The slow isoform of Xenopus troponin I is expressed in developing skeletal muscle but not in the heart

In birds and mammals three isoforms of troponin I (TnI) exist; a slow (TnIs), a fast (TnIf) and a cardiac (TnIc). Although each of these isoforms is expressed in the adult forms of these organisms in a muscle fiber-type-specific manner, the gene encoding TnIs is also expressed within the developing heart of these vertebrates. Herein, our results demonstrate that the developing heart of Xenopus laevis, unlike its counterpart in birds and mammals, does not express the gene encoding the TnIs isoform and that the expression of this gene, as well as the one encoding the Xenopus TnIf isoform, is restricted to skeletal muscle.     

Isolation and cloning by a polymerase chain reaction of a genomic DNA fragment of the human slow skeletal troponin (TNNT1) gene

The genomic 3′ structure of the gene coding for the human slow skeletal troponin T (TNNT1) gene, is reported. An intron of 912 nucleotides containing an Alu-element has been identified and characterized. The complexity of the sequenced region suggests an alternative exon use. The present results may be valuable for further studies on the gene structure of TNNT1 and the related troponin gene family.     

Calcium ion regulation of muscle contraction: The regulatory role of troponin T

The relaxation and contraction in vertebrate skeletal muscle are regulated by Ca2+ through troponin and tropomyosin, which are located in the thin filament. Troponin is composed of three components, troponins C, I and T. In this review article, the Ca2+-regulatory mechanism is discussed with particular reference to the regulatory properties of troponin T.     

中国北方汉族人群sTnT基因单核苷酸多态性分析

目的：研究中国北方汉族人群sTnT基因的单核苷酸多态性（SNP）,观察其在北方汉族人群中的分布。方法：用PCR-RFLP的方法对204名中国北方汉族人群sTnT基因的SNP进行分析,确定其等位基因频率。结果：美国国立生物技术信息中心报告的外显子11上的27916722 A／C未在本项研究人群中检测到。27930097 C／G和的27920978 C／F的等位基因频率与美国国立生物技术信息中心（NCBI）报道均有显著性差异。结论：sTnT基因SNP分布具有种族差异性。     

Genomic sequence and structural organization of mouse slow skeletal muscle troponin T gene

Three muscle type-specific troponin T (TnT) genes are present in vertebrate to encode a number of protein isoforms via alternative mRNA splicing. While the genomic structures of cardiac and fast skeletal muscle TnT genes have been documented, this study cloned and characterized the slow skeletal muscle TnT (sTnT) gene. Complete nucleotide sequence and genomic organization revealed that the mouse sTnT gene spans 11.1kb and contains 14 exons, which is smaller and simpler than the fast skeletal muscle and cardiac TnT genes. Potentially representing a prototype of the TnT gene family, the 5'-region of the sTnT gene contains fewer unsplit large exons, among which two alternatively spliced exons are responsible for the NH2-terminal variation of three sTnT isoforms. The sTnT gene structure shows that the alternatively spliced central segment found in human sTnT cDNAs may be a result from splicing using an alternative acceptor site at the intron 11-exon 12 boundary. Together with the well-conserved protein structure, the highly specific expression of sTnT in slow skeletal muscles indicates a differentiated function of this member of the TnT gene family. The determination of genomic structure and alternative splicing pathways of sTnT gene lays a foundation to further understand the TnT structure-function evolution as well as contractile characteristics of different types of muscle fiber.     

The influence of heart failure co-morbidity on high-sensitivity troponin T levels in COPD exacerbation in a prospective cohort study: data from the Akershus cardiac examination (ACE) 2 study

Context: Troponin (hs-TnT) levels predict mortality after acute exacerbation of COPD (AECOPD). Whether this is independent of heart failure (HF) is not established.

Material and methods: Prospectively included AECOPD patients adjudicated for acute HF categorized into three groups: (A) AECOPD, but acute HF the primary cause for hospitalization; (B) AECOPD the primary cause, but co-existing myocardial dysfunction and (C) AECOPD without myocardial dysfunction.

Results: About 103 AECOPD patients; 18% A, 27% B and 54% C. Hs-TnT level differed between the groups: (ng/l, median) A: 41, B: 25 and C: 15, p?=?0.03 for A versus B and p?=?0.005 for B versus C. During a median 826 days, 47% died. In Cox analysis, hs-TnT levels remained associated with mortality (hazard ratio per 10?ng/l 1.3, p?<?0.0001).

Conclusion: hs-TnT levels are influenced by myocardial dysfunction/HF in AECOPD, but provide independent prognostic information. The prognostic merit of hs-TnT cannot be attributed to HF alone.     

Myofibrillar troponin exists in three states and there is signal transduction along skeletal myofibrillar thin filaments

Activation of striated muscle contraction is a highly cooperative signal transduction process converting calcium binding by troponin C (TnC) into interactions between thin and thick filaments. Once calcium is bound, transduction involves changes in protein interactions along the thin filament. The process is thought to involve three different states of actin-tropomyosin (Tm) resulting from changes in troponin's (Tn) interaction with actin-Tm: a blocked (B) state preventing myosin interaction, a closed (C) state allowing weak myosin interactions and favored by calcium binding to Tn, and an open or M state allowing strong myosin interactions. This was tested by measuring the apparent rate of Tn dissociation from rigor skeletal myofibrils using labeled Tn exchange. The location and rate of exchange of Tn or its subunits were measured by high-resolution fluorescence microscopy and image analysis. Three different rates of Tn exchange were observed that were dependent on calcium concentration and strong cross-bridge binding that strongly support the three-state model. The rate of Tn dissociation in the non-overlap region was 200-fold faster at pCa 4 (C-state region) than at pCa 9 (B-state region). When Tn contained engineered TnC mutants with weakened regulatory TnI interactions, the apparent exchange rate at pCa 4 in the non-overlap region increased proportionately with TnI-TnC regulatory affinity. This suggests that the mechanism of calcium enhancement of the rate of Tn dissociation is by favoring a TnI-TnC interaction over a TnI-actin-Tm interaction. At pCa 9, the rate of Tn dissociation in the overlap region (M-state region) was 100-fold faster than the non-overlap region (B-state region) suggesting that strong cross-bridges increase the rate of Tn dissociation. At pCa 4, the rate of Tn dissociation was twofold faster in the non-overlap region (C-state region) than the overlap region (M-state region) that likely involved a strong cross-bridge influence on TnT's interaction with actin-Tm. At sub-maximal calcium (pCa 6.2-5.8), there was a long-range influence of the strong cross-bridge on Tn to enhance its dissociation rate, tens of nanometers from the strong cross-bridge. These observations suggest that the three different states of actin-Tm are associated with three different states of Tn. They also support a model in which strong cross-bridges shift the regulatory equilibrium from a TnI-actin-Tm interaction to a TnC-TnI interaction that likely enhances calcium binding by TnC.     

Partial characterization of the human beta-myosin heavy-chain gene which is expressed in heart and skeletal muscle

A human myosin heavy-chain gene, cloned in gamma Charon 4A phage (and as a clone designated lambda gMHC-1), was shown to code for a cardiac myosin heavy chain of the beta-type. The 5' end of the 14,200-base-pair genomic DNA clone is located in the head region of the myosin chain. The 3' end was shown to extent to the COOH terminus and includes the 3'-nontranslated sequence of the corresponding mRNA. The identification of lambda gMHC-1 as coding for a cardiac beta-myosin heavy chain was achieved by heteroduplex mapping using genomic cardiac myosin heavy-chain DNA of rabbit as a probe and, furthermore, by DNA sequence analysis of three selected subregions of the clones DNA including the 3'-nontranslated sequence. It was demonstrated by the S1 nuclease protection technique that the beta-myosin heavy-chain gene is transcribed in human heart muscle. In addition, we have found by the same technique that it is also expressed in human skeletal muscle.     

High-sensitivity troponin T in asymptomatic severe aortic stenosis

Background: In asymptomatic severe aortic stenosis (ASAS), treatment decisions are made on an individual basis, and case management presents a clinical conundrum.

Methods: We prospectively phenotyped consecutive patients with ASAS using echocardiography, exercise echocardiography, cardiac MRI and biomarkers (NT-proBNP, high-sensitivity troponin T (hs-TnT) and ST2) (n?=?58). The primary endpoint was a composite of cardiovascular death, new-onset symptoms, cardiac hospitalization, guideline-driven indication for valve replacement and cardiovascular death at 12?months.

Results: During the first year, 46.6% patients met primary endpoint. In multivariable analysis, aortic regurgitation ≥2 (p?=?0.01) and hs-TnT (p?=?0.007) were the only independent predictors of the primary endpoint. The best cutoff value was identified as hs-TnT >10ng/L, which was associated with a ～10-fold greater risk of the primary endpoint (HR, 9.62; 95% CI, 2.27–40.8; p?=?0.002). A baseline predictive model including age, sex and variables showing p?<?0.10 in univariable analyses showed an area under the curve (AUC) of 0.79(0.66–0.91). Incorporation of hs-TnT into this model increased the AUC to 0.90(0.81–0.98) (p?=?0.03). Patient reclassification with the model including hs-TnT yielded an NRI of 1.28(0.46–1.78), corresponding to 43% adequately reclassified patients.

Conclusions: In patients with ASAS, hs-TnT >10ng/L was associated with high risk of events within 12?months. Including hs-TnT in routine ASAS management markedly improved prediction metrics.     

Cardiac troponin T in developing, regenerating and denervated rat skeletal muscle

Fetal rat skeletal muscles express a troponin T (TnT) isoform similar to the TnT isoform expressed in the embryonic heart with respect to electrophoretic mobility and immunoreactivity with cardiac TnT-specific monoclonal antibodies. Immunoblotting analyses reveal that both the embryonic and the adult isoforms of cardiac TnT are transiently expressed during the neonatal stages. In addition, other TnT species, different from both cardiac TnTs and from the TnT isoforms expressed in adult muscles, are present in skeletal muscles during the first two postnatal weeks. By immunocytochemistry, cardiac TnT is detectable at the somitic stage and throughout embryonic and fetal development, and disappears during the first weeks after birth, persisting exclusively in the bag fibers of the muscle spindles. Cardiac TnT is re-expressed in regenerating muscle fibers following a cold injury and in mature muscle fibers after denervation. Developmental regulation of this TnT variant is not coordinated with that of the embryonic myosin heavy chain with respect to timing of disappearance and cellular distribution. No obligatory correlation between the two proteins is likewise found in regenerating and denervated muscles.     

Addition of heart score to high-sensitivity troponin T versus conventional troponin T in risk stratification of patients with chest pain at the coronary emergency rooms

Patients with chest pain have a large impact on available resources in coronary emergency rooms (CER). Clinical judgement, ECG, risk scores and biomarkers guide in risk stratification. We investigated if high-sensitivity troponin T (HsT) and the HEART Score could contribute to risk stratification at the CER. All patients with chest pain, without elevated conventional troponin levels at presentation, were included. HsT levels were determined at admission (T1), at 4–6 h (T2) and 8–10 h after symptom onset (T3). The HEART Score was calculated as risk score for the occurrence of a major adverse cardiac event (MACE). Thirty days after discharge, occurrence of MACE was registered. Eighty-nine patients were included (overall mean age 61 years (range 20–90)). At presentation, 68 patients (76 %) had a HsT below cut-off value of 14 ng/l (mean HEART Score 3.7, range 1–9). Thirty-one of these 68 patients had a HEART Score between 1–3, no MACE occurred in this group. For 3 patients (4 %) HsT levels increased above 14 ng/l. These 3 patients had a HEART Score between 4–6. The majority of patients with chest pain can be safely discharged within 4–6 h after onset of symptoms using HsT and the HEART Score. In contrast, patients with initially normal HsT but a high HEART Score need longer follow-up and repeat HsT determination.     

Circulating plasma serine208‐phosphorylated troponin T levels are indicator of cardiac dysfunction

Heart failure (HF) following myocardial infarction (MI) is characterized by progressive alterations of left ventricular (LV) structure and function, named LV remodelling. Although several risk factors such as infarct size have been identified, HF remains difficult to predict in clinical practice. Recently, using phosphoproteomic technology, we found that serine²⁰⁸‐phosphorylated troponin T (P‐Ser²⁰⁸‐TnT) decreases in LV of HF rats. Our aim was to determine the performance of P‐Ser²⁰⁸‐TnT as plasma biomarker of HF compared to conventional cardiac biomarkers such as B‐type natriuretic peptide (BNP), cardiac troponin I (cTnI), C‐reactive protein (CRP) or tissue inhibitor of metalloproteinase I (TIMP‐1) measured by x‐MAP technology, as well as its capacity to reflect a pharmacological improvement of HF. We observed a significant increase of BNP, TnT and cTnI levels and a significant decrease of P‐Ser²⁰⁸‐TnT and TIMP‐1 in the plasma of 2‐month‐MI rats compared with control rats with no modulation of CRP level. Circulating levels of P‐Ser²⁰⁸‐TnT were shown to be associated with most of the echocardiographic and haemodynamic parameters of cardiac function. We verified that the decrease of P‐Ser²⁰⁸‐TnT was not because of an excess of phosphatase activity in plasma of HF rats. Two‐month‐MI rats treated with the heart rate reducing agent ivabradine had improved LV function and increased plasma levels of P‐Ser²⁰⁸‐TnT. Thus, circulating phosphorylated troponin T is a highly sensitive biological indicator of cardiac dysfunction and has the potentiality of a new biomarker of HF post‐MI, and of a surrogate marker for the efficacy of a successful treatment of HF.     

Developmental changes of cardiac and slow skeletal muscle troponin T expression in chicken cardiac and skeletal muscles

Numerous troponin T (TnT) isoforms are produced by alternative splicing from three genes characteristic of cardiac, fast skeletal, and slow skeletal muscles. Apart from the developmental transition of fast skeletal muscle TnT isoforms, switching of TnT expression during muscle development is poorly understood. In this study, we investigated precisely and comprehensively developmental changes in chicken cardiac and slow skeletal muscle TnT isoforms by two-dimensional gel electrophoresis and immunoblotting with specific antisera. Four major isoforms composed of two each of higher and lower molecular weights were found in cardiac TnT (cTnT). Expression of cTnT changed from high- to low-molecular-weight isoforms during cardiac muscle development. On the other hand, such a transition was not found and only high-molecular-weight isoforms were expressed in the early stages of chicken skeletal muscle development. Two major and three minor isoforms of slow skeletal muscle TnT (sTnT), three of which were newly found in this study, were expressed in chicken skeletal muscles. The major sTnT isoforms were commonly detected throughout development in slow and mixed skeletal muscles, and at developmental stages until hatching-out in fast skeletal muscles. The expression of minor sTnT isoforms varied from muscle to muscle and during development.     

Pyruvate carboxylase is expressed in human skeletal muscle

Pyruvate carboxylase (PC) is a mitochondrial enzyme that catalyses the carboxylation of pyruvate to oxaloacetate thereby allowing supplementation of citric acid cycle intermediates. The presence of PC in skeletal muscle is controversial. We report here, that PC protein is easily detectable by streptavidin blot and describe the presence of considerable amounts of PC in cultured human myotubes and in human muscle tissue.     

The mobility of troponin C and troponin I in muscle

In vertebrate skeletal muscle, contraction is initiated by the elevation of the intracellular Ca²⁺ concentration. The binding of Ca²⁺ to TnC induces a series of conformational changes which ultimately release the inhibition of the actomyosin ATPase activity by Tnl. In this study we have characterized the dynamic behavior of TnC and Tnl in solution, as well as in reconstituted fibers, using EPR and ST-EPR spectroscopy. Cys98 of TnC and Cys133 of Tnl were specifically labeled with malemide spin label (MSL) and indane dione nitroxide spin label (InVSL). In solution, the labeled TnC and Tnl exhibited fast nanosecond motion. MSL-TnC is sensitive to cation binding to the high affinity sites (τ_r increases from 1.5 to 3.7 ns), InVSL-TnC s sensitive to the replacement of Mg²⁺ by Ca²⁺ at these sites (τ_r increase from 1.7 to 6 ns). Upon reconstitution into fibers, the nanosecond mobility is reduced by interactions with other proteins. TnC and Tnl both exhibited microsecond anisotropic motion in fibers similar to that of the actin monomers within the filament. The microsecond motion of TnC was found to be modulated by the binding of Ca²⁺ and by cross-bridge attachment, but this was not the case for the global mobility of Tnl. © 1997 John Wiley & Sons, Ltd.     

Role of troponin T in disease

Several striated muscle myopathies have been directly linked to mutations in contractile and associated proteins. Troponin T (TnT) is one of the three subunits that form troponin (Tn) which together with tropomyosin is responsible for the regulation of striated muscle contraction. All three subunits of cardiac Tn as well as tropomyosin have been associated with hypertrophic cardiomyopathy (HCM). However, TnT accounts for most of the mutations that cause HCM in these regulatory proteins. To date 30 mutations have been identified in the cardiac TnT (CTnT) gene that results in familial HCM (FHC). The CTnT gene has also been associated with familial dilated cardiomyopathy (DCM). CTnT deficiency is lethal due to impaired cardiac development. A recessive nonsense mutation in the gene encoding slow skeletal TnT has been associated with an unusual, severe form of nemaline myopathy among the Old Order Amish. How each mutation leads to the diverse clinical symptoms associated with FHC, DCM or nemaline myopathy is unclear. However, the use of animal model systems, in particular transgenic mice, has significantly increased our knowledge of normal and myopathic muscle physiology. In this review, we focus on the role of TnT in muscle physiology and disease. (Mol Cell Biochem 263: 115–129, 2004)