Isoforms Of Troponin Research Articles

In silico evidence of cardiac troponin non‐classical secretion: new insights on the pathobiology of myocardial injury

Background Cardiac troponin is one of the most important biomarkers in cardiology. Traditionally, it has represented a biochemical sign of necrosis, a characteristic of acute myocardial infarction (AMI). However, with the arrival of high-sensitivity detection assays, cardiac troponin is now detected in healthy people and in a number of uncommon scenarios unrelated to AMI (such as strenuous exercise, rapid atrial pacing and COVID-19). This suggest that troponin could be released as a consequence of alternative processes other than the well-stablished necrotic cell death. To try to explain this phenomenon, multiple mechanisms of troponin release have been postulated, although there is still no consensus on this matter. Furthermore, the term myocardial injury was included in the Fourth Universal Definition of Myocardial Infarction. This entity is understood as a troponin determination above the upper reference limit, not necessarily associated with a specific diagnosis. Nonetheless, the histological or pathobiological process responsible of myocardial injury has not been determined. Here, we studied the possibility of troponin release through non-classical secretion, a pathway specially triggered by cellular stress and inflammation. Methods The protein sequences of the eight different isoforms of troponin (I, T and C of fast-twitch skeletal, slow-twitch skeletal and cardiac muscle) were retrieved from the UniProtKB database. SecretomeP 2.0, a neural networks-based program, was used to predict if any of these proteins undergoes non-classical release. SignalP 5.0 and TMHMM 2.0 were also applied to rule out the possibility of classical (endoplasmic reticulum/Golgi dependent) and Type IV (transmembrane) non-classical secretion. Results and Discussion After analyzing the sequences, the cardiac T isoform was predicted to be non-classically secreted, as well as the I, T and C subunits of the fast-twitch skeletal muscle and the I subunit of the slow-twitch skeletal muscle. Additionally, none of the troponin isoforms was found to be subject to classical or Type IV secretion. The present study provides in silico evidence of cardiac troponin T release by means of non-classical secretion. We believe that this potential new mechanism could contribute to the debate around cardiac troponin high-sensitivity assays. Thus, it might establish a theoretical frame capable of explaining troponin elevations by causes other than myocyte necrosis. Conclusions Troponin T secretion may help in the pathobiological definition of myocardial injury. There are already some experimental studies showing the release of troponin through extracellular microvesicles (a type of non-classical secretion) in various cell lines. Nonetheless, more work is needed to fully understand this intriguing phenomenon in cardiomyocytes.

The haplolethality paradox of the wupA gene in Drosophila.

Haplolethals (HL) are regions of diploid genomes that in one dose are fatal for the organism. Their biological meaning is obscure because heterozygous loss-of-function mutations result in dominant lethality (DL) and, consequently, should be under strong negative selection. We report an in depth study of the HL associated to the gene wings up A (wupA). It encodes 13 transcripts (A-M) that yield 11 protein isoforms (A-K) of Troponin I (TnI). They are functionally diverse in their control of muscle contraction, cell polarity and cell proliferation. Isoform K transfers to the nucleus where it increases transcription of the cell proliferation related genes CDK2, CDK4, Rap and Rab5. The nuclear translocation of isoform K is prevented by the co-expression of A or B isoforms, which illustrates isoform interactions. The corresponding DL mutations are, either DNA rearrangements clustered towards the gene 3’ end, thus affecting the genomic organization of all transcripts, or CRISPR-induced mutations in one of the two ATG sites which eliminate a subset of wupA products. The joint elimination of isoforms C, F, G and H, however, do not cause DL phenotypes. Genetically driven expression of single isoforms rescue neither DL nor any of the mutants known in the gene, suggesting that normal function requires properly regulated expression of specific combinations, rather than single, TnI isoforms. We conclude that the wupA associated HL results from the combined haploinsufficiency of a large set of TnI isoforms. The qualitative and quantitative normal expression of which, requires the chromosomal integrity of the wupA genomic region. Since all fly TnI isoforms are encoded in the same gene, its HL condition becomes unavoidable. These wupA features are comparable to those of dpp, the only other HL studied to some extent, and reveal a scenario of strict dosage dependence with implications for gene expression regulation and splitting.

The Association between Excess Body Mass and Disturbances in Somatic Mineral Levels.

Background: Obesity and excess body weight are significant epidemiological issues, not only because they are costly to treat, but also because they are among the leading causes of death worldwide. In 2016, an estimated 40% of the global population was overweight, reflecting the importance of the issue. Obesity is linked to metabolism malfunction and concomitantly with altered mineral levels in the body. In this paper, we review alterations in somatic levels of iron, calcium, magnesium, copper, iodine, chromium, selenium, and zinc in relation to excess body mass. Methodology: An electronic literature search was performed using PubMed. Our search covered original English research articles published over the past five years, culminating in 63 papers included for study. Results: The reviewed papers presented correlation between obesity and hypomagnesemia and hypozincemia. They also indicated that patients with excess body mass present increased body copper levels. Studies have similarly indicated that obesity appears to be associated with lower selenium levels in both blood and urine, which may be correlated with the decline and weakening of defenses against oxidative stress. It has been found that decreased level of chromium is connected with metabolic syndrome. Chromium supplementation influences body mass, but the effect of the supplementation depends on the chemical form of the chromium. It is hypothesized that obesity poses a risk of iodine deficiency and iodine absorption may be disrupted by increased fat intake in obese women. A range of studies have suggested that obesity is correlated with iron deficiency. On the other hand, some reports have indicated that excess body mass may coexist with iron excess. The relation between obesity and body iron level requires further investigation. Calcium signaling seems to be disturbed in obesity, due to the increased production of reactive oxygen species and low level of fast troponin isoform responsible for mediating calcium sensitivity of muscle relaxation. Correlation between excess body mass and calcium levels needs further research. Conclusions: Excess body mass is associated with alterations in mineral levels in the body, in particular hypomagnesemia and decreased selenium (Se) and zinc (Zn) levels. Chromium (Cr) deficiency is associated with metabolic syndrome. Obese patients are at risk of iodine deficiency. Excess body mass is associated with elevated levels of copper (Cu). Data on the association between obesity and iron (Fe) levels are contradictory. Obesity coexists with disturbed calcium (Ca) signaling pathways. The association between obesity and body Ca levels has not been investigated in detail.

CONGENITAL MYOPATHIES 1 – NEMALINE: P.29 AAV gene therapy for TNNT1-associated Nemaline myopathy

Nemaline myopathy is one of the most common congenital myopathies. The hallmarks of this disorder are early onset muscular weakness, hypotonia, pectus carinatum and rod-like inclusions in muscle cells. The severity spectrum of nemaline myopathy is broad, but early respiratory failure and death are the inevitable outcomes in most of the cases. Several gene mutations have been identified in nemaline myopathy, including a non-sense mutation in exon 11 of TNNT1 gene, encoding for the slow skeletal muscle isoform of troponin T (TnT), which results in selective atrophy of slow-twitch (type I) myofibers and in a unique form of Nemaline myopathy, named Amish Nemaline Myopathy (ANM). Currently there is no treatment for ANM, but the development of an AAV gene therapy is viable and explored in our research. Such an approach requires the restricted expression of TNNT1 to slow-twitch fibers, to avoid potential deleterious effects associated with the ectopic expression of TNNT1 in cardiac and fast-twitch myofibers. We achieved this, in wild-type mice, by using a series of AAV vectors carrying a strong muscle-specific promoter (MHCK7) to express both TNNT1 and suppressive microRNAs (miR208a and miR-133, characteristic of cardiac and fast-twitch myofibers respectively). The next step will be a dose escalation study in Tnnt1−/− mice, to assess TNNT1 expression by western blot and by immunostaining for both TNNT1 and myofiber type specific markers. We will then quantify the changes in skeletal muscle histology (fiber diameter and number of centronucleated fibers) to assess the therapeutic efficacy of our AAV vectors.

Two different troponin isoforms for detecting early myocardial injury after curative resection of oesophageal cancer

BackgroundThe objective of this study was to explore the consistency and correlation of two troponin (cTn) subtypes, troponin I (cTnI) and high-sensitivity troponin T (hs-cTnT), which can be used to judge early myocardial injury after curative resection of oesophageal cancer.MethodsThis study is a secondary analysis of data obtained from a previous randomized controlled trial on postoperative myocardial injury in 70 patients undergoing elective curative resection of oesophageal cancer who were randomly assigned to undergo aggressive body temperature management (nasopharyngeal temperature 36.61 ± 0.18 °C) or standard body temperature management (35.80 ± 0.18 °C, n = 35 in each arm). The serum cTnI and hs-cTnT levels were measured in each patient at the 4 time points: before the operation and 6 h ~ 12 h, 24 h and 48 h after the operation. The diagnostic criteria of myocardial injury followed the third edition ESC/ACCF definition of myocardial infarction. The primary outcomes included the following: (1) the incidence of myocardial injury and the relationship between hs-cTnT and cTn and (2) the consistency and correlation of the two cTn subtypes.ResultsA total of 280 pairs of cTn samples were tested. The incidence of postoperative day 2 myocardial injury was 8.6% (3/35) among patients receiving aggressive body temperature management and 31.4% (11/35) among patients receiving standard body temperature management (P < 0.05). Among 3 patients who experienced myocardial injury in the aggressive body temperature management group, 2 met the diagnostic criteria for cTnI and hs-cTnT and only 1 met the diagnostic criteria for hs-cTnT. Among the 11 patients who experienced myocardial injury in the standard body temperature management group, 7 met the diagnostic criteria for cTnI and hs-cTnT and only 3 met the diagnostic criteria for hs-cTnT; only 1 met the diagnostic criteria for cTnI. The bias of cTnI and hs-cTnT was − 8.82 ± 31.91 ng/L. The consistency limit was − 71.37 ~ 53.73 ng/L. The proportion within the scope of the consistency of its corresponding boundary was 98.57%. The correlation coefficient of cTnI and hs-cTnT was 0.845 (P < 0.05).ConclusionsIn the evaluation of postoperative myocardial injury in patients undergoing curative resection of oesophageal cancer, cTnI and hs-cTnT exhibit high consistency and a good correlation. The combination of cTnI and hs-cTnT can improve the detection rate of myocardial injury, thus providing a better reference than a single measure alone for reducing the risk of perioperative myocardial injury in patients.Trial registrationChiCTR-INR-17011621. Registered June 10, 2017.

Advancing physiological maturation in human induced pluripotent stem cell-derived cardiac muscle by gene editing an inducible adult troponin isoform switch.

Advancing maturation of stem cell-derived cardiac muscle represents a major barrier to progress in cardiac regenerative medicine. Cardiac muscle maturation involves a myriad of gene, protein, and cell-based transitions, spanning across all aspects of cardiac muscle form and function. We focused here on a key developmentally controlled transition in the cardiac sarcomere, the functional unit of the heart. Using a gene-editing platform, human induced pluripotent stem cell (hiPSCs) were engineered with a drug-inducible expression cassette driving the adult cardiac troponin I (cTnI) regulatory isoform, a transition shown to be a rate-limiting step in advancing sarcomeric maturation of hiPSC cardiac muscle (hiPSC-CM) toward the adult state. Findings show that induction of the adult cTnI isoform resulted in the physiological acquisition of adult-like cardiac contractile function in hiPSC-CMs in vitro. Specifically, cTnI induction accelerated relaxation kinetics at baseline conditions, a result independent of alterations in the kinetics of the intracellular Ca2+ transient. In comparison, isogenic unedited hiPSC-CMs had no cTnI induction and no change in relaxation function. Temporal control of adult cTnI isoform induction did not alter other developmentally regulated sarcomere transitions, including myosin heavy chain isoform expression, nor did it affect expression of SERCA2a or phospholamban. Taken together, precision genetic targeting of sarcomere maturation via inducible TnI isoform switching enables physiologically relevant adult myocardium-like contractile adaptations that are essential for beat-to-beat modulation of adult human heart performance. These findings have relevance to hiPSC-CM structure-function and drug-discovery studies in vitro, as well as for potential future clinical applications of physiologically optimized hiPSC-CM in cardiac regeneration/repair.

Non-coronarogenic causes of increased cardiac troponins in clinical practice

Cardiospecific isoforms of troponins are the most sensitive and specific biomarkers for the diagnosis of myocardial infarction. However, though elevated troponin levels indicate myocardial damage, they do not determine the cause and mechanism of the damage. With the new highly sensitive methods, very minor damages of the heart muscle can be detected. Myocardial damage can occur in many non-coronarogenic diseases. In this review, we discuss the mechanisms of elevation, the diagnostic value of cardiac troponins in the renal failure, tachyarrhythmias, endocarditis, myocarditis, pericarditis, sepsis, neurogenic pathologies (stroke), pulmonary embolism. In addition, we pay attention to the main reasons for a false-positive increase of the concentration of cardiac troponins: heterophilic antibodies, rheumatoid factor, alkaline phosphatase, cross-reactions with skeletal muscle troponins.

Phenotypic and transcriptomic changes in zebrafish (Danio rerio) embryos/larvae following cypermethrin exposure

Cypermethrin is one of the widely used type-II pyrethroid and the indiscriminate use of this pesticide leads to life threatening effects and in particular showed developmental effects in sensitive populations such as children and pregnant woman. However, the molecular mechanisms underlying cypermethrin-induced development toxicity is not well defined. To address this gap, the present study was designed to investigate the phenotypic and transcriptomic (next generation RNA-Seq method) impact of cypermethrin in zebrafish embryos as a model system. Zebrafish embryos at two time points, 24 h postfertilization (hpf) and 48 hpf were exposed to cypermethrin at a concentration of 10 μg/L. Respective control groups were maintained. Cypermethrin induced both phenotypic and transcriptomic changes in zebrafish embryos at 48 hpf. The phenotypic anomalies such as delayed hatching rate, increased heartbeat rate and deformed axial spinal curvature in cypermethrin exposed zebrafish embryos at 48 hpf as compared to its respective controls. Transcriptomic analysis indicated that cypermethrin exposure altered genes associated with visual/eye development and gene functional profiling also revealed that cypermethrin stress over a period of 48 h disrupts phototransduction pathway in zebrafish embryos. Interestingly, cypermethrin exposure resulted in up regulation of only one gene, tnnt3b, fast muscle troponin isoform 3T in 24 hpf embryos as compared to its respective controls. The present model system, cypermethrin exposed zebrafish embryos elaborates the toxic consequences of cypermethrin exposure during developmental stages, especially in fishes. The present findings paves a way to understand the visual impairment in sensitive populations such as children exposed to cypermethrin during their embryonic period and further research is warranted.

Non-coronarogenic causes of increased cardiac troponins in the practice of physicians (literature review)

Cardiac isoforms of troponins are the most sensitive and specific biomarkers of myocardial damage and new high-sensitivity methods can reveal very minor damages on the heart muscle. However, elevated troponin levels indicate cardiac damage, but do not define the cause of the damage. Therefore, cardiac troponin elevations are common in many disease states and do not necessarily indicate the presence of acute myocardial infarction. In clinical practice, interpretation of positive troponins may be challenging. In our article, we consider the main non-coronarogenic causes of increased cardiac troponins. The first part of the review discusses the mechanisms of increase and the diagnostic value of cardiac troponins during physical exertion, inflammatory and toxic myocardial injuries (endocarditis, myocarditis, sepsis), and renal failure. The second part of the literature review describes the mechanisms of increase and the diagnostic value of troponins in pulmonary thromboembolism, dissecting aortic aneurysms, neurogenic pathologies (stroke, subarachnoid hemorrhages), and treatment with cardiotoxic drugs (chemotherapy). Special attention is paid to the false positive causes of elevation of troponins.

Host plant defense produces species-specific alterations to flight muscle protein structure and flight-related fitness traits of two armyworms.

Insects manifest phenotypic plasticity in their development and behavior in response to plant defenses, via molecular mechanisms that produce tissue-specific changes. Phenotypic changes might vary between species that differ in their preferred hosts and these effects could extend beyond larval stages. To test this, we manipulated the diet of southern armyworm (SAW; Spodoptera eridania) and fall armyworm (FAW; Spodoptera frugiperda) using a tomato mutant for jasmonic acid plant defense pathway (def1), and wild-type plants, and then quantified gene expression of Troponin t (Tnt) and flight muscle metabolism of the adult insects. Differences in Tnt spliceform ratios in insect flight muscles correlate with changes to flight muscle metabolism and flight muscle output. We found that SAW adults reared on induced def1 plants had a higher relative abundance (RA) of the A isoform of Troponin t (Tnt A) in their flight muscles; in contrast, FAW adults reared on induced def1 plants had a lower RA of Tnt A in their flight muscles compared with adults reared on def1 and controls. Although mass-adjusted flight metabolic rate showed no independent host plant effects in either species, higher flight metabolic rates in SAW correlated with increased RA of Tnt A Flight muscle metabolism also showed an interaction of host plants with Tnt A in both species, suggesting that host plants might be influencing flight muscle metabolic output by altering Tnt This study illustrates how insects respond to variation in host plant chemical defense by phenotypic modifications to their flight muscle proteins, with possible implications for dispersal.

Metabolism of cardiac troponins (literature review)

The review summarizes all recent data on the metabolism of cardiac troponin isoforms. The main mechanisms of troponin release from intact myocardium are described. These mechanisms ensure its baseline levels (less than the 99th percentile) in all healthy individuals. There are various fragments of troponin that circulate in the blood flow as a heterogeneous pool. Their circulation is related to various intracellular and extracellular proteases. In-depth understanding ofthese mechanisms is required to improve the diagnostic process. The article provides new insights into the evaluation of cardiac troponins in other human biological fluids: pericardial, cerebrospinal, amniotic, urine, and oral fluid. The measurements of saliva and urine levels of troponins seem to be promising alternative for non-invasive diagnosis. Recent circadian patterns of high-sensitive cardiac troponin T alterations are reported. These patterns should be taken into account while practicing fast diagnostic algorithms.

Correction to: Roles of the troponin isoforms during indirect flight muscle development in Drosophila

Correction to: Roles of the troponin isoforms during indirect flight muscle development in Drosophila

TNNT2 Missplicing in Skeletal Muscle as a Cardiac Biomarker in Myotonic Dystrophy Type 1 but Not in Myotonic Dystrophy Type 2.

Cardiac involvement is one of the most important manifestations of the multisystemic phenotype of patients affected by myotonic dystrophy (DM) and represents the second cause of premature death. Molecular mechanisms responsible for DM cardiac defects are still unclear; however, missplicing of the cardiac isoform of troponin T (TNNT2) and of the cardiac sodium channel (SCN5A) genes might contribute to the reduced myocardial function and conduction abnormalities seen in DM patients. Since, in DM skeletal muscle, the TNNT2 gene shows the same aberrant splicing pattern observed in cardiac muscle, the principal aim of this work was to verify if the TNNT2 aberrant fetal isoform expression could be secondary to myopathic changes or could reflect the DM cardiac phenotype. Analysis of alternative splicing of TNNT2 and of several genes involved in DM pathology has been performed on muscle biopsies from patients affected by DM type 1 (DM1) or type 2 (DM2) with or without cardiac involvement. Our analysis shows that missplicing of muscle-specific genes is higher in DM1 and DM2 than in regenerating control muscles, indicating that these missplicing could be effectively important in DM skeletal muscle pathology. When considering the TNNT2 gene, missplicing appears to be more evident in DM1 than in DM2 muscles since, in DM2, the TNNT2 fetal isoform appears to be less expressed than the adult isoform. This evidence does not seem to be related to less severe muscle histopathological alterations that appear to be similar in DM1 and DM2 muscles. These results seem to indicate that the more severe TNNT2 missplicing observed in DM1 could not be related only to myopathic changes but could reflect the more severe general phenotype compared to DM2, including cardiac problems that appear to be more severe and frequent in DM1 than in DM2 patients. Moreover, TNNT2 missplicing significantly correlates with the QRS cardiac parameter in DM1 but not in DM2 patients, indicating that this splicing event has good potential to function as a biomarker of DM1 severity and it should be considered in pharmacological clinical trials to monitor the possible effects of different therapeutic approaches on skeletal muscle tissues.

The loss of slow skeletal muscle isoform of troponin T in spindle intrafusal fibres explains the pathophysiology of Amish nemaline myopathy.

The pathogenic mechanism and the neuromuscular reflex-related phenotype (e.g. tremors accompanied by clonus) of Amish nemaline myopathy, as well as of other recessively inherited TNNT1 myopathies, remain to be clarified. The truncated slow skeletal muscle isoform of troponin T (ssTnT) encoded by the mutant TNNT1 gene is unable to incorporate into myofilaments and is degraded in muscle cells. By contrast to extrafusal muscle fibres, spindle intrafusal fibres of normal mice contain a significant level of cardiac TnT and a low molecular weight splice form of ssTnT. Intrafusal fibres of ssTnT-knockout mice have significantly increased cardiac TnT. Rotarod and balance beam tests have revealed abnormal neuromuscular co-ordination in ssTnT-knockout mice and a blunted response to a spindle sensitizer, succinylcholine. The loss of ssTnT and a compensatory increase of cardiac TnT in intrafusal nuclear bag fibres may increase myofilament Ca2+ -sensitivity and tension, impairing spindle function, thus identifying a novel mechanism for the development of targeted treatment. A nonsense mutation at codon Glu180 of TNNT1 gene causes Amish nemaline myopathy (ANM), a recessively inherited disease with infantile lethality. TNNT1 encodes the slow skeletal muscle isoform of troponin T (ssTnT). The truncated ssTnT is unable to incorporate into myofilament and is degraded in muscle cells. The symptoms of ANM include muscle weakness, atrophy, contracture and tremors accompanied by clonus. An ssTnT-knockout (KO) mouse model recapitulates key features of ANM such as atrophy of extrafusal slow muscle fibres and increased fatigability. However, the neuromuscular reflex-related symptoms of ANM have not been explained. By isolating muscle spindles from ssTnT-KO and control mice aiming to examine the composition of myofilament proteins, we found that, in contrast to extrafusal fibres, intrafusal fibres contain a significant level of cardiac TnT and the low molecular weight splice form of ssTnT. Intrafusal fibres from ssTnT-KO mice have significantly increased cardiac TnT. Rotarod and balance beam tests revealed impaired neuromuscular co-ordination in ssTnT-KO mice, indicating abnormality in spindle functions. Unlike the wild-type control, the beam running ability of ssTnT-KO mice had a blunted response to a spindle sensitizer, succinylcholine. Immunohistochemistry detected ssTnT and cardiac TnT in nuclear bag fibres, whereas fast skeletal muscle TnT was detected in nuclear chain fibres, and cardiac α-myosin was present in one of the two nuclear bag fibres. The loss of ssTnT and a compensatory increase of cardiac TnT in nuclear bag fibres would increase myofilament Ca2+ -sensitivity and tension, thus affecting spindle activities. This mechanism provides an explanation for the pathophysiology of ANM, as well as a novel target for treatment.

Full-Size and Partially Truncated Cardiac Troponin Complexes in the Blood of Patients with Acute Myocardial Infarction.

The measurement of cardiac isoforms of troponin I (cTnI) and troponin T (cTnT) is widely used for the diagnosis of acute myocardial infarction (AMI). However, there are conflicting data regarding what forms of cTnI and cTnT are present in the blood of AMI patients. We investigated cTnI and cTnT as components of troponin complexes in the blood of AMI patients. Gel filtration techniques, sandwich fluoroimmunoassays, and Western blotting were used. Plasma samples from patients with AMI contained the following troponin complexes: (a) a cTnI-cTnT-TnC complex (ITC) composed of full-size cTnT of 37 kDa or its 29-kDa fragment and full-size cTnI of 29 kDa or its 27-kDa fragments; (b) ITC with lower molecular weight (LMW-ITC) in which cTnT was truncated to the 14-kDa C-terminal fragments; and (c) a binary cTnI-cTnC complex composed of truncated cTnI of approximately 14 kDa. During the progression of the disease, the amount of ITC in AMI samples decreased, whereas the amounts of LMW-ITC and short 16- to 20-kDa cTnT central fragments increased. Almost all full-size cTnT and a 29-kDa cTnT fragment in AMI plasma samples were the components of ITC. No free full-size cTnT was found in AMI plasma samples. Only 16- to 27-kDa central fragments of cTnT were present in a free form in patient blood. A ternary troponin complex exists in 2 forms in the blood of patients with AMI: full-size ITC and LMW-ITC. The binary cTnI-cTnC complex and free cTnT fragments are also present in patient blood.

Characterization of Contractile Proteins from Skeletal Muscle Using Gel-Based Top-Down Proteomics.

The mass spectrometric analysis of skeletal muscle proteins has used both peptide-centric and protein-focused approaches. The term ‘top-down proteomics’ is often used in relation to studying purified proteoforms and their post-translational modifications. Two-dimensional gel electrophoresis, in combination with peptide generation for the identification and characterization of intact proteoforms being present in two-dimensional spots, plays a critical role in specific applications of top-down proteomics. A decisive bioanalytical advantage of gel-based and top-down approaches is the initial bioanalytical focus on intact proteins, which usually enables the swift identification and detailed characterisation of specific proteoforms. In this review, we describe the usage of two-dimensional gel electrophoretic top-down proteomics and related approaches for the systematic analysis of key components of the contractile apparatus, with a special focus on myosin heavy and light chains and their associated regulatory proteins. The detailed biochemical analysis of proteins belonging to the thick and thin skeletal muscle filaments has decisively improved our biochemical understanding of structure-function relationships within the contractile apparatus. Gel-based and top-down proteomics has clearly established a variety of slow and fast isoforms of myosin, troponin and tropomyosin as excellent markers of fibre type specification and dynamic muscle transition processes.

Through thick and thin: dual regulation of insect flight muscle and cardiac muscle compared

Both insect flight muscle and cardiac muscle contract rhythmically, but the way in which repetitive contractions are controlled is different in the two types of muscle. We have compared the flight muscle of the water bug, Lethocerus, with cardiac muscle. Both have relatively high resting elasticity and are activated by an increase in sarcomere length or a quick stretch. The larger response of flight muscle is attributed to the highly ordered lattice of thick and thin filaments and to an isoform of troponin C that has no exchangeable Ca2+-binding site. The Ca2+ sensitivity of cardiac muscle and flight muscle can be manipulated so that cardiac muscle responds to Ca2+ like flight muscle, and flight muscle responds like cardiac muscle, showing the malleability of regulation. The interactions of the subunits in flight muscle troponin are described; a model of the complex, using the structure of cardiac troponin as a template, shows an overall similarity of cardiac and flight muscle troponin complexes. The dual regulation by thick and thin filaments in skeletal and cardiac muscle is thought to operate in flight muscle. The structure of inhibited myosin heads folded back on the thick filament in relaxed Lethocerus fibres has not been seen in other species and may be an adaptation to the rapid contractions of flight muscle. A scheme for regulation by thick and thin filaments during oscillatory contraction is described. Cardiac and flight muscle have much in common, but the differing mechanical requirements mean that regulation by both thick and thin filaments is adapted to the particular muscle.

Loss of the Slow Skeletal Muscle Isoform of Troponin T Impairs Motor Coordination in Mice

The loss of the slow skeletal muscle isoform of troponin T (ssTnT) encoded by TNNT1 causes lethal myopathies with symptoms including abnormal neuromuscular reflexes. It has been a challenging question that how the loss of one isoform of TnT results in extremely severe phenotypes while the fast isoform of TnT is present in the patient muscles. We investigated ssTnT knockout (KO) mice for the role of ssTnT in muscle spindles that are a mechanosensors critical to normal neuromuscular function. Immunohistological and spindle protein content studies indicated that unlike extrafusal muscle fibers, adult ssTnT-KO mouse spindle intrafusal fibers express high levels of cardiac TnT, which is significantly increased in intrafusal fibers of ssTnT-KO mice. Impaired motor coordination was observed in ssTnT-KO mice with slower walking during a balance beam performance test as compared with age/sex-matched wildtype (WT) control (p<0.05). An interesting observation from the balance beam performance test is that subcutaneous injection of succinylcholine (SCh), a muscle relaxant believed to alter spindle activities, significantly increased the time for WT mice to walk across the balance beam (p<0.01) whereas SCh treatment produced a trend for decreased time across the beam in ssTnT-KO mice (p=0.10). Taken together, our data suggest that the loss of ssTnT in the KO mice impairs spindle intrafusal fiber contractility, identifying a novel therapeutic target. The attenuated effect of SCh treatment on motor coordination in the ssTnT-KO mice may be due to the potentially adaptive increased expression of cardiac TnT in spindle intrafusal fibers. Further testing is currently underway to assessing the hypothesized adaptation of spindle intrafusal fiber TnT isoform expression and function in ssTnT-KO mice during postnatal development.

Fast skeletal troponin I, but not the slow isoform, is increased in patients under statin therapy: a pilot study.

IntroductionStatin therapy is often associated with muscle complaints and increased serum creatine kinase (CK). However, although essential in determining muscle damage, this marker is not specific for skeletal muscle. Recent studies on animal models have shown that slow and fast isoforms of skeletal troponin I (ssTnI and fsTnI, respectively) can be useful markers of skeletal muscle injury. The aim of this study was to evaluate the utility of ssTnI and fsTnI as markers to monitor the statin-induced skeletal muscle damage.Materials and methodsA total of 51 patients (14 using and 37 not using statins) admitted to the intensive care unit of the University of Ferrara Academic Hospital were included in this observational study. Serum activities of CK, aldolase, alanine aminotransferase and myoglobin were determined by spectrophotometric assays or routine laboratory analysis. Isoforms ssTnI and fsTnI were determined by commercially available ELISAs. The creatine kinase MB isoform (CK-MB) and cardiac troponin I (cTnI) were evaluated as biomarkers of cardiac muscle damage by automatic analysers.ResultsAmong the non-specific markers, only CK was significantly higher in statin users (P = 0.027). Isoform fsTnI, but not ssTnI, was specifically increased in those patients using statins (P = 0.009) evidencing the major susceptibility of fast-twitch fibres towards statins. Sub-clinical increase in fsTnI, but not CK, was more frequent in statin users (P = 0.007). Cardiac markers were not significantly altered by statins confirming the selectivity of the effect on skeletal muscle.ConclusionsSerum fsTnI could be a good marker for monitoring statin-associated muscular damage outperforming traditional markers.

SMA THERAPIES II AND BIOMARKERS: P.265Patients with spinal muscular atrophy without cardiac disease show elevated cardiac troponin T

Recent research in adults has shown that skeletal muscles express cardiac isoforms of troponin T (cTnT) in several neuromuscular diseases without cardiomyopathy. We examined whether increased serum cTnT is detectable also in SMA patients and if it correlates with disease severity. The new therapy with Nusinersen has shown to improve motor function. We evaluated whether this treatment has an effect on muscle mass, measured by serum creatinine and whether it changes cTnT. Between 03/2016 and 03/2018 we collected data of 16 patients with SMA (SMA1: 6, SMA 2 or 3: 10). Serum samples were obtained as part of the routine work-up before treatment initiation and prior to each applied dose. The high sensitivity cTnT test (hs cTnT, Roche Cobas) was used. Cardiac disease was excluded clinically, by echocardiography and by measuring cTnI. Serum creatinine (SC) was chosen as surrogate for muscle mass. Disease severity was determined by SMA type and in patients with SMA1 by the need for ventilation within two months after initiating treatment. While all cTnI levels were within the normal range, baseline cTnT in SMA1 patients was 80±39 (range 43-143ng/L) and thus 3 to 10-fold above the upper limit of normal of 14 ng/L. Values of non-ventilated patients were lower than in ventilated patients (p=0,06). SMA2/3 had significantly lower cTnT and cTnT/SC than SMA1 patients (p=0,0002 and p=0,06). After the third application of Nusinersen no significant changes in SC, cTnT, or cTnT/SC could be detected in all SMA groups. As our pilot study shows that cTnT is elevated in all patients with SMA1, it might be useful as a screening tool. As expected, cTnT was correlated with disease severity. We found no support for our hypothesis that Nusinersen improves muscle mass and decreases cTnT. However, small sample size, short follow-up period, and the lack of a control group make a reliable statement impossible. Further studies with larger patient groups including untreated controls are essential.