N-terminal amino acid sequences of three functionally different troponin T isoforms from rabbit fast skeletal muscle

Troponin Variants as Markers of Skeletal Muscle Health and Diseases

Ca2 +-regulated contractility is a key determinant of the quality of muscles. The sarcomeric myofilament proteins are essential players in the contraction of striated muscles. The troponin complex in the actin thin filaments plays a central role in the Ca2+-regulation of muscle contraction and relaxation. Among the three subunits of troponin, the Ca2+-binding subunit troponin C (TnC) is a member of the calmodulin super family whereas troponin I (TnI, the inhibitory subunit) and troponin T (TnT, the tropomyosin-binding and thin filament anchoring subunit) are striated muscle-specific regulatory proteins. Muscle type-specific isoforms of troponin subunits are expressed in fast and slow twitch fibers and are regulated during development and aging, and in adaptation to exercise or disuse. TnT also evolved with various alternative splice forms as an added capacity of muscle functional diversity. Mutations of troponin subunits cause myopathies. Owing to their physiological and pathological importance, troponin variants can be used as specific markers to define muscle quality. In this focused review, we will explore the use of troponin variants as markers for the fiber contents, developmental and differentiation states, contractile functions, and physiological or pathophysiological adaptations of skeletal muscle. As protein structure defines function, profile of troponin variants illustrates how changes at the myofilament level confer functional qualities at the fiber level. Moreover, understanding of the role of troponin modifications and mutants in determining muscle contractility in age-related decline of muscle function and in myopathies informs an approach to improve human health.

Regulation of troponin T expression during muscle development in sea bream Sparus auratus Linnaeus: the potential role of thyroid hormones

In the sea bream Sparus auratus three stage-specific fast troponin T (fTnT) isoforms have been cloned and correspond to embryonic-, larval- and adult-specific isoforms. Characterisation, using database searches, of the putative genomic organisation of Fugu rubripes and Tetraodon nigroviridis fTnT indicates that alternative exon splicing in the 5 region of the gene generates the different isoforms. Moreover, comparison of teleost fTnTs suggests that alternative splicing of fTnT appears to be common in teleosts. A different temporal expression pattern for each fTnT splice varotnt is found during sea bream development and probably relates to differing functional demands, as a highly acidic embryonic form (pI 5.16) is substituted by a basic larval form (pI 9.57). Thyroid hormones (THs), which play an important regulatory role in muscle development in flatfish and tetrapods, appear also to influence TnT gene expression in the sea bream. However, THs have a divergent action on different sea bream TnT genes and although the slow isoform (sTnT1) is TH-responsive, fTnT, sTnT2 and the itronless isoform (iTnT) are unaffected. The present results taken together with those published for flatfish seem to suggest differences may exist in the regulation of larval muscle development in teleosts.

Troponin T isoform expression is modulated during Atlantic halibut metamorphosis

BACKGROUND

Flatfish metamorphosis is a thyroid hormone (TH) driven process which leads to a dramatic change from a symmetrical larva to an asymmetrical juvenile. The effect of THs on muscle and in particular muscle sarcomer protein genes is largely unexplored in fish. The change in Troponin T (TnT), a pivotal protein in the assembly of skeletal muscles sarcomeres and a modulator of calcium driven muscle contraction, during flatfish metamophosis is studied.

RESULTS

In the present study five cDNAs for halibut TnT genes were cloned; three were splice variants arising from a single fast TnT (fTnT) gene; a fourth encoded a novel teleost specific fTnT-like cDNA (AfTnT) expressed exclusively in slow muscle and the fifth encoded the teleost specific sTnT2. THs modified the expression of halibut fTnT isoforms which changed from predominantly basic to acidic isoforms during natural and T4 induced metamorphosis. In contrast, expression of red muscle specific genes, AfTnT and sTnT2, did not change during natural metamorphosis or after T4 treatment. Prior to and after metamorphosis no change in the dorso-ventral symmetry or temporal-spatial expression pattern of TnT genes and muscle fibre organization occurred in halibut musculature.

CONCLUSION

Muscle organisation in halibut remains symmetrical even after metamorphosis suggesting TH driven changes are associated with molecular adaptations. We hypothesize that species specific differences in TnT gene expression in teleosts underlies different larval muscle developmental programs which better adapts them to the specific ecological constraints.

Cardiac Troponin T and Creatine Kinase MB Content in Skeletal Muscle of the Uremic Rat

Background: The assertion that creatine kinase MB (CK-MB) and the developmental isoforms of cardiac troponin T (cTnT) are expressed by skeletal muscle in some clinical settings is an extrapolation from nonuremic rodent studies. We studied the content of CK-MB and cTnT in skeletal muscle of the renal-insufficient rat.

Methods: Skeletal muscles (gastrocnemius) were collected from both five-sixths nephrectomized rats (n = 11) and sham-operated controls (n = 11). cTnT content was analyzed by Elecsys (Roche), immunoblotting, and immunohistochemistry with antibodies M7 and M11-7 (Roche). CK isoenzymes were analyzed electrophoretically.

Results: Trace concentrations of cTnT were detected in some of the skeletal muscle samples [controls (3 of 11) and uremic rats (1 of 11)] at concentrations <0.01% of that detected in heart. By contrast, positive staining appeared in both groups with M11-7 by immunoblotting and immunohistochemistry. No immunoreactivity was detected in skeletal muscle using M7 in the immunoblot format, although immunoreactivity was detected by immunohistochemistry in all samples. The median percentages of CK-MB were 6.0% and 4.1% for the skeletal muscle from control and uremic rats, respectively.

Conclusion: The detection of cTnT and CK-MB in skeletal muscle does not differ for uremic rats compared with sham-operated controls. cTnT isoforms detected by qualitative methods are not detected with the cTnT immunoassay. Observations with rodents should not necessarily be extrapolated to humans.

Mapping the interacting regions between troponins T and C. Binding of TnT and TnI peptides to TnC and NMR mapping of the TnT-binding site on TnC

Muscular contraction is triggered by an increase in calcium concentration, which is transmitted to the contractile proteins by the troponin complex. The interactions among the components of the troponin complex (troponins T, C, and I) are essential to understanding the regulation of muscle contraction. While the structure of TnC is well known, and a model for the binary TnC.TnI complex has been recently published (Tung, C.-S., Wall, M. E., Gallagher, S. C., and Trewhella, J. (2000) Protein Sci. 9, 1312-1326), very little is known about TnT. Using non-denaturing gels and NMR spectroscopy, we have analyzed the interactions between TnC and five peptides from TnT as well as how three TnI peptides affect these interactions. Rabbit fast skeletal muscle peptide TnT-(160-193) binds to TnC with a dissociation constant of 30 +/- 6 microm. This binding still occurs in the presence of TnI-(1-40) but is prevented by the presence of TnI-(56-115) or TnI-(96-139), both containing the primary inhibitory region of TnI. TnT-(228-260) also binds TnC. The binding site for TnT-(160-193) is located on the C-terminal domain of TnC and was mapped to the surface of TnC using NMR chemical shift mapping techniques. In the context of the model for the TnC.TnI complex, we discuss the interactions between TnT and the other troponin subunits.

Evaluation of incidence, clinical significance, and prognostic value of circulating cardiac troponin I and T elevation in hemodynamically stable patients with suspected myocardial contusion after blunt chest trauma

BACKGROUND

The frequency and prognostic influence of myocardial injury in patients with blunt chest trauma is controversial. We investigated the value of cardiac troponin I (cTn-I) and cardiac troponin T (cTn-T), highly specific markers of myocardial injury, to determine whether their measurement would improve the ability to detect myocardial contusion in stable patients with blunt chest trauma in comparison with conventional markers and whether they were associated with significantly worse late clinical outcome.

METHODS AND RESULTS

Over an 18-month period, myocardial contusion was diagnosed in 26 of 94 patients (27.6%) with acute blunt chest trauma (motor vehicle crash; 81%), because of echocardiographic abnormalities (n = 12), electrocardiographic abnormalities (n = 29), or both. Patients with myocardial contusion had a significantly higher Injury Severity Score at the time of admission (p = 0.001) and a significantly longer hospital stay (p = 0.0008). All patients survived admission to hospital and were hemodynamically stable. None of the patients died or had severe in-hospital cardiac complications. The percentage of patients with elevated CK, (CK-MB/total CK) ratio, or CK-MB mass concentration was not significantly different between patients with or without myocardial contusion. However, there were significant differences between the two groups when we applied the commonly used threshold levels of CK-MB activity and myoglobin. The percentage of patients with elevated circulating cTn-I and cTn-T (> or = 0.1 microg/L) was significantly higher in patients with myocardial contusion (23% vs. 3%; p = 0.01 and 12% vs. 0%; p = 0.03, respectively). Complete changes in cTn-I and cTn-T correlated well (r = 0.91, p = 0.0001). Sensitivity, specificity, and negative and positive predictive values of cTn-I and cTn-T in predicting a myocardial contusion in blunt trauma patients were 23%, 97%, and 77%, 75%, and 12%, 100%, and 74%, 100%, respectively. Clinical follow-up was available in 83 patients (88%) (mean, 16 +/- 7.5 months). There were no deaths in either group directly attributed to cardiac complications. None of the patients had any long-term cardiac complications or myocardial failure related to blunt chest trauma.

CONCLUSION

Although improved specificity of cTn-I and cTn-T compared with conventional markers, it should be emphasized that the main problem with cTn-I and cTn-T is low sensitivity as well as low predictive values in diagnosing myocardial contusion. cTn-I and cTn-T measurement is currently not an improved method in diagnosing blunt cardiac injury in hemodynamically stable patients. Moreover, there was no association of postmyocardial contusion cell injury and late outcome in these patients when cTn-I and cTn-T and other conventional markers were considered.

Troponin T as a Marker of Infarction during Coronary Bypass Surgery

To evaluate serum troponin T as a marker of perioperative myocardial infarction, 50 patients undergoing coronary artery bypass grafting were divided into 2 groups. Group A (14 patients) had serum creatine kinase MB-isoenzyme levels above 100 U·L−1and electrocardiographic changes indicative of infarction. Group B (36 patients) had creatine kinase MB levels below 100 U·L−1and no electrocardiographic changes. Blood samples were obtained preoperatively, 6 hours after aortic declamping, and on postoperative day 1, 2, and 3. Following surgery, all patients had increased levels of troponin T and creatine kinase MB. Troponin T was significantly higher in group A compared to group B at 6 hours, day 1, and day 2 postoperatively. Creatine kinase MB levels were significantly higher in group A compared to group B at 6 hours and day 1 postoperatively. The increased levels of troponin T in patients without myocardial infarction suggest that some operative myocardial damage occurred. Patients with perioperative myocardial infarction had significantly higher levels of troponin T up to postoperative day 2, whereas creatine kinase MB levels were almost normal by day 2. This suggests that troponin T may be used up to 2 days postoperatively for detection of myocardial infarction.

Laboratory diagnosis of patients with acute chest pain

The enzyme activities of creatine kinase (CK), its isoenzyme MB (CK-MB) and of lactate dehydrogenase isoenzyme 1 (LD-1) have been used for years in diagnosing patients with chest pain in order to differentiate patients with acute myocardial infarction (AMI) from non-AMI patients. These methods are easy to perform as automated analyses, but they are not specific for cardiac muscle damage. During the early 90's the situation changed. First creatine kinase MB mass (CK-MB mass) replaced the measurement of CK-MB activity. Subsequently cardiac-specific proteins troponin T (cTnT) and troponin I (cTnI) appeared on the scene, displacing LD-1 analysis. However, troponin concentrations in blood increase only from four to six hours after onset of chest pain. Therefore a rapid marker such as myoglobin, fatty acid binding protein or glycogen phosphorylase BB could be used in early diagnosis of AMI. On the other hand, CK-MB isoforms alone may also be useful in rapid diagnosis of cardiac muscle damage. Myoglobin, CK-MB mass, cTnT and cTnI are nowadays widely used in diagnosing patients with acute chest pain. Myoglobin is not cardiac-specific and therefore requires supplementation with some other analyses such as troponins to support the myoglobin value. Troponins are very highly cardiac-specific. Only the sera of some patients with severe renal failure, which requires hemodialysis, have elevated cTnT and/or cTnI without there being any evidence of cardiac damage. On the other hand, the latest studies have shown that elevated troponin levels in sera of hemodialysis patients point to an increased risk of future cardiac events in a similar manner to the elevated troponin values in sera of patients with unstable angina pectoris. In addition, the bedside tests for cTnT and cTnI alone or together with myoglobin and CK-MB mass can be used instead of quantitative analyses in the diagnosis of patients with chest pain. These rapid tests are easy to perform and they do not require expensive instrumentation. For routine clinical laboratory practice we suggest that in diagnosis of patients with chest pain, myoglobin and CK-MB mass measurements should be performed whenever they are requested (24 h/day) and cTnT or cTnI on admission to the hospital and then 4-6 and 12 hours later.

Alternative splicing, muscle calcium sensitivity, and the modulation of dragonfly flight performance

Calcium sensitivity of myosin cross-bridge activation in striated muscles commonly varies during ontogeny and in response to alterations in muscle usage, but the consequences for whole-organism physiology are not well known. Here we show that the relative abundances of alternatively spliced transcripts of the calcium regulatory protein troponin T (TnT) vary widely in flight muscle of Libellula pulchella dragonflies, and that the mixture of TnT splice variants explains significant portions of the variation in muscle calcium sensitivity, wing-beat frequency, and an index of aerodynamic power output during free flight. Two size-distinguishable morphs differ in their maturational pattern of TnT splicing, yet they show the same relationship between TnT transcript mixture and calcium sensitivity and between calcium sensitivity and aerodynamic power output. This consistency of effect in different developmental and physiological contexts strengthens the hypothesis that TnT isoform variation modulates muscle calcium sensitivity and whole-organism locomotor performance. Modulating muscle power output appears to provide the ecologically important ability to operate at different points along a tradeoff between performance and energetic cost.

A new sensitive cardiac Troponin T rapid test (TROPT) for the detection of experimental acute myocardial damage in rats

Cardiac Troponin T (cTnT) is a cardiac structural protein which is released in the circulation during myocardial cell damage. In this study we addressed the question of whether beta-sympathomimetic induced myocardial cell damage in rats can be detected in blood by the TROPT sensitive rapid test strip which is primarily manufactured for the detection of acute myocardial infarction in humans. Sixteen male rats and 16 female animals were treated once with orciprenalinesulphate s.c. to induce tachycardia. The control group which consisted of 16 rats of both sexes received vehicle (physiological saline). The heart rate of two groups of 10 rats (5 of each sex) was measured after orciprenaline or saline treatment. After 1-2 hours we could demonstrate an increased heart rate in the orciprenaline group. Furthermore in this group we could show elevated cTnT levels in peripheral blood measured with the enzyme-linked immunosorbent assay (ELISA) Enzymun-Test Troponin T for troponin T and a positive reaction of the TROPT sensitive test strip. When compared with the ELISA method, the test strip showed a positive reaction at cTnT levels of 0.64 ng/ml upwards. After 24 hours and 96 hours half of the animals were sacrificed for histological examination of the heart tissue. After 24 hours all orciprenaline treated and examined animals showed myofibrillar degeneration of the myocardial cells. The second half of the animals sacrificed 96 hours after treatment with the sympathomimetic drug showed reparative fibrosis of the myocardium. All animals with myocardial damage due to orciprenaline showed a positive test strip result 2 hours after injection. Twenty-four hours after injection only 8 of the 16 animals had a positive test strip result although histological cell damage was demonstrated. All animals treated with physiological saline had negative results of the test strip and showed no signs of myocardial cell damage. We conclude that the TROPT sensitive test strip is a rapid and reliable screening tool for detecting myocardial cell alteration in rats without the need of an expensive and time-consuming ELISA procedure if it is used in early phases of the experiment because all animals with myocardial damage were identified by the test 2 hours after induction of the damage. If the time range between the induction of the injury and the use of the test strip or the ELISA procedure is too wide, in our experiments 24 h, false negative results may occur.

Myoglobin, creatine kinase MB, troponin T, and troponin I - rapid bedside assays in patients with acute chest pain

Three new rapid, qualitative bedside immunoassays were evaluated in the diagnosis of patients with acute chest pain. The subjects, 122 patients in group 1 (bedside tests for myoglobin, creatine kinase MB) and 233 patients in group 2 (bedside tests for troponin I and sensitive troponin T) were admitted to hospital with acute chest pain for less than 12 h. The bedside tests were performed on admission, and 2, 4, and 6 h later. The correlation between the two parts of the rapid creatine kinase MB/myoglobin test during the first 12 h after the onset of chest pain was moderate in all patients (kappa=0.401, 95% confidence interval 0.321-0.483). The highest correlation was seen with the patients with definite and probable myocardial infarction. The correlations were smaller but significant also in other diagnostic groups (unstable angina pectoris, prolonged chest pain, and non-cardiac chest pain). The correlation between the rapid sensitive test for troponin T and rapid test for troponin I was significant in all groups (kappa=0.776, 95% confidence interval 0.711-0.841). The myoglobin part of the rapid creatine kinase MB/myoglobin test may be too non-specific for clinical diagnostic purposes [in non-infarct patients the myoglobin part was significantly more often positive than creatine kinase MB or troponin tests (P<0.001)].

Conformational changes induced in troponin I by interaction with troponin T and actin/tropomyosin

Troponin I (TnI) is the inhibitory component of the striated muscle Ca2+ regulatory protein troponin (Tn). The other two components of Tn are troponin C (TnC), the Ca2+-binding component, and troponin T (TnT), the tropomyosin-binding component. We have used limited chymotryptic digestion to probe the local conformation of TnI in the free state, the binary TnC*TnI complex, the ternary TnC*. TnI*TnT (Tn) complex, and in the reconstituted Tn*tropomyosin*F-actin filament. The digestion of TnI alone or in the TnC*TnI complex produced initially two major fragments via a cleavage of the peptide bond between Phe100 and Asp101 in the so-called inhibitory region. In the ternary Tn complex cleavage occurred at a new site between Leu140 and Lys141. In the absence of Ca2+ this was followed by digestion of the 1-140 fragment at Leu122 and Met116. In the reconstituted thin filament the same fragments as in the case of the ternary complex were produced, but the rate of digestion was slower in the absence than in the presence of Ca2+. These results indicate firstly that in both free TnI and TnI complexed with TnC there is an exposed and flexible site in the inhibitory region. Secondly, TnT affects the conformation of TnI in the inhibitory region and also in the region that contains the 140-141 bond. Thirdly, the 140-141 region of TnI is likely to interact with actin in the reconstituted thin filament when Ca2+ is absent. These findings are discussed in terms of the role of TnI in the mechanism of thin filament regulation, and in light of our previous results [Y. Luo, J.-L. Wu, J. Gergely, T. Tao, Biochemistry 36 (1997) 13449-13454] on the global conformation of TnI.

Structure and evolution of the alternatively spliced fast troponin T isoform gene

The vertebrate fast skeletal muscle troponin T gene, TnTf, produces a complexity of isoforms through differential mRNA splicing. The mechanisms that regulate splicing and the physiological significance of TnTf isoforms are poorly understood. To investigate these questions, we have determined the complete sequence structure of the quail TnTf gene, and we have characterized the developmental expression of alternatively spliced TnTf mRNAs in quail embryonic muscles. We report the following: 1) the quail TnTf gene is significantly larger than the rat TnTf gene and has 8 non-homologous exons, including a pectoral muscle-specific set of alternatively spliced exons; 2) specific sequences are implicated in regulated exon splicing; 3) a 900-base pair sequence element, composed primarily of intron sequence flanking the pectoral muscle-specific exons, is tandemly repeated 4 times and once partially, providing direct evidence that the pectoral-specific TnT exon domain arose by intragenic duplications; 4) a chicken repeat 1 retrotransposon element resides upstream of this repeated intronic/pectoral exon sequence domain and is implicated in transposition of this element into an ancestral genome; and 5) a large set of novel isoforms, produced by regulated exon splicing, is expressed in quail muscles, providing insights into the developmental regulation, physiological function, and evolution of the vertebrate TnTf isoforms.

Characterisation of troponin-T from salmonid fish

Five major troponin-T isoforms were isolated from the myotomal muscles of Atlantic salmon: three from fast muscle (Tn-T1F, Tn-T2F and Tn-T3F) and two from slow muscle (Tn-T1S and Tn-T2S). In addition to their presence in troponin preparations, these proteins were also recognised to be Tn-T on the basis of immunoreaction with anti-troponin-T antibodies and partial amino acid sequence. The electrophoretic mobility in the presence of SDS of the various Tn-Ts increases in the order: 1S < 1F < 2S < 2F < or = 3F. Compositional analysis shows that the higher M(r) forms (1F and 1S) contain considerably more proline, glutamic acid and alanine than the lower-M(r) forms (2F, 3F and 2S). Every isoform lacks cysteine and phosphoserine is present only in isoforms 2F and 3F. All of the Tn-Ts, with the exception of isoform 1F, are N-terminally blocked. CNBr fragments from same cell type Tn-Ts yield identical sequences over at least fifteen Edman cycles. Two full-length cDNA sequences, presumed to represent 1S and 3F, or isoforms that are highly similar, are reported. As documented for higher vertebrate Tn-Ts, the predicted primary structures display a non-uniform distribution of charged amino acids and greater divergence at each end than in the central section. The most striking difference between the two salmonid proteins is the presence of a N-terminal (proline-, glutamic acid- and alanine-rich) extension of about fifty amino acids in Tn-T1s (278 amino acids) that is missing from the fast muscle Tn-T (223 amino acids). The sequences also differ in that 1S lacks the known phosphorylation site while the fast-type isoform contains serine next to the initiating methionine. Of the two, the slow isoform has accumulated the greater number of substitutions.

Tissue-specific distribution of breast-muscle-type and leg-muscle-type troponin T isoforms in birds

In order to show the tissue-specific distribution of troponin T (TnT) isoforms in avian skeletal muscles, their expression was examined by electrophoresis of the breast and leg muscles of seven avian species and immunoblotting with the antiserum against fast skeletal muscle TnT. It has been reported in the chicken that breast-muscle-type (B-type) and leg-muscle-type (L-type) TnT isoforms are expressed specifically in the adult breast and leg muscles, respectively. Their differential expression patterns were confirmed in all birds examined in this study. The expression of a segment encoded by the exon x series of TnT was also examined by immunoblotting with the antiserum against a synthetic peptide derived from the exon x3 sequence, because the segment has been shown to be included exclusively in the B-type, but not in the L-type TnT. The expression of the segment was found only in the breast muscle, but not in the leg muscle of all birds examined. TnT cDNA sequences from the duck breast and leg muscles were determined and showed that only B-type TnT had an exon x-related sequence, suggesting that the expression of B-type TnT containing the exon x-derived segment is conserved consistently in the birds.

Troponin T: genetics, properties and function

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

Training effects on the contractile apparatus

Skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fibre types. Its dynamical nature is reflected by the ability to adapt to altered functional demands by qualitative alterations in fibre type composition. The molecular basis of this versatility is that specific myofibrillar and Ca2+-regulatory protein isoforms are assembled to functionally specialized fibre types. Based on this diversity, adult muscle fibres are capable of changing their molecular composition by altered gene expression. Myosin heavy chain (MHC) isoforms and their unique expression in 'pure' fibres, as well as their coexpression in 'hybrid' 'fibres' represent the best markers of muscle fibre diversity and adaptive changes. Chronic low-frequency stimulation (CLFS) and endurance training represent highly suitable models for studying the effects of increased neuromuscular activity on myofibrillar protein isoform expression and fibre type composition. Generally, both models induce fast-to-slow transitions in myofibrillar protein isoforms and fibre types. However, the responses to endurance training are quantitatively less pronounced than those in muscles exposed to CLFS. Parallel changes in isoforms of specific myofibrillar or Ca2+-regulatory proteins during the induced fast-to-slow transitions point to the existence of fibre type-specific patterns of gene expression. The fast-to-slow transitions do not proceed in abrupt jumps from one extreme to the other, but occur in a gradual and orderly sequential manner. Depending on the basal protein isoform profile, and hence the position within the fast-slow spectrum, the adaptive ranges of different fibre types vary. However, adaptive ranges not only depend on a particular fibre type, but also are influenced by species-specific properties.

Cardiac troponin T in patients with clinically suspected myocarditis

OBJECTIVES

The present study investigated whether myocyte injury can be assessed sensitively by measurement of serum levels of cardiac troponin T (cTnT) in patients with clinically suspected myocarditis and whether cTnT levels may predict the results of histologic and immunohistologic analysis of endomyocardial biopsy specimens.

BACKGROUND

Conventionally used laboratory variables often fail to show myocyte injury in patients with clinically suspected myocarditis, possibly because of a low extent of myocardial injury in these patients. Sensitive variables for myocyte injury have not yet been investigated.

METHODS

Eighty patients with clinically suspected myocarditis were screened for creatine kinase (CK) activity, MB isoform of CK (CK-MB) activity and cTnT. Endomyocardial biopsy specimens were examined histologically and immunohistologically.

RESULTS

cTnT was elevated in 28 of 80 patients with clinically suspected myocarditis, CK in 4 and CK-MB in 1. Histologic analysis alone of the endomyocardial biopsy specimen revealed evidence of myocarditis in only five patients, all with elevated cTnT levels. Twenty-three of 28 patients with elevated cTnT levels had histologically negative findings for myocarditis. Additional immunohistologic analysis revealed evidence of myocarditis in 26 (93%) of 28 patients with elevated cTnT levels and in 23 (44%) of 52 patients with normal cTnT levels. Mean cTnT levels were higher in patients with myocarditis proved histologically or immunohistologically, or both, than in patients without myocarditis (0.59 +/- 1.68 vs. 0.04 +/- 0.05, p < 0.001).

CONCLUSIONS

Measurement of serum levels of cTnT provides evidence of myocyte injury in patients with clinically suspected myocarditis more sensitively than does conventional determination of cardiac enzyme levels. Myocardial cell damage may be present even in the absence of histologic signs of myocarditis. Additional immunohistologic analysis often shows lymphocytic infiltrates in these patients. Elevated levels of cTnT are highly predictive for myocarditis in this group.

Comparison of the troponin T and troponin I ELISA tests, as measured by microplate immunoassay techniques, in diagnosing acute myocardial infarction

We describe an improved procedure using a standard microplate immunoassay reader to measure the concentration of troponin T in human serum. We also describe an immunoassay for troponin I in serum. Only 160 microliters of serum are needed for a single analysis of each troponin. For comparison, creatine kinase MB mass analysis in serum was performed with a commercial luminometric method. From 95 apparently healthy people the following values were obtained: creatine kinase MB mass 2.6 +/- 1.2 micrograms/l, troponin T 0.027 +/- 0.025 microgram/l and troponin I 0.03 +/- 0.031 microgram/l. We compared the results of troponin T and troponin I methods with each other, as well as with those of creatine kinase MB mass measured in 48 patients with verified acute myocardial infarction and in 60 control patients with non-cardiac chest pain. The correlation between troponin T and troponin I values was 0.91 for the total material and 0.94 for 48 patients with acute myocardial infarction. Troponin I showed better earlier sensitivity than troponin T (p = 0.043). In nine patients in the control group, creatine kinase MB mass exceeded the reference limit of 5.0 micrograms/l, while in two patients the cut-off limit of 10.0 micrograms/l was also surpassed, pointing to non-specificity. In the group of infarct patients, the highest serum creatinine value was 193 mumol/l, whereas in the control group it was 406 mumol/l. The sera of patients with impaired renal function without any cardiac failure showed no increase in troponin T and troponin I values. In conclusion, serum creatine kinase MB mass and troponin I seem to confirm an acute myocardial infarction more rapidly than does troponin T; troponin I has the highest cardiac specificity.

Colocalization of the dihydropyridine receptor, the plasma-membrane calcium ATPase isoform 1 and the sodium/calcium exchanger to the junctional-membrane domain of transverse tubules of rabbit skeletal muscle

The subcellular distribution of the calmodulin-stimulated plasma-membrane Ca(2+)-ATPase (PMCA) has been studied in rat and rabbit skeletal muscle cells by indirect (calmodulin gel overlays) and direct (Western blotting with specific antibodies) methods. It has also been studied in situ in immunocytochemistry experiments. The distribution of PMCA has been compared with that of the NA+/Ca2+ exchanger and of the dihydropyridine receptor, which has been studied by Western blotting with specific antibodies. Both PMCA and the Na+/Ca2+ exchanger had a dual localization, i.e., they were found in the plasma membrane and in the transverse-tubule fractions of the two main types of skeletal muscles studied. The pump and the exchanger were not diffusely distributed in the transverse-tubule-membrane system, but specifically confined to the membrane domain where the dihydropyridine receptor was also localized, i.e., the junctional membrane. Experiments with isoform-specific antibodies have shown that the pump isoform expressed in skeletal muscle is PMCA 1.

Developmental genetic analysis of troponin T mutations in striated and nonstriated muscle cells of Caenorhabditis elegans

We have been investigating a set of genes, collectively called mups, that are essential to striated body wall muscle cell positioning in Caenorhabditis elegans. Here we report our detailed characterization of the mup-2 locus, which encodes troponin T (TnT). Mutants for a heat-sensitive allele, called mup-2(e2346ts), and for a putative null, called mup-2(up1), are defective for embryonic body wall muscle cell contraction, sarcomere organization, and cell positioning. Characterizations of the heat-sensitive allele demonstrate that mutants are also defective for regulated muscle contraction in larval and adult body wall muscle, defective for function of the nonstriated oviduct myoepithelial sheath, and defective for epidermal morphogenesis. We cloned the mup-2 locus and its corresponding cDNA. The cDNA encodes a predicted 405-amino acid protein homologous to vertebrate and invertebrate TnT and includes an invertebrate-specific COOH-terminal tail. The mup-2 mutations lie within these cDNA sequences: mup-2(up1) is a termination codon near NH2 terminus (Glu94) and mup-2(e2346ts) is a termination codon in the COOH-terminal invertebrate-specific tail (Trp342). TnT is a muscle contractile protein that, in association with the thin filament proteins tropomyosin, troponin I and troponin C, regulates myosin-actin interaction in response to a rise in intracellular Ca2+. Our findings demonstrate multiple essential functions for TnT and provide a basis to investigate the in vivo functions and protein interactions of TnT in striated and nonstriated muscles.

The prognostic value of serum troponin T in unstable angina

BACKGROUND

Cardiac troponin T is a regulatory contractile protein not normally found in blood. Its detection in the circulation has been shown to be a sensitive and specific marker for myocardial cell damage. We used a newly developed enzyme immunoassay for troponin T to determine whether its presence in the serum of patients with unstable angina was a prognostic indicator.

METHODS

We screened 72 patients with unstable angina (Class III, acute unstable angina) for serum creatine kinase activity, creatine kinase myocardial band (isoenzyme M) activity, and troponin T, every 8 h for 2 days after admission to the hospital. The outcomes of interest during the hospitalization were death and myocardial infarction.

RESULTS

Troponin T was detected in the serum of 24 of the 72 patients (34%) with acute angina at rest. Only four of these patients had elevated creatine kinase M activity. Of the 24 patients who were positive for troponin T, 12 had myocardial infarction, and 6 of these died during hospitalization. In contrast, only 2 of the 48 patients with angina at rest who were negative for troponin T had an acute myocardial infarction, and these patients died. Thus, 12 of the 14 patients with myocardial infarctions had detectable levels of troponin T; only 2 had elevated creatine kinase M activity.

CONCLUSIONS

Cardiac troponin T in serum appears to be a more sensitive indicator of myocardial-cell injury than serum creatine kinase MB activity, and its detection in the circulation may be a useful prognostic indicator in patients with unstable angina.

Physiologically regulated alternative splicing patterns of fast troponin T RNA are conserved in mammals

NH2-terminal isoforms of fast troponin T (TnT) are generated by alternative splicing of fast TnT RNA transcripts. Significantly different estimates for the number of isoforms have been obtained by nucleic acid and protein chemical studies. To resolve this controversy and to determine whether specific 5'-splicing patterns correlate with fiber phenotype, we generated representative populations of 5'-TnT cDNAs from the TnT mRNAs expressed in a set of physiologically and anatomically diverse skeletal muscles. Sequencing and restriction enzyme analyses revealed a total of nine cDNAs that encode the six adult and three perinatal NH2-terminal TnT variants previously identified. Three major 5'-splicing pathways (the TnT1f, TnT2f, and TnT3f patterns) account for more than 90% of the TnT mRNAs and proteins in adult rabbit skeletal muscle. Comparative studies in rats, mice, and humans show that these splicing patterns are conserved and that fast-twitch fibers that are primarily glycolytic utilize the TnT1f and TnT2f patterns preferentially, whereas fast-twitch fibers that are primarily oxidative use the TnT1f and TnT3f patterns preferentially.

Serum cardiac troponin T and creatine kinase-MB elevations in murine autoimmune myocarditis

BACKGROUND

We used a murine model of autoimmune myocarditis to investigate systematically whether serum markers of myocardial cell injury, that is, cardiac troponin T (TnT) and the MB isoenzyme of creatine kinase (CK-MB) are useful for the diagnosis of inflammatory heart disease.

METHODS AND RESULTS

Fifty-two A.SW mice were immunized with cardiac myosin to induce myocarditis. The disease was evident on day 12 after the initial immunization in 14 of 22 immunized mice, on day 16 in 7 of 10 mice, on day 19 in 6 of 10 mice, and on day 23 in 5 of 10. The severity of myocarditis increased between days 12 and 16 and remained constant thereafter. TnT was elevated in a considerable number of mice with myocarditis, resulting in a diagnostic sensitivity (number of marker elevations per number of mice with myocarditis) of 0.43 on day 12, 0.71 on day 16, and 0.50 on day 19. CK-MB elevations were not seen on day 12 but resulted in a diagnostic sensitivity of 0.71 on day 16 and of 0.33 on day 19. No elevations of CK-MB or TnT were observed on day 23. All elevations were specific for the disease, as none of the mice lacking myocarditis showed increased markers.

CONCLUSIONS

In murine autoimmune myocarditis, TnT is a more sensitive marker for the disease than CK-MB. Elevations clearly indicate myocarditis, but negative test results do not exclude the presence of the disease. These data suggest that the determination of CK-MB and, in particular, of TnT, can be useful for the diagnostic evaluation of patients with suspected myocarditis.

Lower cardiac troponin T levels in patients undergoing cardiopulmonary bypass and receiving high-dose aprotinin therapy indicate reduction of perioperative myocardial damage

Nowadays in many European heart centers the activation of the fibrinolytic system, always occurring during cardiopulmonary bypass, is routinely reduced by high-dose application of the proteinase inhibitor aprotinin (total of > 4 million KIU). In this study parameters of myocardial ischemic injury were investigated with the aim of identifying further benefits of aprotinin, particularly the protection of the myocardium during the ischemic period of aortic crossclamping. Forty patients with coronary artery disease who underwent aorta-coronary bypass grafting were randomly and in a double-blind fashion divided into two groups, one that received high-dose aprotinin therapy and one that received only saline solution. Markers such as troponin T, with high specificity for detection of myocardial ischemia and infarction, and markers with more general specificity such as creatine kinase, its isoenzyme, and lactate dehydrogenase showed significantly increased values after ischemia in both groups. In patients who received high-dose aprotinin therapy 3 days after cardiopulmonary bypass all parameters measured showed significantly lower levels compared with those in the control group. Therefore we can presume that the application of high-dose aprotinin provides myocardial protection from perioperative ischemic injury.

Diagnostic efficiency of troponin T measurements in rats with experimental myocardial cell damage

A cardioselective parameter has been available for about 2 years since the development by KATUS of an immunoassay for cardiac Troponin T (TnT). The major advantages of this TnT assay are its cardiospecificity and its sensitivity. The parameters usually determined in toxicity studies in rats to detect alterations in the myocardial cells, e.g. aspartate aminotransferase (ASAT), creatinine kinase (CK) and lactate dehydrogenase (LDH), are either of low sensitivity in this species or give falsely high results as the consequence of stress or haemolysis. We therefore investigated in the present study how well Troponin T, determined with the ELISA Troponin T from Boehringer Mannheim, can detect experimentally induced myocardial lesions in rats. In order to achieve hypoxic damage of the cardiomyocytes in these experiments in rats, male Sprague-Dawley rats were given two doses of 4 mg/kg isoprenaline each (Aludrin from Boehringer Ingelheim, FRG) subcutaneously. The second dose was given 7 h after the start of the experiment. Serum samples were analysed for Troponin T (TnT) levels and, for comparison, aspartate aminotransferase (ASAT), creatine kinase (CK), and lactate dehydrogenase (LDH). Histological examinations of the heart muscle were performed 24 and 96 h after the first injection. As expected, histological examinations of the isoprenaline-treated animals revealed marked myofibrillic degeneration of the myocardium 24 h after the first injection. Markedly elevated serum TnT levels (up to 7.9 ng/ml) were already evident in these animals after 6 h. TnT values decreased with time, but were still statistically significant after 48 h. Of the well-established indicators for diagnosing myocardial infarction, only ASAT showed transient statistically significant increases over 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Overproduction and rapid purification of human fast skeletal beta troponin T using Escherichia coli expression vectors: functional differences between the alpha and beta isoforms

Troponin T (TpnT), an essential component of the Ca(2+)-regulatory troponin complex, is involved in protein-protein interactions with other thin-filament proteins during muscle contraction in vertebrate striated muscle (VSM). The isoforms of TpnT are encoded by members of a multigene family which, by alternate splicing, produces a complex pattern of isoproteins in VSM. The functional domains of TpnT are only tentatively identified and structure-function analysis on this protein is limited due to the heterogeneity of the multiple isoforms. We reasoned that the overproduction and purification of a single TpnT species in Escherichia coli would provide an insight into these studies, besides being useful in crystallizing the protein. We cloned the human fast skeletal beta TpnT-encoding cDNA (beta TpnTf) in three expression vectors. Overexpression was achieved in an E. coli BL21 (DE3) lysogen using a T7 RNA polymerase promoter-based vector, pET17b. The unfused recombinant protein was purified by a simple and rapid procedure in a biologically active and immunoreactive form. This is the first successful synthesis of a complete beta TpnTf polypeptide from any species using an in vitro expression system. Purified human beta TpnTf, a predominant fetal form, was less Ca(2+)-sensitive and exhibited considerably reduced affinity for troponin C and tropomyosin, as compared to the rabbit fast skeletal alpha TpnT, a predominant adult isoform. These results provide a biochemical correlate to the age-related differences in Ca2+ sensitivity of tension development in vertebrate fast skeletal muscles.

Adaptation of the troponin T ELISA test to a microplate immunoassay reader

Troponin T concentration in serum is usually measured by the automated method developed by Boehringer Mannheim for the ES-series of analysers. These instruments need at least 140 microliters of serum and 700 microliters of reagents for a single analysis, which takes 90 min. We describe an alternative procedure, using streptavidin-coated microtitre plates, troponin T reagents of Boehringer Mannheim and an ELISA-reader to measure the concentration of troponin T. The present assay needs only 30 microliters of sample and 200 microliters of reagents, and it takes 75 min; the detection limit is 0.10 microgram/l. We also assessed the microtitre plate method for sensitivity and precision and compared the results with those measured by an ES-300 automatic analyser. Both methods have the same measurement range for troponin T of 0.1 to 15 micrograms/l. For daily routine use of the microtitre plate method we recommend duplicate determinations.

Identification of a fetal exon in the human fast troponin T gene

A developmentally regulated exon has been identified in the 5'-alternatively spliced region of the human fast Troponin T (TnT) gene. Expressed in fetal (but not adult) muscle, this exon is homologous with the fetal exons recently described in the rabbit and rat fast TnT genes. They all exhibit a split codon organization and encode a highly acidic peptide. To determine if the splicing pathways, including the human fetal exon, are also conserved, we defined the major TnT splicing patterns in fetal muscle. They generate fetal TnT 1, fetal TnT 3, and TnT1f, and TnT3f, species previously described in rabbit and rat skeletal muscles.

Role of regulatory proteins (troponin-tropomyosin) in pathologic states

In vertebrate striated muscle, troponin-tropomyosin is responsible, in part, not only for transducing the effect of calcium on contractile protein activation, but also for inhibiting actin and myosin interaction when calcium is absent. The regulatory troponin (Tn) complex displays several molecular and calcium binding variations in cardiac muscles of different species and undergoes genetic changes with development and in various pathologic states. Extensive reviews on the role of tropomyosin (Tm) and Tn in the regulation of striated muscle contraction have been published describing the molecular mechanisms involved in contractile protein regulation. In our studies, we have found an increase in Mg2+ ATPase activity in cardiac myofibrils from dystrophic hamsters and in rats with chronic coronary artery narrowing. The abnormalities in myofibrillar ATPase activity from cardiomyopathic hamsters were largely corrected by recombining the preparations with a TnTm complex isolated from normal hamsters indicating that the TnTm may play a major role in altered myocardial function. We have also observed down regulation of Ca2+ Mg2+ ATPase of myofibrils from hypertrophic guinea pig hearts, myocardial infarcted rats and diabetic-hypertensive rat hearts. In myosin from diabetic rats, this abnormality was substantially corrected by adding troponin-tropomyosin complex from control hearts. All of these disease models are associated with decreased ATPase activities of pure myosin and in the case of rat and hamster models, shifts of myosin heavy chain from alpha to beta predominate. In summary, there are three main troponin subunit components which might alter myofibrillar function however, very few direct links of molecular alterations in the regulatory proteins to physiologic and pathologic function have been demonstrated so far.

Investigation of the effects of phosphorylation of rabbit striated muscle alpha alpha-tropomyosin and rabbit skeletal muscle troponin-T

FPLC has been employed to prepare the phosphorylated and unphosphorylated forms of rabbit striated muscle alpha alpha-tropomyosin (TM), and the major isoform of rabbit fast-skeletal-muscle troponin-T (Tn-T2f) and corresponding chymotryptic fragment T1 (residues 1-158), in order to investigate the effects which these in vivo modifications have on thin filament function. In all instances, no significance could be attributed to the presence of a phosphate moiety on acetyl serine 1 of Tn-T (or fragment T1). As expected, fragment T1 increased the relative viscosities of solutions of unphosphorylated alpha alpha-TM, but this induction was noticeably lower for phosphorylated alpha alpha-TM. In affinity chromatography experiments, fragment T1 bound equally well to either form of alpha alpha-TM, but the interaction between fragment T2 (residues 159-259) and phosphorylated alpha alpha-TM was strengthened relative to the control. In the presence of alpha alpha-TM (unphosphorylated), fragment T1 was found to down regulate the actin-activated myosin-S1 MgATPase activity, indicating that this portion of Tn-T possesses modulatory properties. Under the same conditions, less inhibition was observed with phosphorylated alpha alpha-TM. When the two different forms of alpha alpha-TM were reconstituted into a complete regulatory system, the activation of myosin-S1 was double for those thin filaments containing the phosphorylated molecule. Dephosphorylation of the phospho alpha alpha-TM reduced the rates to control values. In ATPase Ca2+ titrations, these systems exhibited no difference in the co-operativity of activation and little or no difference in the pCa2+ 1/2 value. Developmentally linked changes in the steady-state phosphorylation of alpha alpha-TM could be a mechanism to increase the activating propensity of thin filaments, by modifying the functional properties of the T1 section of Tn-T.

An unusual metal-binding cluster found exclusively in the avian breast muscle troponin T of Galliformes and Craciformes

A repeating metal-binding (Cu2+ > Ni2+ > Zn2+ approximately Co2+) sequence (HE/AEAH)4 has been identified in troponin T isoforms specifically expressed in the breast but not leg muscles of all Galliformes and Craciformes. It is absent in the skeletal and cardiac muscles of mammals and all other avian species investigated. Concentration of the metal-binding sites is adequate to affect free metal levels in the muscle cell and we suggest a possible link between its presence in breast muscle of Galliformes and the high ratio of breast muscle to total body muscle mass and explosive but short-lived flight pattern of these birds. This sequence can be used for a highly selective metal-affinity chromatographic purification of muscle or engineered TnTs even in high salt and/or urea.

Isolation and characterization of human fast skeletal beta troponin T cDNA: comparative sequence analysis of isoforms and insight into the evolution of members of a multigene family

A cDNA encoding human fast skeletal beta troponin T (beta TnTf) has been isolated and characterized from a fetal skeletal muscle library. The cDNA insert is 1,000 bp in length and contains the entire coding region of 777 bp and 5' and 3' untranslated (UT) segments of 12 and 211 bp, respectively. The 3' UT segment shows the predicted stem-loop structure typical of eukaryotic mRNAs. The cDNA-derived amino acid sequence is the first available sequence for human beta TnTf protein. It is encoded by a single-copy gene that is expressed in a tissue-specific manner in fetal and adult fast skeletal muscles. Although the human beta TnTf represents the major fetal isoform, the sequence information indicates that this cDNA and the coded protein are quite distinct from the fetal and neonatal TnTf isoforms reported in other mammalian fetal muscles. The hydropathy plot indicates that human beta TnTf is highly hydrophilic along its entire length. The protein has an extremely high degree of predicted alpha-helical content involving the entire molecule except the carboxy-terminal 30 residues. Comparative sequence analysis reveals that the human beta TnTf shares a high level of sequence similarity in the coding region with other vertebrate TnTf and considerably reduced similarity with slow skeletal and cardiac TnT cDNAs. The TnT isoforms have a large central region consisting of amino acid residues 46-204 which shows a high sequence conservation both at the nucleotide and amino acid levels. This conserved region is flanked by the variable carboxy-terminal and an extremely variable amino-terminal segment. The tropomyosin-binding peptide of TnT, which is represented by amino acid residues 47-151 and also includes a part of troponin I binding region, is an important domain of this central segment. It is suggested that this conserved segment is encoded by an ancestral gene. The variable regions of vertebrate striated TnT isoforms reflect the subsequent addition and modification of genomic sequences to give rise to members of the TnT multigene family.

Forced expression and assembly of rat cardiac troponin T isoforms in cultured muscle and nonmuscle cells

Cardiac troponin T (cTnT), a tropomyosin (TM)-binding subunit of the troponin complex, undergoes a developmentally regulated isoform switch from embryonic form to adult form in the rat heart. To investigate the in vivo assembly of cTnT isoforms, we transiently transfected cDNA clones of either rat cTnT isoform into nonmuscle CHO cells and chick embryo myogenic (CEM) cells. As determined by Western blotting, both isoforms can be expressed in CHO and CEM cells. The expressed proteins had the same mobility as native rat cTnT proteins on SDS polyacrylamide gels and were recognized by anti-TnT antibodies. Conventional and confocal microscopy of transfected cells, double-labelled with antibodies against cTnT and against TM, revealed that neither isoform appears to associate with the nonmuscle TM in CHO cells, although both are able to colocalize with muscle TM-containing microfilament bundles in the myogenic CEM cells. There was no appreciable cTnT isoform-related difference in association with TM, suggesting that the functional significance of isoform variability in rat cTnT does not correspond to an assembly advantage for the maturing cardiac thin filament. To help determine whether cTnT nonassembly in CHO environment is primarily due to the nonmuscle nature of the endogenous TM, or if it involves the absence of other factors specific to muscle, we have isolated several stably-transfected clones of skeletal beta TM-expressing CHO cells which incorporate this muscle TM onto stress fibres. When either isoform of cTnT was transiently expressed in these beta TM-CHO cells, the strictly filamentous beta TM staining pattern was no longer observed. Instead, beta TM codistributed with cTnT in brightly staining aggregates not associated with the intact stress fibres. This suggests that both isoforms of cTnT are interacting with the beta TM in the nonmuscle environment and that other muscle-specific proteins may indeed be required for stable assembly of cTnT onto microfilaments. It also suggests that the interaction between cTnT and muscle TM is stronger than that between muscle TM and nonmuscle microfilaments.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

The prognostic value of serum troponin T in unstable angina

BACKGROUND

Cardiac troponin T is a regulatory contractile protein not normally found in blood. Its detection in the circulation has been shown to be a sensitive and specific marker for myocardial cell damage. We used a newly developed enzyme immunoassay for troponin T to determine whether its presence in the serum of patients with unstable angina was a prognostic indicator.

METHODS

We screened 109 patients with unstable angina (25 with accelerated or subacute angina and 84 with acute angina at rest) for serum creatine kinase activity, creatine kinase isoenzyme MB activity, and troponin T every eight hours for two days after admission to the hospital. The outcomes of interest during the hospitalization were death and myocardial infarction.

RESULTS

Troponin T was detected (range, 0.20 to 3.64 micrograms per liter; mean, 0.78; median, 0.50) in the serum of 33 of the 84 patients (39 percent) with acute angina at rest. Only three of these patients had elevated creatine kinase MB activity (two were positive for troponin T, and one was negative). Of the 33 patients who were positive for troponin T, 10 (30 percent) had myocardial infarction (3 after coronary-artery bypass surgery), and 5 of these died during hospitalization. In contrast, only 1 of the 51 patients with angina at rest who were negative for troponin T had an acute myocardial infarction (P less than 0.001), and this patient died (P = 0.03). Thus, 10 of the 11 patients with myocardial infarctions had detectable levels of troponin T; only 1 had elevated creatine kinase MB activity. Troponin T was not detected in any of the 25 patients with accelerated or subacute angina, and none of these patients died.

CONCLUSIONS

Cardiac troponin T in serum appears to be a more sensitive indicator of myocardial-cell injury than serum creatine kinase MB activity, and its detection in the circulation may be a useful prognostic indicator in patients with unstable angina.

Developmental changes in troponin T isoform expression and tension production in chicken single skeletal muscle fibres

1. The Ca2+ sensitivity of tension development was characterized in single skinned fibres from the slow anterior latissimus dorsi (ALD), fast posterior latissimus dorsi (PLD), and fast pectoralis major (PM) muscles of the chicken at adult and neonatal (2 weeks post-hatch) stages of development. In the adult, the PM was most sensitive, the ALD intermediate, and the PLD least sensitive to Ca2+. 2. PM and PLD fibres were less sensitive to Ca2+ at the neonatal stage of development than in the adult. However, ALD fibres exhibited no age-dependent changes in Ca2+ sensitivity. 3. Characterization of regulatory protein composition indicated that the PM and PLD fibres had identical fast isoforms of troponin C and troponin I at each developmental stage examined, but there were muscle-specific and age-dependent expressions of troponin T isoforms in these fibres. 4. In the ALD fibres, identical slow isoforms of troponin C, troponin I and tropomyosin were found at each stage. In addition, the troponin T isoform that was present did not change with age. 5. The results suggest a relationship between the specific troponin T isoform composition of individual muscle fibres and their calcium sensitivities of tension development.

Troponin T isoform expression in humans. A comparison among normal and failing adult heart, fetal heart, and adult and fetal skeletal muscle

The expression of troponin (Tn) T, a thin-filament regulatory protein, was examined in left ventricular myocardium from normal and from failing adult human hearts. The differences in isoform expression between normal and failing myocardium led us to examine the ontogenic expression of TnT in human striated muscle. Left ventricular samples were obtained from patients with severe heart failure undergoing cardiac transplantation and normal adult organ donors. Fetal muscle was obtained from aborted fetuses after 14-15 weeks of gestation, and adult skeletal muscle was obtained from surgical biopsies. Western blots of normal and failing adult heart proteins demonstrated that two isoforms, TnT1 and TnT2, are expressed in different amounts, with TnT2 being significantly greater in failing hearts (p less than 0.004). Western blots of two-dimensional gels of these proteins resolved two predominant spots of both TnT1 and TnT2 and several minor TnT species. Alkaline phosphatase treatment converted the two major spots of each isoform into the single more basic spots. A comparison of the ATPase activities and the TnT2 percentage of total TnT in individual failing and normal adult hearts demonstrated an inverse and negative relation (r = 0.7, p less than 0.02). In the fetal heart, four TnT isoforms were found, two of which had the same electrophoretic mobilities as the adult cardiac isoforms TnT1 and TnT2. Fetal skeletal muscle expressed two of the four fetal cardiac TnT isoforms, one of which comigrated with adult cardiac TnT1. These cardiac isoforms were expressed in low abundance in fetal skeletal muscle relative to seven fast skeletal muscle TnT isoforms. No cardiac isoforms were present in adult skeletal muscle. Because many etiologies caused heart failure in the transplant patients, we propose that the disease-associated increased expression of the TnT isoform TnT2 is an adaptation to the heart failure state and a partial recapitulation of the fetal expression of cardiac TnT isoforms.

Intracellular compartmentation of cardiac troponin T and its release kinetics in patients with reperfused and nonreperfused myocardial infarction

In a previous study on the diagnostic efficiency of troponin T measurements in patients with suspected acute myocardial infarction (AMI), the authors found a high variability of troponin T serum concentration changes on day 1 in patients with AMI who underwent thrombolytic treatment. Therefore, the aims of the present study were to investigate the intracellular compartmentation of troponin T and to analyze the effects of AMI reperfusion on the appearance kinetics of cardiac troponin T in serum. Cardiac troponin T was measured with a newly developed bideterminant sandwich assay using cardiospecific, affinity-purified polyclonal antibodies and peroxidase-labeled monoclonal antibody. An unbound cytosolic troponin T pool was found in ultracentrifuged homogenates of myocardial tissue of different species ranging from 0.013 to 0.036 mg/g wet weight. The soluble troponin T molecule had electrophoretic properties identical to troponin T compartmented in the myofibrils. The clinical study group comprised 57 patients with AMI undergoing thrombolytic treatment. Blood flow to the infarct zone and point of time of reperfusion were tested by immediate and late angiography. The appearance of troponin T in serum on day 1 after the onset of AMI depended strongly on reperfusion and on duration of ischemia before reperfusion. Thus, in patients with early reperfused AMI, a marked peak in troponin T serum concentrations was found at 14 hours after the onset of pain. This early troponin T peak was absent in patients with AMI reperfusion occurring greater than 5.5 hours after the onset of pain and in patients with nonreperfused AMI. By contrast, the kinetics of troponin T release after the first day after AMI were unaffected by reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Drosophila melanogaster troponin-T mutations engender three distinct syndromes of myofibrillar abnormalities

In vertebrates troponin complexes interact co-operatively with tropomyosin dimers to modulate skeletal muscle contraction. In order further to investigate troponin assembly and function in vivo, we are developing molecular genetic approaches. Here we report characterization of the gene that encodes Drosophila tropinin-T and analyses of muscle defects engendered by several mutant alleles. We found that the Drosophila troponin-T locus specifies at least three proteins having sequences similar to vertebrate troponin-T. All are significantly larger than any avian or mammalian isoforms, however, due to a highly acidic carboxy-terminal extension. Comparisons of the chromosomal arrangements of vertebrate and Drosophila troponin-T genes revealed that the location of one intron-exon boundary is conserved. This observation and the similarity of vertebrate and Drosophila troponin-T primary sequences suggest that the respective proteins are homologous, and that troponin-T pre-dates the divergence of vertebrate and invertebrate organisms. In situ hybridization of the Drosophila troponin-T gene to polytene chromosomes demonstrated that it resides within subdivision 12A of the X chromosome, precisely where upheld and indented thorax flight muscle mutations have been mapped previously. We determined the nucleotide sequences of troponin-T genes in five extant mutants. All have deleterious alterations, directly establishing that upheld and indented thorax muscle abnormalities are due to defective troponin-T. Two of the alleles, upheld2 and upheld3, apparently disrupt RNA splicing and eliminate most or all troponin-T from flight and jump muscles, while the remaining three alleles change the identities of single amino acids of troponin-T. Electron microscopy of mutant muscles revealed that the two null alleles eliminate thin filaments, except where they are bound by electron-dense material presumed to be Z-disc proteins. Two of the point mutations, upheld101 and indented thorax3, do not perturb assembly of myofibrils, but cause their degeneration within days after muscles begin to be utilized. The final mutation, upheldwhu, reduces the diameter of the myofibril lattice by approximately one-half. We propose hypotheses to explain how each troponin-T mutation engenders the observed myofibrillar defects.

Transitions from fetal to fast troponin T isoforms are coordinated with changes in tropomyosin and alpha-actinin isoforms in developing rabbit skeletal muscle

In adult fast skeletal muscle, specific combinations of thin filament and Z-line protein isoforms are coexpressed. To determine whether the expression of these sets of proteins, designated the TnT1f, TnT2f, and TnT3f programs, is coordinated during development, we characterized the transitions in troponin T (TnT), tropomyosin (Tm), and alpha-actinin isoforms that occur in developing fetal and neonatal rabbit skeletal muscle. Two coordinated developmental transitions were identified, and a novel pattern of thin filament expression was found in fetal muscle. In fetal muscle, new TnT species--whose protein and immunochemical properties suggest that they are the products of a new TnT gene--are expressed in combination with beta 2 Tm and alpha-actinin1f/s. This pattern, which is found in both back and hindlimb muscles, is specific to fetal and early neonatal muscle. Just prior to birth, there is a transition from the fetal program to the isoforms that define the TnT3f program, TnT3f, and alpha beta Tm. Like the fetal program, expression of the TnT3f program appears to be a general feature of muscle development, because it occurs in a variety of fast muscles as well as in the slow muscle soleus. The transition to adult patterns of thin filament expression begins at the end of the first postnatal week. Based on studies of erector spinae, the isoforms comprising the TnT2f program, TnT2f, alpha 2 Tm, and alpha-actinin2f, appear and increase coordinately at this time. The transitions, first to the TnT3f program, and then to adult patterns of expression indicate that synthesis of the isoforms comprising each program is coordinated during muscle specialization and throughout muscle development. In addition, these observations point to a dual role for the TnT3f program, which is the major thin filament program in some adult muscles, but appears to bridge the transition from developmentally to physiologically regulated patterns of thin filament expression during the late fetal and early neonatal development.

Changes in myofibrillar activation and troponin C Ca2+ binding associated with troponin T isoform switching in developing rabbit heart

Postnatal development of the mammalian heart is associated with changes in the population of isoforms of the thin filament proteins. We correlated the change in thin filament proteins, which occur in rabbit hearts between 5 days and 22 days of age, with changes in Ca2+ dependence of myofibrillar ATPase activity, force generation, and troponin C Ca2+ binding. The preparations derived from the 5-day-old animals exhibited a high molecular weight isoform of troponin T not found in the hearts of the 22-day-old animals. Other troponin T isoforms were also found to be present in different relative amounts. No other major differences in thin filament protein composition could be identified. Compared with the 5-day-old rabbit heart preparations, the ATPase activity of myofibrils from 22-day-old rabbit hearts exhibited a reduced Ca2+ sensitivity. The pCa50 (negative log of the half-maximal-activity free Ca2+) of the MgATPase activity was shifted by 0.15 pCa units with maturation. Maturation of the myofibrils was also associated with an increased effect of Mg2+ on pCa50. On increasing the Mg2+ from 2 to 10 mM at constant MgATP2-, the pCa50 of 5-day myofibrils was increased (shifted to the right) by 0.39 pCa units for 5-day-old rabbit hearts and 0.45 pCa units for 22-day-old rabbit hearts. Although similar changes in pCa50 of force developed by myofibrils were marginally significant, fibers from hearts of 5-day-old rabbits exhibited a greater Hill coefficient than hearts from 22-day-old rabbits (3.0 vs. 2.1). Despite the increased sensitivity of 5-day-old rabbit hearts to Ca2+, these hearts exhibited significantly less Ca2+ bound to myofibrillar troponin C than did the 22-day-old rabbit hearts. Moreover, the models that best described the Ca2+ binding data are different for the two age groups. Our data indicate that the Ca2+ activation and Ca2+ binding properties of myofibrillar troponin C are altered in developing cardiac myofibrils and that the changes in these properties may be influenced by changes in the troponin T isoforms present in the myofibril.