Molecular cloning and developmental expression of the rat cardiac-specific isoform of troponin I

Knockout of tnni1b in zebrafish causes defects in atrioventricular valve development via the inhibition of the myocardial wnt signaling pathway

The proper development of atrioventricular (AV) valves is critical for heart morphogenesis and for the formation of the cardiac conduction system. Defects in AV valve development are the most common type of congenital heart defect. Cardiac troponin I ( ctnni), a structural and regulatory protein involved in cardiac muscle contraction, is a subunit of the troponin complex, but the functions and molecular mechanisms of ctnni during early heart development remain unclear. We created a knockout zebrafish model in which troponin I type 1b ( tnni1b) ( Tnni-HC, heart and craniofacial) was deleted using the clustered regularly interspaced short palindromic repeat/clustered regularly interspaced short palindromic repeat-associated protein system. In the homozygous mutant, the embryos showed severe pericardial edema, malformation of the heart tube, reduction of heart rate without contraction and with almost no blood flow, heart cavity congestion, and lack of an endocardial ring or valve leaflet, resulting in 88.8 ± 6.0% lethality at 7 d postfertilization. Deletion of tnni1b caused the abnormal expression of several markers involved in AV valve development, including bmp4, cspg2, has2, notch1b, spp1, and Alcam. Myocardial re-expression of tnni1b in mutants partially rescued the pericardial edema phenotype and AV canal (AVC) developmental defects. We further showed that tnni1b knockout in zebrafish and ctnni knockdown in rat h9c2 myocardial cells inhibited cardiac wnt signaling and that myocardial reactivation of wnt signaling partially rescued the abnormal expression of AVC markers caused by the tnni1b deletion. Taken together, our data suggest that tnni1b plays a vital role in zebrafish AV valve development by regulating the myocardial wnt signaling pathway.-Cai, C., Sang, C., Du, J., Jia, H., Tu, J., Wan, Q., Bao, B., Xie, S., Huang, Y., Li, A., Li, J., Yang, K., Wang, S., Lu, Q. Knockout of tnni1b in zebrafish causes defects in atrioventricular valve development via the inhibition of myocardial wnt signaling pathway.

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Inhibition of G9a Histone Methyltransferase Converts Bone Marrow Mesenchymal Stem Cells to Cardiac Competent Progenitors

The G9a histone methyltransferase inhibitor BIX01294 was examined for its ability to expand the cardiac capacity of bone marrow cells. Inhibition of G9a histone methyltransferase by gene specific knockdown or BIX01294 treatment was sufficient to induce expression of precardiac markers Mesp1 and brachyury in bone marrow cells. BIX01294 treatment also allowed bone marrow mesenchymal stem cells (MSCs) to express the cardiac transcription factors Nkx2.5, GATA4, and myocardin when subsequently exposed to the cardiogenic stimulating factor Wnt11. Incubation of BIX01294-treated MSCs with cardiac conditioned media provoked formation of phase bright cells that exhibited a morphology and molecular profile resembling similar cells that normally form from cultured atrial tissue. Subsequent aggregation and differentiation of BIX01294-induced, MSC-derived phase bright cells provoked their cardiomyogenesis. This latter outcome was indicated by their widespread expression of the primary sarcomeric proteins muscle α-actinin and titin. MSC-derived cultures that were not initially treated with BIX01294 exhibited neither a commensurate burst of phase bright cells nor stimulation of sarcomeric protein expression. Collectively, these data indicate that BIX01294 has utility as a pharmacological agent that could enhance the ability of an abundant and accessible stem cell population to regenerate new myocytes for cardiac repair.

Single amino acid sequence polymorphisms in rat cardiac troponin revealed by top-down tandem mass spectrometry

Heterotrimeric cardiac troponin (cTn) is a critical component of the thin filament regulatory complex in cardiac muscle. Two of the three subunits, cTnI and cTnT, are subject to post-translational modifications such as proteolysis and phosphorylation, but linking modification patterns to function remains a major challenge. To obtain a global view of the biochemical state of cTn in native tissue, we performed high resolution top-down mass spectrometry of cTn heterotrimers from healthy adult rat hearts. cTn heterotrimers were affinity purified, desalted and then directly subjected to mass spectrometry using a 7 Tesla Thermo LTQ-FT-ICR instrument equipped with an ESI source. Molecular ions for N-terminally processed and acetylated cTnI and cTnT were readily detected as were other post-translationally modified forms of these proteins. cTnI was phosphorylated with a distribution of un-, mono- and bisphosphorylated forms of 41 +/- 3%, 46 +/- 1%, 13 +/- 3%, respectively. cTnT was predominantly monophosphorylated and partially proteolyzed at the Glu(29)-Pro(30) peptide bond. Also observed in high resolution spectra were 'shadow' peaks of similar intensity to 'parent' peaks exhibiting masses of cTnI+16 Da and cTnT+128 Da, subsequently shown by tandem mass spectrometry (MS/MS) to be single amino acid polymorphisms. Intact and protease-digested cTn subunits were fragmented by electron capture dissociation or collision activated dissociation to localize an Ala/Ser polymorphism at residue 7 of cTnI. Similar analysis of cTnT localized an additional Gln within a three residue alternative splice site beginning at residue 192. Besides being able to provide unique insights into the global state of post-translational modification of cTn subunits, high resolution top-down mass spectrometry readily revealed naturally occurring single amino acid sequence variants including a genetic polymorphism at residue 7 in cTnI, and an alternative splice isoform that affects a putative hinge region around residue 192 of cTnT, all of which co-exist within a single rat heart.

Identification of rat lung--prominent genes by a parallel DNA microarray hybridization

Background

The comparison of organ transcriptomes is an important strategy for understanding gene functions. In the present study, we attempted to identify lung-prominent genes by comparing the normal transcriptomes of rat lung, heart, kidney, liver, spleen, and brain. To increase the efficiency and reproducibility, we first developed a novel parallel hybridization system, in which 6 samples could be hybridized onto a single slide at the same time.

Results

We identified the genes prominently expressed in the lung (147) or co-expressed in lung-heart (23), lung-liver (37), lung-spleen (203), and lung-kidney (98). The known functions of the lung-prominent genes mainly fell into 5 categories: ligand binding, signal transducer, cell communication, development, and metabolism. Real-time PCR confirmed 13 lung-prominent genes, including 5 genes that have not been investigated in the lung, vitamin D-dependent calcium binding protein (Calb3), mitogen activated protein kinase 13 (Mapk13), solute carrier family 29 transporters, member 1 (Slc29a1), corticotropin releasing hormone receptor (Crhr1), and lipocalin 2 (Lcn2).

Conclusion

The lung-prominent genes identified in this study may provide an important clue for further investigation of pulmonary functions.

An R111C polymorphism in wild turkey cardiac troponin I accompanying the dilated cardiomyopathy-related abnormal splicing variant of cardiac troponin T with potentially compensatory effects

Cardiac muscle contraction is regulated by Ca(2+) through the troponin complex consisting of three subunits: troponin C (TnC), troponin T (TnT), and troponin I (TnI). We reported previously that the abnormal splicing of cardiac TnT in turkeys with dilated cardiomyopathy resulted in a greater binding affinity to TnI. In the present study, we characterized a polymorphism of cardiac TnI in the heart of wild turkeys. cDNA cloning and sequencing of the novel turkey cardiac TnI revealed a single amino acid substitution, R111C. Arg(111) in avian cardiac TnI corresponds to a Lys in mammals. This residue is conserved in cardiac and skeletal muscle TnIs across the vertebrate phylum, implying a functional importance. In the partial crystal structure of cardiac troponin, this amino acid resides in an alpha-helix that directly contacts with TnT. Structural modeling indicates that the substitution of Cys for Arg or Lys at this position would not disrupt the global structure of troponin. To evaluate the functional significance of the different size and charge between the Arg and Cys side chains, protein-binding assays using purified turkey cardiac TnI expressed in Escherichia coli were performed. The results show that the R111C substitution lowered binding affinity to TnT, which is potentially compensatory to the increased TnI-binding affinity of the cardiomyopathy-related cardiac TnT splicing variant. Therefore, the fixation of the cardiac TnI Cys(111) allele in the wild turkey population and the corresponding functional effect reflect an increased fitness value, suggesting a novel target for the treatment of TnT myopathies.

Is there a potential role for serum cardiac troponin I as a marker for myocardial dysfunction in pediatric patients receiving anthracycline-based therapy? A pilot study

Serum cardiac troponin I (cTnI) levels have been reported to have high specificity and sensitivity to acute myocardial infarction and coronary ischemic syndromes in adult patients. Our goal was to evaluate the usefulness of serum cTnI in the early diagnosis of cardiac injury from anthracyclines, and to compare these values with echocardiographic findings of cardiac dysfunction. In this prospective study, children being treated on several Children's Cancer Group protocols underwent measurement of shortening fraction (SF), ejection fraction (EF), and serum cTnI levels prior to anthracycline therapy. Sequential serum cTnI levels were then measured along with regularly scheduled echocardiograms with progressively increasing doses of anthracyclines. Fifteen children with median age of 5.75 years (range, 15 months to 15.5 years) at diagnosis were evaluated. Anthracycline doses ranged from 11.72 mg/kg (in patients < 3 years of age) to 375 mg/m2. All but one patient had normal cTnI levels. His level measured at 1.7 ng/ml after 315 mg/m2, but was normal on follow-up testing. Initial SF ranged from 32 to 48%, and EF from 60 to 80%. On follow-up, SF and EF ranged from 30 to 41% and 55 to 70%, respectively. Both SF and EF were significantly lower (p < 0.001) as compared to the initial values. Despite this, all patients remained clinically asymptomatic from the cardiac standpoint. We did not observe elevations of serum cTnI levels in clinically asymptomatic children who received anthracycline therapy up to doses of 375 mg/m2. Does this mean that cardiac injury has not occurred? The possibility of assay sensitivity and the timing of serum sampling and echocardiograms may be important. In addition, larger sample size or longer follow-up may be helpful to determine if higher doses or symptomatic patients potentially have elevations in cTnI levels.

A proteolytic NH2-terminal truncation of cardiac troponin I that is up-regulated in simulated microgravity

In a tail suspension rat model, we investigated changes in myofilament protein during cardiac adaptation in simulated microgravity. Contractile force and velocity of cardiac muscle were decreased in the tail suspension rats as compared with the control. Ca(2+)-dependent actomyosin ATPase activity was also decreased; however, sensitivity of cardiac muscle to Ca(2+) activation was unchanged. There was no change in expression of myosin heavy chain, tropomyosin, troponin T, or troponin I isoforms in hearts of tail suspension rats. A novel finding is a fragment of cardiac troponin I (cTnI) that had increased amounts in the heart of tail suspension rats. Binding of this cTnI fragment by a monoclonal antibody that specifically recognizes the COOH terminus indicates an intact COOH terminus. NH(2)-terminal sequence analysis of the cTnI fragment revealed truncations primarily of amino acids 1-26 and 1-27 and smaller amounts of 1-30, including Ser(23) and Ser(24), which are substrates of protein kinase A phosphorylation. This cTnI fragment is present in normal cardiac muscle and incorporated into myofibrils, indicating a role in regulating contractility. This proteolytic modification of cTnI up-regulated during simulated microgravity suggests a potential role of the NH(2)-terminal segment of cTnI in functional adaptations of cardiac muscle.

The highly conserved COOH terminus of troponin I forms a Ca2+-modulated allosteric domain in the troponin complex

The primary structure of the COOH-terminal region of troponin I (TnI) is highly conserved among the cardiac, slow, and fast skeletal muscle TnI isoforms and across species. Although no binding site for the other thin filament proteins is found at the COOH terminus of TnI, truncations of the last 19-23 amino acid residues reduce the activity of TnI in the inhibition of actomyosin ATPase and result in cardiac muscle malfunction. We have developed a specific monoclonal antibody (mAb), TnI-1, against the conserved COOH terminus of TnI. Using this mAb, isolation of the troponin complex by immunoaffinity chromatography from muscle homogenate and immunofluorescence microscopic staining of myofibrils indicate that the COOH terminus of TnI forms an exposed structure in the muscle thin filament. Binding of this mAb to the COOH terminus of cardiac TnI induced extensive conformational changes in the protein, suggesting an allosteric role of this region in the functional integrity of troponin. In the absence of Ca2+, the binding of troponin C and troponin T to TnI had very little effect on the conformation of the COOH terminus of TnI as indicated by the unaffected mAb affinity for the TnI-1 epitope. However, Ca2+ significantly increased the accessibility of the TnI-1 epitope on TnI in the presence of troponin C and troponin T. The results provide evidence that the COOH terminus is an essential structure in TnI and participates in the allosteric switch during Ca2+ activation of contraction.

Cardiac troponin I in acute coronary ischemic syndromes. Epidemiological and clinical correlates

The present study was aimed to investigate the variability of cardiac troponin I (cTnI) in the first week of acute myocardial infarction (AMI) course with regard to some epidemiological and clinical parameters and in patients with non-AMI acute coronary ischemic disease. Serum cTnI was assayed in 82 patients, 42 affected with AMI and 40 with non-AMI acute coronary ischemic disease, on admission in coronary care unit, within 6 h after the onset of symptoms, and, in AMI group, on 24 and 48 h and 7th day of illness course. cTnI is increased within the first 6 h, remaining above normal until 7th day. However, some distinctive features in the subgroups scheduled for this study are present. (1) The mean values of cTnI in AMI patients who died, >60 years old and with anterolateral necrosis are constantly higher than in survivors, <60 years old and with inferoposterior necrosis, respectively. (2) The cTnI concentration is already returned in normal range at 7th day of illness course in survivors and in patients with inferoposterior AMI. (3) The 24-h peak level of cTnI is significantly higher in fibrinolysed than in patients who didn't undergo fibrinolysis. (4) A direct correlation between the cTnI value and the Killip class is present either in the whole group or in any subset of patients and the progressive decrease of the cTnI concentration along the AMI course doesn't occur in Killip>2 group. (5) cTnI is higher in unstable than in stable anginous patients and normal subjects but not in stable angina with respect to healthy controls.

CONCLUSIONS

(1, 2) The less increase and the early return in normal range of cTnI serum levels which occur in AMI subgroups with a better prognosis could be regarded as favourable prognostic signs. (3) The persistent higher values of cTnI in fibrinolysed subjects being associated with the angiographic finding of patent coronary arteries, it can be suggested that the large and persistent relase of cTnI from myocardium represents a reliable biochemical marker following the wash-out associated to a successful reperfusion. (4) The persistent increase of cTnI in AMI patients with advanced Killip class suggests that the high cTnI values are not only a strong index of myocardial necrosis but also of ongoing myocyte injury and hemodynamic impairment predictive of poor outcome. (5) The hypothesis can be reasonably advanced that the higher values of cTnI in unstable angina are due to focal areas of myocardial necrosis undetectable by the conventional serum markers or to a clinically silent AMI occurred in the week or so before in-hospital admission.

Detection of myocardial injury during transvenous implantation of automatic cardioverter-defibrillators

OBJECTIVES

The present study was designed to assess the extent of myocardial injury in patients undergoing transvenous implantation of an automatic implantable cardioverter-defibrillator (ICD) using cardiac troponin I (cTNI), which is a highly specific marker of structural cardiac injury.

BACKGROUND

During ICD implantation, repetitive induction and termination of ventricular fibrillation (VF) via endocardial direct current shocks is required to demonstrate the correct function of the device. Transthoracic electrical shocks can cause myocardial cell injury.

METHODS

Measurements of total creatine kinase (CK), CK-MB, myoglobin, cardiac troponin T (cTNT) and cTNI were obtained before and after ICD implantation in 49 consecutive patients. Blood samples were drawn before and 2, 4, 8, and 24 h after implantation.

RESULTS

Elevations of CK, CK-MB, myoglobin, cTNT and cTNI above cut-off level were found in 25%, 6%, 76%, 37% and 14% of patients, respectively, with peak cTNI concentrations ranging from 1.7 to 5.5 ng/ml. Cumulative defibrillation energy (DFE), mean DFE, cumulative VF time, number of shocks as well as prior myocardial infarction (MI) were found to be significantly related to a rise of cTNI. Mean DFE > or = 18 J and a recent MI were identified as strong risk factors for cTNI rise.

CONCLUSIONS

During transvenous ICD implantation myocardial injury as assessed by cTNI rise occurs in about 14% of the patients. Peak cTNI concentrations are only minimally elevated reflecting subtle myocardial cell damage. Patients with a recent MI and a mean DFE > or = 18 J seem to be prone to cTNI rise.

Cardiac troponin I and Q-wave perioperative myocardial infarction after coronary artery bypass surgery

OBJECTIVE

To monitor cardiac troponin I (cTnI), a newly developed biochemical index for cardiac damage, in patients during and after coronary artery bypass surgery (CABS) to determine whether the measurement of the serum levels of this marker could be of value in formulating an early diagnosis of Q-wave perioperative myocardial infarction (PMI).

DESIGN

Prospective study with sequential measurements of biological markers in a selected surgical patient group.

SETTING

University research laboratory and general university hospital (Cardiac Surgery Unit and Anesthesiology and Reanimation Unit).

PATIENTS

Forty-two patients undergoing elective CABS without concomitant valvular replacement.

INTERVENTIONS

There were no interventions required for this study. However, patients entered into the study had CABS, sequential arterial blood samples, ECG recordings, and echocardiograms performed.

MEASUREMENTS AND MAIN RESULTS

Pre-, intra-, and postoperative (up to 48 hrs) measurements of cardiac troponin I, MB-CK, and total creatine kinase, as well as serial electrocardiograms and echocardiograms. Perioperative infarction was assessed as the development of new persistent regional wall motion abnormalities in echocardiography together with electrocardiographic alterations and MB-CK increases. Eight patients had Q-wave PMI. All PMI patients had elevated peak cTnI values (all >9.2 ng/mL), whereas the 34 nonPMI patients had peak values <9.0 ng/mL; therefore, sensitivity and specificity (with a 9.0 ng/mL cut-off value) are 100%. MB-CK measurement peak values did not demonstrate such a high specificity and sensitivity.

CONCLUSIONS

Because of its high specificity and sensitivity, serial measurements of cTnI provide a rapid and accurate method for confirming or excluding the diagnosis of perioperative myocardial injury. cTnI evaluation can therefore be used both as an independent prognostic marker for patients undergoing cardiac surgery and as a powerful tool for detecting smaller PMIs often missed with standard PMI diagnostic criteria.

Cardiac troponin I as a predictor of major cardiac events in emergency department patients with acute chest pain

OBJECTIVES

We sought to evaluate the diagnostic and prognostic value of cardiac troponin I (cTnI) in emergency department (ED) patients with chest pain.

BACKGROUND

Although cTnI has been shown to correlate with an increased risk for complications in patients with unstable angina, the prognostic significance of this assay in the heterogeneous population of patients who present to the ED with chest pain is unclear.

METHODS

cTnI and creatine kinase-MB fraction (CK-MB) mass concentration were collected serially during the first 48 h from onset of symptoms in 1,047 patients > or =30 years old admitted for acute chest pain. Sensitivity, specificity and receiver operating characteristic curves were calculated for cTnI and CK-MB collected in the first 24 h.

RESULTS

The sensitivity, specificity and positive predictive value of cTnI for major cardiac events were 47%, 80% and 19%, respectively. Among patients were who ruled out for myocardial infarction, cTnI was elevated in 26% who had major cardiac complications compared with 5% for CK-MB; the positive predictive value for an abnormal cTnI result was 8%. Elevated cTnI in the presence of ischemia on the electrocardiogram was associated with an adjusted odds ratio of 1.8 (95% confidence interval 1.1 to 2.9) for major cardiac events within 72 h. Among patients without a myocardial infarction or unstable angina, cTnI was not an independent correlate of complications.

CONCLUSIONS

In patients presenting to the ED with acute chest pain, cTnI was an independent predictor of major cardiac events, However, the positive predictive value of an abnormal assay result was not high in this heterogeneous cohort.

Postnatal changes in contractile time parameters, calcium regulatory proteins, and phosphatases

Compared with isolated electrically driven neonatal ventricular preparations, the total time of contraction, the time to peak tension, and the time of relaxation were decreased to approximately 50% in adult ventricular preparations. The expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) was increased to 133% at the protein level and to 154% at the mRNA level in adult vs. neonatal ventricular preparations, whereas phospholamban was unchanged at both the protein and mRNA levels. Moreover, Ca2+ uptake was increased to 180% in adult vs. neonatal ventricular preparations. Phospholamban phosphorylation was enhanced in adult vs. neonatal ventricular preparations. In adult ventricular preparations, phosphatase activity was reduced to 53% of neonatal preparations, the protein levels of the immunologically detectable catalytic subunits of protein phosphatase types 1 and 2A were reduced to 28 and 61% of neonatal preparations, respectively, and the mRNA levels of type 1alpha, 1beta, 1gamma, 2Aalpha, and 2Abeta phosphatase isoforms were decreased to 69, 68, 54, 67, and 63%, respectively. We conclude that in the adult rat heart, the shortened time parameters of contraction can be explained by an elevated expression of SERCA. In addition, an increased phosphorylation state of phospholamban due to reduced phosphatase activity may be involved.

Regulation of the rat cardiac troponin I gene by the transcription factor GATA-4

Troponin I is a thin-filament contractile protein expressed in striated muscle. There are three known troponin I genes which are expressed in a muscle-fibre-type-specific manner in mature animals. Although the slow skeletal troponin I isoform is expressed in fetal and neonatal heart, the cardiac isoform is restricted in its expression to the myocardium at all developmental stages. To study the regulation of this cardiac-specific and developmentally regulated gene in vitro, the rat cardiac troponin I gene was cloned. Transient transfection assays were performed with troponin I-luciferase fusion plasmids to characterize the regulatory regions of the gene. Proximal regions of the upstream sequence were sufficient to support high levels of expression of the reporter gene in cardiocytes and relatively low levels in other cell types. The highest luciferase activity in the cardiocytes was noted with a plasmid that included the region spanning -896 to +45 of the troponin I genomic sequence. Co-transfection of GATA-4, a recently identified cardiac transcription factor, with troponin I-luciferase constructs permitted high levels of luciferase expression in non-cardiac cells. Electrophoretic mobility-shift assays demonstrated specific binding of GATA-4 to oligonucleotides representative of multiple sites of the troponin I sequence. Mutation of a proximal GATA-4 DNA-binding site decreased transcriptional activation in transfected cardiocytes. These results indicate that the proximal cardiac troponin I sequence is sufficient to support high levels of cardiac-specific gene expression and that the GATA-4 transcription factor regulates troponin I-luciferase expression in vitro.

Cardiac-like musculature of the intrapulmonary venous wall of the long-clawed shrew (Sorex unguiculatus), common tree shrew (Tupaia glis) and common marmoset (Callithrix jacchus)

BACKGROUND

The cardiac-like musculature is distributed not only in the heart wall but also in the intrapulmonary venous wall in a few species of insectivores. It has been suggested that the evolutionary origin of venous cardiac-like musculature may be traceable to a basic stock of certain mammalian lines of descent. So, it is important to clarify whether the musculature may be a common structure in lower mammals from insectivores to primates and to examine the functional significance of the structure.

METHODS

The distribution of cardiac-like musculature in the intrapulmonary venous wall of the long-clawed shrew (Sorex unguiculatus), common tree shrew (Tupaia glis), and common marmoset (Callithrix jacchus) was observed by light and electron microscopy. The presence of atrial natriuretic polypeptide (ANP) was examined in the musculature by immunohistochemistry.

RESULTS

All three species contained cardiac-like myocytes in the tunica media of intrapulmonary venous wall. In the common tree shrew and the common marmoset, cardiac-like musculature was found in the small intrapulmonary vein of 150-200 microns in diameter, while, in the long-clawed shrew, it was distributed even in the intrapulmonary vein of 30 microns in diameter. Ultrastructure of myocytes was fundamentally similar to that of atrial myocytes in the long-clawed shrew and the common tree shrew. The presence of atrial natriuretic polypeptide (ANP) was also demonstrated by immunohistochemistry in the intrapulmonary venous walls of common tree shrews.

CONCLUSIONS

The results indicate that the pulmonary venous cardiac-like musculature occurs in the lower eutherian mammals from insectivores to primitive primates. The musculature is thought to act as a regulatory pump in pulmonary circulation and as an endocrine apparatus of ANP.

Strong evolutionary conservation of broadly expressed protein isoforms in the troponin I gene family and other vertebrate gene families

It is well established that different protein classes undergo molecular evolution at different rates, presumably reflecting differing functional constraints. However, it is also the case that different isoforms of the "same" protein, encoded by a multigene family, may evolve at different rates. Here I report a relationship within gene families between isoform evolutionary rate and gene expression profile: Broadly expressed isoforms show stronger sequence conservation than do narrowly expressed isoforms. This observation emerged initially from cDNA cloning and sequencing studies, described here, of a vertebrate gene family encoding three differentially expressed isoforms of the muscle protein troponin I. However, the expression breadth/sequence conservation relationship applies to vertebrate gene families in general. In a broad and arbitrary survey sampling of sequence data on well-characterized vertebrate gene families, I found that in 14/15 families the most strongly conserved isoform was the most broadly expressed isoform, or one of several similarly broadly expressed isoforms. Broadly expressed isoforms are presumably subjected to greater negative selection pressure because they must function in a more diverse biochemical environment than do narrowly expressed isoforms. The expression breadth/evolutionary rate relationship has several interesting implications regarding the overall process of gene family evolution by duplication/divergence from ancestral genes.

Improved detection of cardiac contusion with cardiac troponin I

Detecting cardiac injury in patients with chest trauma is difficult because the level of the MB isoenzyme of creatine kinase (MBCK) can be elevated from skeletal muscle injury alone. However, the level of cardiac troponin I (cTnl) is not elevated by skeletal muscle injury. To determine whether its measurement would improve the ability to detect cardiac injury in patients with blunt chest trauma, 44 patients were studied. Serial echocardiograms and serial blood samples were obtained. Six patients had evidence of cardiac injury by echocardiography; all had elevations of MBCK and cTnl. One patient had elevations of both MBCK and cTnl with only a pericardial effusion. Twenty-six of the 37 patients without contusion had elevations of MBCK; none had elevations of cTnl. The ratio of MBCK to total creatine kinase improved specificity at the expense of sensitivity. Measurement of cTnl accurately detects cardiac injury in patients with blunt chest trauma and should facilitate the diagnosis and management of such patients.

On the ontogeny of cardiac gene transcripts

As a prerequisite to investigating the specification and differentiation of cardiac tissue in vitro, the ontogeny of a number of putative cardiac-specific, and striated muscle-specific gene transcripts has been studied. The probes used include cDNAs of alpha-actins, myosin heavy chains, myosin light chains, alpha-tropomyosin, troponin-T and atrial natriuretic factor. The expression of these genes was monitored by Northern analysis of heart and various other tissues at three developmental ages, viz, adult, neonatal and mid-foetal. The aim of this exercise was to confirm the efficacy of a number of markers to represent a cardiac-specific subset of gene expression in our mammalian model, the guinea pig. Our results indicate predominantly cardiac expression for the mRNA transcripts of cardiac alpha-actin (c alpha-actin), cardiac myosin heavy chain-alpha (MHC alpha), cardiac myosin heavy chain-beta (MHC beta), myosin light chain-1A (MLC1A), myosin light chain-1V (MLC1V), alpha-tropomyosin (alpha TM), cardiac troponin-T (cTnT) and atrial natriuretic factor (ANF). Furthermore, cardiac-specific expression at the midfoetal time point was observed for five gene transcripts, MLC1V, MHC alpha, MHC beta, striated alpha TM and ANF. No genes were expressed exclusively in cardiac tissue; for example, expression of the genes for c alpha-actin, both cardiac MHCs, both MLCs, alpha TM and cTnT was evident in skeletal and vascular smooth muscles at some stages of development. An interesting difference between this species and those of previous studies was the minor contribution of skeletal alpha-actin to cardiac phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Diagnosis of perioperative myocardial infarction with measurement of cardiac troponin I

BACKGROUND

Perioperative myocardial infarction is the most common cause of morbidity and mortality in patients who have had noncardiac surgery, but its diagnosis can be difficult. The present study was designed to determine whether the measurement of serum levels of cardiac troponin I, a highly sensitive and specific marker for cardiac injury, would help establish the diagnosis of myocardial infarction.

METHODS

We obtained preoperative measurements of MB creatine kinase, total creatine kinase, and cardiac troponin I, in addition to base-line electrocardiograms and two-dimensional echocardiograms, in 96 patients undergoing vascular surgery and 12 undergoing spinal surgery. Blood samples were obtained every 6 hours for at least the first 36 hours after surgery, and electrocardiograms were obtained daily; a second echocardiogram was obtained approximately three days after surgery. The appearance of a new abnormality in segmental-wall motion on the postoperative echocardiogram (that is, an abnormality that had not been seen on the preoperative echocardiogram) was considered to be indicative of perioperative infarction.

RESULTS

Eight patients who underwent vascular surgery had new abnormalities in segmental-wall motion and received a diagnosis of perioperative infarction. All eight had elevations of cardiac troponin I, and six had elevations of MB creatine kinase. Of the 100 patients without perioperative infarction detected by echocardiography, 19 had elevations of MB creatine kinase, and 1 had a slight elevation of cardiac troponin I.

CONCLUSIONS

The measurement of cardiac troponin I is a sensitive and specific method for the diagnosis of perioperative myocardial infarction. It avoids the high incidence of false diagnoses associated with the use of MB creatine kinase as a diagnostic marker.

Targeting gene expression to specific cardiovascular cell types in transgenic mice

Transgenic techniques, which allow the introduction of exogenous genes into the genome of experimental animals, promise to bridge the gap between the in vitro observations made by molecular and cellular biologists on cardiac and vascular cells in tissue culture and the physiology and pathology of the whole organ system. One such application of these techniques is tissue targeting: by genetic manipulation to direct expression of a protein--such as a signaling peptide, a growth factor receptor, or an oncogene involved in cell growth--to a tissue where it normally would not be expressed (or where expression is tightly controlled) by fusing it to the transcriptional control sequences of another gene normally expressed in that tissue. In the cardiovascular system, regulatory sequences for cardiomyocyte-specific proteins, vascular endothelium-specific proteins, and smooth muscle-specific proteins can be used to target heterologous genes to their respective tissues in transgenic animals. The effects that such perturbations have on organ physiology and intracellular and intercellular communication can be observed by applying established physiological and molecular approaches. In this review, we highlight some tissue-specific genes from cardiac and vascular cell types whose regulatory sequences may be used to target heterologous proteins; we discuss neutral "reporter" proteins and signal transduction components as paradigms for the application of this technique; and we briefly touch on the potentials and pitfalls of transgenic approaches to molecular physiology.

Cardiac troponin I. A marker with high specificity for cardiac injury

BACKGROUND

Levels of MBCK can be increased in patients with skeletal muscle injury or renal failure in the absence of myocardial injury, causing diagnostic confusion. This study was designed to determine whether measurement of cardiac troponin I (cTnI), a myocardial regulatory protein with comparable sensitivity to MBCK, has sufficient specificity to clarify the etiology of MBCK elevations in patients with acute or chronic skeletal muscle disease or renal failure.

METHODS AND RESULTS

Of the patients (n = 215) studied, 37 had acute skeletal muscle injury, 10 had chronic muscle disease, nine were marathon runners, and 159 were chronic dialysis patients. Patients were evaluated clinically, by ECG, and by two-dimensional echocardiography. Total creatine kinase (normal, < 170 IU/L) was determined spectrophotometrically, and cTnI (normal, < 3.1 ng/mL) and MBCK (normal, < 6.7 ng/mL) were determined with specific monoclonal antibodies. Values above the upper reference limit were considered "elevated." Elevations of total creatine kinase were common, and elevations of MBCK occurred in 59% of patients with acute muscle injury, 78% of patients with chronic muscle disease and marathon runners, and 3.8% of patients with chronic renal failure. Some of the patients were critically ill; five patients were found to have had myocardial infarctions and one had a myocardial contusion. cTnI was elevated only in these patients.

CONCLUSIONS

Elevations of cTnI are highly specific for myocardial injury. Use of cTnI should facilitate distinguishing whether elevations of MBCK are due to myocardial or skeletal muscle injury.

Troponin I gene expression during human cardiac development and in end-stage heart failure

Recent reports have demonstrated the presence of two isoforms of troponin I in the human fetal heart, namely, cardiac troponin I and slow skeletal muscle troponin I. Structural and physiological considerations indicate that these isoforms would confer differing contractile properties on the myocardium, particularly on the phosphorylation-mediated regulation of contractility by adrenergic agonists. We have investigated the developmental expression of these isoforms in the human heart from 9 weeks of gestation to 9 months of postnatal life, using Western blots revealed with troponin I antibodies to detect troponin protein isoforms and Northern blots to detect the corresponding mRNAs. The results show the following: 1) Slow skeletal muscle troponin I is the predominant isoform throughout fetal life. 2) After birth, the slow skeletal isoform is lost, with cardiac troponin I being the only isoform detectable by 9 months of postnatal development. 3) The protein isoforms and their corresponding mRNAs follow the same pattern of accumulation, suggesting that the transition in troponin expression is regulated at the level of gene transcription. The developmental transition in troponin I isoform content has implications for contractility of the fetal and postnatal myocardium. We further analyzed right and left ventricular muscle samples from 17 hearts in end-stage heart failure resulting from pulmonary hypertension, ischemic heart disease, or dilated cardiomyopathy. Cardiac troponin I mRNA remained abundant in each case, and slow skeletal muscle troponin I mRNA was not detectable in any of sample. We conclude that alterations in troponin I isoform content do not therefore contribute to the altered contractile characteristics of the adult failing ventricle.

Ordered phosphorylation of a duplicated minimal recognition motif for cAMP-dependent protein kinase present in cardiac troponin I

Cardiac troponin I contains two adjacent serines in sequence after three arginine residues thus making up a minimally duplicated recognition motif for cAMP-dependent protein kinase. In a synthetic peptide, PVRRRSSANY, the two serine residues are phosphorylated sequentially with the intermediate formation of a monophosphorylated species according to the following reaction sequence: Peptide k1----Peptide-P k2----Peptide-P2. The calculated rat constants are: k1 = 0.435.min-1 and k2 = 0.034.min-1. Sequence analyses of the monophosphopeptide and its tryptic fragments show that the predominant monophosphoform carries phosphate at the second serine.

Identification and functional significance of troponin I isoforms in neonatal rat heart myofibrils

We investigated the mechanism(s) responsible for differences in the effects of acidic pH on Ca2+ activation of the activity of adult and neonatal rat heart myofilaments. Studies on preparations of myofilaments reconstituted with adult troponin-tropomyosin (Tn-Tm) and either adult or neonatal thick filaments indicated that the difference in effect of acidic pH is related to differences in Tn-Tm and not other myofilament proteins. Immunoblotting analysis showed that development of the rat heart myofibrils is associated with isoform switching from slow skeletal TnI to cardiac TnI and from a slow mobility isoform of TnT (TnT1) to a faster Mr isoform (TnT2. Expression of slow skeletal TnI was associated with a relative insensitivity of myofilament Ca2+ activation to deactivation by acidic pH. Moreover, the effect of acidic pH on Ca2+ activation of ATPase activity of soleus myofibrils, which contain cardiac TnC and slow skeletal TnI, was essentially the same as the effect of acidic pH on rat cardiac myofibrils in the early neonatal period. Neonatal myofilaments also contained a relative abundance of a set of polypeptides copurifying with the thin filaments. We have identified these proteins as histones. The relative amount of histones among a variety of preparations from different species was not correlated with the pH sensitivity of myofibrillar Ca2+ activation. Shifts in TnT isoforms among these species were also not correlated with an altered response to acidic pH. Our data provide evidence in support of the hypothesis that the relative insensitivity of neonatal myofilament activity to acidic pH is due to the presence of slow skeletal TnI in the thin-filament regulatory complex.