that the NH2-terminal variability-induced conformational changes can alter TnT's binding affinity for tropomyosin and troponin I. The results suggest a functional modulation of TnT through the configuration of the NH2-terminal domain, and this novel mechanism may mediate the physiological significance of the TnT isoform regulation.

Phosphorylation-specific antibodies for human cardiac troponin-I

A panel of seven monoclonal antibodies (mAbs) raised against cardiac troponin-I (CdTnI) isolated from canine and human hearts, which have been shown to be cardiac-specific but cross-species reactive [Cummins, B., Aukland, M. L. & Cummins, P. (1987) Amer. Heart J. 113, 1333-1344], were used in this study. These mAbs were tested against recombinant wild-type and mutant human CdTnI proteins to assess their value as probes for the phosphorylation status of CdTnI. Four mAbs were found to react positively with the recombinant wild-type protein and their epitopes were contained in residues 31-210 of the human cardiac protein. Two of these mAbs appeared to be directed against the same epitope site within this region. The remaining three mAbs only reacted against the recombinant wild-type protein when it was phosphorylated, showing that these three antibodies were directed against the phosphate group(s) on Ser23 and/or Ser24. In order to investigate this further, a series of single and double mutants of CdTnI were used in which either Ala (to direct the enzymatic phosphorylation) or Asp (to mimic the phosphate group) replaced the Ser23 and/or Ser24. It was found the all three mAbs were able to react with the mono-phosphorylated form of the [Ala23]CdTnI single mutant but not the mono-phosphorylated form of the [Ala24]CdTnI single mutant, showing that they specifically required phosphorylation at Ser24. Experiments with a synthetic peptide composed of residues 1-29 of human CdTnI confirmed these data. Two of the three phosphorylation-specific mAbs were able to react with mutants containing either two Asp residues replacing Ser23 and Ser24 or one Asp residue instead of Ser24, indicating that a negative charge at position Ser24 is sufficient to invoke a reaction. The other mAb was more specific in that it would only react with CdTnI species with a phosphate group on Ser24.

Amino acid sequence of troponin-I from Akazara scallop striated adductor muscle

The complete amino acid sequence of Akazara scallop, Chlamys nipponensis akazara, troponin-I was determined by automated Edman degradation. It is composed of 292 amino acid residues with a blocked N-terminus. The Mr is calculated to be 34,678, about 14,000 larger than that of vertebrate skeletal troponin-I but significantly smaller than the 52,000 that had been estimated by SDS-polyacrylamide gel electrophoresis. The homologous sequence to vertebrate and arthropoda troponin-Is is found in the C-terminal region. In particular, the sequence of the regions essential for binding to actin and troponin-C is highly conserved. On the other hand, Akazara scallop troponin-I has 100-133 extra residues at the N-terminus compared with vertebrate troponin-I. This extra region is rich in Glu and Arg and has a unique sequence, that shows in part a high sequence homology with the tropomyosin-binding site of troponin-T and caldesmon.

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

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

Removal of endotoxin from recombinant protein preparations

OBJECTIVES

To develop an effective method to remove endotoxin from large scale E. coli recombinant protein purifications.

DESIGN AND METHODS

Triton X-114 phase separation, affinity chromatography utilizing immobilized polymyxin B or immobilized histidine, were used to remove endotoxin from purified preparations of recombinant CK-BB, CK-MB, CK-MM, myoglobin, and cardiac troponin I. Endotoxin levels were measured by a Limulus Amebocyte Lysate gel-clot assay. The immunoactivity of these protein preparations was determined by BIAcore analysis using a panel of in-house generated monoclonal antibodies and by a Stratus Fluorometric Analyzer. In the case of troponin I, the BIAcore was also utilized to measure troponin C interactions.

RESULTS

Phase separation with Triton X-114 was the most effective method in reducing the amount of endotoxin present in the protein preparations compared to either polymyxin B or histidine affinity chromatography. With Triton X-114, the reduction in endotoxin levels was greater than 99% and recovery of the proteins after endotoxin removal was greater than 90%. All three procedures for removing endotoxin had no deleterious effects on the immunoactivity of majority proteins when tested with a panel of monoclonal antibodies. Troponin I also retained its ability to bind to troponin C in the presence of Ca2+. Recombinant CK-BB and CK-MM which were expressed in the soluble fraction of E. coli cell lysates, contained significantly higher endotoxin levels than recombinant CK-MB, myoglobin and cardiac troponin I which were expressed in the form of inclusion bodies.

CONCLUSION

Of the three methods tested, Triton X-114 phase separation was the most effective way of removing endotoxin from recombinant proteins.

Recombinant troponin I substitution and calcium responsiveness in skinned cardiac muscle

Using treatment with vanadate solutions, we extracted native cardiac troponin I and troponin C (cTnI and cTnC) from skinned fibers of porcine right ventricles. These proteins were replaced by exogenously supplied TnI and TnC isoforms, thereby restoring Ca2+-dependent regulation. Force then depended on the negative logarithm of Ca2+ concentration (pCa) in a sigmoidal manner, the pCa for 50% force development, pCa50, being about 5.5. For reconstitution we used fast-twitch rabbit skeletal muscle TnI and TnC (sTnI and sTnC), bovine cTnI and cTnC or recombinant sTnIs that were altered by site-directed mutagenesis. Incubation with TnI inhibited isometric tension in TnI-extracted fibers in the absence of Ca2+, but restoration of Ca2+ dependence required incubation with both TnI and TnC. Relaxation at low Ca2+ levels and the steepness of the force/pCa relation depended on the concentration of exogenously supplied TnI in the reconstitution solution (range 20-150 "mu"M), while Ca2+ sensitivity, i.e. the pCa50, was dependent on the isoform, and also on the concentration of TnC in the reconstitution solution. At pH 6.7, skinned fibers reconstituted with optimal concentrations of sTnC and sTnI (120 "mu"M and 150 "mu"M, respectively) were more sensitive to Ca2+ than those reconstituted with cTnC and cTnI (difference in pCa50 approx. 0.2 units). Rabbit sTnI was cloned and expressed in Escherichia coli using a high yield expression plasmid. We introduced point mutations into the TnI inhibitory region comprising the sequence of the minimal common TnC/actin binding site (-G104-K-F-K-R-P-P-L-R-R-V-R115-). The four mutants produced by substitution of T for P110, G for P110, G for L111, and G for K105 were chosen, based on previous work with synthetic peptides showing that single amino acid substitution in this region diminished the capacity of these peptides to inhibit acto-S1 ATPase or contraction of skinned fibers. Therefore, all amino acid residues of the inhibitory region are thought to contribute to biological activity of TnI. However, each of the recombinant TnIs could substitute for endogenous TnI. In combination with exogenous TnC, Ca2+ dependence could be restored when gly110sTnI, thr110sTnI or gly111sTnI was used for reconstitution. The mutant gly105sTnI, on the other hand, reduced the ability of skinned fibers to relax at low Ca2+ concentrations and it caused an increase in Ca2+ sensitivity.