The localization of the different forms of troponin I in skeletal and cardiac muscle cells

Diagnostic Role and Methods of Detection of Cardiac Troponins: An Opinion from Historical and Current Points of View

The laboratory methods for the determination of cardiac troponins (cTnI, cTnT) used nowadays are extremely diverse, which has a significant impact on our understanding of the biology and diagnostic the value of cTnI and cTnT as biomarkers. The main classification of methods for the determination of cTnI and cTnT is based on the sensitivity of the immunoassay. Low- and moderately sensitive detection methods are known to be relatively less sensitive, which leads to a relatively late confirmation of cardiomyocyte death. Due to the new highly sensitive methods used to determine cTnI and cTnT, designated as a highly or ultrasensitive immunoassays (hs-TnT and hs-TnT), we received new, revised data about the biology of cardiac troponin molecules. In particular, it became clear that they can be considered products of normal myocardium metabolism since hs-TnT and hs-TnT are detected in almost all healthy patients. It also turned out that hs-TnT and hs-TnT differ by gender (in men, troponin concentration in the blood is higher than in women), age (in elderly patients, the levels of troponins are higher than in young ones) and circadian cycles (morning concentrations of troponins are higher than in the evening). A large variety of methods for determining cTnI and cTnT, differing in their diagnostic capabilities, creates the need for tests to perform an unbiased assessment of the analytical characteristics of each method. This review focuses on the most pressing issues related to the discussion of the biological characteristics of cardiac troponin and the analytical characteristics of troponin immunoassays from a historical and contemporary point of view.

Cardiac Troponins: Contemporary Biological Data and New Methods of Determination

Laboratory diagnosis plays one of the key roles in the diagnosis of many diseases, including cardiovascular diseases (CVD). The methods underlying the in vitro study of many CVD biomarkers, including cardiac troponins (cTnI and cTnT), are imperfect and are continually being improved to enhance their analytical performance, with sensitivity and specificity being the most important. Recently developed improved cTnI and cTnT detection methods, referred to as highly sensitive methods (hs-cTnI, hs-cTnT), have changed many of our ideas about the biology of cardiac troponins and opened up a number of additional diagnostic capabilities for practical healthcare. This article systematizes some relevant data on the biology of cardiac troponins as well as on methods for determining cTnI and cTnT with an analysis of the diagnostic value of their analytical characteristics (limit of blank, limit of detection, 99th percentile, coefficient of variation, and others). Data on extracardiac expression of cTnI and cTnT, mechanisms of formation and potential clinical significance of gender, age, and circadian characteristics of hs-cTnI and hs-cTnT content in serum are discussed. Considerable attention is paid to the discussion of new diagnostic capabilities of hs-cTnI, hs-cTnT, including consideration of promising possibilities for their study in biological fluids that can be obtained by non-invasive methods. Also, some possibilities of using hs-cTnI and hs-cTnT as prognostic laboratory biomarkers in healthy people (for example, to assess the risk of developing CVD) and in patients suffering from a number of pathological conditions that cause damage to cardiomyocytes are examined, and the potential mechanisms underlying the increase in hs-cTnI and hs-cTnT are discussed.

High-sensitivity cardiac troponins: detection and central analytical characteristics

Recently, there have been some important changes in the laboratory diagnosis of patients with acute coronary syndrome, due to the introduction into routine practice of new high- and ultra-sensitive techniques for detection of myocardial damage biomarkers — cardiac troponins. Each method for cardiac troponins’ detection, among the existing wide variety of troponin immunoassays, has different analytical characteristics and allows the detection of different concentrations of troponins in the same patient. With an increasing number of companies developing high-sensitivity troponin immunoassays receiving regulatory approval, there is an urgent need for independent analytical and clinical evaluation of each method. This article discusses high- and ultrasensitive techniques for detection of cardiac troponins. The modern data on biochemical and metabolic characteristics of troponins, obtained using high- and ultra-sensitive techniques, are described: sex, age, circadian features and potential for detecting troponins in other biological fluids. Considerable attention is paid to the analytical characteristics of troponin immunoassays: limit of blank, limit of detection and limit of quantitation, coefficient of variation, as well as the 99th percentile and factors influencing it.

Characterization of troponin I levels post synchronized direct current cardioversion of atrial arrhythmias in patients with and without cardiomyopathy

BACKGROUND

Cardiac-specific markers of myocardial injury, such as troponin I (TnI), are often elevated following procedures that stimulate the myocardium. This study aimed to determine the effect of synchronized direct current (DC) cardioversion of atrial arrhythmias on myocardial injury 6-h post-procedure, as measured by cardiac TnI in patients with and without cardiomyopathy.

METHODS

Seventy-three individuals (59 M:14 F) participated in this study. Inclusion criteria were subjects 18 and older undergoing DC cardioversion for an atrial arrhythmia, including elective and non-elective admissions. Exclusion criteria included MI or CABG within the past month, cardioversion for a ventricular arrhythmia, or recent shock by implantable internal cardioverter defibrillator. Patients underwent standard DC cardioversion procedure with blood work (TnI and CRP) prior to and 6-h post-cardioversion. Primary outcome was change in TnI. Secondary outcomes included changes in CRP, correlation of TnI with cumulative energy and LVM, and a sub-group analysis in patients with cardiomyopathy.

RESULTS

There was no significant change in TnI following cardioversion (20.4 ± 7.9 vs. 17.5 ± 6.5 ng/L, F(1,72) = 2.651, p = 0.108). When stratified by cardiomyopathy status, there was a statistically significant reduction in TnI following cardioversion in the non-cardiomyopathy group (6.7 ± 3.7 ng/L vs. 6.2 ± 3.2 ng/L, F(1,58) = 6.481, p = 0.014) and a clinically significant reduction in the cardiomyopathy group (74.4 ± 136.7 ng/L vs. 54.6 ± 104.3 ng/L, F(1,13) = 3.676, p = 0.07). There was no significant relationship between change in TnI and cumulative energy or LVM (r = 0.137, p = 0.306 and r = 0.125, p = 0.412 respectively).

CONCLUSIONS

Synchronized DC cardioversion of an atrial arrhythmia did not cause myocardial injury 6-h post-cardioversion. Sub-group analysis suggests that cardioversion of patients with cardiomyopathy may result in normalization of TnI levels.

Acquisition of a quantitative, stoichiometrically conserved ratiometric marker of maturation status in stem cell-derived cardiac myocytes

There is no consensus in the stem cell field as to what constitutes the mature cardiac myocyte. Thus, helping formalize a molecular signature for cardiac myocyte maturation would advance the field. In the mammalian heart, inactivation of the “fetal” TNNI gene, TNNI1 (ssTnI), together in temporal concert with its stoichiometric replacement by the adult TNNI gene product, TNNI3 (cTnI), represents a quantifiable ratiometric maturation signature. We examined the TNNI isoform transition in human induced pluripotent stem cell (iPSC) cardiac myocytes (hiPSC-CMs) and found the fetal TNNI signature, even during long-term culture. Rodent stem cell-derived and primary myocytes, however, transitioned to the adult TnI profile. Acute genetic engineering of hiPSC-CMs enabled a rapid conversion toward the mature TnI profile. While there is no single marker to denote the mature cardiac myocyte, we propose that tracking the cTnI:ssTnI protein isoform ratio provides a valuable maturation signature to quantify myocyte maturation status across laboratories.

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The TNNI gene switch is a quantitative maturation signal for hiPSC-CMs

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TnI isoform ratio is necessary, but not sufficient, to establish the mature state

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TNNI protein isoform switching is stalled in hiPSC-CMs

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Gene transfer enables acquisition of the mature TNNI signature in hiPSC-CMs

Epitope mapping and targeted quantitation of the cardiac biomarker troponin by SID-MRM mass spectrometry

The absolute quantitation of the targeted protein using MS provides a promising method to evaluate/verify biomarkers used in clinical diagnostics. In this study, a cardiac biomarker, troponin I (TnI), was used as a model protein for method development. The epitope peptide of TnI was characterized by epitope excision followed with LC/MS/MS method and acted as the surrogate peptide for the targeted protein quantitation. The MRM-based MS assay using a stable internal standard that improved the selectivity, specificity, and sensitivity of the protein quantitation. Also, plasma albumin depletion and affinity enrichment of TnI by anti-TnI mAb-coated microparticles reduced the sample complexity, enhanced the dynamic range, and further improved the detecting sensitivity of the targeted protein in the biological matrix. Therefore, quantitation of TnI, a low abundant protein in human plasma, has demonstrated the applicability of the targeted protein quantitation strategy through its epitope peptide determined by epitope mapping method.

Myo/Nog cell regulation of bone morphogenetic protein signaling in the blastocyst is essential for normal morphogenesis and striated muscle lineage specification

Cells that express MyoD mRNA, the G8 antigen and the bone morphogenetic protein (BMP) inhibitor noggin (Nog) are present in the epiblast before gastrulation. Ablation of "Myo/Nog" cells in the blastocyst results in an expansion of canonical BMP signaling and prevents the expression of noggin and follistatin before and after the onset of gastrulation. Once eliminated in the epiblast, they are neither replaced nor compensated for as development progresses. Older embryos lacking Myo/Nog cells exhibit severe axial malformations. Although Wnts and Sonic hedgehog are expressed in ablated embryos, skeletal muscle progenitors expressing Pax3 are missing in the somites. Pax3+ cells do emerge adjacent to Wnt3a+ cells in vitro; however, few undergo skeletal myogenesis. Ablation of Myo/Nog cells also results in ectopically placed cardiac progenitors and cardiomyocytes in the somites. Reintroduction of Myo/Nog cells into the epiblast of ablated embryos restores normal patterns of BMP signaling, morphogenesis and skeletal myogenesis, and inhibits the expression of cardiac markers in the somites. This study demonstrates that Myo/Nog cells are essential regulators of BMP signaling in the early epiblast and are indispensable for normal morphogenesis and striated muscle lineage specification.

New potential uses for cardiac troponins

This review briefly synthesizes the molecular biology of troponin, which is currently the best biochemical marker for the detection of cardiac injury and, thus, acute myocardial infarction as well. Potential new uses for the marker based on these insights, with a specific interest in cardiac troponin fragments that potentially could be linked to distinct clinical conditions, are described. Some of the clinical problems clinicians are faced with including how to use the markers in renal failure and the difficulties associated with the heterogeneity of current troponin assays are also discussed. Finally, we present the possibility of specific cardiac troponin fragments resulting from modification or degradation, associated with distinct pathological processes, as new potential uses for this biomarker.

Use of biomarkers in the emergency department and chest pain unit

The use of biomarkers of cardiac injury in the emergency department (ED) and observation unit settings has several nuances that are different and, therefore, worthy of its own set of use guidelines. The markers that are used, however, are the same. The primary marker of choice continues to be cardiac troponin (Tn). Other markers that have been used because of the need in the ED for rapid triage have been myoglobin and fatty acid binding protein. In addition, some centers still prefer less sensitive and less specific markers such as creatine kinase myocardial band (CK-MB). More recently, a push has occurred to develop markers of ischemia, such as ischemia modified albumin (IMA),to determine which patients have ischemia, even in the absence of cardiac injury. As troponin assays become more sensitive and method for use becomes better understood, the use of these other markers are being relegated to lesser and lesser roles. Markers of ischemia are useful, but at present, despite some enthusiasm, are not ready for routine use. Before describing the recommendations for clinical use of biomarkers in the ED, a basic understanding of some of the science and measurement issues related to these analytes is helpful.

Perinuclear localization of slow troponin C m RNA in muscle cells is controlled by a cis-element located at its 3' untranslated region

The process of mRNA localization within a specific cytoplasmic region is an integral aspect of the regulation of gene expression. Furthermore, colocalization of mRNAs and their respective translation products may facilitate the proper assembly of multi-subunit complexes like the thick and thin filaments of muscle. This postulate was tested by investigating the cytoplasmic localization of three mRNAs-the alpha-actin, slow troponin C (sTnC), and slow troponin I (sTnI), which encode different poly-peptide partners of the thin filament. Using in situ hybridization we showed that all three thin filament mRNAs are localized in the perinuclear cytoplasm of cultured C2C12 muscle cells. Their localization differs from that of the nonmuscle beta-actin mRNA, which is localized in the peripheral region of both proliferating nondifferentiated myoblasts and the differentiated myocytes. Analysis of the localization signal of the sTnC mRNA showed that a 40-nucleotide-long region of the sTnC mRNA 3' UTR is sufficient to confer the perinuclear localization on a heterologous reporter beta-Gal mRNA. This localization signal showed tissue specificity and worked only in the differentiated myocytes, but not in the proliferating myoblasts or in HeLa cells. The predicted secondary structure of the localization signal suggests the presence of multiple stem and loop structures in this region of the 3' UTR. Mutations within the stem region of the localization signal, which abolish the base pairing in this region, significantly reduced its perinuclear mRNA localization activity. Using UV-induced photo-cross-linking of RNA and proteins we found that a myotube-specific 42-kDa polypeptide binds to the localization signal.

Immunocytochemical characteristics of elbow, knee and ankle muscles of the five-toed jerboa (Allactaga elater)

Biochemical adaptations of limb myofibres to intensive bipedal hopping were investigated using the five-toed jerboa Allactaga elater as a model in comparison with the rat. Immunofluorescence methods included immunoreactivity to anti-fast and anti-slow MHC and troponin I. There is no specialization of triceps caput mediale for postural function in the minute non-locomotor forelimbs, unlike quadruped mammals. The various elbow extensor heads and the flexor muscles are alike with regard to fibre type population and cross-sectional areas of each type of fibre. The extensor muscle in the elongated hindlimbs of the five-toed jerboa, at both the knee and the ankle joints, differ from each other extensively. One head, made up of an extremely high percentage of type I, fatigue-resistant fibres, is suited to postural function. Two extensor heads at each joint contain a very high percentage of type IIB fibres (having the greatest maximal velocity of contraction) and are able to produce the powerful acceleration needed to trigger the leap. The relative cross-sectional areas of the myofibres are characteristic of hopping locomotion: predominance in number of one type of myofibre in a muscle accompanies greater cross-sectional area, which increases muscle efficiency in either postural or accelerative function of the muscle.

Cardiac markers protocols in a chest pain observation unit

The use of cardiac markers to identify high-risk patients in the observation unit is undeniable. As the literature reviewed here reveals, the history and ECG miss a significant portion of patients with acute cardiac ischemia. It appears that acute MI and some high-risk "unstable angina" observation unit patients can be identified within 6 hours of hospital presentation using a combination of cardiac markers. Testing these patients soon after symptom onset or on arrival in the ED for myoglobin, CK-MB subforms, or CK-MB delta appears to provide the best diagnostic usefulness. For testing later in the clinical course, CK-MB troponin I, or troponin T are of clear diagnostic and prognostic value. The markers currently used are unable to identify the significant subset of patients with "non-AMI" coronary syndromes, however. These patients require further testing with appropriate noninvasive or invasive diagnostic studies.

Cardiac myofibrillar proteins: biochemical markers to estimate myocardial injury

Ischaemic heart disease represents the most common of the serious health problems in the contemporary society and acute myocardial infarction (AMI) is the major cause of cardiovascular morbidity and death. The accurate localization and determination of the infarct size and the volume of myocardium at risk at the time of insult is crucial and vital for the choice of treatment. Initially the ischaemic cells are reversibly injured. However, if these changes are not reverted at the earliest, it results in the death of the myocyte. This irreversible myocyte necrosis travels transmurally towards epicardium in the form of a wavefront. A timely intervention during evolving infarct could reduce and delimit the infarct and preserve the left ventricular function. Enzyme analysis and electrocardiography (ECG) along with the clinical history of the patient is still considered to constitute a reliable triad in the diagnosis of myocardial infarction (MI). Efforts have been made to relate infarct size with the serum enzyme level changes without much success. In addition, a number of specialist techniques such as planar radioisotope imaging, single photon emission computed tomography (SPECT), positron emission tomography (PET), Echocardiography, Ventriculography and nuclear magnetic resonance (NMR) imaging have been devised to support diagnosis in the patients who show ambiguous symptoms and ECG findings. However most of these procedures are unavailable to the patients due to economic reasons while others have suffered due to non-availability of ideal radiopharmaceuticals. Major advances have been made in the methods based on immunological techniques to improve the detection and estimation of infarct. These methods are exclusively based upon the production and availability of specific antibodies against intracellular, cardiac specific components.

Radiolabelled monoclonal antibodies (McAb): an alternate approach to the conventional methods for the assessment of cardiomyocyte damage in an experimental brain-death pig model

The present study was carried out to determine the possible use of cTn-I in the cardiac myofibrillar architecture, as a potential target for in vivo radioimmunodetection of cardiac damage in a brain death pig model. Radioiodination of the anti-cTn-I 5F4 McAb was carried out by lactoperoxidase method. The percentage iodine incorporation achieved was 70-75%. The radioiodinated McAbs were purified on Sephadex G-25 column and characterised by Paper chromatography, Phast Gel electrophoresis and electroimmunoblotting. Radioiodinated anti-cTn-I 5F4 McAbs were employed alongside Pyrophosphate (Tc99m-PPi) and Thallium201 chloride (Tl201) in 24 landrace pigs (brain-dead = 18 & sham-operated = 6). The percentage cardiac uptake of the radiolabelled antibody injected dose was significantly higher in the brain dead animals (0.196%) as compared to that of sham-operated animals (0.11%). Specific in vivo localization of radiolabelled McAbs in the infarcted cardiac tissue was confirmed by computer-aided reconstruction of 3-D images of the isolated heart. The preliminary results of the study revealed preferential uptake of radiolabelled antibody at the site of myocyte damage resulting from artificially induced brain death.

Immunohistochemical differentiation of fiber types in human skeletal muscle using monoclonal antibodies to slow and fast isoforms of troponin I subunit

The cDNA sequence of troponin I (TnI), one of the subunits of the skeletal muscle regulatory protein, differs between slow-twitch muscle and fast-twitch muscle. We prepared monoclonal antibodies to the slow and fast isoforms of human TnI for the purpose of differentiating muscle fiber types in human neuromuscular disorders. Slow TnI antibody was labeled with tetramethylrhodamine isothiocyanate while fast TnI antibody was labeled with fluorescein isothiocyanate; then these two antibodies were mixed. This mixture was then used to stain biopsied muscle from patients with neuromuscular disorders. It was possible to differentiate muscle fibers into slow, fast and intermediate fibers having various contents of slow and fast TnI. In tissue composed of small muscle fibers, this method facilitated differentiation of types of muscle fibers by allowing staining of only a single section. The usefulness of our technique using slow and fast TnI antibodies is discussed in comparison with ATPase staining. Because our staining method can distinguish slow and fast fiber components, it is useful for clinical application.

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.

Expression pattern of skeletal muscle troponin T isoforms is fixed in cell lineage

The expression of fast-muscle-type troponin T isoforms in chicken skeletal muscles was studied by two-dimensional SDS-polyacrylamide gel electrophoresis and immunoblotting. According to the pattern of troponin T isoform expression, chicken fast muscle was classified into two groups: One group expressed breast-fast-muscle-type troponin T in addition to leg-fast-muscle-type troponin T, the other expressed only leg-fast-muscle-type troponin T. To the former group belong breast and wing fast muscles and some of the back fast muscles, and to the latter group belong the fast muscles in leg, abdomen, and neck. Transplantation of breast muscle into leg was performed in order to change the physical environment and to investigate the mechanism of isoform expression. Histological observation of the transplant revealed severe degeneration of muscle cells, followed by differentiation of myoblasts in which breast-muscle-type troponin T was eventually expressed. The results showed that the pattern of troponin T isoform expression is primarily fixed in the cell lineage, although nerves modulate it.

Developmental expression of troponin I isoforms in fetal human heart

We have used antibodies specific for troponin I proteins to examine human cardiac development and have detected a transiently expressed developmental isoform. This isoform is distinct from adult cardiac troponin I (TnIc) but is indistinguishable, on the basis of electrophoretic mobility and antibody reactivity, from the isoform found in slow skeletal muscle (TnIs). Furthermore, we show that mRNA for TnIs is present in fetal, but not adult, heart. Analysis of a developmental series of fetal samples indicates that there is a transition in expression from TnIs to TnIc which occurs between 20 weeks fetal and 9 months postnatal development.

Inter- and intra-specific variation in myosin light chain and troponin I composition in fast muscle fibres from two species of fish (genus Oreochromis) which have different temperature-dependent contractile properties

The contractile properties and myofibrillar protein composition of fast muscle have been characterized in pure strains of two tropical fish Oreochromis niloticus and O. andersoni. Single fast muscle fibres were isolated from the abdominal myotomes and chemically skinned. The maximum tension-temperature relationships of fibres were similar at 25-30 degrees C, but diverged below 17 degrees C. At 10 degrees C, maximum tension was around 60% higher in O. andersoni (160 +/- 15 kN m-2) than O. niloticus (105 +/- 13 kN m-2) (mean +/- SD). The myofibrillar protein composition of fast fibres was investigated using one-dimensional and two-dimensional gel electrophoresis and peptide mapping. The two Oreochromis species differed with respect to the composition of myosin light chains, troponin I and myosin heavy chains (V8 protease and chymotrypsin peptide maps). An unexpected finding was the presence of two isoforms of myosin light chain 1 in O. andersoni, with apparent molecular masses of 27.5 kDa (LC1f1) and 26.9 kDa (LC1f2). Individuals with LC1f1 (n = 20) and LC1f1 + LC1f2 (n = 12) were represented in the population studied. The myosin light chain 3 (LC3f) content of fibres was similar in both cases. Breeding experiments established that these intra-specific variations in isoform composition were heritable. Fast muscle from O. niloticus and O. andersoni contain two isoforms of troponin I (TNIfl + TNIf2) which were both expressed in single fibres. The identity of TNI was confirmed using a stationary phase troponin-C affinity column. Of the 20 O. niloticus studied seven contained only TNIf1.(ABSTRACT TRUNCATED AT 250 WORDS)

Coordinate accumulation of troponin subunits in chicken breast muscle

The accumulation of troponin subunits in developing chicken breast muscle was determined by two-dimensional gel electrophoresis and an image analyzing system. Many troponin T isoforms, including those hidden behind creatine kinase, were detected on the two-dimensional pattern by the addition of 6 M urea in the second dimension. These troponin T isoforms were classified into four types by developmental order, isoelectric point, and molecular weight: leg-muscle type (L), neonatal breast-muscle type (BN), young chicken breast-muscle type (BC), and adult breast-muscle type (BA). The L-, BN-, and BC-type troponin Ts were transiently expressed at specific developmental stages. Quantitative analysis of two-dimensional patterns of troponin subunits including troponin I and troponin C showed moderate coordination in accumulation among the three subunits throughout postnatal development, when the total amount of all isoforms of troponin T was taken into account.

Identification and pattern of expression of a developmental isoform of troponin I in chicken and rat cardiac muscle

A monoclonal antibody that reacts with all known isoforms of troponin I detected a single isoform of cardiac troponin I in both atrial and ventricular chambers of adult chicken and rat hearts in an immunoblotting analysis. Another isoform of troponin I in addition to the adult cardiac form, however, was present in all chambers of the heart during early development in both species. This developmental isoform appeared to have the same electrophoretic mobility on SDS tris glycine polyacrylamide gels as that observed for the adult slow skeletal muscle isoform. In the rat, only the developmental isoform of troponin I was present in the early foetal heart and small amounts of the adult cardiac isoform were not apparent until late in gestation, whereas the developmental and adult isoforms were expressed in approximately equal amounts throughout embryonic development in the chicken. The level of developmental isoform of troponin I in both the chicken and the rat hearts gradually decreased so that only small amounts of this variant were detectable two weeks after birth or post hatching.

Acetylcholine receptors in human thymic myoid cells in situ: an immunohistological study

Myoid cells were studied by double immunofluorescence in sections of thymus from 47 patients with myasthenia gravis and 15 control subjects, using polyclonal sheep anti-troponin T and monoclonal antibodies to troponin I, striated muscle myosin, and acetylcholine receptor (AChR). The myoid cells were rare and located mainly in the medulla, and most were clearly positive for AChR; labeling was similar with four individual monoclonal antibodies specific for extrajunctional AChR and five that also recognize endplate AChR. They were mostly keratin-positive and consistently HLA-DR-negative. In the myasthenia gravis samples, the myoid cells were similar but largely confined to medullary epithelial areas; AChR labeling was slightly weaker, but otherwise they did not differ noticeably from those of control subjects. A preliminary finding was of even rarer AChR-positive/HLA-DR-positive antigen-presenting (possibly) cells seen in 9 of 9 myasthenia gravis samples and in none of 9 control samples. Although myoid-cell AChR appears antigenically similar to extrajunctional muscle AChR, and must therefore express the epitopes that myasthenics' antibodies recognize, these cells do not appear to be foci of immunological stimulation in myasthenia gravis.

Monoclonal antibody that detects human type I muscle fibres in routinely fixed wax embedded sections

A murine monoclonal antibody, F7, which selectively shows type I fibres in human skeletal muscle is reported. The antibody reacts with frozen sections and with formalin fixed wax embedded material. It should prove useful in the retrospective and prospective study of muscle pathology.

Cross reactive identification of types 1 and 2C fibers in human skeletal muscles with monoclonal anti-neurofilament (200 kd) antibody

Types 1 and 2C fibers in human skeletal muscle were cross-reactively identified with monoclonal anti-bovine neurofilament (200 kd) antibody. Thirty seven biopsy samples including sixteen vastus lateralis muscles, twelve lumbar paravertebral muscles, six gluteus medius muscles, two flexor carpi ulnaris muscles, and one flexor pollicis longus muscle, were examined. Serial transverse sections were stained histochemically with myofibrillar ATPase (pH 10.4, 4.6, 4.3) and DPNH-tetrazolium reductase reactions, and immunochemically using the avidin-biotin-peroxidase complex with the primary antibodies of monoclonal anti-bovine neurofilament (200 kd, 160 kd, 70 kd) antibodies and anti-bovine glial filament acidic protein antibody. The immunochemical reaction with anti-NF (200 kd) antibody could distinguish two kinds of fibers; positive and negative in all of the specimens. No fiber was recognized with other antibodies. Myosin ATPase reactions in serial sections proved that the positively stained fibers with anti-NF (200 kd) antibody were types 1 and 2C fibers and negative fibers types 2A and 2B fibers. At present, it is not known what substance is responsible for the cross-reaction with the monoclonal anti-NF (200 kd) antibody in types 1 and 2C fibers, but this unique antibody would be valuable in two aspects: one concerns the problem of the evolution of fiber types, and the other the utility as another supplemental method to conventional myosin ATPase scheme.

Distribution of isoforms of the myofibrillar proteins in myoid cells of thymus

Myoid cells of calf and rat thymus have been identified by staining with a monoclonal antibody to the heavy chain of myosin that is not isoform specific. Heterogeneity in the protein composition of myoid cells has been demonstrated by staining with antibodies to the skeletal muscle isoforms of the myosin heavy chain, C-protein and components of the troponin complex. The immunochemical studies suggest that the myoid cells contain proteins closely resembling if not identical with those present in the myofibrils of skeletal muscle. The slow and fast skeletal muscle isoforms of the myofibrillar proteins are present in a large proportion of the myoid cells. A fraction of the myoid cells contains only the fast isoforms of the myofibrillar proteins but there is no sharp compartmentalization of the isoforms as occurs in type 1 and type 2 fibres of skeletal muscle. In general the pattern of gene expression is similar to that of developing skeletal muscle.

Selective synthesis and degradation of slow skeletal myosin heavy chains in developing muscle fibers

During fetal development of fast skeletal muscles in the rat, three types of cells could be identified using a monoclonal antibody to slow skeletal muscle myosin heavy chain. Presumptive type I cells stained positive for slow forms of skeletal myosin heavy chain and a previously described protein of an apparent molecular weight of 100 KD, whereas presumptive type 2B cells did not stain for either of these peptides. Presumptive type 2A cells, on the other hand, did not stain for slow isoform of 100-K protein, but did stain positive for slow skeletal myosin heavy chain. There was a progressive suppression or degradation of slow skeletal myosin heavy chain in presumptive type 2A cells during subsequent fetal development, so that it was almost undetectable in most animals at birth. Soleus, a slow muscle, however, did not show clear differentiation into presumptive type I and type 2 cells until 4 days after birth.

The isoforms of C protein and their distribution in mammalian skeletal muscle

A monoclonal antibody that is specific for the slow skeletal muscle isoform of C protein of rabbit muscle has been prepared by immunizing mice with a crude preparation of human myosin. It reacted with the X protein fraction of rabbit skeletal muscle and stained all type I cells in this tissue. It also stained a fraction of the type II cells with varying intensities. The type II cells staining with antibody to slow C protein also stained with a polyclonal antibody prepared against rabbit fast muscle C protein. The type II cells not staining with antibody to slow C protein stained strongly with antibody to fast C protein. In the human skeletal muscle antibody to slow C protein stained all cells whereas antibody to fast C stained only type II cells. It is concluded that the distribution of the isoforms of C protein in adult vertebrate skeletal muscle is more complex than is the case with proteins such as components of the troponin complex.

Identification of multiple variants of fast muscle troponin T in the chicken using monoclonal antibodies

Two monoclonal antibodies, T1/7 and T1/61, have been prepared which are specific for chicken fast muscle troponin T. Both are of the IgG gamma 1 subclass. Both antibodies cross-react strongly with human fast and chicken cardiac troponin T, but while T1/7 reacts weakly with rabbit fast troponin T, T1/61 does not. The antibodies can be used for fibre typing of both chicken and human muscle. The antibodies have been used to identify fast troponin T on two-dimensional maps of proteins from a variety of chicken muscles by electrophoretic transfer to nitrocellulose followed by immunoperoxidase staining. Using this technique five variant forms of fast troponin T have been identified. Two variants, fBT1 and fBT2, are expressed in breast muscle, while the other three, fLT1, fLT2 and fLT3 are expressed in leg muscle. Of the leg muscle variants, fLT1 and fLT2 correspond to the two forms described previously [Wilkinson, J.M., Moir, A.J.G. and Waterfield, M.D. (1984) Eur. J. Biochem. 143, 47-56]. The third variant, fLT3, has not been described before and is expressed in muscles which have a high content of slow fibres. In addition to these clearly defined variant forms immunostaining reveals multiple minor variants of troponin T present in leg muscle which may reflect complex RNA processing of the troponin T gene transcript.

Initiation of differentiation into skeletal muscle fiber types

A monoclonal antibody directed against a slow isoform of a 100-K myofibrillar protein has been used to study the differentiation of muscle fibers in the fast and slow muscles of the rat. Immunohistochemical studies have shown that initiation of differentiation into fast and slow muscle fibers occurs very early during development. The distribution pattern of cell types in different muscles is unique and is also determined very early during fetal muscle development. It is concluded that motor innervation is probably not essential to initiate differentiation into distinct muscle fiber types.

Factors determining the subunit composition of tropomyosin in mammalian skeletal muscle

Adult rat fast-twitch skeletal muscle such as extensor digitorum longus contains alpha- and beta-tropomyosin subunits, as is the case in the corresponding muscles of rabbit. Adult rat soleus muscle contains beta-, gamma- and delta-tropomyosins, but no significant amounts of alpha-tropomyosin. Evidence for the presence of phosphorylated forms of at least three of the four tropomyosin subunit isoforms was obtained, particularly in developing muscle. Immediately after birth alpha- and beta-tropomyosins were the major components of skeletal muscle, in both fast-twitch and slow-twitch muscles. Differentiation into slow-twitch skeletal muscles was accompanied by a fall in the amount of alpha-tropomyosin subunit and its replacement with gamma- and delta-subunits. After denervation and during regeneration after injury, the tropomyosin composition of slow-twitch skeletal muscle changed to that associated with fast-twitch muscle. Thyroidectomy slowed down the changes in tropomyosin composition resulting from the denervation of soleus muscle. The results suggest that the 'ground state' of tropomyosin-gene expression in the skeletal muscle gives rise to alpha- and beta-tropomyosin subunits. Innervation by a 'slow-twitch' nerve is essential for the expression of the genes controlling gamma- and delta-subunits. There appears to be reciprocal relationship between expression of the gene controlling the synthesis of alpha-tropomyosin and those controlling the synthesis of gamma- and delta-tropomyosin subunits.

Comparison of the structure of two cardiac troponin T isoforms

Two isoforms of troponin T have been isolated from bovine cardiac muscle. One isoform has an Mr of 31000 and a pI at about 7.1, the corresponding values for the second isoform being 33000 and 6.5. Both isoforms have identical C- and N-terminal sequences, and, according to the data from tryptic-peptide mapping, a similar structure of the central and C-terminal domains. The large N-terminal peptides of troponin T isoforms differ in the content of glutamine/glutamic acid and alanine. It is concluded that the isoform with Mr 33000 has an additional peptide enriched with glutamic acid and alanine that is inserted between the N-terminal pentapeptide and the cysteine located 40-60 residues from the N-terminus.

Different muscle-specific forms of rabbit skeletal muscle alpha-actinin

The structures and functions of the two alpha-actinin isoforms [R. Kobayashi et al. (1983) Eur. J. Biochem. 133, 607-611] isolated from rabbit longissimus dorsi and psoas muscles were compared. One-dimensional and two-dimensional electrophoretic analyses showed that the two alpha-actinins were different from each other in their subunit chain weights and isoelectric points. The Stokes' radius of the longissimus dorsi and psoas alpha-actinins was 7.4 nm and 7.0 nm, respectively. The amino acid analyses showed that, although the two alpha-actinins are similar in their amino acid compositions, longissimus dorsi alpha-actinin contains more aspartic acid and isoleucine than psoas alpha-actinin but fewer glycine and valine residues. Analysis of the soluble tryptic peptides by two-dimensional mapping revealed that the two alpha-actinins had major differences. These data suggested that the two isoforms are the products of at least two different genes. Despite these differences, both alpha-actinins share a number of common properties. Both alpha-actinins contain a 55-kDa peptide resistant to trypsin. The two proteins show no differences in actomyosin turbidity assays. ATPase assays and F-actin binding assays of alpha-actinin activity. Immunological examination indicates that the two alpha-actinins share antigenic determinants in common.

Changes in the distribution of fast and slow forms of troponin I in some neuromuscular disorders

As judged from the atrophy of the muscle cells, the distribution of slow and fast troponin I in type I and type II cells in peripheral neuropathies in the adults (24-67 years of age) was unaffected for long periods after denervation. Reinnervation by fast or slow nerve in chronic neuropathies usually resulted in typical fibre type grouping with complete segregation of slow and fast troponin I in type I and type II cells, respectively. Intermediate fibres (slow and fast troponin I in the same cell) were present during the course of transformation. Unlike peripheral neuropathies, the number of cells staining for slow troponin I in the very atrophic fascicles in spinal muscular atrophy was considerably reduced. The synthesis of fast troponin I was stimulated in original type I cells containing slow troponin I. These observations support the experimental data (Dhoot and Perry 1982b) that the suppression of fast troponin I in type I or presumptive type I cells requires innervation by the slow nerve. Its absence results in an increased expression of fast troponin I in cells that originally contained only slow troponin I. Once suppressed, fast troponin I is absent in type I cells even after long periods of experimental denervation or cases of human peripheral neuropathies but not so in the cases of spinal muscular atrophy.

Transformation of fibre types in muscular dystrophies

In the normal human skeletal muscle, slow and fast forms of troponin I are segregated in type I and type II cells, respectively. Muscle biopsies from different dystrophies showed a large number of intermediate cells that stained with antibodies to both fast and slow troponin I. Intermediate cells of variable size were scattered at random and did not show a motor unit distribution as generally seen in some neuromuscular disorders. The preponderance of a particular cell type depended on the type of dystrophy. Except for dystrophia myotonica, all cases showed presence of small cells that looked like regenerating cells.

Neural influences on the distribution of troponin I isotypes in the cat

An immunohistochemical technique was used to study changes in the distribution of fast and slow forms of troponin I (TN-I) in response to alterations in the nerve supply. Hind limb muscles from normal, spinal-isolated, and cordotomized cats, one leg of which had undergone cross innervation between slow (soleus) and fast (flexor hallucis longus, FHL) muscles, were examined. At 8 months after cross innervation of normal soleus by the FHL nerve, the number of fast TN-I-positive cells had increased from 0.26 to 22.1%. At 8 months after cross innervation of normal FHL muscle with the soleus nerve, the number of fast TN-I-positive cells had decreased from 86.5 to 30.5%. The number of intermediate cells staining for both fast and slow TN-I, increased considerably after cross innervation of both soleus and FHL muscles. Spinal isolation by itself had a dramatic effect on the distribution of fast and slow TN-I, converting almost all the originally slow fibers in the FHL and 60.0% of the soleus fibers to fast TN-I-positive cells in 8 months. Cordotomy, in contrast, produced an increase of only 15.6% in the soleus, and did not change the FHL. There was no quantitative difference in the crossed-and uncrossed muscles of spinal isolated cats. In cordotomized cats, cross innervation of the soleus by the FHL nerve resulted in 32.3% fast TN-I-positive cells, with some fiber type grouping. Thus, distribution of fast and slow forms of TN-I changed after each neural manipulation which altered amounts and patterns of muscle contraction and stretch.

The effect of heart and skeletal muscle troponin complexes and calmodulin on the Ca2+-dependent reactions of phosphorylase kinase isoenzymes

The dephosphorylated form of phosphorylase kinase was purified 700-fold from rabbit heart extract. The purified enzyme had a pH 6.8/pH 8.2 activity ratio of 0.04-0.08 and was completely dependent on Ca2+ with an apparent Ka value for Ca2+ of 2.59 microM at pH 6.8. At free Ca2+ concentrations between 0.057 microM and 400 microM, 1.5 microM rabbit heart troponin complex had no significant effect on the reaction. However, 1.5 microM rabbit skeletal muscle troponin complex stimulated the reaction 1.5-2-fold with a concomitant decrease in the Ka value for Ca2+ to 1.40 microM. No differences in the effects of these troponin complexes were observed when heart-type and skeletal muscle-type phosphorylase b isoenzymes from either rabbit or pig were used as substrate. Similar effects of heart and skeletal muscle troponin complexes were observed on the Ca2+-dependent reaction of the dephosphorylated form of phosphorylase kinase partially purified from rabbit skeletal muscle. A saturating concentration (1.36 microM) of bovine brain calmodulin stimulated 2-5-fold the Ca2+-dependent reaction of skeletal muscle phosphorylase kinase, but not the reaction of heart phosphorylase kinase. Heart troponin complex (12 microM) suppressed 80-100% the stimulatory effect of skeletal muscle troponin complex on the reactions of phosphorylase kinase isoenzymes, but had no significant effect on the stimulation by calmodulin of skeletal muscle phosphorylase kinase reaction.

Effect of denervation at birth on the development of skeletal muscle cell types in the rat

In the newborn rat all cells of soleus, extensor digitorum longus (EDL), and tibialis anterior (TA) muscles stained for fast troponin I. A proportion of the cells, that was much higher in the soleus, also stained for slow troponin I. Fast and slow troponin I were segregated in different cell types in all three muscles 10 to 12 days after birth. No subsequent changes in the distribution of the two forms of troponin I occurred with further growth of EDL and TA muscles. The number of type I cells in soleus steadily increased with increasing age to 24 weeks. Three weeks after denervation at birth, almost all cells in soleus muscle stained for fast troponin I but less than 5% stained significantly dark for slow troponin I. All cells stained for myosin ATPase after alkaline preincubation, but very few after acid preincubation. Three weeks after denervation of EDL and to a lesser extent with TA muscle, fast and slow troponin I were still segregated in different cells. After alkaline preincubation all cells stained equally dark for myosin ATPase but only those positive for slow troponin I stained for myosin ATPase after acid preincubation.

The effect of denervation on the distribution of the polymorphic forms of troponin components in fast and slow muscles of the adult rat

The structure of a proprioceptor in the lateral hypodermal chords of Denotostoma californicum has been studied by light and electron microscopy. It is comprised of a sensory cell provided with a cilium situated in a terminal invagination. An accompanying dendrite forms a synaptic junction at the distal end of the sensory cell. This is the first fine structural description of this proprioceptor in the Enoplida.

Changes in the forms of the components of the troponin complex during regeneration of injured skeletal muscle

Using the immunoperoxidase technique, antibodies to the fast components of the troponin complex stained all regenerating cells after localized alcohol injury to rat skeletal muscle. Antibodies to slow troponin components stained only some of these cells. About 6 weeks after injury with the nerve intact, the fast and slow forms of the troponin components were located in different cells. During the later stages of regeneration, staining for myosin ATPase correlated with the staining with antibodies to fast and slow troponin components. A similar staining pattern was also observed in the early stages of regeneration of muscle denervated at the time of injury. In this case, antibodies to fast skeletal muscle troponin components continued to stain all the cells 10 weeks after injury. Injured denervated muscle cells stained equally dark by myosin ATPase after preincubation at pH 9.4 over this period. None of the regenerating myotubes in denervated muscle stained for myosin ATPase after preincubation at pH 4.3.

Differentiation of troponin in cardiac and skeletal muscles in chicken embryos as studied by immunofluorescence microscopy

The differentiation of troponin (TN) in cardiac and skeletal muscles of chicken embryos was studied by indirect immunofluorescence microscopy. Serial sections of embryos were stained with antibodies specific to TN components (TN-T, -I, and -C) from adult chicken cardiac and skeletal muscles. Cardiac muscle began to be stained with antibodies raised against cardiac TN components in embryos after stage 10 (Hamburger and Hamilton numbering, 1951, J. Morphol. 88:49-92). It reacted also with antiskeletal TN-I from stage 10 to hatching. Skeletal muscle was stained with antibodies raised against skeletal TN components after stage 14. It also reacted with anticardiac TN-T and C from stage C from stage 14 to hatching. It is concluded that, during embryonic development, cardiac muscle synthesizes TN-T and C that possess cardiac-type antigenicity and TN-I that has antigenic determinants similar to those present in cardiac as well as in skeletal muscles. Embryonic skeletal muscle synthesizes TN-I that possesses antigenicity for skeletal muscle and TN-T and C which share the antigenicities for both cardiac and skeletal muscles. Thus, in the development of cardiac and skeletal muscles, a process occurs in which the fiber changes its genomic programming: it ceases synthesis of the TN components that are immunologically indistinguishable from one another and synthesizes only tissue-type specific proteins after hatching.