Troponin T: genetics, properties and function

Adeno-associated virus: a key to the human genome?

Adeno-associated viruses (AAV) are widely spread throughout the human population, yet no pathology has been associated with infection. This fact, together with the availability of simple molecular techniques to alter the packaged viral genome, has made AAV a serious contender in the search for an ideal gene therapy delivery vehicle. However, our understanding of the intriguing features of this virus is far from exhausted and it is likely that the mechanisms underlying the viral lifestyle will reveal possible novel strategies that can be employed in future clinical approaches. One such aspect is the unique approach AAV has evolved in order to establish latency. In the absence of a cellular milieu that will support productive viral replication, wild-type AAV can integrate its genome site specifically into a locus on human chromosome 19 (termed AAVS1), where it resides without apparent effects on the host cell until cellular conditions are changed by outside influences, such as adenovirus super-infection, which will lead to the rescue of the viral genome and productive replication. This article will introduce the biology of AAV, the unique viral strategy of targeted genome integration and address relevant questions within the context of attempts to establish therapeutic approaches that will utilize targeted gene addition to the human genome.

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

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

The role of tropomyosin isoforms and phosphorylation in force generation in thin-filament reconstituted bovine cardiac muscle fibres

The thin filament extraction and reconstitution protocol was used to investigate the functional roles of tropomyosin (Tm) isoforms and phosphorylation in bovine myocardium. The thin filament was extracted by gelsolin, reconstituted with G-actin, and further reconstituted with cardiac troponin together with one of three Tm varieties: phosphorylated alphaTm (alphaTm.P), dephosphorylated alphaTm (alphaTm.deP), and dephosphorylated betaTm (betaTm.deP). The effects of Ca, phosphate, MgATP and MgADP concentrations were examined in the reconstituted fibres at pH 7.0 and 25 degrees C. Our data show that Ca(2+) sensitivity (pCa(50): half saturation point) was increased by 0.19 +/- 0.07 units when betaTm.deP was used instead of alphaTm.deP (P < 0.05), and by 0.27 +/- 0.06 units when phosphorylated alphaTm was used (P < 0.005). The cooperativity (Hill factor) decreased (but insignificantly) from 3.2 +/- 0.3 (5) to 2.8 +/- 0.2 (7) with phosphorylation. The cooperativity decreased significantly from 3.2 +/- 0.3 (5) to 2.1 +/- 0.2 (9) with isoform change from alphaTm.deP to betaTm.deP. There was no significant difference in isometric tension or stiffness between alphaTm.P, alphaTm.deP, and betaTm.deP muscle fibres at saturating [Ca(2+)] or after rigor induction. Based on the six-state cross-bridge model, sinusoidal analysis indicated that the equilibrium constants of elementary steps differed up to 1.7x between alphaTm.deP and betaTm.deP, and up to 2.0x between alphaTm.deP and alphaTm.P. The rate constants differed up to 1.5x between alphaTm.deP and betaTm.deP, and up to 2.4x between alphaTm.deP and alphaTm.P. We conclude that tension and stiffness per cross-bridge are not significantly different among the three muscle models.

Structure and sequence of the human fast skeletal troponin T (TNNT3) gene: insight into the evolution of the gene and the origin of the developmentally regulated isoforms

We describe the cloning, sequencing and structure of the human fast skeletal troponin T (TNNT3) gene located on chromosome 11p15.5. The single-copy gene encodes 19 exons and 18 introns. Eleven of these exons, 1–3, 9–15 and 18, are constitutively spliced, whereas exons 4–8 are alternatively spliced. The gene contains an additional subset of developmentally regulated and alternatively spliced exons, including a foetal exon located between exon 8 and 9 and exon 16 or α (adult) and 17 or β (foetal and neonatal). Exon phasing suggests that the majority of the alternatively spliced exons located at the 5′ end of the gene may have evolved as a result of exon shuffling, because they are of the same phase class. In contrast, the 3′ exons encoding an evolutionarily conserved heptad repeat domain, shared by both TnT and troponin I (TnI), may be remnants of an ancient ancestral gene. The sequence of the 5′ flanking region shows that the putative promoter contains motifs including binding sites for MyoD, MEF-2 and several transcription factors which may play a role in transcriptional regulation and tissue-specific expression of TnT. The coding region of TNNT3 exhibits strong similarity to the corresponding rat sequence. However, unlike the rat TnT gene, TNNT3 possesses two repeat regions of CCA and TC. The exclusive presence of these repetitive elements in the human gene indicates divergence in the evolutionary dynamics of mammalian TnT genes. Homologous muscle-specific splicing enhancer motifs are present in the introns upstream and downstream of the foetal exon, and may play a role in the developmental pattern of alternative splicing of the gene. The genomic correlates of TNNT3 are relevant to our understanding of the evolution and regulation of expression of the gene, as well as the structure and function of the protein isoforms. The nucleotide sequence of TNNT3 has been submitted to EMBL/GenBank under Accession No. AF026276.

Mutual rescues between two dominant negative mutations in cardiac troponin I and cardiac troponin T

Troponin T (TnT) and troponin I (TnI) are two evolutionarily and functionally linked subunits of the troponin complex that regulates striated muscle contraction. We previously reported a single amino acid substitution in the highly conserved TnT-binding helix of cardiac TnI (cTnI) in wild turkey hearts in concurrence with an abnormally spliced myopathic cardiac TnT (cTnT) (Biesiadecki, B. J., Schneider, K. L., Yu, Z. B., Chong, S. M., and Jin, J. P. (2004) J. Biol. Chem. 279, 13825-13832). To investigate the functional effect of this cTnI mutation and its potential value in compensating for the cTnT abnormality, we developed transgenic mice expressing the mutant cTnI (K118C) in the heart with or without the deletion of the endogenous cTnI gene to mimic the homozygote and heterozygote of wild turkeys. Double and triple transgenic mice were created by crossing the cTnI-K118C lines with transgenic mice overexpressing the myopathic cTnT (exon 7 deletion). Functional studies of ex vivo working hearts found that cTnI-K118C alone had a dominantly negative effect on diastolic function and blunted the inotropic responses of cardiac muscle to beta-adrenergic stimuli without abolishing the protein kinase A-dependent phosphorylation of cTnI. When co-expressed with the cTnT mutation, cTnI-K118C corrected the significant depression of systolic function caused by cTnT exon 7 deletion, and the co-existence of exon 7-deleted cTnT minimized the diastolic abnormality of cTnI-K118C. Characterization of this naturally selected pair of mutually rescuing mutations demonstrated that TnI-TnT interaction is a critical link in the Ca(2+) signaling and beta-adrenergic regulation in cardiac muscle, suggesting a potential target for the treatment of troponin cardiomyopathies and heart failure.

Localization of the two tropomyosin-binding sites of troponin T

Troponin T (TnT) binds to tropomyosin (Tm) to anchor the troponin complex in the thin filament, and it thus serves as a vital link in the Ca(2+) regulation of striated muscle contraction. Pioneer work three decades ago determined that the T1 and T2 chymotryptic fragments of TnT each contains a Tm-binding site. A more precise localization of the two Tm-binding sites of TnT remains to be determined. In the present study, we tested serial deletion constructs of TnT and carried out monoclonal antibody competition experiments to show that the T1 region Tm-binding site involves mainly a 39 amino acids segment in the N-terminal portion of the conserved middle region of TnT. We further employed another set of TnT fragments to locate the T2 region Tm-binding site to a segment of 25 amino acids near the beginning of the T2 fragment. The localization of the two Tm-binding sites of TnT provided new information for the structure-function relationship of TnT and the anchoring of troponin complex on muscle thin filament.

Cardiac troponin mutations and restrictive cardiomyopathy

Mutations in sarcomeric proteins have recently been established as heritable causes of Restrictive Cardiomyopathy (RCM). RCM is clinically characterized as a defect in cardiac diastolic function, such as, impaired ventricular relaxation, reduced diastolic volume and increased end-diastolic pressure. To date, mutations have been identified in the cardiac genes for desmin, α-actin, troponin I and troponin T. Functional studies in skinned muscle fibers reconstituted with troponin mutants have established phenotypes consistent with the clinical findings which include an increase in myofilament Ca²⁺ sensitivity and basal force. Moreover, when RCM mutants are incorporated into reconstituted myofilaments, the ability to inhibit the ATPase activity is reduced. A majority of the mutations cluster in specific regions of cardiac troponin and appear to be mutational “hot spots”. This paper highlights the functional and clinical characteristics of RCM linked mutations within the troponin complex.

Coexistence of cardiac troponin T variants reduces heart efficiency

Corresponding to the synchronized contraction of the myocardium and rhythmic pumping function of the heart, a single form of cardiac troponin T (cTnT) is present in the adult cardiac muscle of humans and most other vertebrate species. Alternative splicing variants of cTnT are found in failing human hearts and animal dilated cardiomyopathies. Biochemical analyses have shown that these cTnT variants are functional and produce shifted myofilament Ca(2+) sensitivity. We proposed a hypothesis that the coexistence of two or more functionally distinct TnT variants in the adult ventricular muscle that is normally activated as a syncytium may decrease heart function and cause cardiomyopathy (Huang et al., Am J Physiol Cell Physiol 294: C213-C222, 2008). In the present study, we studied transgenic mouse hearts expressing one or two cTnT variants in addition to normal adult cTnT to investigate whether desynchronized myofilament activation decreases ventricular efficiency. The function of ex vivo working hearts was examined in the absence of systemic neurohumoral influence. The results showed that the transgenic mouse hearts produced lower maximum left ventricular pressure, slower contractile and relaxation velocities, and decreased stroke volume compared with wild-type controls. Ventricular pumping efficiency, calculated by the ejection integral versus total systolic integral and cardiac work versus oxygen consumption, was significantly lower in transgenic mouse hearts and corresponded to the number of cTnT variants present. The results indicated a pathogenic mechanism in which the coexistence of functionally different cTnT variants in cardiac muscle reduces myocardial efficiency due to desynchronized thin filament activation.

Deletion of a genomic segment containing the cardiac troponin I gene knocks down expression of the slow troponin T gene and impairs fatigue tolerance of diaphragm muscle

The loss of slow skeletal muscle troponin T (TnT) results in a recessive nemaline myopathy in the Amish featured with lethal respiratory failure. The genes encoding slow TnT and cardiac troponin I (TnI) are closely linked. Ex vivo promoter analysis suggested that the 5'-enhancer region of the slow TnT gene overlaps with the structure of the upstream cardiac TnI gene. Using transgenic expression of exogenous cardiac TnI to rescue the postnatal lethality of a mouse line in which the entire cardiac TnI gene was deleted, we investigated the effect of enhancer deletion on slow TnT gene expression in vivo and functional consequences. The levels of slow TnT mRNA and protein were significantly reduced in the diaphragm muscle of adult double transgenic mice. The slow TnT-deficient (ssTnT-KD) diaphragm muscle exhibited atrophy and decreased ratios of slow versus fast isoforms of TnT, TnI, and myosin. Consistent with the changes toward more fast myofilament contents, ssTnT-KD diaphragm muscle required stimulation at higher frequency for optimal tetanic force production. The ssTnT-KD diaphragm muscle also exhibited significantly reduced fatigue tolerance, showing faster and more declines of force with slower and less recovery from fatigue as compared with the wild type controls. The natural switch to more slow fiber contents during aging was partially blunted in the ssTnT-KD skeletal muscle. The data demonstrated a critical role of slow TnT in diaphragm function and in the pathogenesis and pathophysiology of Amish nemaline myopathy.

Muscle differentiation in a colonial ascidian: organisation, gene expression and evolutionary considerations

BACKGROUND

Ascidians are tunicates, the taxon recently proposed as sister group to the vertebrates. They possess a chordate-like swimming larva, which metamorphoses into a sessile adult. Several ascidian species form colonies of clonal individuals by asexual reproduction. During their life cycle, ascidians present three muscle types: striated in larval tail, striated in the heart, and unstriated in the adult body-wall.

RESULTS

In the colonial ascidian Botryllus schlosseri, we investigated organisation, differentiation and gene expression of muscle beginning from early buds to adults and during zooid regression. We characterised transcripts for troponin T (BsTnT-c), adult muscle-type (BsMA2) and cytoplasmic-type (BsCA1) actins, followed by in situ hybridisation (ISH) on sections to establish the spatio-temporal expression of BsTnT-c and BsMA2 during asexual reproduction and in the larva. Moreover, we characterised actin genomic sequences, which by comparison with other metazoans revealed conserved intron patterns.

CONCLUSION

Integration of data from ISH, phalloidin staining and TEM allowed us to follow the phases of differentiation of the three muscle kinds, which differ in expression pattern of the two transcripts. Moreover, phylogenetic analyses provided evidence for the close relationship between tunicate and vertebrate muscle genes. The characteristics and plasticity of muscles in tunicates are discussed.

A proteomics analysis of the effects of chronic hemiparetic stroke on troponin T expression in human vastus lateralis

Stroke disability is attributed to upper motor neuron deficits resulting from ischemic brain injury. We have developed proteome maps of the Vastus lateralis to examine the effects of ischemic brain injury on paretic skeletal muscle myofilament proteins. Proteomics analyses from seven hemiparetic stroke patients have detected a decrease of three troponin T isoforms in the paretic muscle suggesting that myosin-actin interactions may be attenuated. We propose that ischemic brain injury may prevent troponin T participation in complex formation thereby affecting the protein interactions associated with excitation-contraction coupling. We have also detected a novel skeletal troponin T isoform that has a C-terminal variation. Our data suggest that the decreased slow troponin T isoform pools in the paretic limb may contribute to the gait deficit after stroke. The complexity of the neurological deficit on Vastus lateralis is suggested by the multiple changes in proteins detected by our proteomics mapping.

Myofilament incorporation determines the stoichiometry of troponin I in transgenic expression and the rescue of a null mutation

The highly organized contractile machinery in skeletal and cardiac muscles requires an assembly of myofilament proteins with stringent stoichiometry. To understand the maintenance of myofilament protein stoichiometry under dynamic protein synthesis and catabolism in muscle cells, we investigated the equilibrium of troponin I (TnI) in mouse cardiac muscle during developmental isoform switching and in under- and over-expression models. Compared with the course of developmental TnI isoform switching in normal hearts, the postnatal presence of slow skeletal muscle TnI lasted significantly longer in the hearts of cardiac TnI (cTnI) knockout (cTnI-KO) mice, in which the diminished synthesis was compensated by prolonging the life of myofilamental TnI. Transgenic postnatal expression of an N-terminal truncated cTnI (cTnI-ND) using alpha-myosin heavy chain promoter effectively rescued the lethality of cTnI-KO mice and shortened the postnatal presence of slow TnI in cardiac muscle. cTnI-KO mice rescued with different levels of cTnI-ND over-expression exhibited similar levels of myocardial TnI comparable to that in wild type hearts, demonstrating that excessive synthesis would not increase TnI stoichiometry in the myofilaments. Consistently, haploid under-expression of cTnI in heterozygote cTnI-KO mice was sufficient to sustain the normal level of myocardial cTnI, indicating that cTnI is synthesized in excess in wild type cardiomyocytes. Altogether, these observations suggest that under wide ranges of protein synthesis and turnover, myofilament incorporation determines the stoichiometry of troponin subunits in muscle cells.

Nonmyofilament-associated troponin T fragments induce apoptosis

Troponin T (TnT) is a striated muscle-specific protein and an abundant component of the myofilaments. Nonmyofilament-associated TnT is rapidly degraded in myocytes, implying an importance in the maintenance of the cellular environment. However, if the level of nonmyofilament-associated TnT or TnT fragments exceeds the degradation capacity, it may cause cytotoxicity. To investigate this hypothesis, we constructed bicistronic vectors to express different portions of TnT polypeptide chain, together with nonfusion green fluorescent protein as a tracer for the transfection. Cytotoxicity of the TnT fragments was studied through forced expression in C(2)C(12) myoblasts and human embryonic kidney-293 nonmuscle cells and examination of the viability of the transfected cells. The results demonstrated that, in the absence of myofilaments, the conserved COOH-terminal and middle fragments of TnT were highly effective on inducing cell death via apoptosis, whereas the NH(2)-terminal variable region was not. As combined effects, nonmyofilament-associated intact cardiac TnT and a COOH-terminal truncated slow TnT fragment found in Amish nemaline myopathy exhibited intermediate cytotoxicity. A particular significance of this finding is that peak releases of TnT or TnT fragments from decomposition of a large number of myofibrils in acute myocardial infarction may breach the cellular protection of proteolytic degradation and result in apoptosis as a potential cause for the loss of cardiomyocytes.

To investigate protein evolution by detecting suppressed epitope structures

Material remains of ancestor nucleotides and proteins are largely unavailable, thus sequence comparison among homologous genes in present-day organisms forms the core of current knowledge of molecular evolution. Variation in protein three-dimensional structure is a basis for functional diversity. To study the evolution of three-dimensional structures in related proteins would significantly improve our understanding of protein evolution and function. A protein may contain ancestor conformations that have been allosterically suppressed by evolutionarily additive structures. Using monoclonal antibody probes to detect such conformation in proteins after removing the suppressor structure, our study demonstrates three-dimensional structure evidence for the evolutionary relationship between troponin I and troponin T, two subunits of the troponin complex in the Ca(2+)-regulatory system of striated muscle, and among their muscle type-specific isoforms. The experimental data show the feasibility of detecting evolutionarily suppressed history-telling structural states in proteins by removing conformational modulator segments added during evolution. In addition to identifying structural modifications that were critical to the emergence of diverged proteins, investigating this novel mode of evolution will help us to understand the origin and functional potential of protein structures.

Comparative transcriptional and biochemical studies in muscle of myotonic dystrophies (DM1 and DM2)

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (proximal muscular myopaty/DM2) are caused by similar dynamic mutations at two distinct genetic loci. The two diseases also lead to similar phenotypes but different clinical severity. Dysregulation of alternative splicing has been suggested as the common pathogenic mechanism. Here, we investigate the molecular differences between DM1 and DM2 using reverse transcriptase-polymerase chain reaction of troponin T (TnT) and the insulin receptor (IR), as well as immunoblotting of TnT in muscle biopsies from DM1 and DM2 patients. We found that: (a) slow TnT was encoded by two different transcripts in significantly different ratios in DM1 and DM2 muscles; (b) DM2 muscles exhibited a higher degree of alternative splicing dysregulation for fast TnT transcripts when compared to DM1 muscles; (c) the distribution of TnT proteins was significantly skewed towards higher molecular weight species in both diseases; (d) the RNA for the insulin-independent IR-A isoform was significantly increased and appeared related to the fibre-type composition in the majority of the cases examined. On the whole, these data should give a better insight on pathogenesis of DM1 and DM2.

Isoform-specific variation in the intrinsic disorder of troponin I

Various intrinsic disorder (ID) prediction algorithms were applied to the three tissue isoforms of troponin I (TnI). The results were interpreted in terms of the known structure and dynamics of troponin. In line with previous results, all isoforms of TnI were predicted to have large stretches of ID. The predictions show that the C-termini of all isoforms are extensively disordered as is the N-terminal extension of the cardiac isoform. Cardiac TnI likely belongs to the group of intrinsically disordered signalling hub proteins. For a given portion of the protein sequence, most ID prediction approaches indicate isoform-dependent variations in the probability of disorder. Comparison of machine learning and physically based approaches suggests the ID variations are only partially attributable to local variations in the ratio of charged to hydrophobic residues. The VSL2B algorithm predicts the largest variations in ID across the isoforms, with the cardiac isoform having the highest probability of structured regions, and the fast-skeletal isoform having no intrinsic structure. The region corresponding to residues 57-95 of the fast-skeletal isoform, known to form a coiled coil substructure with troponin T, was highly variable between isoforms. The isoform-specific ID variations may have mechanistic significance, modulating the extent to which conformational fluctuations in tropomyosin are communicated to the troponin complex. We discuss structural mechanisms for this communication. Overall, the results motivate the development of predictors designed to address relative levels of disorder between highly similar proteins.

Close proximity of myosin loop 3 to troponin determined by triangulation of resonance energy transfer distance measurements

Cooperative activation of the thin filament is known to be influenced by the tight binding of myosin to actin, but the molecular mechanism underlying this contribution of myosin is not well understood. To better understand the structural relationship of myosin with the regulatory troponin complex, resonance energy transfer measurements were used to map the location of troponin relative to a neighboring myosin bound to actin using atomic models. Using a chicken troponin T isoform that contains a single cysteine near the binding interface between troponins T, I, and C, this uniquely labeled cysteine on troponin was found to be remarkably near loop 3 of myosin. This loop has previously been localized near the actin and myosin interface by chemical cross-linking methods, but its functional contributions have not been established. The implications of this close proximity are examined by molecular modeling, which suggests that only restricted conformations of actomyosin can accommodate the presence of troponin at this location near the cross-bridge. This potential for interaction between troponin and myosin heads that bind near it along the thin filament raises the possibility of models in which direct myosin and troponin interactions may play a role in the regulatory mechanism.

Present and Future Biochemical Markers for Detection of Acute Coronary Syndrome

The use of biochemical markers in the diagnosis and management of patients with acute coronary syndrome has increased continually in recent decades. The development of highly sensitive and cardiac-specific troponin assays has changed the view on diagnosis of myocardial infarction and also extended the role of biochemical markers of necrosis into risk stratification and guidance for treatment. The consensus definition of myocardial infarction places increased emphasis on cardiac marker testing, with cardiac troponin replacing creatine kinase MB as the "gold standard" for diagnosis of myocardial infarction. Along with advances in the use of more cardiac-specific markers of myocardial necrosis, biochemical markers that are involved in the progression of atherosclerotic plaques to the vulnerable state or that signal the presence of vulnerable plaques have recently been identified. These markers have variable abilities to predict the risk of an individual for acute coronary syndrome. The aim of this review is to provide an overview of the well-established markers of myocardial necrosis, with a special focus on cardiac troponin I, together with a summary of some of the potential future markers of inflammation, plaque instability, and ischemia.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Adaptation by alternative RNA splicing of slow troponin T isoforms in type 1 but not type 2 Charcot-Marie-Tooth disease

Slow troponin T (TnT) plays an indispensable role in skeletal muscle function. Alternative RNA splicing in the NH(2)-terminal region produces high-molecular-weight (HMW) and low-molecular-weight (LMW) isoforms of slow TnT. Normal adult slow muscle fibers express mainly HMW slow TnT. Charcot-Marie-Tooth disease (CMT) is a group of inherited peripheral polyneuropathies caused by various neuronal defects. We found in the present study that LMW slow TnT was significantly upregulated in demyelination form type 1 CMT (CMT1) but not axonal form type 2 CMT (CMT2) muscles. Contractility analysis showed an increased specific force in single fibers isolated from CMT1 but not CMT2 muscles compared with control muscles. However, an in vitro motility assay showed normal velocity of the myosin motor isolated from CMT1 and CMT2 muscle biopsies, consistent with their unchanged myosin isoform contents. Supporting a role of slow TnT isoform regulation in contractility change, LMW and HMW slow TnT isoforms showed differences in the molecular conformation in conserved central and COOH-terminal regions with changed binding affinity for troponin I and tropomyosin. In addition to providing a biochemical marker for the differential diagnosis of CMT, the upregulation of LMW slow TnT isoforms under the distinct pathophysiology of CMT1 demonstrates an adaptation of muscle function to neurological disorders by alternative splicing modification of myofilament proteins.

Skeletal muscle contractile protein function is preserved in human heart failure

Skeletal muscle weakness is a common finding in patients with chronic heart failure (CHF). This functional deficit cannot be accounted for by muscle atrophy alone, suggesting that the syndrome of heart failure induces a myopathy in the skeletal musculature. To determine whether decrements in muscle performance are related to alterations in contractile protein function, biopsies were obtained from the vastus lateralis muscle of four CHF patients and four control patients. CHF patients exhibited reduced peak aerobic capacity and knee extensor muscle strength. Decrements in whole muscle strength persisted after statistical control for muscle size. Thin filaments and myosin were isolated from biopsies and mechanically assessed using the in vitro motility assay. Isolated skeletal muscle thin-filament function, however, did not differ between CHF patients and controls with respect to unloaded shortening velocity, calcium sensitivity, or maximal force. Similarly, no difference in maximal force or unloaded shortening velocity of isolated myosin was observed between CHF patients and controls. From these results, we conclude that skeletal contractile protein function is unaltered in CHF patients. Other factors, such as a decrease in total muscle myosin content, are likely contributors to the skeletal muscle strength deficit of heart failure.

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.

Co-expression of skeletal and cardiac troponin T decreases mouse cardiac function

In contrast to skeletal muscles that simultaneously express multiple troponin T (TnT) isoforms, normal adult human cardiac muscle contains a single isoform of cardiac TnT. To understand the significance of myocardial TnT homogeneity, we examined the effect of TnT heterogeneity on heart function. Transgenic mouse hearts overexpressing a fast skeletal muscle TnT together with the endogenous cardiac TnT was investigated in vivo and ex vivo as an experimental system of concurrent presence of two classes of TnT in the adult cardiac muscle. This model of myocardial TnT heterogeneity produced pathogenic phenotypes: echocardiograph imaging detected age-progressive reductions of cardiac function; in vivo left ventricular pressure analysis showed decreased myocardial contractility; ex vivo analysis of isolated working heart preparations confirmed an intrinsic decrease of cardiac function in the absence of neurohumoral influence. The transgenic mice also showed chronic myocardial hypertrophy and degeneration. The dominantly negative effects of introducing a fast TnT into the cardiac thin filaments to produce two classes of Ca(2+) regulatory units in the adult myocardium suggest that TnT heterogeneity decreases contractile function by disrupting the synchronized action during ventricular contraction that is normally activated as an electrophysiological syncytium.

Regulation of troponin T expression during muscle development in sea bream Sparus auratus Linnaeus: the potential role of thyroid hormones

In the sea bream Sparus auratus three stage-specific fast troponin T (fTnT) isoforms have been cloned and correspond to embryonic-, larval- and adult-specific isoforms. Characterisation, using database searches, of the putative genomic organisation of Fugu rubripes and Tetraodon nigroviridis fTnT indicates that alternative exon splicing in the 5 region of the gene generates the different isoforms. Moreover, comparison of teleost fTnTs suggests that alternative splicing of fTnT appears to be common in teleosts. A different temporal expression pattern for each fTnT splice varotnt is found during sea bream development and probably relates to differing functional demands, as a highly acidic embryonic form (pI 5.16) is substituted by a basic larval form (pI 9.57). Thyroid hormones (THs), which play an important regulatory role in muscle development in flatfish and tetrapods, appear also to influence TnT gene expression in the sea bream. However, THs have a divergent action on different sea bream TnT genes and although the slow isoform (sTnT1) is TH-responsive, fTnT, sTnT2 and the itronless isoform (iTnT) are unaffected. The present results taken together with those published for flatfish seem to suggest differences may exist in the regulation of larval muscle development in teleosts.

Aberrant splicing of an alternative exon in the Drosophila troponin-T gene affects flight muscle development

During myofibrillogenesis, many muscle structural proteins assemble to form the highly ordered contractile sarcomere. Mutations in these proteins can lead to dysfunctional muscle and various myopathies. We have analyzed the Drosophila melanogaster troponin T (TnT) up1 mutant that specifically affects the indirect flight muscles (IFM) to explore troponin function during myofibrillogenesis. The up1 muscles lack normal sarcomeres and contain "zebra bodies," a phenotypic feature of human nemaline myopathies. We show that the up(1) mutation causes defective splicing of a newly identified alternative TnT exon (10a) that encodes part of the TnT C terminus. This exon is used to generate a TnT isoform specific to the IFM and jump muscles, which during IFM development replaces the exon 10b isoform. Functional differences between the 10a and 10b TnT isoforms may be due to different potential phosphorylation sites, none of which correspond to known phosphorylation sites in human cardiac TnT. The absence of TnT mRNA in up1 IFM reduces mRNA levels of an IFM-specific troponin I (TnI) isoform, but not actin, tropomyosin, or troponin C, suggesting a mechanism controlling expression of TnT and TnI genes may exist that must be examined in the context of human myopathies caused by mutations of these thin filament proteins.

Troponin T isoform expression is modulated during Atlantic halibut metamorphosis

BACKGROUND

Flatfish metamorphosis is a thyroid hormone (TH) driven process which leads to a dramatic change from a symmetrical larva to an asymmetrical juvenile. The effect of THs on muscle and in particular muscle sarcomer protein genes is largely unexplored in fish. The change in Troponin T (TnT), a pivotal protein in the assembly of skeletal muscles sarcomeres and a modulator of calcium driven muscle contraction, during flatfish metamophosis is studied.

RESULTS

In the present study five cDNAs for halibut TnT genes were cloned; three were splice variants arising from a single fast TnT (fTnT) gene; a fourth encoded a novel teleost specific fTnT-like cDNA (AfTnT) expressed exclusively in slow muscle and the fifth encoded the teleost specific sTnT2. THs modified the expression of halibut fTnT isoforms which changed from predominantly basic to acidic isoforms during natural and T4 induced metamorphosis. In contrast, expression of red muscle specific genes, AfTnT and sTnT2, did not change during natural metamorphosis or after T4 treatment. Prior to and after metamorphosis no change in the dorso-ventral symmetry or temporal-spatial expression pattern of TnT genes and muscle fibre organization occurred in halibut musculature.

CONCLUSION

Muscle organisation in halibut remains symmetrical even after metamorphosis suggesting TH driven changes are associated with molecular adaptations. We hypothesize that species specific differences in TnT gene expression in teleosts underlies different larval muscle developmental programs which better adapts them to the specific ecological constraints.

Role of myofibril-inducing RNA in cardiac TnT expression in developing Mexican axolotl

The Mexican axolotl, Ambystoma mexicanum, has been a useful animal model to study heart development and cardiac myofibrillogenesis. A naturally-occurring recessive mutant, gene "c", for cardiac non-function in the Mexican axolotl causes a failure of myofibrillogenesis due to a lack of tropomyosin expression in homozygous mutant (c/c) embryonic hearts. Myofibril-inducing RNA (MIR) rescues mutant hearts in vitro by promoting tropomyosin expression and myofibril formation thereafter. We have studied the effect of MIR on the expression of various isoforms of cardiac troponin T (cTnT), a component of the thin filament that binds with tropomyosin. Four alternatively spliced cTnT isoforms have been characterized from developing axolotl heart. The expression of various cTnT isoforms in normal, mutant, and mutant hearts corrected with MIR, is evaluated by real-time RT-PCR using isoform specific primer pairs; MIR affects the total transcription as well as the splicing of the cTnT in axolotl heart.

Troponin T core structure and the regulatory NH2-terminal variable region

The conserved central and COOH-terminal regions of troponin T (TnT) interact with troponin C, troponin I, and tropomyosin to regulate striated muscle contraction. Phylogenic data show that the NH2-terminal region has evolved as an addition to the conserved core structure of TnT. This NH2-terminal region does not bind other thin filament proteins, and its sequence is hypervariable between fiber type and developmental isoforms. Previous studies have demonstrated that NH2-terminal modifications alter the COOH-terminal conformation of TnT and thin filament Ca2+-activation, yet the functional core structure of TnT and the mechanism of NH2-terminal modulation are not well understood. To define the TnT core structure and investigate the regulatory role of the NH2-terminal variable region, we investigated two classes of model TnT molecules: (1) NH2-terminal truncated cardiac TnT and (2) chimera proteins consisting of an acidic or basic skeletal muscle TnT NH2-terminus spliced to the cardiac TnT core. Deletion of the TnT hypervariable NH2-terminus preserved binding to troponin I and tropomyosin and sustained cardiac muscle contraction in the heart of transgenic mice. Further deletion of the conserved central region diminished binding to tropomyosin. The reintroduction of differently charged NH2-terminal domains in the chimeric molecules produced long-range conformational changes in the central and COOH-terminal regions to alter troponin I and tropomyosin binding. Similar NH2-terminal charge effects are demonstrated in naturally occurring cardiac TnT isoforms, indicating a physiological significance. These results suggest that the hypervariable NH2-terminal region modulates the conformation and function of the TnT core structure to fine-tune muscle contractility.

Fluorescence spectroscopic analysis of the proximity changes between the central helix of troponin C and the C-terminus of troponin T from chicken skeletal muscle

Recent structural studies of the troponin (Tn) core complex have shown that the regulatory head containing the N-lobe of TnC is connected to the IT arm by a flexible linker of TnC. The IT arm is a long coiled-coil formed by alpha-helices of TnI and TnT, plus the C-lobe of TnC. The TnT is thought to play a pivotal role in the linking of Ca(2+) -triggered conformational changes in thin filament regulatory proteins to the activation of cross-bridge cycling. However, a functional domain at the C-terminus of TnT is missing from the Tn core complex. In this study, we intended to determine the proximity relationship between the central helix of TnC and the TnT C-terminus in the binary and the ternary complex with and without Ca2+ by using pyrene excimer fluorescence spectroscopy and fluorescence resonance energy transfer. Chicken fast skeletal TnC contains a Cys102 at the E helix, while TnT has a Cys264 at its C-terminus. These two cysteines were specifically labeled with sulfhydryl-reactive fluorescence probes. The measured distance in the binary complex was about 19 Angstroms and slightly increased when they formed the ternary complex with TnI (20 Angstroms). Upon Ca2+ binding the distance was not affected in the binary complex but increased by approximately 4 Angstroms in the ternary complex. These results suggest that TnI plays an essential role in the Ca(2+) -mediated change in the spatial relationship between the C-lobe of TnC and the C-terminus of TnT.

Tissue-specific interactions of TNI isoforms with other TN subunits and tropomyosins in C. elegans: the role of the C- and N-terminal extensions

The aim of this study is to investigate the function of the C-terminal extension of three troponin I isoforms, that are unique to the body wall muscles of Caenorhabditis elegans and to understand the molecular interactions within the TN complex between troponin I with troponin C/T, and tropomyosin. We constructed several expression vectors to generate recombinant proteins of three body wall and one pharyngeal troponin I isoforms in Escherichia coli. Protein overlay assays and Western blot analyses were performed using antibodies. We demonstrated that pharyngeal TNI-4 interacted with only the pharyngeal isoforms of troponin C/T and tropomyosin. In contrast, the body wall TNI-2 bound both the body wall and pharyngeal isoforms of these components. Similar to other invertebrates, the N-terminus of troponin I contributes to interactions with troponin C. Full-length troponin I was essential for interactions with tropomyosin isoforms. Deletion of the C-terminal extension had no direct effect on the binding of the body wall troponin I to other muscle thin filament troponin C/T and tropomyosin isoforms.

Molecular and immunohistochemical analyses of cardiac troponin T during cardiac development in the Mexican axolotl,Ambystoma mexicanum

The Mexican axolotl, Ambystoma mexicanum, is an excellent animal model for studying heart development because it carries a naturally occurring recessive genetic mutation, designated gene c, for cardiac nonfunction. The double recessive mutants (c/c) fail to form organized myofibrils in the cardiac myoblasts resulting in hearts that fail to beat. Tropomyosin expression patterns have been studied in detail and show dramatically decreased expression in the hearts of homozygous mutant embryos. Because of the direct interaction between tropomyosin and troponin T (TnT), and the crucial functions of TnT in the regulation of striated muscle contraction, we have expanded our studies on this animal model to characterize the expression of the TnT gene in cardiac muscle throughout normal axolotl development as well as in mutant axolotls. In addition, we have succeeded in cloning the full-length cardiac troponin T (cTnT) cDNA from axolotl hearts. Confocal microscopy has shown a substantial, but reduced, expression of TnT protein in the mutant hearts when compared to normal during embryonic development.

Computational simulation of hypertrophic cardiomyopathy mutations in Troponin I: Influence of increased myofilament calcium sensitivity on isometric force, ATPase and [Ca2+]i

Familial hypertrophic cardiomyopathy (FHC) is an inherited disease that is characterized by ventricular hypertrophy, cardiac arrhythmias and increased risk of premature sudden death. FHC is caused by autosomal-dominant mutations in genes for a number of sarcomeric proteins; many mutations in Ca(2+)-regulatory proteins of the cardiac thin filament are associated with increased Ca(2+) sensitivity of myofilament function. Computational simulations were used to investigate the possibility that these mutations could affect the Ca(2+) transient and mechanical response of a myocyte during a single cardiac cycle. We used existing experimental data for specific mutations of cardiac troponin I that exhibit increased Ca(2+) sensitivity in physiological and biophysical assays. The simulated Ca(2+) transients were used as input for a three-dimensional half-sarcomere biomechanical model with filament compliance to predict the resulting force. Mutations with the highest Ca(2+) affinity (lowest K(m)) values, exhibit the largest decrease in peak Ca(2+) assuming a constant influx of Ca(2+) into the cytoplasm; they also prolong Ca(2+) removal but have little effect on diastolic Ca(2+). Biomechanical model results suggest that these cTnI mutants would increase peak force despite the decrease in peak [Ca(2+)](i). There is a corresponding increase in net ATP hydrolysis, with no change in tension cost (ATP hydrolyzed per unit of time-integrated tension). These simulations suggest that myofilament-initiated hypertrophic signaling could be associated with decreased [Ca(2+)](i), increased stress/strain, and/or increased ATP flux.

Differential regulation of myofilament protein isoforms underlying the contractility changes in skeletal muscle unloading

Weight-bearing skeletal muscles change phenotype in response to unloading. Using the hindlimb suspension rat model, we investigated the regulation of myofilament protein isoforms in correlation to contractility. Four weeks of continuous hindlimb unloading produced progressive atrophy and contractility changes in soleus but not extensor digitorum longus muscle. The unloaded soleus muscle also had decreased fatigue resistance. Along with the decrease of myosin heavy chain isoform I and IIa and increase of IIb and IIx, coordinated regulation of thin filament regulatory protein isoforms were observed: gamma- and beta-tropomyosin decreased and alpha-tropomyosin increased, resulting in an alpha/beta ratio similar to that in normal fast twitch skeletal muscle; troponin I and troponin T (TnT) both showed decrease in the slow isoform and increases in the fast isoform. The TnT isoform switching began after 7 days of unloading and TnI isoform showed detectable changes at 14 days while other protein isoform changes were not significant until 28 days of treatment. Correlating to the early changes in contractility, especially the resistance to fatigue, the early response of TnT isoform regulation may play a unique role in the adaptation of skeletal muscle to unloading. When the fast TnT gene expression was upregulated in the unloaded soleus muscle, alternative RNA splicing switched to produce more high molecular weight acidic isoforms, reflecting a potential compensation for the decrease of slow TnT that is critical to skeletal muscle function. The results demonstrate that differential regulation of TnT isoforms is a sensitive mechanism in muscle adaptation to functional demands.

Selective deletion of the NH2-terminal variable region of cardiac troponin T in ischemia reperfusion by myofibril-associated mu-calpain cleavage

The structure of the NH2-terminal region of troponin T (TnT) is hypervariable among the muscle type-specific isoforms and is also regulated by alternative RNA splicing. This region does not contain binding sites for other thin filament proteins, but alteration of its structure affects the Ca2+ regulation of muscle contraction. Here we report a truncated cardiac TnT produced during myocardial ischemia reperfusion. Amino acid sequencing and protein fragment reconstruction determined that it is generated by a posttranslational modification selectively removing the NH2-terminal variable region and preserving the conserved core structure of TnT. Triton X-100 extraction of cardiac muscle fibers promoted production of the NH2-terminal truncated cardiac TnT (cTnT-ND), indicating a myofibril-associated proteolytic activity. Mu-calpain is a myofibril-associated protease and is known to degrade TnT. Supporting a role of mu-calpain in producing cTnT-ND in myocardial ischemia reperfusion, calpain inhibitors decreased the level of cTnT-ND in Triton-extracted myofibrils. Mu-calpain treatment of the cardiac myofibril and troponin complex specifically reproduced cTnT-ND. In contrast, mu-calpain treatment of isolated cardiac TnT resulted in nonspecific degradation, suggesting that this structural modification is relevant to physiological structures of the myofilament. Triton X-100 treatment of transgenic mouse cardiac myofibrils overexpressing fast skeletal muscle TnT produced similar NH2-terminal truncations of the endogenous and exogenous TnT, despite different amino acid sequences at the cleavage site. With the functional consequences of removing the NH2-terminal variable region of TnT, the mu-calpain-mediated proteolytic modification of TnT may act as an acute mechanism to adjust muscle contractility under stress conditions.

Troponin phosphorylation and regulatory function in human heart muscle: dephosphorylation of Ser23/24 on troponin I could account for the contractile defect in end-stage heart failure

We made quantitative measurements of phosphorylation in troponin isolated from 6 non-failing donor hearts and 6 explanted hearts with end-stage heart failure in SDS-PAGE gels using Pro-Q Diamond phosphoprotein stain. The troponin T phosphorylation level was the same in troponin from failing and non-failing heart (3.1 mol Pi/mol). However, troponin I phosphorylation was significantly lower in failing (0.37+/-0.18 mol Pi/mol) compared with non-failing heart troponin (2.25+/-0.36 mol Pi/mol). Levels of troponin I PKA-dependent phosphorylation, measured with a phosphoserine 23/24-specific antibody, were also significantly lower in failing heart troponin (0.19+/-0.06 mol Pi/mol) compared to non-failing troponin (1.14+/-0.09 mol Pi/mol). We calculate that there is phosphorylation in addition to serine 23/24 of 1.11+/-0.34 mol Pi/mol in non-failing reduced to 0.18+/-0.17 mol Pi/mol in failing heart troponin, attributed to phosphorylation on the PKC sites. To test for the functional role of troponin I phosphorylation, the native troponin I from either non-failing or failing heart troponin was exchanged for a recombinant (unphosphorylated) human cardiac troponin I. Thin filament Ca(2+)-regulatory function was studied with the quantitative in vitro motility assay: thin filaments containing the replaced troponin I resulted in a failing phenotype of a 17-26% reduced sliding speed and an increased Ca(2+)-sensitivity relative to non-failing troponin (EC(50) TnI-exchanged/non-failing=0.57, p<0.001). When exchanged with troponin I phosphorylated with PKA motility parameters reverted to a pattern indistinguishable from non-failing troponin (p=0.35-0.75). We suggest that changes in troponin function can account for the contractile abnormality in failing heart muscle and that the functional changes in troponin are due to reduced phosphorylation of troponin I at the PKA sites.

Alterations of tension-dependent ATP utilization in a transgenic rat model of hypertrophic cardiomyopathy

Although it is established that familial hypertrophic cardiomyopathy (FHC) is caused by mutations in several sarcomeric proteins, including cardiac troponin T (TnT), its pathogenesis is still not completely understood. Previously, we established a transgenic rat model of FHC expressing a human TnT molecule with a truncation mutation (DEL-TnT). This study investigated whether contractile dysfunction and electrical vulnerability observed in DEL-TnT rats might be due to alterations of intracellular Ca(2+) homeostasis, myofibrillar Ca(2+) sensitivity, and/or myofibrillar ATP utilization. Simultaneous measurements of the force of contraction and intracellular Ca(2+) transients were performed in right ventricular trabeculae of DEL-TnT hearts at 0.25 and 1.0 Hz. Rats expressing wild-type human TnT as well as nontransgenic rats served as controls. In addition, calcium-dependent ATPase activity and tension development were investigated in skinned cardiac muscle fibers. Force of contraction was significantly decreased in DEL-TnT compared with nontransgenic rats and TnT. Time parameters of Ca(2+) transients were unchanged at 0.25 Hz but prolonged at 1.0 Hz in DEL-TnT. The amplitude of the fura-2 transient was similar in all groups investigated, whereas diastolic and systolic fura-2 ratios were found elevated in rats expressing nontruncated human troponin T. In DEL-TnT rats, myofibrillar Ca(2+)-dependent tension development as well as Ca(2+) sensitivity of tension were significantly decreased, whereas tension-dependent ATP consumption ("tension cost") was markedly increased. Thus, a C-terminal truncation of the cardiac TnT molecule impairs the force-generating capacity of the cycling cross-bridges resulting in increased tension-dependent ATP utilization. Taken together, our data support the hypothesis of energy compromise as a contributing factor in the pathogenesis of FHC.

Differences in myofilament calcium sensitivity in rat psoas fibers reconstituted with troponin T isoforms containing the alpha- and beta-exons

The carboxy terminus of fast skeletal muscle troponin T (fsTnT) is highly conserved. However, mutually exclusive splicing of exons 16 and 17 in the fsTnT gene results in the expression of either the alpha- or beta-fsTnT isoform. The alpha-isoform is expressed only in adult fast skeletal muscle, whereas the beta-isoform is expressed in varying quantities throughout muscle development. Reconstitution of detergent-skinned adult rat psoas muscle fibers with rat fast skeletal troponin complexes containing either fsTnT isoform demonstrated that reconstitution with alpha-fsTnT resulted in greater myofilament Ca(2+) sensitivity than reconstitution with beta-fsTnT, without changes to Ca(2+)-activated maximal tension, ATPase activity or tension cost. The observed isoform-specific differences in myofilament Ca(2+) sensitivity may be due to changes in the transition of the thin-filament regulatory unit from the off to the on state, possibly due to altered interactions of the C-terminus of fsTnT with troponins I and/or C.

Myocardial regulatory proteins and heart failure

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

Sarcomeric proteins and familial hypertrophic cardiomyopathy: linking mutations in structural proteins to complex cardiovascular phenotypes

Hypertrophic Cardiomyopathy (HCM) is a relatively common primary cardiac disorder defined as the presence of a hypertrophied left ventricle in the absence of any other diagnosed etiology. HCM is the most common cause of sudden cardiac death in young people which often occurs without precedent symptoms. The overall clinical phenotype of patients with HCM is broad, ranging from a complete lack of cardiovascular symptoms to exertional dyspnea, chest pain, and sudden death, often due to arrhythmias. To date, 270 independent mutations in nine sarcomeric protein genes have been linked to Familial Hypertrophic Cardiomyopathy (FHC), thus the clinical variability is matched by significant genetic heterogeneity. While the final clinical phenotype in patients with FHC is a result of multiple factors including modifier genes, environmental influences and genotype, initial screening studies had suggested that individual gene mutations could be linked to specific prognoses. Given that the sarcomeric genes linked to FHC encode proteins with known functions, a vast array of biochemical, biophysical and physiologic experimental approaches have been applied to elucidate the molecular mechanisms that underlie the pathogenesis of this complex cardiovascular disorder. In this review, to illustrate the basic relationship between protein dysfunction and disease pathogenesis we focus on representative gene mutations from each of the major structural components of the cardiac sarcomere: the thick filament (beta MyHC), the thin filament (cTnT and Tm) and associated proteins (MyBP-C). The results of these studies will lead to a better understanding of FHC and eventually identify targets for therapeutic intervention.

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.

Troponin T modulates sarcomere length-dependent recruitment of cross-bridges in cardiac muscle

The heterogenic nature of troponin T (TnT) isoforms in fast skeletal and cardiac muscle suggests important functional differences. Dynamic features of rat cardiac TnT (cTnT) and rat fast skeletal TnT (fsTnT) reconstituted cardiac muscle preparations were captured by fitting the force response of small amplitude (0.5%) muscle length changes to the recruitment-distortion model. The recruitment of force-bearing cross-bridges (XBs) by increases in muscle length was favored by cTnT. The recruitment magnitude was approximately 1.5 times greater for cTnT- than for fsTnT-reconstituted muscle fibers. The speed of length-mediated XB recruitment (b) in cTnT-reconstituted muscle fiber was 0.50-0.57 times as fast as fsTnT-reconstituted muscle fibers (3.05 vs. 5.32 s(-1) at sarcomere length, SL, of 1.9 microm and 4.16 vs. 8.36 s(-1) at SL of 2.2 microm). Due to slowing of b in cTnT-reconstituted muscle fibers, the frequency of minimum stiffness (f(min)) was shifted to lower frequencies of muscle length changes (at SL of 1.9 microm, 0.64 Hz, and 1.16 Hz for cTnT- and fsTnT-reconstituted muscle fibers, respectively; at SL of 2.2 microm, 0.79 Hz, and 1.11 Hz for cTnT- and fsTnT-reconstituted muscle fibers, respectively). Our model simulation of the data implicates TnT as a participant in the process by which SL- and XB-regulatory unit cooperative interactions activate thin filaments. Our data suggest that the amino-acid sequence differences in cTnT may confer a heart-specific regulatory role. cTnT may participate in tuning the heart muscle by decreasing the speed of XB recruitment so that the heart beats at a rate commensurate with f(min).