A direct regulatory role for troponin T and a dual role for troponin C in the Ca2+ regulation of muscle contraction

Morphologic, cytometric, quantitative transcriptomic and functional characterisation provide insights into the haemocyte immune responses of Pacific abalone (Haliotis discus hannai)

In recent years, the abalone aquaculture industry has been threatened by the bacterial pathogens. The immune responses mechanisms underlying the phagocytosis of haemocytes remain unclear in Haliotis discus hannai. It is necessary to investigate the immune mechanism in response to these bacterial pathogens challenges. In this study, the phagocytic activities of haemocytes in H. discus hannai were examined by flow cytometry combined with electron microscopy and transcriptomic analyses. The results of Vibrio parahaemolyticus, Vibrio alginolyticus and Staphylococcus aureu challenge using electron microscopy showed a process during phagosome formation in haemocytes. The phagocytic rate (PP) of S. aureus was higher than the other five foreign particles, which was about 63%. The PP of Vibrio harveyi was about 43%, the PP peak of V. alginolyticus in haemocyte was 63.7% at 1.5 h. After V. parahaemolyticus and V. alginolyticus challenge, acid phosphatase, alkaline phosphatase, total superoxide dismutase, lysozyme, total antioxidant capacity, catalase, nitric oxide synthase and glutathione peroxidase activities in haemocytes were measured at different times, differentially expressed genes (DEGs) were identified by quantitative transcriptomic analysis. The identified DEGs after V. parahaemolyticus challenge included haemagglutinin/amebocyte aggregation factor-like, supervillin-like isoform X4, calmodulin-like and kyphoscoliosis peptidase-like; the identified DEGs after V. alginolyticus challenge included interleukin-6 receptor subunit beta-like, protein turtle homolog B-like, rho GTPase-activating protein 6-like isoform X2, leukocyte surface antigen CD53-like, calponin-1-like, calmodulin-like, troponin C, troponin I-like isoform X4, troponin T-like isoform X18, tumor necrosis factor ligand superfamily member 10-like, rho-related protein racA-like and haemagglutinin/amebocyte aggregation factor-like. Some immune-related KEGG pathways were significantly up-regulated or down-regulated after challenge, including thyroid hormone synthesis, Th17 cell differentiation signalling pathway, focal adhesion, melanogenesis, leukocyte transendothelial migration, inflammatory mediator regulation of TRP channels, ras signalling pathway, rap1 signalling pathway. This study is the first step towards understanding the H. discus hannai immune system by adapting several tools to gastropods and providing a first detailed morpho-functional study of their haemocytes.

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Case report: A new de novo mutation of the Troponin T2 gene in a Chinese patient with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a cardiovascular disease characterized by persistent ventricular dilatation and systolic dysfunction. DCM has a variety of causes, including myocarditis; exposure to narcotics, alcohol, or other toxins; and metabolic or endocrine disorders. Genetic factors play a dominant role in 30%-40% of DCM cases. Here, we report a case of DCM with very severe heart failure. Because of the severity of heart failure, the patient underwent heart transplantation. We speculated that the patient's DCM might be due to a mutation; hence, we performed whole-exome sequencing of the patient and their parents, which showed a de novo heterozygous mutation (NM_001001431.2c.769G>A:p.E257K) in TNNT2, which was considered pathogenic according to the ACMG pathogenicity assessment. This finding expands the genetic map of DCM and TNNT2 and will be important for future studies on the genetic and disease relationships between DCM and TNNT2.

Modulation of Structure and Dynamics of Cardiac Troponin by Phosphorylation and Mutations Revealed by Molecular Dynamics Simulations

Adrenaline acts on β1 receptors in the heart muscle to enhance contractility, increase the heart rate, and increase the rate of relaxation (lusitropy) via activation of the cyclic AMP-dependent protein kinase, PKA. Phosphorylation of serines 22 and 23 in the N-terminal peptide of cardiac troponin I is responsible for lusitropy. Mutations associated with cardiomyopathy suppress the phosphorylation-dependent change. Key parts of troponin responsible for this modulatory system are disordered and cannot be resolved by conventional structural approaches. We performed all-atom molecular dynamics simulations (5 × 1.5 μs runs) of the troponin core (419 amino acids) in the presence of Ca2+ in the bisphosphorylated and unphosphorylated states for both wild-type troponin and the troponin C (cTnC) G159D mutant. PKA phosphorylation affects troponin dynamics. There is significant rigidification of the structure involving rearrangement of the cTnI(1-33)-cTnC interaction and changes in the distribution of the cTnC helix A/B angle, troponin I (cTnI) switch peptide (149-164) docking, and the angle between the regulatory head and ITC arm domains. The familial dilated cardiomyopathy cTnC G159D mutation whose Ca2+ sensitivity is not modulated by cTnI phosphorylation exhibits a structure inherently more rigid than the wild type, with phosphorylation reversing the direction of all metrics relative to the wild type.

Cardiomyocyte Dysfunction in Inherited Cardiomyopathies

Inherited cardiomyopathies form a heterogenous group of disorders that affect the structure and function of the heart. Defects in the genes encoding sarcomeric proteins are associated with various perturbations that induce contractile dysfunction and promote disease development. In this review we aimed to outline the functional consequences of the major inherited cardiomyopathies in terms of myocardial contraction and kinetics, and to highlight the structural and functional alterations in some sarcomeric variants that have been demonstrated to be involved in the pathogenesis of the inherited cardiomyopathies. A particular focus was made on mutation-induced alterations in cardiomyocyte mechanics. Since no disease-specific treatments for familial cardiomyopathies exist, several novel agents have been developed to modulate sarcomere contractility. Understanding the molecular basis of the disease opens new avenues for the development of new therapies. Furthermore, the earlier the awareness of the genetic defect, the better the clinical prognostication would be for patients and the better the prevention of development of the disease.

Troponin Variants in Congenital Myopathies: How They Affect Skeletal Muscle Mechanics

The troponin complex is a key regulator of muscle contraction. Multiple variants in skeletal troponin encoding genes result in congenital myopathies. TNNC2 has been implicated in a novel congenital myopathy, TNNI2 and TNNT3 in distal arthrogryposis (DA), and TNNT1 and TNNT3 in nemaline myopathy (NEM). Variants in skeletal troponin encoding genes compromise sarcomere function, e.g., by altering the Ca²⁺ sensitivity of force or by inducing atrophy. Several potential therapeutic strategies are available to counter the effects of variants, such as troponin activators, introduction of wild-type protein through AAV gene therapy, and myosin modulation to improve muscle contraction. The mechanisms underlying the pathophysiological effects of the variants in skeletal troponin encoding genes are incompletely understood. Furthermore, limited knowledge is available on the structure of skeletal troponin. This review focusses on the physiology of slow and fast skeletal troponin and the pathophysiology of reported variants in skeletal troponin encoding genes. A better understanding of the pathophysiological effects of these variants, together with enhanced knowledge regarding the structure of slow and fast skeletal troponin, will direct the development of treatment strategies.

Genetic Control of Muscle Diversification and Homeostasis: Insights from Drosophila

In the fruit fly, Drosophila melanogaster, the larval somatic muscles or the adult thoracic flight and leg muscles are the major voluntary locomotory organs. They share several developmental and structural similarities with vertebrate skeletal muscles. To ensure appropriate activity levels for their functions such as hatching in the embryo, crawling in the larva, and jumping and flying in adult flies all muscle components need to be maintained in a functionally stable or homeostatic state despite constant strain. This requires that the muscles develop in a coordinated manner with appropriate connections to other cell types they communicate with. Various signaling pathways as well as extrinsic and intrinsic factors are known to play a role during Drosophila muscle development, diversification, and homeostasis. In this review, we discuss genetic control mechanisms of muscle contraction, development, and homeostasis with particular emphasis on the contractile unit of the muscle, the sarcomere.

Molecular characterization of troponin C (TnC) in Scylla paramamosain and its role in white spot syndrome virus and Vibrio alginolyticus infection

Troponin C (TnC) is one member of the EF-hand superfamily. In many species, this gene had been identified and related functions had been elucidated. The TnC gene was still blank in the Scylla paramamosain. We obtained the TnC gene for the first time in the S. paramamosain. And we systematically analyzed the possible role of this gene in the innate immunity of S. paramamosain while infected with white spot syndrome virus (WSSV) or Vibrio alginolyticus. The full-length 1427 bp sequence of TnC contains a 453 bp open reading frame (ORF) for encoding a 151 amino acid protein. Detection of tissue specificity of gene expression showed that the TnC was primarily expressed in muscle tissue. The expression of TnC was successfully inhibited by RNA interference technology, and several immune genes were affected. The activity of phenoloxidase and superoxide dismutase increased, and the total hemocytes counts increased after RNAi of TnC. It was found that after infection with V. alginolyticus and WSSV, the expression of TnC in hemocytes decreased. Infected with V. alginolyticus and WSSV, the cumulative mortality and apoptotic rate of hemocytes increased after silencing the TnC gene. Our results indicate that TnC takes participate in the innate immunity of S. paramamosain and may plays a different role in the antiviral and antibacterial immune response.

The intrinsically disordered C terminus of troponin T binds to troponin C to modulate myocardial force generation

Aberrant regulation of myocardial force production represents an early biomechanical defect associated with sarcomeric cardiomyopathies, but the molecular mechanisms remain poorly defined. Here, we evaluated the pathogenicity of a previously unreported sarcomeric gene variant identified in a pediatric patient with sporadic dilated cardiomyopathy, and we determined a molecular mechanism. Trio whole-exome sequencing revealed a de novo missense variant in TNNC1 that encodes a p.I4M substitution in the N-terminal helix of cardiac troponin C (cTnC). Reconstitution of this human cTnC variant into permeabilized porcine cardiac muscle preparations significantly decreases the magnitude and rate of isometric force generation at physiological Ca2+-activation levels. Computational modeling suggests that this inhibitory effect can be explained by a decrease in the rates of cross-bridge attachment and detachment. For the first time, we show that cardiac troponin T (cTnT), in part through its intrinsically disordered C terminus, directly binds to WT cTnC, and we find that this cardiomyopathic variant displays tighter binding to cTnT. Steady-state fluorescence and NMR spectroscopy studies suggest that this variant propagates perturbations in cTnC structural dynamics to distal regions of the molecule. We propose that the intrinsically disordered C terminus of cTnT directly interacts with the regulatory N-domain of cTnC to allosterically modulate Ca2+ activation of force, perhaps by controlling the troponin I switching mechanism of striated muscle contraction. Alterations in cTnC-cTnT binding may compromise contractile performance and trigger pathological remodeling of the myocardium.

The homozygous K280N troponin T mutation alters cross-bridge kinetics and energetics in human HCM

Hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomeric proteins, but the pathogenic mechanism is unclear. Piroddi et al. find impairment of cross-bridge kinetics and energetics in human sarcomeres with a TNNT2 mutation, suggesting that HCM involves inefficient ATP utilization.

Hypertrophic cardiomyopathy (HCM) is a genetic form of left ventricular hypertrophy, primarily caused by mutations in sarcomere proteins. The cardiac remodeling that occurs as the disease develops can mask the pathogenic impact of the mutation. Here, to discriminate between mutation-induced and disease-related changes in myofilament function, we investigate the pathogenic mechanisms underlying HCM in a patient carrying a homozygous mutation (K280N) in the cardiac troponin T gene (TNNT2), which results in 100% mutant cardiac troponin T. We examine sarcomere mechanics and energetics in K280N-isolated myofibrils and demembranated muscle strips, before and after replacement of the endogenous troponin. We also compare these data to those of control preparations from donor hearts, aortic stenosis patients (LVH_ao), and HCM patients negative for sarcomeric protein mutations (HCM_smn). The rate constant of tension generation following maximal Ca²⁺ activation (k_ACT) and the rate constant of isometric relaxation (slow k_REL) are markedly faster in K280N myofibrils than in all control groups. Simultaneous measurements of maximal isometric ATPase activity and Ca²⁺-activated tension in demembranated muscle strips also demonstrate that the energy cost of tension generation is higher in the K280N than in all controls. Replacement of mutant protein by exchange with wild-type troponin in the K280N preparations reduces k_ACT, slow k_REL, and tension cost close to control values. In donor myofibrils and HCM_smn demembranated strips, replacement of endogenous troponin with troponin containing the K280N mutant increases k_ACT, slow k_REL, and tension cost. The K280N TNNT2 mutation directly alters the apparent cross-bridge kinetics and impairs sarcomere energetics. This result supports the hypothesis that inefficient ATP utilization by myofilaments plays a central role in the pathogenesis of the disease.

Characterizing metal-binding sites in proteins with X-ray crystallography

Metals have crucial roles in many physiological, pathological, toxicological, pharmaceutical, and diagnostic processes. Proper handling of metal-containing macromolecule samples for structural studies is not trivial, and failure to handle them properly is often a source of irreproducibility caused by issues such as pH changes, incorporation of unexpected metals, or oxidization/reduction of the metal. This protocol outlines the guidelines and best practices for characterizing metal-binding sites in protein structures and alerts experimenters to potential pitfalls during the preparation and handling of metal-containing protein samples for X-ray crystallography studies. The protocol features strategies for controlling the sample pH and the metal oxidation state, recording X-ray fluorescence (XRF) spectra, and collecting diffraction data sets above and below the corresponding metal absorption edges. This protocol should allow experimenters to gather sufficient evidence to unambiguously determine the identity and location of the metal of interest, as well as to accurately characterize the coordinating ligands in the metal binding environment within the protein. Meticulous handling of metal-containing macromolecule samples as described in this protocol should enhance experimental reproducibility in biomedical sciences, especially in X-ray macromolecular crystallography. For most samples, the protocol can be completed within a period of 7-190 d, most of which (2-180 d) is devoted to growing the crystal. The protocol should be readily understandable to structural biologists, particularly protein crystallographers with an intermediate level of experience.

Tropomyosin Must Interact Weakly with Actin to Effectively Regulate Thin Filament Function

Elongated tropomyosin, associated with actin-subunits along the surface of thin filaments, makes electrostatic interactions with clusters of conserved residues, K326, K328, and R147, on actin. The association is weak, permitting low-energy cost regulatory movement of tropomyosin across the filament during muscle activation. Interestingly, acidic D292 on actin, also evolutionarily conserved, lies adjacent to the three-residue cluster of basic amino acids and thus may moderate the combined local positive charge, diminishing tropomyosin-actin interaction and facilitating regulatory-switching. Indeed, charge neutralization of D292 is connected to muscle hypotonia in individuals with D292V actin mutations and linked to congenital fiber-type disproportion. Here, the D292V mutation may predispose tropomyosin-actin positioning to a myosin-blocking state, aberrantly favoring muscle relaxation, thus mimicking the low-Ca²⁺ effect of troponin even in activated muscles. To test this hypothesis, interaction energetics and in vitro function of wild-type and D292V filaments were measured. Energy landscapes based on F-actin-tropomyosin models show the mutation localizes tropomyosin in a blocked-state position on actin defined by a deeper energy minimum, consistent with augmented steric-interference of actin-myosin binding. In addition, whereas myosin-dependent motility of troponin/tropomyosin-free D292V F-actin is normal, motility is dramatically inhibited after addition of tropomyosin to the mutant actin. Thus, D292V-induced blocked-state stabilization appears to disrupt the delicately poised energy balance governing thin filament regulation. Our results validate the premise that stereospecific but necessarily weak binding of tropomyosin to F-actin is required for effective thin filament function.

Toxic responses of Perna viridis hepatopancreas exposed to DDT, benzo(a)pyrene and their mixture uncovered by iTRAQ-based proteomics and NMR-based metabolomics

Dichlorodiphenyltrichloroethane (DDT) and benzo(a)pyrene (BaP) are environmental estrogens (EEs) that are ubiquitous in the marine environment. In the present study, we integrated isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomic and nuclear magnetic resonance (NMR)-based metabolomic approaches to explore the toxic responses of green mussel hepatopancreas exposed to DDT (10μg/L), BaP (10μg/L) and their mixture. The metabolic responses indicated that BaP primarily disturbed energy metabolism and osmotic regulation in the hepatopancreas of the male green mussel P. viridis. Both DDT and the mixture of DDT and BaP perturbed the energy metabolism and osmotic regulation in P. viridis. The proteomic responses revealed that BaP affected the proteins involved in energy metabolism, material transformation, cytoskeleton, stress responses, reproduction and development in green mussels. DDT exposure could change the proteins involved in primary metabolism, stress responses, cytoskeleton and signal transduction. However, the mixture of DDT and BaP altered proteins associated with material and energy metabolism, stress responses, signal transduction, reproduction and development, cytoskeleton and apoptosis. This study showed that iTRAQ-based proteomic and NMR-based metabolomic approaches could effectively elucidate the essential molecular mechanism of disturbances in hepatopancreas function of green mussels exposed to environmental estrogens.

Overexpression of miRNA-9 Generates Muscle Hypercontraction Through Translational Repression of Troponin-T in Drosophila melanogaster Indirect Flight Muscles

MicroRNAs (miRNAs) are small noncoding endogenous RNAs, typically 21-23 nucleotides long, that regulate gene expression, usually post-transcriptionally, by binding to the 3'-UTR of target mRNA, thus blocking translation. The expression of several miRNAs is significantly altered during cardiac hypertrophy, myocardial ischemia, fibrosis, heart failure, and other cardiac myopathies. Recent studies have implicated miRNA-9 (miR-9) in myocardial hypertrophy. However, a detailed mechanism remains obscure. In this study, we have addressed the roles of miR-9 in muscle development and function using a genetically tractable model system, the indirect flight muscles (IFMs) of Drosophila melanogaster Bioinformatics analysis identified 135 potential miR-9a targets, of which 27 genes were associated with Drosophila muscle development. Troponin-T (TnT) was identified as major structural gene target of miR-9a. We show that flies overexpressing miR-9a in the IFMs have abnormal wing position and are flightless. These flies also exhibit a loss of muscle integrity and sarcomeric organization causing an abnormal muscle condition known as "hypercontraction." Additionally, miR-9a overexpression resulted in the reduction of TnT protein levels while transcript levels were unaffected. Furthermore, muscle abnormalities associated with miR-9a overexpression were completely rescued by overexpression of TnT transgenes which lacked the miR-9a binding site. These findings indicate that miR-9a interacts with the 3'-UTR of the TnT mRNA and downregulates the TnT protein levels by translational repression. The reduction in TnT levels leads to a cooperative downregulation of other thin filament structural proteins. Our findings have implications for understanding the cellular pathophysiology of cardiomyopathies associated with miR-9 overexpression.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Troponin C Mutations Partially Stabilize the Active State of Regulated Actin and Fully Stabilize the Active State When Paired with Δ14 TnT

Striated muscle contraction is regulated by the actin-associated proteins tropomyosin and troponin. The extent of activation of myosin ATPase activity is lowest in the absence of both Ca²⁺ and activating cross-bridges (i.e., S1-ADP or rigor S1). Binding of activating species of myosin to actin at a saturating Ca²⁺ concentration stabilizes the most active state (M state) of the actin-tropomyosin-troponin complex (regulated actin). Ca²⁺ binding alone produces partial stabilization of the active state. The extent of stabilization at a saturating Ca²⁺ concentration depends on the isoform of the troponin subunits, the phosphorylation state of troponin, and, in the case of cardiac muscle, the presence of hypertrophic cardiomyopathy-producing mutants of troponin T and troponin I. Cardiac dysfunction is also associated with mutations of troponin C (TnC). Troponin C mutants A8V, C84Y, and D145E increase the Ca²⁺ sensitivity of ATPase activity. We show that these mutants change the distribution of regulated actin states. The A8V and C84Y TnC mutants decreased the inactive B state distribution slightly at low Ca²⁺ concentrations, but the D145E mutants had no effect on that state. All TnC mutants increased the level of the active M state compared to that of the wild type, at a saturating Ca²⁺ concentration. Troponin complexes that contained two mutations that stabilize the active M state, A8V TnC and Δ14 TnT, appeared to be completely in the active state in the presence of only Ca²⁺. Because Ca²⁺ gives full activation, in this situation, troponin must be capable of positioning tropomyosin in the active M state without the need for rigor myosin binding.

Myofilaments: Movers and Rulers of the Sarcomere

Striated cardiac and skeletal muscles play very different roles in the body, but they are similar at the molecular level. In particular, contraction, regardless of the type of muscle, is a precise and complex process involving the integral protein myofilaments and their associated regulatory components. The smallest functional unit of muscle contraction is the sarcomere. Within the sarcomere can be found a sophisticated ensemble of proteins associated with the thick filaments (myosin, myosin binding protein-C, titin, and obscurin) and thin myofilaments (actin, troponin, tropomyosin, nebulin, and nebulette). These parallel thick and thin filaments slide across one another, pulling the two ends of the sarcomere together to regulate contraction. More specifically, the regulation of both timing and force of contraction is accomplished through an intricate network of intra- and interfilament interactions belonging to each myofilament. This review introduces the sarcomere proteins involved in striated muscle contraction and places greater emphasis on the more recently identified and less well-characterized myofilaments: cardiac myosin binding protein-C, titin, nebulin, and obscurin. © 2017 American Physiological Society. Compr Physiol 7:675-692, 2017.

Cardiac Troponin and Tropomyosin: Structural and Cellular Perspectives to Unveil the Hypertrophic Cardiomyopathy Phenotype

Inherited myopathies affect both skeletal and cardiac muscle and are commonly associated with genetic dysfunctions, leading to the production of anomalous proteins. In cardiomyopathies, mutations frequently occur in sarcomeric genes, but the cause-effect scenario between genetic alterations and pathological processes remains elusive. Hypertrophic cardiomyopathy (HCM) was the first cardiac disease associated with a genetic background. Since the discovery of the first mutation in the β-myosin heavy chain, more than 1400 new mutations in 11 sarcomeric genes have been reported, awarding HCM the title of the “disease of the sarcomere.” The most common macroscopic phenotypes are left ventricle and interventricular septal thickening, but because the clinical profile of this disease is quite heterogeneous, these phenotypes are not suitable for an accurate diagnosis. The development of genomic approaches for clinical investigation allows for diagnostic progress and understanding at the molecular level. Meanwhile, the lack of accurate in vivo models to better comprehend the cellular events triggered by this pathology has become a challenge. Notwithstanding, the imbalance of Ca²⁺ concentrations, altered signaling pathways, induction of apoptotic factors, and heart remodeling leading to abnormal anatomy have already been reported. Of note, a misbalance of signaling biomolecules, such as kinases and tumor suppressors (e.g., Akt and p53), seems to participate in apoptotic and fibrotic events. In HCM, structural and cellular information about defective sarcomeric proteins and their altered interactome is emerging but still represents a bottleneck for developing new concepts in basic research and for future therapeutic interventions. This review focuses on the structural and cellular alterations triggered by HCM-causing mutations in troponin and tropomyosin proteins and how structural biology can aid in the discovery of new platforms for therapeutics. We highlight the importance of a better understanding of allosteric communications within these thin-filament proteins to decipher the HCM pathological state.

Toward Understanding the Molecular Bases of Stretch Activation: A STRUCTURAL COMPARISON OF THE TWO TROPONIN C ISOFORMS OF LETHOCERUS

Muscles are usually activated by calcium binding to the calcium sensory protein troponin-C, which is one of the three components of the troponin complex. However, in cardiac and insect flight muscle activation is also produced by mechanical stress. Little is known about the molecular bases of this calcium-independent activation. In Lethocerus, a giant water bug often used as a model system because of its large muscle fibers, there are two troponin-C isoforms, called F1 and F2, that have distinct roles in activating the muscle. It has been suggested that this can be explained either by differences in structural features or by differences in the interactions with other proteins. Here we have compared the structural and dynamic properties of the two proteins and shown how they differ. We have also mapped the interactions of the F2 isoform with peptides spanning the sequence of its natural partner, troponin-I. Our data have allowed us to build a model of the troponin complex and may eventually help in understanding the specialized function of the F1 and F2 isoforms and the molecular mechanism of stretch activation.

In situ time-resolved FRET reveals effects of sarcomere length on cardiac thin-filament activation

During cardiac thin-filament activation, the N-domain of cardiac troponin C (N-cTnC) binds to Ca(2+) and interacts with the actomyosin inhibitory troponin I (cTnI). The interaction between N-cTnC and cTnI stabilizes the Ca(2+)-induced opening of N-cTnC and is presumed to also destabilize cTnI-actin interactions that work together with steric effects of tropomyosin to inhibit force generation. Recently, our in situ steady-state FRET measurements based on N-cTnC opening suggested that at long sarcomere length, strongly bound cross-bridges indirectly stabilize this Ca(2+)-sensitizing N-cTnC-cTnI interaction through structural effects on tropomyosin and cTnI. However, the method previously used was unable to determine whether N-cTnC opening depends on sarcomere length. In this study, we used time-resolved FRET to monitor the effects of cross-bridge state and sarcomere length on the Ca(2+)-dependent conformational behavior of N-cTnC in skinned cardiac muscle fibers. FRET donor (AEDANS) and acceptor (DDPM)-labeled double-cysteine mutant cTnC(T13C/N51C)AEDANS-DDPM was incorporated into skinned muscle fibers to monitor N-cTnC opening. To study the structural effects of sarcomere length on N-cTnC, we monitored N-cTnC opening at relaxing and saturating levels of Ca(2+) and 1.80 and 2.2-μm sarcomere length. Mg(2+)-ADP and orthovanadate were used to examine the structural effects of noncycling strong-binding and weak-binding cross-bridges, respectively. We found that the stabilizing effect of strongly bound cross-bridges on N-cTnC opening (which we interpret as transmitted through related changes in cTnI and tropomyosin) become diminished by decreases in sarcomere length. Additionally, orthovanadate blunted the effect of sarcomere length on N-cTnC conformational behavior such that weak-binding cross-bridges had no effect on N-cTnC opening at any tested [Ca(2+)] or sarcomere length. Based on our findings, we conclude that the observed sarcomere length-dependent positive feedback regulation is a key determinant in the length-dependent Ca(2+) sensitivity of myofilament activation and consequently the mechanism underlying the Frank-Starling law of the heart.

Pathogenesis associated with a restrictive cardiomyopathy mutant in cardiac troponin T is due to reduced protein stability and greatly increased myofilament Ca2+ sensitivity

BACKGROUND

Dilated and hypertrophic cardiomyopathy mutations in troponin can blunt effects of protein kinase A (PKA) phosphorylation of cardiac troponin I (cTnI), decreasing myofilament Ca2+-sensitivity; however this effect has never been tested for restrictive cardiomyopathy (RCM) mutants. This study explores whether an RCM cardiac troponin T mutant (cTnT-ΔE96) interferes with convergent PKA regulation and if TnT instability contributes to greatly enhanced Ca2+-sensitivity in skinned fibers.

METHODS

Force of contraction in skinned cardiac porcine fiber and spectroscopic studies were performed.

RESULTS

A decrease of -0.26 and -0.25 pCa units in Ca2+-sensitivity of contraction after PKA incubation was observed for skinned fibers incorporated with WT or cTnT-ΔE96, respectively. To further assess whether cTnT-ΔE96 interferes solely with transmission of cTnI phosphorylation effects, skinned fibers were reconstituted with PKA pseudo-phosphorylated cTnI (cTnI-SS/DD.cTnC). Fibers displaced with cTnT-WT, reconstituted with cTnI-SS/DD.cTnC decreased Ca2+-sensitivity of force (pCa50=5.61) compared to control cTnI-WT.cTnC (pCa50=5.75), similarly affecting cTnT-ΔE96 (pCa50=6.03) compared to control \cTnI-WT.cTnC (pCa50=6.14). Fluorescence studies measuring cTnC(IAANS) Ca2+-affinity changes due to cTnT-ΔE96 indicated that higher complexity (thin filament) better recapitulates skinned fiber Ca2+ sensitive changes. Circular dichroism revealed reduced α-helicity and earlier thermal unfolding for cTnT-ΔE96 compared to WT.

CONCLUSIONS

Although ineffective in decreasing myofilament Ca2+-sensitivity to normal levels, cTnT-ΔE96 does not interfere with PKA cTnI phosphorylation mediated effects; 2) cTnT-ΔE96 requires actin to increase cTnC Ca2+-affinity; and 3) deletion of E96 reduces cTnT stability, likely disrupting crucial thin filament interactions.

GENERAL SIGNIFICANCE

The pathological effect of cTnT-ΔE96 is largely manifested by dramatic myofilament Ca2+-sensitization which still persists even after PKA phosphorylation mediated Ca2+-desensitization.

Cardiac troponin T (TNNT2) mutations in chinese dilated cardiomyopathy patients

BACKGROUND

Dilated cardiomyopathy (DCM) is one of the leading causes of heart failure with high morbidity and mortality. Although more than 40 genes have been reported to cause DCM, the role of genetic testing in clinical practice is not well defined. Mutations in the troponin T (TNNT2) gene represent an important subset of known disease-causing mutations associated with DCM. Therefore, the aim of the present study was to determine the genetic variations in TNNT2 and the associations of those variations with DCM in Chinese patients.

METHODS

An approximately 4 kb fragment of the TNNT2 gene was isolated from 103 DCM patients and 192 healthy controls and was analyzed by DNA sequence analysis for genetic variations.

RESULTS

A total of 6 TNNT2 mutations were identified in 99 patients, including a G321T missense mutation (Leu84Phe) and 5 novel intronic mutations. Alleles of two novel SNPs (c.192 + 353 C>A, OR = 0.095, 95% CI: 0.013-0.714, P = 0.022; c.192 + 463 G>A, OR = 0.090, 95% CI: 0.012-0.675, P = 0.019) and SNP rs3729843 (OR = 1.889, 95% CI: 1.252-2.852; P = 0.002) were significantly correlated with DCM.

CONCLUSIONS

These results suggest that the missense mutation (Leu84Phe) and two novel SNPs (c.192 + 353 C>A, c.192 + 463 G>A) in TNNT2 gene might be associated with DCM in the Chinese population.

FRET study of the structural and kinetic effects of PKC phosphomimetic cardiac troponin T mutants on thin filament regulation

FRET was used to investigate the structural and kinetic effects that PKC phosphorylations exert on Ca(2+) and myosin subfragment-1 dependent conformational transitions of the cardiac thin filament. PKC phosphorylations of cTnT were mimicked by glutamate substitution. Ca(2+) and S1-induced distance changes between the central linker of cTnC and the switch region of cTnI (cTnI-Sr) were monitored in reconstituted thin filaments using steady state and time resolved FRET, while kinetics of structural transitions were determined using stopped flow. Thin filament Ca(2+) sensitivity was found to be significantly blunted by the presence of the cTnT(T204E) mutant, whereas pseudo-phosphorylation at additional sites increased the Ca(2+)-sensitivity. The rate of Ca(2+)-dissociation induced structural changes was decreased in the C-terminal end of cTnI-Sr in the presence of pseudo-phosphorylations while remaining unchanged at the N-terminal end of this region. Additionally, the distance between cTnI-Sr and cTnC was decreased significantly for the triple and quadruple phosphomimetic mutants cTnT(T195E/S199E/T204E) and cTnT(T195E/S199E/T204E/T285E), which correlated with the Ca(2+)-sensitivity increase seen in these same mutants. We conclude that significant changes in thin filament Ca(2+)-sensitivity, structure and kinetics are brought about through PKC phosphorylation of cTnT. These changes can either decrease or increase Ca(2+)-sensitivity and likely play an important role in cardiac regulation.

Complex tropomyosin and troponin T isoform expression patterns in orbital and global fibers of adult dog and rat extraocular muscles

We reported marked differences in the myosin heavy and light chain (MHC and MLC) isoform composition of fast and slow fibers between the global and orbital layers of dog extraocular muscles. Many dog extraocular fibers, especially orbital fibers, have MHC and MLC isoform patterns that are distinct from those in limb skeletal muscles. Additional observations suggested possible differences in the tropomyosin (Tm) and troponin T (TnT) isoform composition of global and orbital fibers. Therefore, we tested, using SDS-PAGE and immunoblotting, whether differences in Tm and TnT isoform expression do, in fact, exist between global and orbital layers of dog and rat EOMs and to compare expression patterns among identified fast and slow single fibers from both muscle layers. The Tm isoforms expressed in global fast and slow fibers are the same as in limb fast (α-Tm and β-Tm) and slow (γ-Tm and β-Tm) fibers, respectively. Orbital slow orbital fibers, on the other hand, each co-express all three sarcomeric Tm isoforms (α, β and γ). The results indicate that fast global and orbital fibers express only fast isoforms of TnT, but the relative amounts of the individual isoforms are different from those in limb fast muscle fibers and an abundant fast TnT isoform in the orbital layer was not detected in fast limb muscles. Slow fibers in both layers express slow TnT isoforms and the relative amounts also differ from those in limb slow fibers. Unexpectedly, significant amounts of cardiac TnT isoforms were also detected in slow fibers, especially in the orbital layer in both species. TnI and TnC isoform patterns are the same as in fast and slow fibers in limb muscles. These results expand the understanding of the elaborate diversity in contractile protein isoform expression in mammalian extraocular muscle fibers and suggest that major differences in calcium-activation properties exist among these fibers, based upon Tm and TnT isoform expression patterns.

TNNT2 gene polymorphisms are associated with susceptibility to idiopathic dilated cardiomyopathy in the Han Chinese population

Background. Idiopathic dilated cardiomyopathy (DCM) is characterized by ventricular chamber enlargement and systolic dysfunction. The pathogenesis of DCM remains uncertain, and the TNNT2 gene is potentially associated with DCM. To assess the role of TNNT2 in DCM, we examined 10 tagging single nucleotide polymorphisms (SNPs) in the patients. Methods. A total of 97 DCM patients and 189 control subjects were included in the study, and all SNPs were genotyped by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Results. In the TNNT2 gene, there was a significant association between DCM and genotype for the tagging SNPs rs3729547 (χ² = 6.63, P = 0.036, OR = 0.650, and 95% CI = 0.453–0.934) and rs3729843 (χ² = 9.787, P = 0.008, OR = 1.912, and 95% CI = 1.265–2.890) in the Chinese Han population. Linkage disequilibrium (LD) analysis showed that the SNPs rs7521796, rs2275862, rs3729547, rs10800775, and rs1892028, which are approximately 6 kb apart, were in high LD (D′ > 0.80) in the DCM patients. Conclusion. These results suggest that the TNNT2 polymorphisms might play an important role in susceptibility to DCM in the Chinese Han population.

The tropomyosin binding region of cardiac troponin T modulates crossbridge recruitment dynamics in rat cardiac muscle fibers

The cardiac muscle comprises dynamically interacting components that use allosteric/cooperative mechanisms to yield unique heart-specific properties. An essential protein in this allosteric/cooperative mechanism is cardiac muscle troponin T (cTnT), the central region (CR) and the T2 region of which differ significantly from those of fast skeletal muscle troponin T (fsTnT). To understand the biological significance of such sequence heterogeneity, we replaced the T1 or T2 domain of rat cTnT (RcT1 or RcT2) with its counterpart from rat fsTnT (RfsT1or RfsT2) to generate RfsT1-RcT2 and RcT1-RfsT2 recombinant proteins. In addition to contractile function measurements, dynamic features of RfsT1-RcT2- and RcT1-RfsT2-reconstituted rat cardiac muscle fibers were captured by fitting the recruitment-distortion model to the force response of small-amplitude (0.5%) muscle length changes. RfsT1-RcT2 fibers showed a 40% decrease in tension and a 44% decrease in ATPase activity, but RcT1-RfsT2 fibers were unaffected. The magnitude of length-mediated increase in crossbridge (XB) recruitment (E0) decreased by ~33% and the speed of XB recruitment (b) increased by ~100% in RfsT1-RcT2 fibers. Our data suggest the following: (1) the CR of cTnT modulates XB recruitment dynamics; (2) the N-terminal end region of cTnT has a synergistic effect on the ability of the CR to modulate XB recruitment dynamics; (3) the T2 region is important for tuning the Ca(2+) regulation of cardiac thin filaments. The combined effects of CR-tropomyosin interactions and the modulating effect of the N-terminal end of cTnT on CR-tropomyosin interactions may lead to the emergence of a unique property that tunes contractile dynamics to heart rates.

Myofibrillar Ca(2+) sensitivity is uncoupled from troponin I phosphorylation in hypertrophic obstructive cardiomyopathy due to abnormal troponin T

AIMS

We studied the relationship between myofilament Ca(2+) sensitivity and troponin I (TnI) phosphorylation by protein kinase A at serines 22/23 in human heart troponin isolated from donor hearts and from myectomy samples from patients with hypertrophic obstructive cardiomyopathy (HOCM).

METHODS AND RESULTS

We used a quantitative in vitro motility assay. With donor heart troponin, Ca(2+) sensitivity is two- to three-fold higher when TnI is unphosphorylated. In the myectomy samples from patients with HOCM, the mean level of TnI phosphorylation was low: 0.38 ± 0.19 mol Pi/mol TnI compared with 1.60 ± 0.19 mol Pi/mol TnI in donor hearts, but no difference in myofilament Ca(2+) sensitivity was observed. Thus, troponin regulation of thin filament Ca(2+) sensitivity is abnormal in HOCM hearts. HOCM troponin (0.29 mol Pi/mol TnI) was treated with protein kinase A to increase the level of phosphorylation to 1.56 mol Pi/mol TnI. No difference in EC(50) was found in thin filaments containing high and low TnI phosphorylation levels. This indicates that Ca(2+) sensitivity is uncoupled from TnI phosphorylation in HOCM heart troponin. Coupling could be restored by replacing endogenous troponin T (TnT) with the recombinant TnT T3 isoform. No difference in Ca(2+) sensitivity was observed if TnI was exchanged into HOCM heart troponin or if TnT was exchanged into the highly phosphorylated donor heart troponin. Comparison of donor and HOCM heart troponin by mass spectrometry and with adduct-specific antibodies did not show any differences in TnT isoform expression, phosphorylation or any post-translational modifications.

CONCLUSION

An abnormality in TnT is responsible for uncoupling myofibrillar Ca(2+) sensitivity from TnI phosphorylation in the septum of HOCM patients.

Cardiomyopathy-Related Mutations in Cardiac Troponin C, L29Q and G159D, Have Divergent Effects on Rat Cardiac Myofiber Contractile Dynamics

Previous studies of cardiomyopathy-related mutations in cardiac troponin C (cTnC)-L29Q and G159D-have shown diverse findings. The link between such mutant effects and their divergent impact on cardiac phenotypes has remained elusive due to lack of studies on contractile dynamics. We hypothesized that a cTnC mutant-induced change in the thin filament will affect global myofilament mechanodynamics because of the interactions of thin filament kinetics with both Ca(2+) binding and crossbridge (XB) cycling kinetics. We measured pCa-tension relationship and contractile dynamics in detergent-skinned rat cardiac papillary muscle fibers reconstituted with the recombinant wild-type rat cTnC (cTnC(WT)), cTnC(L29Q), and cTnC(G159D) mutants. cTnC(L29Q) fibers demonstrated a significant decrease in Ca(2+) sensitivity, but cTnC(G159D) fibers did not. Both mutants had no effect on Ca(2+)-activated maximal tension. The rate of XB recruitment dynamics increased in cTnC(L29Q) (26%) and cTnC(G159D) (25%) fibers. The rate of XB distortion dynamics increased in cTnC(G159D) fibers (15%). Thus, the cTnC(L29Q) mutant modulates the equilibrium between the non-cycling and cycling pool of XB by affecting the on/off kinetics of the regulatory units (Tropomyosin-Troponin); whereas, the cTnC(G159D) mutant increases XB cycling rate. Different effects on contractile dynamics may offer clue regarding how cTnC(L29Q) and cTnC(G159D) cause divergent effects on cardiac phenotypes.

Disease causing mutations of troponin alter regulated actin state distributions

Striated muscle contraction is regulated primarily through the action of tropomyosin and troponin that are bound to actin. Activation requires Ca(2+) binding to troponin and/or binding of high affinity myosin complexes to actin. Mutations within components of the regulatory complex may lead to familial cardiomyopathies and myopathies. In several cases examined, either physiological or pathological changes in troponin alter the distribution among states of actin-tropomyosin-troponin that differ in their abilities to stimulate myosin ATPase activity. These observations open possibilities for managing disorders of the troponin complex. Furthermore, analyses of mutant forms of troponin give insights into the regulation of striated muscle contraction.

Fetal cardiac troponin isoforms rescue the increased Ca2+ sensitivity produced by a novel double deletion in cardiac troponin T linked to restrictive cardiomyopathy: a clinical, genetic, and functional approach

A novel double deletion in cardiac troponin T (cTnT) of two highly conserved amino acids (Asn-100 and Glu-101) was found in a restrictive cardiomyopathic (RCM) pediatric patient. Clinical evaluation revealed the presence of left atrial enlargement and marked left ventricle diastolic dysfunction. The explanted heart examined by electron microscopy revealed myofibrillar disarray and mild fibrosis. Pedigree analysis established that this mutation arose de novo. The patient tested negative for six other sarcomeric genes. The single and double recombinant cTnT mutants were generated, and their functional consequences were analyzed in porcine skinned cardiac muscle. In the adult Tn environment (cTnT3 + cardiac troponin I), the single cTnT3-ΔN100 and cTnT3-ΔE101 mutations had opposing effects on the Ca(2+) sensitivity of force development compared with WT, whereas the double deletion cTnT3-ΔN100/ΔE101 increased the Ca(2+) sensitivity + 0.19 pCa units. In addition, cTnT3-ΔN100/ΔE101 decreased the cooperativity of force development, suggesting alterations in intrafilament protein-protein interactions. In the fetal Tn environment, (cTnT1 + slow skeletal troponin I), the single (cTnT1-ΔN110) and double (cTnT1-ΔN110/ΔE111) deletions did not change the Ca(2+) sensitivity compared with control. To recreate the patient's heterozygous genotype, we performed a reconstituted ATPase activity assay. Thin filaments containing 50:50 cTnT3-ΔN100/ΔE101:cTnT3-WT also increased the myofilament Ca(2+) sensitivity compared with WT. Co-sedimentation of thin filament proteins indicated that no significant changes occurred in the binding of Tn containing the RCM cTnT mutation to actin-Tm. This report reveals the protective role of Tn fetal isoforms as they rescue the increased Ca(2+) sensitivity produced by a cTnT-RCM mutation and may account for the lack of lethality during gestation.

Interaction between troponin and myosin enhances contractile activity of myosin in cardiac muscle

Ca(2+) signaling in striated muscle cells is critically dependent upon thin filament proteins tropomyosin (Tm) and troponin (Tn) to regulate mechanical output. Using in vitro measurements of contractility, we demonstrate that even in the absence of actin and Tm, human cardiac Tn (cTn) enhances heavy meromyosin MgATPase activity by up to 2.5-fold in solution. In addition, cTn without Tm significantly increases, or superactivates sliding speed of filamentous actin (F-actin) in skeletal motility assays by at least 12%, depending upon [cTn]. cTn alone enhances skeletal heavy meromyosin's MgATPase in a concentration-dependent manner and with sub-micromolar affinity. cTn-mediated increases in myosin ATPase may be the cause of superactivation of maximum Ca(2+)-activated regulated thin filament sliding speed in motility assays relative to unregulated skeletal F-actin. To specifically relate this classical superactivation to cardiac muscle, we demonstrate the same response using motility assays where only cardiac proteins were used, where regulated cardiac thin filament sliding speeds with cardiac myosin are >50% faster than unregulated cardiac F-actin. We additionally demonstrate that the COOH-terminal mobile domain of cTnI is not required for this interaction or functional enhancement of myosin activity. Our results provide strong evidence that the interaction between cTn and myosin is responsible for enhancement of cross-bridge kinetics when myosin binds in the vicinity of Tn on thin filaments. These data imply a novel and functionally significant molecular interaction that may provide new insights into Ca(2+) activation in cardiac muscle cells.

Redox state of troponin C cysteine in the D/E helix alters the C-domain affinity for the thin filament of vertebrate striated muscle

BACKGROUND

Despite a broad spectrum of structural studies, it is not yet clear whether the D/E helix of troponin C (TnC) contributes to the interaction of TnC with troponin I (TnI). Redox modifications at Cys 98 in the D/E helix were explored for clues to TnC binding to the thin filament off-state, using recombinant wild-type TnC and an engineered mutant without Cys (Cys98Leu).

METHODS

Recombinant proteins and rabbit psoas skinned fibres were reduced with dithiothreitol (DTT) and variously recombined. Changes in affinity of reduced or oxidised TnC for the thin filament were evaluated via TnC binding and dissociation, using a standardized test for maximal force as an index of fibre TnC content.

RESULTS

All oxidation and reduction effects observed were reversible and led to changes in TnC content. Oxidation (H(2)O(2)) reduced TnC affinity for the filament; reduction (DTT) increased it. Reducing other fibre proteins had no effect. Binding of the Cys-less TnC mutant was not altered by DTT, nor was dissociation of wild-type TnC from reconstituted hybrids (skeletal TnC in cardiac trabeculae). Thus when Cys 98 in the D/E helix of TnC is fully reduced, its binding affinity for the thin filament of skeletal muscle is enhanced and helps to anchor it to the filament.

GENERAL SIGNIFICANCE

Signal transmission between TnC and the other proteins of the regulatory complex is sensitive to the redox state of Cys 98.

Predicting cardiomyopathic phenotypes by altering Ca2+ affinity of cardiac troponin C

Cardiac diseases associated with mutations in troponin subunits include hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and restrictive cardiomyopathy (RCM). Altered calcium handling in these diseases is evidenced by changes in the Ca(2+) sensitivity of contraction. Mutations in the Ca(2+) sensor, troponin C (TnC), were generated to increase/decrease the Ca(2+) sensitivity of cardiac skinned fibers to create the characteristic effects of DCM, HCM, and RCM. We also used a reconstituted assay to determine the mutation effects on ATPase activation and inhibition. One mutant (A23Q) was found with HCM-like properties (increased Ca(2+) sensitivity of force and normal levels of ATPase inhibition). Three mutants (S37G, V44Q, and L48Q) were identified with RCM-like properties (a large increase in Ca(2+) sensitivity, partial loss of ATPase inhibition, and increased basal force). Two mutations were identified (E40A and I61Q) with DCM properties (decreased Ca(2+) sensitivity, maximal force recovery, and activation of the ATPase at high [Ca(2+)]). Steady-state fluorescence was utilized to assess Ca(2+) affinity in isolated cardiac (c)TnCs containing F27W and did not necessarily mirror the fiber Ca(2+) sensitivity. Circular dichroism of mutant cTnCs revealed a trend where increased alpha-helical content correlated with increased Ca(2+) sensitivity in skinned fibers and vice versa. The main findings from this study were as follows: 1) cTnC mutants demonstrated distinct functional phenotypes reminiscent of bona fide HCM, RCM, and DCM mutations; 2) a region in cTnC associated with increased Ca(2+) sensitivity in skinned fibers was identified; and 3) the F27W reporter mutation affected Ca(2+) sensitivity, maximal force, and ATPase activation of some mutants.

A dilated cardiomyopathy troponin C mutation lowers contractile force by reducing strong myosin-actin binding

In this study we explore the mechanisms by which a double mutation (E59D/D75Y) in cardiac troponin C (CTnC) associated with dilated cardiomyopathy reduces the Ca(2+)-activated maximal tension of cardiac muscle. Studying the single mutants (i.e. E59D or D75Y) indicates that D75Y, but not E59D, causes a reduction in the calcium affinity of CTnC in troponin complex, regulated thin filaments (RTF), and the Ca(2+) sensitivity of contraction and ATPase in cardiac muscle preparations. However, both D75Y and E59D are required to reduce the actomyosin ATPase activity and maximal force in muscle fibers, indicating that E59D enhances the effects of D75Y. Part of the reduction in force/ATPase may be due to a defect in the interactions between CTnC and cardiac troponin T, which are known to be necessary for ATPase activation. An additional mechanism for the reduction in force/ATPase comes from measurements of the binding stoichiometry of myosin subfragment-1 (S-1) to the RTF. Using wild type RTFs, 4.8 mol S-1 was bound per mol filament (seven actins), whereas with E59D/D75Y RTFs, the number of binding sites was reduced by approximately 23% to 3.7. Altogether, these results suggest that the reduction in force and ATPase activation is possibly due to a thin filament conformation that promotes fewer accessible S-1-binding sites. In the absence of any family segregation data, the functional results presented here support the concept that this is likely a dilated cardiomyopathy-causing mutation.

Electron microscopy and three-dimensional reconstruction of native thin filaments reveal species-specific differences in regulatory strand densities

Throughout the animal kingdom striated muscle contraction is regulated by the thin filament troponin-tropomyosin complex. Homologous regulatory components are shared among vertebrate and arthropod muscles; however, unique protein extensions and/or components characterize the latter. The Troponin T (TnT) isoforms of Drosophila indirect flight and tarantula femur muscle for example contain distinct C-terminal extensions and are approximately 20% larger overall than their vertebrate counterpart. Using electron microscopy and three-dimensional helical reconstruction of native Drosophila, tarantula and frog muscle thin filaments we have identified species-specific differences in tropomyosin regulatory strand densities. The strands on the arthropod thin filaments were significantly larger in diameter than those from vertebrates, although not significantly different from each other. These findings reflect differences in the regulatory troponin-tropomyosin complex, which are likely due to the larger TnT molecules aligning and extending along much of the tropomyosin strands' length. Such an arrangement potentially alters the physical properties of the regulatory strands and may help establish contractile characteristics unique to certain arthropod muscles.

Differences between cardiac and skeletal troponin interaction with the thin filament probed by troponin exchange in skeletal myofibrils

Troponin (Tn) is the calcium-sensing protein of the thin filament. Although cardiac troponin (cTn) and skeletal troponin (sTn) accomplish the same function, their subunit interactions within Tn and with actin-tropomyosin are different. To further characterize these differences, myofibril ATPase activity as a function of pCa and labeled Tn exchange in rigor myofibrils was used to estimate Tn dissociation rates from the nonoverlap and overlap region as a function of pCa. Measurement of ATPase activity showed that skeletal myofibrils containing >96% cTn had a higher pCa 9 ATPase activity than, but similar pCa 4 activity to, sTn-containing myofibrils. Analysis of the pCa-ATPase activity relation showed that cTn myofibrils were more calcium sensitive but less cooperative (pCa50 = 6.14, nH = 1.46) than sTn myofibrils (pCa50= 5.90, nH = 3.36). The time course of labeled Tn exchange at pCa 9 and 4 were quite different between cTn and sTn. The apparent cTn dissociation rates were approximately 2-10-fold faster than sTn under all the conditions studied. The apparent dissociation rates for cTn were 5 x 10(-3) min(-1), 150 x 10(-3) min(-1), and 260 x 10(-3) min(-1), whereas for sTn they were 0.6 x 10(-3) min(-1), 88 x 10(-3) min(-1), and 68 x 10(-3) min(-1) for the nonoverlap region at pCa 9, nonoverlap region at pCa 4, and overlap region at pCa 4, respectively. Normalization of the apparent dissociation rates gives 1:30:50 for cTn compared with 1:150:110 for sTn (nonoverlap at pCa 9:nonoverlap at pCa 4:overlap at pCa 4) suggesting that calcium has a smaller influence, whereas strong cross-bridges have a larger influence on cTn dissociation compared with sTn. The higher cTn dissociation rate in the nonoverlap region and ATPase activity at pCa 9 suggest that it gives a less off or inactive thin filament. Analysis of the intensity ratio (after a short time of exchange) as a function of pCa showed that cTn had greater calcium sensitivity but lower cooperativity than sTn. In addition, the magnitude of the change in intensity ratio going from pCa 9 to 4 was less for cTn than sTn. These data suggest that the influence of calcium on cTn exchange is less than sTn even though calcium can activate ATPase activity to a similar extent in cTn compared with sTn myofibrils. This may be explained partially by cTn being less off or inactive at pCa 9. Modeling of the intensity profiles obtained after Tn exchange at pCa 5.8 suggest that the profiles are best explained by a model that includes a long-range cross-bridge effect that grades with distance from the rigor cross-bridge for both cTn and sTn.

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.

A functional and structural study of troponin C mutations related to hypertrophic cardiomyopathy

Recently four new hypertrophic cardiomyopathy mutations in cardiac troponin C (cTnC) (A8V, C84Y, E134D, and D145E) were reported, and their effects on the Ca(2+) sensitivity of force development were evaluated (Landstrom, A. P., Parvatiyar, M. S., Pinto, J. R., Marquardt, M. L., Bos, J. M., Tester, D. J., Ommen, S. R., Potter, J. D., and Ackerman, M. J. (2008) J. Mol. Cell. Cardiol. 45, 281-288). We performed actomyosin ATPase and spectroscopic solution studies to investigate the molecular properties of these mutations. Actomyosin ATPase activity was measured as a function of [Ca(2+)] utilizing reconstituted thin filaments (TFs) with 50% mutant and 50% wild type (WT) and 100% mutant cardiac troponin (cTn) complexes: A8V, C84Y, and D145E increased the Ca(2+) sensitivity with only A8V demonstrating lowered Ca(2+) sensitization at the 50% ratio when compared with 100%; E134D was the same as WT at both ratios. Of these four mutants, only D145E showed increased ATPase activation in the presence of Ca(2+). None of the mutants affected ATPase inhibition or the binding of cTn to the TF measured by co-sedimentation. Only D145E increased the Ca(2+) affinity of site II measured by 2-(4'-(2''-iodoacetamido)phenyl)aminonaphthalene-6-sulfonic acid fluorescence in isolated cTnC or the cTn complex. In the presence of the TF, only A8V was further sensitized to Ca(2+). Circular dichroism measurements in different metal-bound states of the isolated cTnCs showed changes in the secondary structure of A8V, C84Y, and D145E, whereas E134D was the same as WT. PyMol modeling of each cTnC mutant within the cTn complex revealed potential for local changes in the tertiary structure of A8V, C84Y, and D145E. Our results indicate that 1) three of the hypertrophic cardiomyopathy cTnC mutants increased the Ca(2+) sensitivity of the myofilament; 2) the effects of the mutations on the Ca(2+) affinity of isolated cTnC, cTn, and TF are not sufficient to explain the large Ca(2+) sensitivity changes seen in reconstituted and fiber assays; and 3) changes in the secondary structure of the cTnC mutants may contribute to modified protein-protein interactions along the sarcomere lattice disrupting the coupling between the cross-bridge and Ca(2+) binding to cTnC.

Structural basis for the activation of muscle contraction by troponin and tropomyosin

The molecular regulation of striated muscle contraction couples the binding and dissociation of Ca(2+) on troponin (Tn) to the movement of tropomyosin on actin filaments. In turn, this process exposes or blocks myosin binding sites on actin, thereby controlling myosin crossbridge dynamics and consequently muscle contraction. Using 3D electron microscopy, we recently provided structural evidence that a C-terminal extension of TnI is anchored on actin at low Ca(2+) and competes with tropomyosin for a common site to drive tropomyosin to the B-state location, a constrained, relaxing position on actin that inhibits myosin-crossbridge association. Here, we show that release of this constraint at high Ca(2+) allows a second segment of troponin, probably representing parts of TnT or the troponin core domain, to promote tropomyosin movement on actin to the Ca(2+)-induced C-state location. With tropomyosin stabilized in this position, myosin binding interactions can begin. Tropomyosin appears to oscillate to a higher degree between respective B- and C-state positions on troponin-free filaments than on fully regulated filaments, suggesting that tropomyosin positioning in both states is troponin-dependent. By biasing tropomyosin to either of these two positions, troponin appears to have two distinct structural functions; in relaxed muscles at low Ca(2+), troponin operates as an inhibitor, while in activated muscles at high Ca(2+), it acts as a promoter to initiate contraction.

Myofilament Ca2+ sensitization causes susceptibility to cardiac arrhythmia in mice

In human cardiomyopathy, anatomical abnormalities such as hypertrophy and fibrosis contribute to the risk of ventricular arrhythmias and sudden death. Here we have shown that increased myofilament Ca2+ sensitivity, also a common feature in both inherited and acquired human cardiomyopathies, created arrhythmia susceptibility in mice, even in the absence of anatomical abnormalities. In mice expressing troponin T mutants that cause hypertrophic cardiomyopathy in humans, the risk of developing ventricular tachycardia was directly proportional to the degree of Ca2+ sensitization caused by the troponin T mutation. Arrhythmia susceptibility was reproduced with the Ca2+-sensitizing agent EMD 57033 and prevented by myofilament Ca2+ desensitization with blebbistatin. Ca2+ sensitization markedly changed the shape of ventricular action potentials, resulting in shorter effective refractory periods, greater beat-to-beat variability of action potential durations, and increased dispersion of ventricular conduction velocities at fast heart rates. Together these effects created an arrhythmogenic substrate. Thus, myofilament Ca2+ sensitization represents a heretofore unrecognized arrhythmia mechanism. The protective effect of blebbistatin provides what we believe to be the first direct evidence that reduction of Ca2+ sensitivity in myofilaments is antiarrhythmic and might be beneficial to individuals with hypertrophic cardiomyopathy.

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.

Global and comparative protein profiles of the pronotum of the southern pine beetle, Dendroctonus frontalis

The southern pine beetle (Dendroctonus frontalis Zimmermann) kills all pines within its range and is among the most important forest pest species in the US. Using a specialized mycangium surrounded by gland cells in the pronotum, adult females culture, transport, and inoculate two fungi into beetle galleries during oviposition. These fungal symbionts, to varying degrees, exclude antagonistic fungi and provide nutrients to larvae. However, the mechanisms (e.g. secreted antibiotic chemicals or nutrients, proteins or pathways) by which this relationship is maintained are not known. Here we present the first global and differential proteome profile of the southern pine beetle pronotum. Two-dimensional polyacrylamide electrophoresis, tandem mass spectrometry, and database searches revealed that the majority of pronotal proteins were related to energy-yielding metabolism, contractile apparati, cell structure, and defence. The identified proteins provide important insights into the molecular and biochemical processes of, and candidates for functional genomics to understand mycangia and pronotum functions in, the southern pine beetle.