Amino acid sequence of rabbit cardiac troponin T

Effect of the N-terminal extension in myosin essential light chain A1 on the mechanism of actomyosin ATP hydrolysis

Myosin essential light chains A1 and A2 are identical isoforms except for an extension of ∼40 amino acids at the N terminus of A1 that binds F-actin. The extension has no bearing on the burst hydrolysis rate (M-ATP → M-ADP-Pi) as determined by chemical quench flow (100 μM isoenzyme). Whereas actomyosin-S1A2 steady state MgATPase (low ionic strength, 20 °C) is hyperbolically dependent on concentration: Vmax 7.6 s-1, Kapp 6.4 μM (F-actin) and Vmax 10.1 s-1, Kapp 5.5 μM (native thin filaments, pCa 4), the relationship for myosin-S1A1 is bimodal; an initial rise at low concentration followed by a decline to one-third the Vmax of S1A2, indicative of more than one rate-limiting step and A1-enforced flux through the slower actomyosin-limited hydrolysis pathway. In double-mixing stopped-flow with an indicator, Ca(II)-mediated activation of Pi dissociation (regulatedAM-ADP-Pi → regulatedAM-ADP + Pi) is attenuated by A1 attachment to thin filaments (pCa 4). The maximum accelerated rates of Pi dissociation are: 81 s-1 (S1A1, Kapp 8.9 μM) versus 129 s-1 (S1A2, Kapp 58 μM). To investigate apomyosin-S1-mediated activation, thin filaments (EGTA) are premixed with a given isomyosin-S1 and double-mixing is repeated with myosin-S1A1 in the first mix. Similar maximum rates of Pi dissociation are observed, 44.5 s-1 (S1A1) and 47.1 s-1 (S1A2), which are lower than for Ca(II) activation. Overall, these results biochemically demonstrate how the longer light chain A1 can contribute to slower contraction and higher force and the shorter version A2 to faster contraction and lower force, consistent with their distribution in different types of striated muscle.

Clinical Utility of Cardiac Troponin I and Cardiac Troponin T Measurements

The measurement of CK-MB remains the test of choice for confirmation or exclusion of AMI and probably will remain the test of choice for routine diagnosis in the near future. Nowadays determination of cardiac troponin T (cTnT) and cardiac troponin I (cTnI) as a method relatively expensive and time-consuming should be restricted to clinical settings that really require their high specificity.

Cardiac troponins: from myocardial infarction to chronic disease

Elucidation of the physiologically distinct subunits of troponin in 1973 greatly facilitated our understanding of cardiac contraction. Although troponins are expressed in both skeletal and cardiac muscle, there are isoforms of troponin I/T expressed selectively in the heart. By exploiting cardiac-restricted epitopes within these proteins, one of the most successful diagnostic tests to date has been developed: cardiac troponin (cTn) assays. For the past decade, cTn has been regarded as the gold-standard marker for acute myocardial necrosis: the pathological hallmark of acute myocardial infarction (AMI). Whilst cTn is the cornerstone for ruling-out AMI in patients presenting with a suspected acute coronary syndrome (ACS), elevated cTn is frequently observed in those without clinical signs indicative of AMI, often reflecting myocardial injury of 'unknown origin'. cTn is commonly elevated in acute non-ACS conditions, as well as in chronic diseases. It is unclear why these elevations occur; yet they cannot be ignored as cTn levels in chronically unwell patients are directly correlated to prognosis. Paradoxically, improvements in assay sensitivity have meant more differential diagnoses have to be considered due to decreased specificity, since cTn is now more easily detected in these non-ACS conditions. It is important to be aware cTn is highly specific for myocardial injury, which could be attributable to a myriad of underlying causes, emphasizing the notion that cTn is an organ-specific, not disease-specific biomarker. Furthermore, the ability to detect increased cTn using high-sensitivity assays following extreme exercise is disconcerting. It has been suggested troponin release can occur without cardiomyocyte necrosis, contradicting conventional dogma, emphasizing a need to understand the mechanisms of such release. This review discusses basic troponin biology, the physiology behind its detection in serum, its use in the diagnosis of AMI, and some key concepts and experimental evidence as to why cTn can be elevated in chronic diseases.

Ca2+-induced movement of tropomyosin on native cardiac thin filaments revealed by cryoelectron microscopy

Muscle contraction relies on the interaction of myosin motors with F-actin, which is regulated through a translocation of tropomyosin by the troponin complex in response to Ca²⁺ The current model of muscle regulation holds that at relaxing (low-Ca²⁺) conditions tropomyosin blocks myosin binding sites on F-actin, whereas at activating (high-Ca²⁺) conditions tropomyosin translocation only partially exposes myosin binding sites on F-actin so that binding of rigor myosin is required to fully activate the thin filament (TF). Here we used a single-particle approach to helical reconstruction of frozen hydrated native cardiac TFs under relaxing and activating conditions to reveal the azimuthal movement of the tropomyosin on the surface of the native cardiac TF upon Ca²⁺ activation. We demonstrate that at either relaxing or activating conditions tropomyosin is not constrained in one structural state, but rather is distributed between three structural positions on the surface of the TF. We show that two of these tropomyosin positions restrain actomyosin interactions, whereas in the third position, which is significantly enhanced at high Ca²⁺, tropomyosin does not block myosin binding sites on F-actin. Our data provide a structural framework for the enhanced activation of the cardiac TF over the skeletal TF by Ca²⁺ and lead to a mechanistic model for the regulation of the cardiac TF.

Cardiac Damage in Rodents after Exposure to Bis(2-chloroethoxy)methane

We report that an environmental agent, bis(2-chloroethoxy)methane (CEM), caused cardiac toxicity in male and female F344 rats and B6C3F1 mice exposed to the chemical by dermal administration at doses of 0, 50, 100, 200, 400 or 600 mg/kg 5 days a week for up to 14 weeks. Treatment-related deaths occurred in 10/10 male and 10/10 female rats at 600 mg/kg, in 2/10 female rats at 400 mg/kg, and in 3/10 female mice at 600 mg/kg. The heart lesions were more severe in rats than mice, and more severe in females than males. In rats, the no-observed-adverse-effect level (NOAEL) for the heart lesions was 200 mg/kg for males and 100 mg/kg for females; in mice, it was more than 600 mg/kg for males and 200 mg/kg for females. Multifocal, widespread vacuolization of the myocytes comprised the main morphological feature of the lesions, and only in rats was it accompanied by mononuclear cell infiltration, myocytic necrosis and atrial thrombosis. Hearts from male rats were immunohistochemically stained for troponin T (cTnT) protein. Loss of cytoplasmic cTnT correlated with histopathological damage only in the 600 mg/kg animals. CEM is metabolized to thiodiglycolic acid, a chemical that causes mitochondrial dysfunction. It is hypothesized that mitochondrial damage leads to the heart toxicity from bis(2-chloroethoxy)methane.

Prognostic value of plasma biomarkers in patients with acute coronary syndrome: a review of advances in the past decade

Acute coronary syndrome (ACS), especially myocardial infarction, commonly known as a heart attack, is a serious life-threatening cardiovascular disease. Despite dramatic therapeutic advances, there have still been more than 20% patients with ACS suffering recurrent adverse cardiovascular events 3 years after disease onset. Therefore, the aim to prevent cardiac death caused by the heart attack remains challenging. Plasma biomarkers, originally developed to complement clinical assessment and electrocardiographic examination for the diagnosis of ACS, have been reported to play important prognostic roles in predicting adverse outcomes. These biomarkers mirror different pathophysiological mechanisms in association with ACS. In this review, we focus on advances of prognostic biomarkers in the past decade for short- and long-term risk assessment and management of patients with ACS.

Structure, Expression, and Function of ICAM-5

Cell adhesion is of utmost importance in normal development and cellular functions. ICAM-5 (intercellular adhesion molecule-5, telencephalin, TLN) is a member of the ICAM family of adhesion proteins. As a novel cell adhesion molecule, ICAM-5 shares many structural similarities with the other members of IgSF, especially the ICAM subgroup; however, ICAM-5 has several unique properties compared to the other ICAMs. With its nine extracellular Ig domains, ICAM-5 is the largest member of ICAM subgroup identified so far. Therefore, it is much more complex than the other ICAMs. The expression of ICAM-5 is confined to the telencephalic neurons of the central nervous system whereas all the other ICAM members are expressed mostly by cells in the immune and blood systems. The developmental appearance of ICAM-5 parallels the time of dendritic elongation and branching, and synapse formation in the telencephalon. As a somatodendrite-specific adhesion molecule, ICAM-5 not only participates in immune-nervous system interactions, it could also participate in neuronal activity, Dendrites' targeting signals, and cognition. It would not be surprising if future investigations reveal more binding partners and other related functions of ICAM-5.

New features of troponin testing in different clinical settings

Cardiac troponin levels are routinely measured for diagnosing acute myocardial infarction. Cardiac troponin measurements also provide information concerning prognosis and the effect of early intervention in patients with acute coronary syndromes. The recent development of highly sensitive cardiac troponin assays permits detection of very low circulating levels. Use of sensitive troponin assays improves overall diagnostic accuracy in patients with suspected acute coronary syndromes, and these assays provide strong prognostic information in stable coronary artery disease and chronic heart failure. However, increased sensitivity comes with a cost of decreased specificity, and serial testing, as well as clinical context and judgment, is likely to become increasingly important in the interpretation of troponin assay results.

Mechanism of regulation of native cardiac muscle thin filaments by rigor cardiac myosin-S1 and calcium

We have studied the mechanism of activation of native cardiac thin filaments by calcium and rigor myosin. The acceleration of the rate of 2'-deoxy-3'-O-(N-methylanthraniloyl)ADP (mdADP) dissociation from cardiac myosin-S1-mdADP-P(i) and cardiac myosin-S1-mdADP by native cardiac muscle thin filaments was measured using double mixing stopped-flow fluorescence. Relative to inhibited thin filaments (no bound calcium or rigor S1), fully activated thin filaments (with both calcium and rigor-S1 bound) increase the rate of product dissociation from the physiologically important pre-power stroke myosin-mdADP-P(i) by a factor of ∼75. This can be compared with only an ∼6-fold increase in the rate of nucleotide diphosphate dissociation from nonphysiological myosin-mdADP by the fully activated thin filaments relative to the fully inhibited thin filaments. These results show that physiological levels of regulation are not only dependent on the state of the thin filament but also on the conformation of the myosin. Less than 2-fold regulation is due to a change in affinity of myosin-ADP-P(i) for thin filaments such as would be expected by a simple "steric blocking" of the myosin-binding site of the thin filament by tropomyosin. Although maximal activation requires both calcium and rigor myosin-S1 bound to the cardiac filament, association with a single ligand produces ∼70% maximal activation. This can be contrasted with skeletal thin filaments in which calcium alone only activated the rate of product dissociation ∼20% of maximum, and rigor myosin produces ∼30% maximal activation.

Myofibrillogenesis in the developing zebrafish heart: A functional study of tnnt2

Various hypotheses have been proposed to explain the molecule processes of sarcomere assembly, partially due to the lack of systematic genetic studies of sarcomeric genes in an in vivo model. Towards the goal of developing zebrafish as a vertebrate model for this purpose, we characterized myofibrillogenesis in a developing zebrafish heart and went on to examine the functions of cardiac troponin T (tnnt2). We found that sarcomere assembly in zebrafish heart was initiated from a non-striated actin filament network at the perimembrane region, whereas sarcomeric myosin is independently assembled into thick filaments of variable length before integrating into the thin filament network. Compared to Z-discs that are initially aligned to form shorter periodic dots and expanded longitudinally at a later time, M-lines assemble later and have a constant length. Depletion of full-length tnnt2 disrupted the striation of thin filaments and Z-bodies, which sequentially affects the striation of thick filaments and M-lines. Conversely, truncation of a C-terminal troponin complex-binding domain did not affect the striation of these sarcomere sub-structures, but resulted in reduced cardiomyocyte size. In summary, our data indicates that zebrafish are a valuable in vivo model for studying both myofibrillogenesis and sarcomere-based cardiac diseases.

Myocardial regulatory proteins and heart failure

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

Troponin as a marker of myocardiac damage in drug-induced cardiotoxicity

Cardiac troponins T and I (cTnT and cTnI) are becoming the serum biomarkers of choice for monitoring potential drug-induced myocardial injury in both clinical and preclinical studies. The utility of cardiac troponins has been mainly demonstrated following the administration of antineoplastic drugs and beta-sympathomimetics, although the routine use of these markers in the monitoring in patients who received anthracyclines therapy is far from settled. Unlike the previous markers, which suffered from numerous shortages, the main advantages of cardiac troponins are their high specificity and sensitivity, wide diagnostic window and the possibility to use commercially available assays in clinical settings as well as in a broad range of laboratory animals. Nevertheless, in spite of vigorous research in this area, a number of questions are still unanswered and these are discussed in this review. The main problems seem to be the lack of standardisation of variety of troponin immunoassays, the assessment of suitable cutoff for drug-induced cardiotoxicity and determination of critical diagnostic window related to the optimal timing of sample collection, which may be drug-dependent.

Review Article. Cardiac troponins and renal disease

Cardiovascular disease is the most common cause of death in patients with renal failure. Patients with renal failure are at greater risk of atypical presentations of myocardial ischaemia. Traditional markers of myocardial damage are often increased in renal failure in the absence of clinically suspect myocardial ischaemia. The cardiac troponins are specific markers of myocardial injury. Large-scale trials, excluding patients with renal disease, have shown the importance of the cardiac troponins in predicting adverse outcome and in guiding both therapy and intervention in acute coronary syndromes. Cardiac Troponin T and cardiac Troponin I are increased in patients with renal failure and this is likely to represent multifactorial pathology including cardiac dysfunction, left ventricular hypertrophy and cardiac microinfarctions. Increases in serum troponin from baseline, in patients with renal disease with acute coronary syndromes, may represent a poor prognosis. Small studies of patients with renal failure have suggested that elevation of the cardiac troponins is associated with an increased risk of cardiac death.

Cardiac troponin T isoforms affect the Ca2+ sensitivity and inhibition of force development. Insights into the role of troponin T isoforms in the heart

At least four isoforms of troponin T (TnT) exist in the human heart, and they are expressed in a developmentally regulated manner. To determine whether the different N-terminal isoforms are functionally distinct with respect to structure, Ca(2+) sensitivity, and inhibition of force development, the four known human cardiac troponin T isoforms, TnT1 (all exons present), TnT2 (missing exon 4), TnT3 (missing exon 5), and TnT4 (missing exons 4 and 5), were expressed, purified, and utilized in skinned fiber studies and in reconstituted actomyosin ATPase assays. TnT3, the adult isoform, had a slightly higher alpha-helical content than the other three isoforms. The variable region in the N terminus of cardiac TnT was found to contribute to the determination of the Ca(2+) sensitivity of force development in a charge-dependent manner; the greater the charge the higher the Ca(2+) sensitivity, and this was primarily because of exon 5. These studies also demonstrated that removal of either exon 4 or exon 5 from TnT increased the cooperativity of the pCa force relationship. Troponin complexes reconstituted with the four TnT isoforms all yielded the same maximal actin-tropomyosin-activated myosin ATPase activity. However, troponin complexes containing either TnT1 or TnT2 (both containing exon 5) had a reduced ability to inhibit this ATPase activity when compared with wild type troponin (which contains TnT3). Interestingly, fibers containing these isoforms also showed less relaxation suggesting that exon 5 of cardiac TnT affects the ability of Tn to inhibit force development and ATPase activity. These results suggest that the different N-terminal TnT isoforms would produce different functional properties in the heart that would directly affect myocardial contraction.

Cardiac troponins

Cardiac troponins I and T are proteins integral to the function of cardiac muscle. They are very sensitive markers for the detection of myocardial damage, and the ability to assay their serum levels accurately and quickly have revolutionized the concepts of minor myocardial injury and infarction. They are also powerful prognostic indicators of future adverse cardiac events. Limitations, more of troponin T than I, include decreased specificity in renal failure and skeletal muscle disease. Rapid, whole blood assays are now available that can be done at the patient's bedside. This review discusses the cardiac troponins, their biochemistry, the assays for them currently available, and their roles in the evaluation of cardiac disease in the Emergency Department (ED).

Cardiac troponin and beta-type myosin heavy chain concentrations in patients with polymyositis or dermatomyositis

Cardiac troponin T (cTnT), cardiac troponin I (cTnI), myosin heavy chains (MHC), myoglobin, creatine kinase (CK), and creatine kinase isoenzyme MB (CKMB), were measured in blood samples from 39 polymyositis (PM) or dermatomyositis (DM) patients without clinical evidence for cardiac involvement to evaluate their clinical usefulness in this patient population. MHC, myoglobin, and CKMB were frequently elevated and correlated with each other and with disease severity. Undetectable cTnI in all but one patient indicated that MHC was released from skeletal muscle, thereby providing the first laboratory evidence of frequent slow-twitch muscle fibre-necrosis in patients with PM or DM. CKMB was elevated in 51%, cTnT in 41%, and cTnI in only 2.5% of patients. cTnI did not correlate with other markers or with disease severity scores. The close correlations found between cTnT and skeletal muscle damage markers and the relationship between cTnT with disease severity without clinical evidence for myocardial damage suggest a release of cTnT from skeletal muscle. The relationship of cTnT with disease severity indicates a possible role of the marker for risk stratification. However, the prognostic values of cardiac troponins and other muscle damage markers in PM/DM patients remain to be compared in prospective outcome trials.

Clinical and Experimental Results on Cardiac Troponin Expression in Duchenne Muscular Dystrophy

Background: Because of controversial earlier studies, the purpose of this study was to provide novel experimental and additional clinical data regarding the possible reexpression of cardiac troponin T (cTnT) in regenerating skeletal muscle in Duchenne muscular dystrophy (DMD).

Methods: Plasma from 14 patients (mean age, 7.5 years; range, 5.7–19.4 years) with DMD was investigated for creatine kinase (CK), the CK MB isoenzyme (CKMB), cTnT and cardiac troponin I (cTnI), and myoglobin. cTnT concentrations were measured by an ELISA (second-generation assay; Roche) using the ES 300 Analyzer. cTnI, myoglobin, and CKMB were measured by an ELISA using the ACCESS System (Beckman Diagnostics). Troponin isoform expression was studied by Western blot analysis in remnants of skeletal muscle biopsies of three patients with DMD and in an animal model of DMD (mdx mice; n = 6).

Results: There was no relation of cTnT and cTnI to clinical evidence for cardiac failure. cTnI concentrations remained below the upper reference limit in all patients. cTnT was increased (median, 0.11 μg/L; range, 0.06–0.16 μg/L) in 50% of patients. The only significant correlation was found for CK (median, 3938 U/L; range, 2763–5030 U/L) with age (median, 7.5 years; range, 6.8–10.9 years; r = −0.762; P = 0.042). Western blot analysis of human or mouse homogenized muscle specimens showed no evidence for cardiac TnT and cTnI expression, despite strong signals for skeletal muscle troponin isoforms.

Conclusions: We found no evidence for cTnT reexpression in human early-stage DMD and in mdx mouse skeletal muscle biopsies. Discrepancies of cTnT and cTnI in plasma samples of DMD patients were found, but neither cTnT nor cTnI plasma concentrations were related with other clinical evidence for cardiac involvement.

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.

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.

Correlation between serum levels of cardiac troponin-T and the severity of the chronic cardiomyopathy induced by doxorubicin

PURPOSE

To investigate, over a wide range of cumulative doxorubicin doses, the feasibility of using serum concentrations of cardiac troponin-T (cTnT) as a biomarker for doxorubicin-induced myocardial damage.

MATERIALS AND METHODS

Groups of spontaneously hypertensive rats (SHR) were given 1 mg/kg doxorubicin weekly for 2 to 12 weeks. Cardiomyopathy scores were assessed according to the method of Billingham and serum levels of cTnT were quantified by a noncompetitive immunoassay. Myocardial localization of cTnT was studied by immunohistochemical staining and confocal microscopy.

RESULTS

Increases in serum levels of cTnT (0.03 to 0.05 ng/mL) and myocardial lesions (cardiomyopathy scores of 1 or 1.5) were found in one out of five and two out of five SHR given 2 and 4 mg/kg doxorubicin, respectively. All animals given 6 mg/kg or more of doxorubicin had increases in serum cTnT and myocardial lesions. The average cTnT levels and the cardiomyopathy scores correlated with the cumulative dose of doxorubicin (0.13 v 0.4 ng/mL cTnT and scores of 1.4 v 3.0 in SHR given 6 and 12 mg/kg doxorubicin, respectively). Decreased staining for cTnT was observed in cardiac tissue from SHR receiving cumulative doses that caused only minimal histologic alterations (scores of 1 to 1.5). Staining for cTnT decreased simultaneously with increases in the severity of the cardiomyopathy scores.

CONCLUSION

cTnT is released from doxorubicin-damaged myocytes. Measurements of serum levels of this protein seem to provide a sensitive means for assessing the early cardiotoxicity of doxorubicin.

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.

Cardiac troponin T composition in normal and regenerating human skeletal muscle

Cardiac troponin T (cTnT), measurement of which has been recommended for diagnosing myocardial infarction, was initially believed to be specific for the heart. However, recent publications have reported cTnT in sera of patients without cardiac disease; therefore, we investigated whether cTnT could be found in human skeletal muscle tissues. Using immunohistochemistry, Western blot, and quantitative cTnT ELISA, we assayed human heart (n = 3), normal human skeletal muscle (n = 6), and diseased skeletal muscle samples from patients with polymyositis (PM, n = 13) and Duchenne muscular dystrophy (DMD, n = 6). All heart specimens contained cTnT, but the expression of cTnT in normal skeletal muscle samples varied widely, ranging from no expression (quadriceps femoris) to expression by up to 20% of the muscle fibers (diaphragm). Immunohistochemistry detected cTnT in skeletal muscle of 8 of the PM patients and all of the DMD patients. Mean myofibrillar cTnT concentrations (mg/g myofibrillar protein) were: cardiac = 10.0, normal skeletal = 0.8, PM skeletal = 0.7, and DMD skeletal = 4.37, confirming the results of immunohistochemistry. Western blot analysis also confirmed the expression of cTnT in muscle from DMD patients. These findings provide evidence that cTnT is not 100% cardiac-specific but also is expressed in regenerating (PM and DMD) as well as in normal (nonregenerating) skeletal muscle.

Cardiac troponin I and troponin T: are enzymes still relevant as cardiac markers?

Creatine kinase (CK) MB and lactate dehydrogenase (LDH) isoenzyme 1 are not heart-specific. By contrast, the regulatory proteins troponin I and troponin T are expressed in three different isoforms, one for slow-twitch skeletal muscle fibers, one for fast-twitch skeletal muscle fibers, and one for cardiac muscle (cTnI, cTnT). cTnI and cTnT are usually not detectable in patients without myocardial damage, which is a prerequisite for high diagnostic performance. After acute myocardial infarction (AMI) cTnI, cTnT, and CKMB mass have a comparable early sensitivity. cTnI and cTnT usually peak in parallel except for patients without reperfusion in whom cTnI peaks about 1 day and cTnT approximately 3-4 days after onset of AMI. Both stay increased for at least 4-5 days. cTnT tends to stay increased longer than cTnI. Because the sensitivities of cTnI and cTnT for myocardial injury are comparable, their specificities are the main topic of current debate. Recent reports on mismatches of cTnI and cTnT in patients with renal failure and myopathy without other evidence for myocardial injury suggest that cTnT could be reexpressed similar to CKMB and LDH-1 in chronically damaged human skeletal muscle. In contrast to cTnT, CKMB, and LDH-1, cTnI is not expressed in skeletal muscle during fetal development. So far, an increase in cTnI has been reported only after myocardial damage. Because of currently higher costs, troponin measurement should be restricted at present to clinical settings that really require their high specificity. Based on its distinct functional association with the metabolism of acute ischemic myocardium and according to initial clinical results, glycogen phosphorylase isoenzyme BB is a promising enzyme for the early detection of ischemic myocardial damage.

Progress in myocardial damage detection: new biochemical markers for clinicians

New clinical requirements for triaging chest pain patients challenge the abilities of the current cardiac markers. Serial measurements of myoglobin, creatine kinase (CK) isoenzyme MB (CKMB) mass, or CK isoforms in emergency rooms help to rapidly rule out acute myocardial infarction (AMI). However, within the first 3 to 4 h from chest pain onset, their sensitivities are too low to contribute significantly to AMI diagnosis during this period. CKMB and lactate dehydrogenase (LDH) isoenzyme 1 are not heart-specific, which hampers reliable diagnosis in patients with concomitant skeletal muscle damage. By contrast, the regulatory proteins troponin I and troponin T are expressed in three different isoforms: one for slow-twitch skeletal muscle fibers, one for fast-twitch skeletal muscle fibers, and one for cardiac muscle (cTnI, cTnT); cardiac-specific cTnI and cTnT assays are already available for routine use. cTnT and cTnI are the most promising markers for risk stratification in patients with unstable angina pectoris. Recent reports on increased cTnT in patients with renal failure or myopathy without evidence of myocardial injury and undetectable cTnI suggest that cTnT could be reexpressed similar to CKMB and LDH-1 in chronically damaged human skeletal muscle. Therefore, cTnI is probably the most heart-specific marker. Among the recently proposed new markers for early AMI diagnosis: glycogen phosphorylase isoenzyme BB (GPBB), fatty acid binding protein, phosphoglyceric acid mutase isoenzyme MB, enolase isoenzyme alpha beta, S100a0, and annexin V, GPBB is the most promising because it increases as early as 1 to 4 h from chest pain onset and its early release appears to be essentially dependent on ischemic myocardial injury.

Distinct troponin T genes are expressed in embryonic/larval tail striated muscle and adult body wall smooth muscle of ascidian

During development of the ascidian Halocynthia roretzi, the tadpole larva hatched from the tailbud embryo metamorphoses to the sessile adult with a body wall muscle. Although the adult body wall muscle is morphologically nonsarcomeric smooth muscle, it contains troponin complex consisting of three subunits (T, I, and C) as do vertebrate striated muscles. Different from vertebrate troponins, however, the smooth muscle troponin promotes actomyosin Mg2+-ATPase activity in the presence of high concentration of Ca2+, and this promoting property is attributable to troponin T. To address whether the embryonic/larval tail striated muscle and the adult smooth muscle utilize identical or different regulatory machinery, we cloned troponin T cDNAs from each cDNA library. The embryonic and the adult troponin Ts were encoded by distinct genes and shared only <60% identity with each other. Northern blotting and whole mount in situ hybridization revealed that these isoforms were specifically expressed in the embryonic/larval tail striated muscle and the adult smooth muscle, respectively. These results may imply that these isoforms regulate actin-myosin interaction in different manners. The adult troponin T under forced expression in mouse fibroblasts was unexpectedly located in the nuclei. However, a truncated protein with a deletion including a cluster of basic amino acids colocalized with tropomyosin on actin filaments. Thus, complex formation with troponin I and C immediately after the synthesis is likely to be essential for the protein to properly localize on the thin filaments.

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.

Expression of cDNAs encoding mouse cardiac troponin T isoforms: characterization of a large sample of independent clones

We have isolated 52 mouse cardiac troponin-T-encoding cDNA clones (TnT) by specific antibody screening of a lambda ZAPII expression library. Sequencing data from the large sample of independent cDNAs demonstrated relationships among the expression of four alternatively-spliced exons of the cardiac TnT gene, producing seven classes of cDNAs encoding four protein isoforms differing in two variable regions. In the N-terminal variable region and next to the embryonic-specific exon 4, an alternatively spliced exon 3a was identified in 20% of the adult isoforms. The alternatively spliced exon 12, corresponding to a central variable region between the two functional domains of TnT, was found in approx. 79% of the 52 mouse cardiac TnT cDNAs with a single base mutation completely abolishing the splicing at an internal acceptor site. Three novel alternative splicing acceptor sites in the 5'-untranslated portion of exon 2 have been identified with different frequencies.

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)

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

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

Mutations in the genes for cardiac troponin T and alpha-tropomyosin in hypertrophic cardiomyopathy

BACKGROUND

Familial hypertrophic cardiomyopathy can be caused by mutations in the genes for beta cardiac myosin heavy chain, alpha-tropomyosin, or cardiac troponin T. It is not known how often the disease is caused by mutations in the tropomyosin and troponin genes, and the associated clinical phenotypes have not been carefully studied.

METHODS

Linkage between polymorphisms of the alpha-tropomyosin gene or the cardiac troponin T gene and hypertrophic cardiomyopathy was assessed in 27 families. In addition, 100 probands were screened for mutations in the alpha-tropomyosin gene, and 26 were screened for mutations in the cardiac troponin T gene. Life expectancy, the incidence of sudden death, and the extent of left ventricular hypertrophy were compared in patients with different mutations.

RESULTS

Genetic analyses identified only one alpha-tropomyosin mutation, identical to one previously described. Five novel mutations in cardiac troponin were identified, as well as a further example of a previously described mutation. The clinical phenotype of four troponin T mutations in seven unrelated families was similar and was characterized by a poor prognosis (life expectancy, approximately 35 years) and a high incidence of sudden death. The mean (+/- SD) maximal thickness of the left ventricular wall in subjects with cardiac troponin T mutations (16.7 +/- 5.5 mm) was significantly less than that in subjects with beta cardiac myosin heavy-chain mutations (23.7 +/- 7.7 mm, P < 0.001).

CONCLUSIONS

Mutations in alpha-tropomyosin are a rare cause of familial hypertrophic cardiomyopathy, accounting for approximately 3 percent of cases. Mutations in cardiac troponin T account for approximately 15 percent of cases of familial hypertrophic cardiomyopathy in this referral-center population. These mutations are characterized by relatively mild and sometimes subclinical hypertrophy but a high incidence of sudden death. Genetic testing may therefore be especially important in this group.

Human cardiac troponin T: cloning and expression of new isoforms in the normal and failing heart

Troponin T, like many myofibrillar proteins, exists as multiple isoforms encoded by distinct genes or generated by splicing of the same primary RNA transcript. We have previously cloned the first human cardiac troponin T (cTnT) cDNA and showed the differential expression of cTnT in cardiac and skeletal muscle during ontogenic development. In this work we located the human cTnT gene by means of fluorescent in situ hybridization to 1q32 and, by sequencing thirteen cDNAs isolated from a human fetal heart cDNA library, identified three new isoforms resulting from specific combinations of three variable regions in human cTnT cDNA. The first variable region is a 30-bp box located at the 5' end of the cDNA, which can be excised either totally or only from the first 3 bp onwards; the second is a codon which can be completely excised; and the third is a 9-bp box in the 3' half of the cDNA, which can also be excised either totally or only from the first 3 bp. The existence of the corresponding RNAs in fetal and adult ventricles was confirmed by RNase protection studies. No accumulation of the fetal isoforms was found in failing ventricles compared with controls.

Alpha-tropomyosin and cardiac troponin T mutations cause familial hypertrophic cardiomyopathy: a disease of the sarcomere

We demonstrate that missense mutations (Asp175Asn; Glu180Gly) in the alpha-tropomyosin gene cause familial hypertrophic cardiomyopathy (FHC) linked to chromosome 15q2. These findings implicated components of the troponin complex as candidate genes at other FHC loci, particularly cardiac troponin T, which was mapped in this study to chromosome 1q. Missense mutations (Ile79Asn; Arg92Gln) and a mutation in the splice donor sequence of intron 15 of the cardiac troponin T gene are also shown to cause FHC. Because alpha-tropomyosin and cardiac troponin T as well as beta myosin heavy chain mutations cause the same phenotype, we conclude that FHC is a disease of the sarcomere. Further, because the splice site mutation is predicted to function as a null allele, we suggest that abnormal stoichiometry of sarcomeric proteins can cause cardiac hypertrophy.

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.

Troponin T as a marker of ischemic myocardial injury

A study was undertaken to evaluate the clinical relevance of serum troponin T (TnT) as a marker of ischemic myocardial injury, using a new automated enzyme immunoassay. The reference range for serum TnT was established by measuring serum TnT concentrations in blood obtained from 262 healthy subjects. The serum concentration of TnT was compared to serum creatine kinase activity, creatine kinase MB (mass and activity), myoglobin concentration, and lactate dehydrogenase activity: in 77 patients with myocardial infarction (55 received thrombolytic treatment); in 32 patients with unstable angina; in 30 patients with nonischemic heart diseases; and in 40 patients with skeletal muscle injuries. Our findings showed that: a) 99% of healthy blood donors had TnT concentrations < 0.10 micrograms/L; b) the test had a high clinical efficiency in the diagnosis of acute myocardial infarction, with a sensitivity of 1.0 and a specificity of 0.88 at a decision level of 0.20 micrograms/L; c) serum TnT had a later peak value (8-38 h), but a wider diagnostic window (> 126 h) than the traditional markers considered in the study; d) serum TnT had an excellent sensitivity in the detection of microinfarctions in patients with unstable angina pectoris; e) the release patterns of serum TnT were qualitatively different in perfused versus nonperfused patients. Peak serum TnT values and time to peak values were statistically different (p = 0.0336 and p = 0.0001) in reperfused and nonreperfused AMI patients, respectively; f) a ratio of serum TnT at 16 h to serum TnT at 32 h after chest pain > 1 provided a good indication of reperfusion in thrombolytic treatment (94% efficiency).

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.

An ICAM-related neuronal glycoprotein, telencephalin, with brain segment-specific expression

Telencephalin (TLN) is a 130 kd glycoprotein expressed exclusively in neurons of the telencephalon, the most rostral brain segment. In the neurons, TLN is localized to soma-dendritic membrane but not to axonal membrane. In this study, we have cloned cDNA encoding rabbit and mouse TLN. The cDNA-derived primary structure of TLN predicts an integral membrane protein with nine tandem immunoglobulin-like domains in an extra-cellular region, a transmembrane domain, and a short cytoplasmic tail. The distal eight immunoglobulin-like domains of TLN show highest homology with the immunoglobulin-like domains of intracellular adhesion molecules (ICAMs) 1, 2, and 3/R. The structural similarity of TLN with ICAMs provides a new and strong link between immunoglobulin superfamily molecules in the nervous and immune systems. TLN is an example of a dendrite-associated cell adhesion molecule involved in the brain's segmental organization, cell-cell interactions during dendritic development, and maintenance of functional neuronal networks.

Molecular basis of cardiac troponin T isoform heterogeneity in rabbit heart

In the rabbit heart, multiple isoforms of cardiac troponin T (cTnT1 through cTnT5, from largest in size to smallest), a protein essential for calcium-regulated myofibrillar ATPase activity, have been identified, and a correlation has been found between these isoforms and myofilament sensitivity to calcium. We have sought to establish the molecular basis of this diversity. Restriction-digest analysis of genomic DNA has indicated that the rabbit cTnT gene is a single-copy gene. cTnT cDNA clones were isolated from cDNA libraries, yielding a consensus sequence for the protein. Newborn rabbit heart cDNAs, obtained using the reverse-transcriptase polymerase chain reaction (RT-PCR), were amplified using primers derived from this cDNA. Three full-length cDNAs that differed by the inclusion or exclusion of three short nucleotide sequences within the cDNAs were obtained. Amplification in the 5' half of the cDNAs confirmed that multiple cTnT products arose because of the variable inclusion of an 18- and a 30-nt sequence. The 30-nt sequence has homology with previously described alternatively spliced exons in rat and chicken cTnT, whereas the 18-nt sequence has not been described previously. RT-PCR in the 3' half of the cDNAs confirmed an additional region of heterogeneity: the presence, in part or in full, or absence of a 9-nt region, which matches the alternatively spliced exon 12 described for rat cTnT. In vitro transcription and translation of four cDNA clones containing both the 18- and 30-nt sequences, the 30-nt sequence, the 18-nt sequence, or neither generated protein isoforms that comigrated with cTnT1, cTnT2, cTnT3, and cTnT4, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning and developmental expression of human cardiac troponin T

We have isolated a full-size cDNA coding for cardiac troponin T (cTnT) from a human adult heart library, using a slow skeletal TnT probe. This cDNA detected a 1.2 kb mRNA in fetal and post-natal human heart, the amount of which increased during ontogenic development. Interestingly, a similar transcript was coexpressed in fetal skeletal muscle, together with the 0.9 kb slow skeletal muscle mRNA, and its expression was down-regulated during further development.