The C terminus of cardiac troponin I is essential for full inhibitory activity and Ca2+ sensitivity of rat myofibrils

The role of the troponin T interactions with actin in regulation of cardiac thin filament revealed by the troponin T pathogenic variant Ile79Asn

Cardiac muscle contraction/relaxation cycle depends on the rising and falling Ca2+ levels in sarcomeres that control the extent of interactions between myosin-based thick and actin-based thin filaments. Cardiac thin filament (cTF) consists of actin, tropomyosin (Tm) that regulates myosin binding to actin, and troponin complex that governs Tm position upon Ca2+-binding. Troponin has three subunits - Ca2+-binding troponin C (TnC), Tm stabilizing troponin T (TnT), and inhibitory troponin I (TnI). TnT N-terminus (TnT1) interactions with actin stabilize the inhibited state of cTF. TnC, TnI, and Tm work in concert to control actomyosin interactions. Cryo-electron microscopy (cryo-EM) provided factual structures of healthy cTF, but structures of cTF carrying missense mutations linked to human cardiomyopathy are unknown. Variant Ile79Asn in human cardiac TnT (TnT-I79N) increases myofilament Ca2+ sensitivity and slows cross-bridge kinetics, leading to severe hypertrophic/restrictive cardiomyopathy. Here, we used TnT-I79N mutation as a tool to examine the role of TnT1 in the complex mechanism of cTF regulation. Comparison of the cryo-EM structures of murine wild type and TnT-I79N native cTFs at systolic Ca2+ levels (pCa = 5.8) demonstrates that TnT-I79N causes 1) dissociation of the TnT1 loop from its actin interface that results in Tm release to a more activated position, 2) reduced interaction of TnI C-terminus with actin-Tm, and 3) increased frequency of Ca2+-bound regulatory units. Our data indicate that the TnT1 loop is a crucial element of the allosteric regulatory network that couples Tn subunits and Tm to maintain adequate cTF response to physiological Ca2+ levels during a heartbeat.

Troponin Structural Dynamics in the Native Cardiac Thin Filament Revealed by Cryo Electron Microscopy

Cardiac muscle contraction occurs due to repetitive interactions between myosin thick and actin thin filaments (TF) regulated by Ca2+ levels, active cross-bridges, and cardiac myosin-binding protein C (cMyBP-C). The cardiac TF (cTF) has two nonequivalent strands, each comprised of actin, tropomyosin (Tm), and troponin (Tn). Tn shifts Tm away from myosin-binding sites on actin at elevated Ca2+ levels to allow formation of force-producing actomyosin cross-bridges. The Tn complex is comprised of three distinct polypeptides - Ca2+-binding TnC, inhibitory TnI, and Tm-binding TnT. The molecular mechanism of their collective action is unresolved due to lack of comprehensive structural information on Tn region of cTF. C1 domain of cMyBP-C activates cTF in the absence of Ca2+ to the same extent as rigor myosin. Here we used cryo-EM of native cTFs to show that cTF Tn core adopts multiple structural conformations at high and low Ca2+ levels and that the two strands are structurally distinct. At high Ca2+ levels, cTF is not entirely activated by Ca2+ but exists in either partially or fully activated state. Complete dissociation of TnI C-terminus is required for full activation. In presence of cMyBP-C C1 domain, Tn core adopts a fully activated conformation, even in absence of Ca2+. Our data provide a structural description for the requirement of myosin to fully activate cTFs and explain increased affinity of TnC to Ca2+ in presence of active cross-bridges. We suggest that allosteric coupling between Tn subunits and Tm is required to control actomyosin interactions.

The cardiomyocyte origins of diastolic dysfunction: cellular components of myocardial "stiffness"

The impaired ability of the heart to relax and stretch to accommodate venous return is generally understood to represent a state of "diastolic dysfunction" and often described using the all-purpose noun "stiffness." Despite the now common qualitative usage of this term in fields of cardiac patho/physiology, the specific quantitative concept of stiffness as a molecular and biophysical entity with real practical interpretation in healthy and diseased hearts is sometimes obscure. The focus of this review is to characterize the concept of cardiomyocyte stiffness and to develop interpretation of "stiffness" attributes at the cellular and molecular levels. Here, we consider "stiffness"-related terminology interpretation and make links between cardiomyocyte stiffness and aspects of functional and structural cardiac performance. We discuss cross bridge-derived stiffness sources, considering the contributions of diastolic myofilament activation and impaired relaxation. This includes commentary relating to the role of cardiomyocyte Ca2+ flux and Ca2+ levels in diastole, the troponin-tropomyosin complex role as a Ca2+ effector in diastole, the myosin ADP dissociation rate as a modulator of cross bridge attachment and regulation of cross-bridge attachment by myosin binding protein C. We also discuss non-cross bridge-derived stiffness sources, including the titin sarcomeric spring protein, microtubule and intermediate filaments, and cytoskeletal extracellular matrix interactions. As the prevalence of conditions involving diastolic heart failure has escalated, a more sophisticated understanding of the molecular, cellular, and tissue determinants of cardiomyocyte stiffness offers potential to develop imaging and molecular intervention tools.

Pathogenic variants in TNNC2 cause congenital myopathy due to an impaired force response to calcium

Troponin C (TnC) is a critical regulator of skeletal muscle contraction; it binds Ca2+ to activate muscle contraction. Surprisingly, the gene encoding fast skeletal TnC (TNNC2) has not yet been implicated in muscle disease. Here, we report 2 families with pathogenic variants in TNNC2. Patients present with a distinct, dominantly inherited congenital muscle disease. Molecular dynamics simulations suggested that the pathomechanisms by which the variants cause muscle disease include disruption of the binding sites for Ca2+ and for troponin I. In line with these findings, physiological studies in myofibers isolated from patients' biopsies revealed a markedly reduced force response of the sarcomeres to [Ca2+]. This pathomechanism was further confirmed in experiments in which contractile dysfunction was evoked by replacing TnC in myofibers from healthy control subjects with recombinant, mutant TnC. Conversely, the contractile dysfunction of myofibers from patients was repaired by replacing endogenous, mutant TnC with recombinant, wild-type TnC. Finally, we tested the therapeutic potential of the fast skeletal muscle troponin activator tirasemtiv in patients' myofibers and showed that the contractile dysfunction was repaired. Thus, our data reveal that pathogenic variants in TNNC2 cause congenital muscle disease, and they provide therapeutic angles to repair muscle contractility.

Anesthetic Agents Isoflurane and Propofol Decrease Maximal Ca2+-Activated Force and Thus Contractility in the Failing Myocardium

In the normal heart, frequently used anesthetics such as isoflurane and propofol can reduce inotropy. However, the impact of these agents on the failing myocardium is unclear. Here, we examined whether and how isoflurane and propofol influence cardiac contractility in intact cardiac muscles from rats treated with monocrotaline to induce heart failure. We measured force and intracellular Ca2+ ([Ca2 +]i) in trabeculae from the right ventricles of the rats in the absence or presence of propofol or isoflurane. At low to moderate concentrations, both propofol and isoflurane dose-dependently depressed cardiac force generation in failing trabeculae without altering [Ca2+]i At high doses, propofol (but not isoflurane) also decreased amplitude of [Ca2+]i transients. During steady-state activation, both propofol and isoflurane impaired maximal Ca2+-activated force (Fmax) while increasing the amount of [Ca2+]i required for 50% of maximal activation (Ca50). These events occurred without apparent change in the Hill coefficient, suggesting no impairment of cooperativity. Exposing these same muscles to the anesthetics after fiber skinning resulted in a similar decrement in Fmax and rise in Ca50 but no change in the myofibrillar ATPase-Ca2+ relationship. Thus, our study demonstrates that challenging the failing myocardium with commonly used anesthetic agents such as propofol and isoflurane leads to reduced force development as a result of lowered myofilament responsiveness to Ca2+ SIGNIFICANCE STATEMENT: Commonly used anesthetics such as isoflurane and propofol can impair myocardial contractility in subjects with heart failure by lowering myofilament responsiveness to Ca2+. High doses of propofol can also reduce the overall amplitude of the intracellular Ca2+ transient. These findings may have important implications for the safety and quality of intra- and perioperative care of patients with heart failure and other cardiac disorders.

Conformation switching of single native proteins revealed by nanomechanical probing without a pulling force

Protein conformational changes are essential to biological function, and the heterogeneous nature of the corresponding protein states provokes an interest to measure conformational changes at the single molecule level. Here we demonstrate that conformational changes in single native proteins can be revealed by non-covalent antibody-targeting of specific domains within the protein, using nanomechanical probing without an applied pulling force. The protein of interest was captured between a particle and a substrate and three properties were quantified: the twist amplitude related to an applied torque, torsional compliance related to rotational Brownian motion, and translational Brownian displacement. Calcium-dependent conformation switching was studied in native human cardiac troponin, a heterotrimer protein complex that regulates the contraction and relaxation of heart muscle cells and is also a key biomarker for diagnosing myocardial infarction. The data reveal a change in mechanical properties upon conformation switching from the non-saturated to the calcium-saturated state, which in cardiomyocytes gives myosin motor proteins access to actin filaments. A clear increase was observed in the molecular stiffness for the calcium-saturated protein conformation. Using libraries of monoclonal antibodies, the nanomechanical probing of conformation by antibody targeting opens avenues for characterizing single native protein complexes for research as well as for diagnostic applications.

Fropofol decreases force development in cardiac muscle

Supranormal contractile properties are frequently associated with cardiac diseases. Anesthetic agents, including propofol, can depress myocardial contraction. We tested the hypothesis that fropofol, a propofol derivative, reduces force development in cardiac muscles via inhibition of cross-bridge cycling and may therefore have therapeutic potential. Force and intracellular Ca²⁺ concentration ([Ca²⁺]_i) transients of rat trabecular muscles were determined. Myofilament ATPase, actin-activated myosin ATPase, and velocity of actin filaments propelled by myosin were also measured. Fropofol dose dependently decreased force without altering [Ca²⁺]_i in normal and pressure-induced hypertrophied-hypercontractile muscles. Similarly, fropofol depressed maximum Ca²⁺-activated force ( F_max) and increased the [Ca²⁺]_i required for 50% of F_max (Ca₅₀) at steady state without affecting the Hill coefficient in both intact and skinned cardiac fibers. The drug also depressed cardiac myofibrillar and actin-activated myosin ATPase activity. In vitro actin sliding velocity was significantly reduced when fropofol was introduced during rigor binding of cross-bridges. The data suggest that the depressing effects of fropofol on cardiac contractility are likely to be related to direct targeting of actomyosin interactions. From a clinical standpoint, these findings are particularly significant, given that fropofol is a nonanesthetic small molecule that decreases myocardial contractility specifically and thus may be useful in the treatment of hypercontractile cardiac disorders.-Ren, X., Schmidt, W., Huang, Y., Lu, H., Liu, W., Bu, W., Eckenhoff, R., Cammarato, A., Gao, W. D. Fropofol decreases force development in cardiac muscle.

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca²⁺-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca²⁺] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1-167) and cTnI(1-193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca²⁺-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1-167) truncation diminished the SL-induced increase in Ca²⁺-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca²⁺-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1-167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.

Distal arthrogryposis with variable clinical expression caused by TNNI2 mutation

Distal arthrogryposis (DA) is a clinically and genetically heterogeneous disorder with multiple joint contractures. We describe a female DA patient with hand and foot deformities, and right-sided torticollis. Using exome sequencing, we identified a novel TNNI2 mutation (c.485>A, p.Arg162Lys) in the patient and her father. The father has no typical DA but hip dysplasia. This may explain the clinical features of DA2B in this family, but with variable clinical expression.

Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca(2+) depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca(2+)-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (∼0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (∼1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca(2+) was mostly (61%), but not completely eliminated with cTnI1-163. TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin.

Cardiac troponin structure-function and the influence of hypertrophic cardiomyopathy associated mutations on modulation of contractility

Cardiac troponin (cTn) acts as a pivotal regulator of muscle contraction and relaxation and is composed of three distinct subunits (cTnC: a highly conserved Ca(2+) binding subunit, cTnI: an actomyosin ATPase inhibitory subunit, and cTnT: a tropomyosin binding subunit). In this mini-review, we briefly summarize the structure-function relationship of cTn and its subunits, its modulation by PKA-mediated phosphorylation of cTnI, and what is known about how these properties are altered by hypertrophic cardiomyopathy (HCM) associated mutations of cTnI. This includes recent work using computational modeling approaches to understand the atomic-based structural level basis of disease-associated mutations. We propose a viewpoint that it is alteration of cTnC-cTnI interaction (rather than the Ca(2+) binding properties of cTn) per se that disrupt the ability of PKA-mediated phosphorylation at cTnI Ser-23/24 to alter contraction and relaxation in at least some HCM-associated mutations. The combination of state of the art biophysical approaches can provide new insight on the structure-function mechanisms of contractile dysfunction resulting cTnI mutations and exciting new avenues for the diagnosis, prevention, and even treatment of heart diseases.

Molecular mechanism of anesthetic-induced depression of myocardial contraction

Isoflurane and propofol are known to depress cardiac contraction, but the molecular mechanisms involved are not known. In this study, we determined whether decreasing myofilament Ca(2+) responsiveness underlies anesthesia-induced depression of contraction and uncovered the molecular targets of isoflurane and propofol. Force and intracellular Ca(2+) ([Ca(2+)]i) were measured in rat trabeculae superfused with Krebs-Henseleit solution, with or without propofol or isoflurane. Photoaffinity labeling of myofilament proteins with meta-Azi-propofol (AziPm) and Azi-isoflurane (Azi-iso) and molecular docking were also used. Both propofol and isoflurane dose dependently depressed force from low doses (propofol, 27 ± 6 μM; isoflurane, 1.0 ± 0.1%) to moderate doses (propofol, 87 ± 4 μM; isoflurane, 3.0 ± 0.25%), without significant alteration [Ca(2+)]i During steady-state activations in both intact and skinned preparations, propofol and isoflurane depressed maximum Ca(2+)-activated force and increased the [Ca(2+)]i required for 50% of activation. Myofibrils photolabeled with AziPm and Azi-iso identified myosin, actin, and myosin light chain as targets of the anesthetics. Several adducted residues in those proteins were located in conformationally sensitive regions that underlie contractile function. Thus, propofol and isoflurane decrease force development by directly depressing myofilament Ca(2+) responsiveness and have binding sites in key regions for contraction in both actin and myosin.-Meng, T., Bu, W., Ren, X., Chen, X., Yu, J., Eckenhoff, R. G., Gao, W. D. Molecular mechanism of anesthetic-induced depression of myocardial contraction.

Green Tea Catechin Normalizes the Enhanced Ca2+ Sensitivity of Myofilaments Regulated by a Hypertrophic Cardiomyopathy-Associated Mutation in Human Cardiac Troponin I (K206I)

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiovascular disease characterized by thickening of ventricular walls and decreased left ventricular chamber volume. The majority of HCM-associated mutations are found in genes encoding sarcomere proteins. Herein, we set out to functionally characterize a novel HCM-associated mutation (K206I-TNNI3) and elucidate the mechanism of dysfunction at the level of myofilament proteins.

METHODS AND RESULTS

The male index case was diagnosed with HCM after an out-of-hospital cardiac arrest, which was followed by comprehensive clinical evaluation, transthoracic echocardiography, and clinical genetic testing. To determine molecular mechanism(s) of the mutant human cardiac troponin I (K206I), we tested the Ca(2+) dependence of thin filament-activated myosin-S1-ATPase activity in a reconstituted, regulated, actomyosin system comparing wild-type human troponin complex, 50% mix of K206I/wildtype, or 100% K206I. We also exchanged native troponin detergent extracted fibers with reconstituted troponin containing either wildtype or a 65% mix of K206I/wildtype and measured force generation. The Ca(2+) sensitivity of the myofilaments containing the K206I variant was significantly increased, and when treated with 20 µmol/L (-)-epigallocatechin gallate (green tea) was restored back to wild-type levels in ATPase and force measurements. The K206I mutation impairs the ability of the troponin I to inhibit ATPase activity in the absence of calcium-bound human cardiac troponin C. The ability of calcium-bound human cardiac troponin C to neutralize the inhibition of K206I was greater than with wild-type TnI.

CONCLUSIONS

Compromised interactions of K206I with actin and hcTnC may lead to impaired relaxation and HCM.

Structure and function of cardiac troponin C (TNNC1): Implications for heart failure, cardiomyopathies, and troponin modulating drugs

In striated muscle, the protein troponin complex turns contraction on and off in a calcium-dependent manner. The calcium-sensing component of the complex is troponin C, which is expressed from the TNNC1 gene in both cardiac muscle and slow-twitch skeletal muscle (identical transcript in both tissues) and the TNNC2 gene in fast-twitch skeletal muscle. Cardiac troponin C (cTnC) is made up of two globular EF-hand domains connected by a flexible linker. The structural C-domain (cCTnC) contains two high affinity calcium-binding sites that are always occupied by Ca(2+) or Mg(2+) under physiologic conditions, stabilizing an open conformation that remains anchored to the rest of the troponin complex. In contrast, the regulatory N-domain (cNTnC) contains a single low affinity site that is largely unoccupied at resting calcium concentrations. During muscle activation, calcium binding to cNTnC favors an open conformation that binds to the switch region of troponin I, removing adjacent inhibitory regions of troponin I from actin and allowing muscle contraction to proceed. Regulation of the calcium binding affinity of cNTnC is physiologically important, because it directly impacts the calcium sensitivity of muscle contraction. Calcium sensitivity can be modified by drugs that stabilize the open form of cNTnC, post-translational modifications like phosphorylation of troponin I, or downstream thin filament protein interactions that impact the availability of the troponin I switch region. Recently, mutations in cTnC have been associated with hypertrophic or dilated cardiomyopathy. A detailed understanding of how calcium sensitivity is regulated through the troponin complex is necessary for explaining how mutations perturb its function to promote cardiomyopathy and how post-translational modifications in the thin filament affect heart function and heart failure. Troponin modulating drugs are being developed for the treatment of cardiomyopathies and heart failure.

Mechanism of the calcium-regulation of muscle contraction--in pursuit of its structural basis

The author reviewed the research that led to establish the structural basis for the mechanism of the calcium-regulation of the contraction of striated muscles. The target of calcium ions is troponin on the thin filaments, of which the main component is the double-stranded helix of actin. A model of thin filament was generated by adding tropomyosin and troponin. During the process to provide the structural evidence for the model, the troponin arm was found to protrude from the calcium-depleted troponin and binds to the carboxyl-terminal region of actin. As a result, the carboxyl-terminal region of tropomyosin shifts and covers the myosin-binding sites of actin to block the binding of myosin. At higher calcium concentrations, the troponin arm changes its partner from actin to the main body of calcium-loaded troponin. Then, tropomyosin shifts back to the position near the grooves of actin double helix, and the myosin-binding sites of actin becomes available to myosin resulting in force generation through actin-myosin interactions.

New insights into the functional significance of the acidic region of the unique N-terminal extension of cardiac troponin I

Previous structural studies indicated a special functional role for an acidic region composed of residues 1-10 in the unique N-terminal peptide of cardiac troponin I (cTnI). Employing LC-MS/MS, we determined the presence of phosphorylation sites at S5/S6 in cTnI from wild type mouse hearts as well as in hearts of mice chronically expressing active protein kinase C-ε (PKCε) and exhibiting severe dilated cardiomyopathy (DCM). To determine the functional significance of these phosphorylations, we cloned and expressed wild-type cTnI, (Wt), and cTnI variants expressing pseudo-phosphorylation cTnI-(S5D), cTnI(S6D), as well as cTnI(S5A) and cTnI(S6A). We exchanged native Tn of detergent-extracted (skinned) fiber bundles with Tn reconstituted with the variant cTnIs and measured tension and cross-bridge dynamics. Compared to controls, myofilaments controlled by cTnI with pseudo-phosphorylation (S6D) or Ala substitution (S6A) demonstrated a significant depression in maximum tension, ATPase rate, and ktr, but no change in half-maximally activating Ca(2+). In contrast, pseudo-phosphorylation at position 5 (S5D) had no effects, although S5A induced an increase in Ca(2+)-sensitivity with no change in maximum tension or ktr. We further tested the impact of acidic domain modifications on myofilament function in studies examining the effects of cTnI(A2V), a mutation linked to DCM. This mutation significantly altered the inhibitory activity of cTnI as well as cooperativity of activation of myofilament tension, but not when S23/S24 were pseudo-phosphorylated. Our data indicate a new functional and pathological role of amino acid modifications in the N-terminal acidic domain of cTnI that is modified by phosphorylations at cTnI(S23/S24). This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Structural and functional consequences of cardiac troponin C L57Q and I61Q Ca(2+)-desensitizing variants

Two cTnC variants, L57Q and I61Q, both of which are located on helix C within the N domain of cTnC, were originally reported in the skeletal muscle system [Tikunova, Davis, J. Biol. Chem. 279 (2004) 35341-35352], as the analogous L58Q and I62Q sTnC, and demonstrated a decreased Ca(2+) binding affinity. Here, we provide detailed characterization of structure-function relationships for these two cTnC variants, to determine if they behave differently in the cardiac system and as a framework for determining similarities and differences with other cTnC mutations that have been associated with DCM. We have used an integrative approach to study the structure and function of these cTnC variants both in solution and in silico, to understand how the L57Q and I61Q mutations influence Ca(2+) binding at site II, the subsequent effects on the interaction with cTnI, and the structural changes which are associated with these changes. Steady-state and stopped flow fluorescence spectroscopy confirmed that a decrease in Ca(2+) affinity for recombinant cTnC and cTn complexes containing the L57Q or I61Q variants. The L57Q variant was intermediate between WT and I61Q cTnC and also did not significantly alter cTnC-cTnI interaction in the absence of Ca(2+), but did decrease the interaction in the presence of Ca(2+). In contrast, I61Q decreased the cTnC-cTnI interaction in both the absence and presence of Ca(2+). This difference in the absence of Ca(2+) suggests a greater structural change in cNTnC may occur with the I61Q mutation than the L57Q mutation. MD simulations revealed that the decreased Ca(2+) binding induced by I61Q may result from destabilization of the Ca(2+) binding site through interruption of intra-molecular interactions when residue 61 forms new hydrogen bonds with G70 on the Ca(2+) binding loop. The experimentally observed interruption of the cTnC-cTnI interaction caused by L57Q or I61Q is due to the disruption of key hydrophobic interactions between helices B and C in cNTnC. This study provides a molecular basis of how single mutations in the C helix of cTnC can reduce Ca(2+) binding affinity and cTnC-cTnI interaction, which may provide useful insights for a better understanding of cardiomyopathies and future gene-based therapies.

Structural dynamics of troponin I during Ca2+-activation of cardiac thin filaments: a multi-site Förster resonance energy transfer study

A multi-site, steady-state Förster resonance energy transfer (FRET) approach was used to quantify Ca²⁺-induced changes in proximity between donor loci on human cardiac troponin I (cTnI), and acceptor loci on human cardiac tropomyosin (cTm) and F-actin within functional thin filaments. A fluorescent donor probe was introduced to unique and key cysteine residues on the C- and N-termini of cTnI. A FRET acceptor probe was introduced to one of three sites located on the inner or outer domain of F-actin, namely Cys-374 and the phalloidin-binding site on F-actin, and Cys-190 of cTm. Unlike earlier FRET analyses of protein dynamics within the thin filament, this study considered the effects of non-random distribution of dipoles for the donor and acceptor probes. The major conclusion drawn from this study is that Ca²⁺ and myosin S1-binding to the thin filament results in movement of the C-terminal domain of cTnI from the outer domain of F-actin towards the inner domain, which is associated with the myosin-binding. A hinge-linkage model is used to best-describe the finding of a Ca²⁺-induced movement of the C-terminus of cTnI with a stationary N-terminus. This dynamic model of the activation of the thin filament is discussed in the context of other structural and biochemical studies on normal and mutant cTnI found in hypertrophic cardiomyopathies.

Significance of troponin dynamics for Ca2+-mediated regulation of contraction and inherited cardiomyopathy

Ca(2+) dissociation from troponin causes cessation of muscle contraction by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca(2+) binding in the NH(2)-lobe of subunit troponin C (TnC) was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange (HDX)-MS. The findings demonstrate the interrelationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca(2+) slowed HDX up to 2 orders of magnitude within the NH(2)-lobe and the NH(2)-lobe-associated TnI switch helix, implying that Ca(2+) greatly stabilizes this troponin regulatory region. HDX of the TnI COOH terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca(2+) and actin. Ca(2+)-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1 and to the COOH terminus of the TnT-TnI coiled-coil. Ca(2+) destabilized rather than stabilized specific TnI segments within the coiled-coil and destabilized a region not previously implicated in Ca(2+)-mediated regulation: the coiled-coil's NH(2)-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within influentially dynamic regions of troponin, and many sites were Ca(2+)-sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca(2+), including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca(2+) release triggers an intramolecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

Insights into restrictive cardiomyopathy from clinical and animal studies

Cardiomyopathies are diseases that primarily affect the myocardium, leading to serious cardiac dysfunction and heart failure. Out of the three major categories of cardiomyopathies (hypertrophic, dilated and restrictive), restrictive cardiomyopathy (RCM) is less common and also the least studied. However, the prognosis for RCM is poor as some patients dying in their childhood. The molecular mechanisms behind the disease development and progression are not very clear and the treatment of RCM is very difficult and often ineffective. In this article, we reviewed the recent progress in RCM research from the clinical studies and the translational studies done on diseased transgenic animal models. This will help for a better understanding of the mechanisms underlying the etiology and development of RCM and for the design of better treatments for the disease.

Disease-related cardiac troponins alter thin filament Ca2+ association and dissociation rates

The contractile response of the heart can be altered by disease-related protein modifications to numerous contractile proteins. By utilizing an IAANS labeled fluorescent troponin C, [Formula: see text], we examined the effects of ten disease-related troponin modifications on the Ca(2+) binding properties of the troponin complex and the reconstituted thin filament. The selected modifications are associated with a broad range of cardiac diseases: three subtypes of familial cardiomyopathies (dilated, hypertrophic and restrictive) and ischemia-reperfusion injury. Consistent with previous studies, the majority of the protein modifications had no effect on the Ca(2+) binding properties of the isolated troponin complex. However, when incorporated into the thin filament, dilated cardiomyopathy mutations desensitized (up to 3.3-fold), while hypertrophic and restrictive cardiomyopathy mutations, and ischemia-induced truncation of troponin I, sensitized the thin filament to Ca(2+) (up to 6.3-fold). Kinetically, the dilated cardiomyopathy mutations increased the rate of Ca(2+) dissociation from the thin filament (up to 2.5-fold), while the hypertrophic and restrictive cardiomyopathy mutations, and the ischemia-induced truncation of troponin I decreased the rate (up to 2-fold). The protein modifications also increased (up to 5.4-fold) or decreased (up to 2.5-fold) the apparent rate of Ca(2+) association to the thin filament. Thus, the disease-related protein modifications alter Ca(2+) binding by influencing both the association and dissociation rates of thin filament Ca(2+) exchange. These alterations in Ca(2+) exchange kinetics influenced the response of the thin filament to artificial Ca(2+) transients generated in a stopped-flow apparatus. Troponin C may act as a hub, sensing physiological and pathological stimuli to modulate the Ca(2+)-binding properties of the thin filament and influence the contractile performance of the heart.

Structural and functional consequences of the cardiac troponin C L48Q Ca(2+)-sensitizing mutation

Calcium binding to the regulatory domain of cardiac troponin C (cNTnC) causes a conformational change that exposes a hydrophobic surface to which troponin I (cTnI) binds, prompting a series of protein-protein interactions that culminate in muscle contraction. A number of cTnC variants that alter the Ca(2+) sensitivity of the thin filament have been linked to disease. Tikunova and Davis engineered a series of cNTnC mutations that altered Ca(2+) binding properties and studied the effects on the Ca(2+) sensitivity of the thin filament and contraction [Tikunova, S. B., and Davis, J. P. (2004) J. Biol. Chem. 279, 35341-35352]. One of the mutations they engineered, the L48Q variant, resulted in a pronounced increase in the cNTnC Ca(2+) binding affinity and Ca(2+) sensitivity of cardiac muscle force development. In this work, we sought structural and mechanistic explanations for the increased Ca(2+) sensitivity of contraction for the L48Q cNTnC variant, using an array of biophysical techniques. We found that the L48Q mutation enhanced binding of both Ca(2+) and cTnI to cTnC. Nuclear magnetic resonance chemical shift and relaxation data provided evidence that the cNTnC hydrophobic core is more exposed with the L48Q variant. Molecular dynamics simulations suggest that the mutation disrupts a network of crucial hydrophobic interactions so that the closed form of cNTnC is destabilized. The findings emphasize the importance of cNTnC's conformation in the regulation of contraction and suggest that mutations in cNTnC that alter myofilament Ca(2+) sensitivity can do so by modulating Ca(2+) and cTnI binding.

Recurrent and founder mutations in the Netherlands: cardiac Troponin I (TNNI3) gene mutations as a cause of severe forms of hypertrophic and restrictive cardiomyopathy

BACKGROUND

About 2-7% of familial cardiomyopathy cases are caused by a mutation in the gene encoding cardiac troponin I (TNNI3). The related clinical phenotype is usually severe with early onset. Here we report on all currently known mutations in the Dutch population and compared these with those described in literature.

METHODS

TheTNNI3 gene was screened for mutations in all coding exons and flanking intronic sequences in a large cohort of cardiomyopathy patients. All Dutch index cases carrying a TNNI3 mutation that are described in this study underwent extensive cardiological evaluation and were listed by their postal codes.

RESULTS

In 30 families, 14 different mutations were identified. Three TNNI3 mutations were found relatively frequently in both familial and non-familial cases of hypertrophic cardiomyopathy (HCM) or restrictive cardiomyopathy (RCM). Haplotype analysis showed that p.Arg145Trp and p.Ser166Phe are founder mutations in the Netherlands, while p.Glu209Ala is not. The majority of Dutch TNNI3 mutations were associated with a HCM phenotype. Mean age at diagnosis was 36.5 years. Mutations causing RCM occurred less frequently, but were identified in very young children with a poor prognosis.

CONCLUSION

In line with previously published data, we found TNNI3 mutations to be rare and associated with early onset and severe clinical presentation.

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.

Dual regulatory functions of the thin filament revealed by replacement of the troponin I inhibitory peptide with a linker

Striated muscles are relaxed under low Ca(2+) concentration conditions due to actions of the thin filament protein troponin. To investigate this regulatory mechanism, an 11-residue segment of cardiac troponin I previously termed the inhibitory peptide region was studied by mutagenesis. Several mutant troponin complexes were characterized in which specific effects of the inhibitory peptide region were abrogated by replacements of 4-10 residues with Gly-Ala linkers. The mutations greatly impaired two of troponin's actions under low Ca(2+) concentration conditions: inhibition of myosin subfragment 1 (S1)-thin filament MgATPase activity and cooperative suppression of myosin S1-ADP binding to thin filaments with low myosin saturation. Inhibitory peptide replacement diminished but did not abolish the Ca(2+) dependence of the ATPase rate; ATPase rates were at least 2-fold greater when Ca(2+) rather than EGTA was present. This residual regulation was highly cooperative as a function of Ca(2+) concentration, similar to the degree of cooperativity observed with WT troponin present. Other effects of the mutations included 2-fold or less increases in the apparent affinity of the thin filament regulatory Ca(2+) sites, similar decreases in the affinity of troponin for actin-tropomyosin regardless of Ca(2+), and increases in myosin S1-thin filament ATPase rates in the presence of saturating Ca(2+). The overall results indicate that cooperative myosin binding to Ca(2+)-free thin filaments depends upon the inhibitory peptide region but that a cooperatively activating effect of Ca(2+) binding does not. The findings suggest that these two processes are separable and involve different conformational changes in the thin filament.

Low temperature dynamic mapping reveals unexpected order and disorder in troponin

Troponin is a pivotal regulatory protein that binds Ca(2+) reversibly to act as the muscle contraction on-off switch. To understand troponin function, the dynamic behavior of the Ca(2+)-saturated cardiac troponin core domain was mapped in detail at 10 °C, using H/D exchange-mass spectrometry. The low temperature conditions of the present study greatly enhanced the dynamic map compared with previous work. Approximately 70% of assessable peptide bond hydrogens were protected from exchange sufficiently for dynamic measurement. This allowed the first characterization by this method of many regions of regulatory importance. Most of the TnI COOH terminus was protected from H/D exchange, implying an intrinsically folded structure. This region is critical to the troponin inhibitory function and has been implicated in thin filament activation. Other new findings include unprotected behavior, suggesting high mobility, for the residues linking the two domains of TnC, as well as for the inhibitory peptide residues preceding the TnI switch helix. These data indicate that, in solution, the regulatory subdomain of cardiac troponin is mobile relative to the remainder of troponin. Relatively dynamic properties were observed for the interacting TnI switch helix and TnC NH(2)-domain, contrasting with stable, highly protected properties for the interacting TnI helix 1 and TnC COOH-domain. Overall, exchange protection via protein folding was relatively weak or for a majority of peptide bond hydrogens. Several regions of TnT and TnI were unfolded even at low temperature, suggesting intrinsic disorder. Finally, change in temperature prominently altered local folding stability, suggesting that troponin is an unusually mobile protein under physiological conditions.

Sarcomere control mechanisms and the dynamics of the cardiac cycle

This review focuses on recent developments in the molecular mechanisms by which Ca activates cardiac sarcomeres and how these mechanisms play out in the cardiac cycle. I emphasize the role of mechanisms intrinsic to the sarcomeres as significant determinants of systolic elastance and ventricular stiffening during ejection. Data are presented supporting the idea that processes intrinsic to the thin filaments may promote cooperative activation of the sarcomeres and be an important factor in maintaining and modifying systolic elastance. Application of these ideas to translational medicine and rationale drug design forms an important rationale for detailed understanding of these processes.

Switch action of troponin on muscle thin filament as revealed by spin labeling and pulsed EPR

We have used pulsed electron-electron double resonance (PELDOR) spectroscopy to measure the distance between spin labels at Cys(133) of the regulatory region of TnI (TnI133) and a native or genetically substituted cysteine of TnC (TnC44, TnC61, or TnC98). In the +Ca(2+) state, the TnC44-TnI133-T distance was 42 A, with a narrow distribution (half-width of 9 A), suggesting that the regulatory region binds the N-lobe of TnC. Distances for TnC61-TnI133 and TnC98-TnI133 were also determined to be 38 A (width of 12 A) and 22 A (width of 3.4 A), respectively. These values were all consistent with recently published crystal structure (Vinogradova, M. V., Stone, D. B., Malanina, G. G., Karatzaferi, C., Cooke, R., Mendelson, R. A., and Fletterick, R. J. (2005) Proc. Natl Acad. Sci. U.S.A. 102, 5038-5043). Similar distances were obtained with the same spin pairs on a reconstituted thin filament in the +Ca(2+) state. In the -Ca(2+) state, the distances displayed broad distributions, suggesting that the regulatory region of TnI was physically released from the N-lobe of TnC and consequently fluctuated over a variety of distances on a large scale (20-80 A). The interspin distance appeared longer on the filament than on troponin alone, consistent with the ability of the region to bind actin. These results support a concept that the regulatory region of TnI, as a molecular switch, binds to the exposed hydrophobic patch of TnC and traps the inhibitory region of TnI away from actin in Ca(2+) activation of muscle.

Increased cross-bridge cycling kinetics after exchange of C-terminal truncated troponin I in skinned rat cardiac muscle

The precise mechanism of cardiac troponin I (cTnI) proteolysis in myocardial stunning is not fully understood. Accordingly, we determined the effect of cTnI C terminus truncation on chemo-mechanical transduction in isolated skinned rat trabeculae. Recombinant troponin complex (cTn), containing either mouse cTnI-(1-193) or human cTnI-(1-192) was exchanged into skinned cardiac trabeculae; Western blot analysis confirmed that 60-70% of the endogenous cTn was replaced by recombinant Tn. Incorporation of truncated cTnI induced significant reductions ( approximately 50%) in maximum force and cooperative activation as well as increases ( approximately 50%) in myofilament Ca(2+) sensitivity and tension cost. Similar results were obtained with either mouse or human truncated cTn. Presence of truncated cTnI increased maximum actin-activated S1 ATPase activity as well as its Ca(2+) sensitivity in vitro. Partial exchange (50%) for truncated cTnI resulted in similar reductions in maximum force and cooperativity; tension cost was increased in proportion to truncated cTnI content. In vitro, to determine the molecular mechanism responsible for the enhanced myofilament Ca(2+) sensitivity, we measured Ca(2+) binding to cTn as reported using a fluorescent probe. Incorporation of truncated cTnI did not affect Ca(2+) binding affinity to cTn alone. However, when cTn was incorporated into thin filaments, cTnI truncation induced a significant increase in Ca(2+) binding affinity to cTn. We conclude that cTnI truncation induces depressed myofilament function. Decreased cardiac function after ischemia/reperfusion injury may directly result, in part, from proteolytic degradation of cTnI, resulting in alterations in cross-bridge cycling kinetics.

Tropomyosin and the steric mechanism of muscle regulation

Contraction in all muscles must be precisely regulated and requisite control systems must be able to adjust to changes in physiological and myopathic stimuli. In this chapter, we outline the structural evidence for a steric mechanism that governs muscle activity. The mechanism involves calcium and myosin induced changes in the position of tropomyosin along actin-based thin filaments. This process either blocks or uncovers myosin crossbridge binding sites on actin and consequently regulates crossbridge cycling on thin filaments, the sliding of thin and thick filaments and muscle shortening and force production.

Nitroxyl increases force development in rat cardiac muscle

Donors of nitroxyl (HNO), the reduced congener of nitric oxide (NO), exert positive cardiac inotropy/lusitropy in vivo and in vitro, due in part to their enhancement of Ca(2+) cycling into and out of the sarcoplasmic reticulum. Here we tested whether the cardiac action of HNO further involves changes in myofilament-calcium interaction. Intact rat trabeculae from the right ventricle were mounted between a force transducer and a motor arm, superfused with Krebs-Henseleit (K-H) solution (pH 7.4, room temperature) and loaded iontophoretically with fura-2 to determine [Ca(2+)](i). Sarcomere length was set at 2.2-2.3 microm. HNO donated by Angeli's salt (AS; Na(2)N(2)O(3)) dose-dependently increased both twitch force and [Ca(2+)](i) transients (from 50 to 1000 microm). Force increased more than [Ca(2+)](i) transients, especially at higher doses (332 +/- 33% versus 221 +/- 27%, P < 0.01 at 1000 microm). AS/HNO (250 microm) increased developed force without changing Ca(2+) transients at any given [Ca(2+)](o) (0.5-2.0 mm). During steady-state activation, AS/HNO (250 microm) increased maximal Ca(2+)-activated force (F(max), 106.8 +/- 4.3 versus 86.7 +/- 4.2 mN mm(-2), n = 7-8, P < 0.01) without affecting Ca(2+) required for 50% activation (Ca(50), 0.44 +/- 0.04 versus 0.52 +/- 0.04 microm, not significant) or the Hill coefficient (4.75 +/- 0.67 versus 5.02 +/- 1.1, not significant). AS/HNO did not alter myofibrillar Mg-ATPase activity, supporting an effect on the myofilaments themselves. The thiol reducing agent dithiothreitol (DTT, 5.0 mm) both prevented and reversed HNO action, confirming AS/HNO redox sensitivity. Lastly, NO (from DEA/NO) did not mimic AS/HNO cardiac effects. Thus, in addition to reported changes in Ca(2+) cycling, HNO also acts as a cardiac Ca(2+) sensitizer, augmenting maximal force without altering actomyosin ATPase activity. This is likely to be due to modulation of myofilament proteins that harbour reactive thiolate groups that are targets of HNO.

Structural basis for calcium-regulated relaxation of striated muscles at interaction sites of troponin with actin and tropomyosin

In summary, we have shown that the TnI-TnC-TnT2 ternary complex (-52 kDa) has a mobile actin-binding domain (-6.1 kDa) that tumbles independently of the core domain. By docking the mobile domain and the core domain into the cryo-EM map obtained for thin filaments at low Ca2+, a model for actin-troponin interaction has been obtained. This model shows the atomic details of interactions of actin with the mobile domain and suggests the mechanism by which troponin generates a shift in the azimuthal position of tropomyosin in response to changes in Ca2+ levels. In this model the mobile domain of troponin interacts with three actins and one troponin interacts with four actin molecules. The relationship between myosin and the mobile domain suggests that the latter may work as a fail-safe latch to secure a relaxed state. The model also provides insights into many mutations associated with human cardiomyopathy and has implications for the function of other actin-binding proteins. Coordinates of the mobile domain have been deposited in the Protein Data Bank under accession codes 1VDI (low Ca2+) and 1VDJ (high Ca2+). Chemical shifts of the mobile domain have been deposited in the BMRB under accession ID 18140.

Mutations in fast skeletal troponin I, troponin T, and β‐tropomyosin that cause distal arthrogryposis all increase contractile function

Distal arthrogryposes (DAs) are a group of disorders characterized by congenital contractures of distal limbs without overt neurological or muscle disease. Unexpectedly, mutations in genes encoding the fast skeletal muscle regulatory proteins troponin T (TnT), troponin I (TnI), and beta-tropomyosin (beta-TM) have been shown to cause autosomal dominant DA. We tested how these mutations affect contractile function by comparing wild-type (WT) and mutant proteins in actomyosin ATPase assays and in troponin-replaced rabbit psoas fibers. We have analyzed all four reported mutants: Arg63His TnT, Arg91Gly beta-TM, Arg174Gln TnI, and a TnI truncation mutant (Arg156ter). Thin filaments, reconstituted using actin and WT troponin and beta-TM, activated myosin subfragment-1 ATPase in a calcium-dependent, cooperative manner. Thin filaments containing either a troponin or beta-TM DA mutant produced significantly enhanced ATPase rates at all calcium concentrations without alternating calcium-sensitivity or cooperativity. In troponin-exchanged skinned fibers, each mutant caused a significant increase in Ca2+ sensitivity, and Arg156ter TnI generated significantly higher maximum force. Arg91Gly beta-TM was found to have a lower actin affinity than WT and form a less stable coiled coil. We propose the mutations cause increased contractility of developing fast-twitch skeletal muscles, thus causing muscle contractures and the development of the observed limb deformities.

Impaired Diastolic Function After Exchange of Endogenous Troponin I With C-Terminal Truncated Troponin I in Human Cardiac Muscle

The specific and selective proteolysis of cardiac troponin I (cTnI) has been proposed to play a key role in human ischemic myocardial disease, including stunning and acute pressure overload. In this study, the functional implications of cTnI proteolysis were investigated in human cardiac tissue for the first time. The predominant human cTnI degradation product (cTnI 1–192 ) and full-length cTnI were expressed in Escherichia coli , purified, reconstituted with the other cardiac troponin subunits, troponin T and C, and subsequently exchanged into human cardiac myofibrils and permeabilized cardiomyocytes isolated from healthy donor hearts. Maximal isometric force and kinetic parameters were measured in myofibrils, using rapid solution switching, whereas force development was measured in single cardiomyocytes at various calcium concentrations, at sarcomere lengths of 1.9 and 2.2 μm, and after treatment with the catalytic subunit of protein kinase A (PKA) to mimic β-adrenergic stimulation. One-dimensional gel electrophoresis, Western immunoblotting, and 3D imaging revealed that approximately 50% of endogenous cTnI had been homogeneously replaced by cTnI 1–192 in both myofibrils and cardiomyocytes. Maximal tension was not affected, whereas the rates of force activation and redevelopment as well as relaxation kinetics were slowed down. Ca 2+ sensitivity of the contractile apparatus was increased in preparations containing cTnI 1–192 (pCa 50 : 5.73±0.03 versus 5.52±0.03 for cTnI 1–192 and full-length cTnI, respectively). The sarcomere length dependency of force development and the desensitizing effect of PKA were preserved in cTnI 1–192 -exchanged cardiomyocytes. These results indicate that degradation of cTnI in human myocardium may impair diastolic function, whereas systolic function is largely preserved.

A point mutation (R192H) in the C-terminus of human cardiac troponin I causes diastolic dysfunction in transgenic mice

Cardiac troponin I (cTnI) mutations have been linked to the development of restrictive cardiomyopathy (RCM) in human patients. We modeled one mutation in human cTnI C-terminus, arginine192-->histidine (R192H) by cardiac specific expression of the mutated protein (cTnI(193His) in mouse sequence) in transgenic mice. Heart tissue sections revealed neither significant hypertrophy nor ventricular dilation in cTnI(193His) mice. The main functional alteration detected in cTnI(193His) mice by ultrasound cardiac imaging examinations was impaired cardiac relaxation manifested by a decreased left ventricular end diastolic dimension (LVEDD) and an increased end diastolic dimension in both atria. The cardiac ejection fraction (EF) was not significant changed in 6- to 8-week-old cTnI(193His) mice, however, the EF was significantly decreased in cTnI(193His) mice at age of 11 months. These data indicate that individual genetic conditions and environmental factors participate together in the development of the cTnI mutation based-cardiac muscle disorders. This mouse model provides us with a tool to further investigate the pathophysiology and the development of RCM.

Increased Ca2+ affinity of cardiac thin filaments reconstituted with cardiomyopathy-related mutant cardiac troponin I

To understand the molecular mechanisms whereby cardiomyopathy-related cardiac troponin I (cTnI) mutations affect myofilament activity, we have investigated the Ca2+ binding properties of various assemblies of the regulatory components that contain one of the cardiomyopahty-related mutant cTnI. Acto-S1 ATPase activities in reconstituted systems were also determined. We investigated R145G and R145W mutations from the inhibitory region and D190H and R192H mutations from the second actin-tropomyosin-binding site. Each of the four mutations sensitized the acto-S1 ATPase to Ca2+. Whereas the mutations from the inhibitory region increased the basal level of ATPase activity, those from the second actin-tropomyosin-binding site did not. The effects on the Ca2+ binding properties of the troponin ternary complex and the troponin-tropomyosin complex with one of four mutations were either desensitization or no effect compared with those with wild-type cTnI. All of the mutations, however, affected the Ca2+ sensitivities of the reconstituted thin filaments in the same direction as the acto-S1 ATPase activity. Also the thin filaments with one of the mutant cTnIs bound Ca2+ with less cooperativity compared with those with wild-type cTnI. These data indicate that the mutations found in the inhibitory region and those from the second actin-tropomyosin site shift the equilibrium of the states of the thin filaments differently. Moreover, the increased Ca2+ bound to myofilaments containing the mutant cTnIs may be an important factor in triggered arrhythmias associated with the cardiomyopathy.

Calcium-dependent movement of troponin I between troponin C and actin as revealed by spin-labeling EPR

We measured EPR spectra from a spin label on the Cys133 residue of troponin I (TnI) to identify Ca(2+)-induced structural states, based on sensitivity of spin-label mobility to flexibility and tertiary contact of a polypeptide. Spectrum from Tn complexes in the -Ca(2+) state showed that Cys133 was located at a flexible polypeptide segment (rotational correlation time tau=1.9ns) that was free from TnC. Spectra of both Tn complexes alone and those reconstituted into the thin filaments in the +Ca(2+) state showed that Cys133 existed on a stable segment (tau=4.8ns) held by TnC. Spectra of reconstituted thin filaments (-Ca(2+) state) revealed that slow mobility (tau=45ns) was due to tertiary contact of Cys133 with actin, because the same slow mobility was found for TnI-actin and TnI-tropomyosin-actin filaments lacking TnC, T or tropomyosin. We propose that the Cys133 region dissociates from TnC and attaches to the actin surface on the thin filaments, causing muscle relaxation at low Ca(2+) concentrations.

Drastic Ca2+ sensitization of myofilament associated with a small structural change in troponin I in inherited restrictive cardiomyopathy

Six missense mutations in human cardiac troponin I (cTnI) were recently found to cause restrictive cardiomyopathy (RCM). We have bacterially expressed and purified these human cTnI mutants and examined their functional and structural consequences. Inserting the human cTnI into skinned cardiac muscle fibers showed that these mutations had much greater Ca2+-sensitizing effects on force generation than the cTnI mutations in hypertrophic cardiomyopathy (HCM). The mutation K178E in the second actin-tropomyosin (Tm) binding region showed a particularly potent Ca2+-sensitizing effect among the six RCM-causing mutations. Circular dichroism and nuclear magnetic resonance spectroscopy revealed that this mutation does not extensively affect the structure of the whole cTnI molecule, but induces an unexpectedly subtle change in the structure of a region around the mutated residue. The results indicate that the K178E mutation has a localized effect on a structure that is critical to the regulatory function of the second actin-Tm binding region of cTnI. The present study also suggests that both HCM and RCM involving cTnI mutations share a common feature of increased Ca2+ sensitivity of cardiac myofilament, but more severe change in Ca2+ sensitivity is associated with the clinical phenotype of RCM.

Structural basis for Ca2+-regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin

Troponin and tropomyosin on actin filaments constitute a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle through a series of conformational changes within the actin-based thin filament. Troponin consists of three subunits: an inhibitory subunit (TnI), a Ca2+-binding subunit (TnC), and a tropomyosin-binding subunit (TnT). Ca2+-binding to TnC is believed to weaken interactions between troponin and actin, and triggers a large conformational change of the troponin complex. However, the atomic details of the actin-binding sites of troponin have not been determined. Ternary troponin complexes have been reconstituted from recombinant chicken skeletal TnI, TnC, and TnT2 (the C-terminal region of TnT), among which only TnI was uniformly labelled with 15N and/or 13C. By applying NMR spectroscopy, the solution structures of a "mobile" actin-binding domain (approximately 6.1 kDa) in the troponin ternary complex (approximately 52 kDa) were determined. The mobile domain appears to tumble independently of the core domain of troponin. Ca2+-induced changes in the chemical shift and line shape suggested that its tumbling was more restricted at high Ca2+ concentrations. The atomic details of interactions between actin and the mobile domain of troponin were defined by docking the mobile domain into the cryo-electron microscopy (cryo-EM) density map of thin filament at low [Ca2+]. This allowed the determination of the 3D position of residue 133 of TnI, which has been an important landmark to incorporate the available information. This enabled unique docking of the entire globular head region of troponin into the thin filament cryo-EM map at a low Ca2+ concentration. The resultant atomic model suggests that troponin interacted electrostatically with actin and caused the shift of tropomyosin to achieve muscle relaxation. An important feature is that the coiled-coil region of troponin pushed tropomyosin at a low Ca2+ concentration. Moreover, the relationship between myosin and the mobile domain on actin filaments suggests that the latter works as a fail-safe latch.

Calcium, thin filaments, and the integrative biology of cardiac contractility

Although well known as the location of the mechanism by which the cardiac sarcomere is activated by Ca2+ to generate force and shortening, the thin filament is now also recognized as a vital component determining the dynamics of contraction and relaxation. Molecular signaling in the thin filament involves steric, allosteric, and cooperative mechanisms that are modified by protein phosphorylation, sarcomere length and load, the chemical environment, and isoform composition. Approaches employing transgenesis and mutagenesis now permit investigation of these processes at the level of the systems biology of the heart. These studies reveal that the thin filaments are not merely slaves to the levels of Ca2+ determined by membrane channels, transporters and exchangers, but are actively involved in beat to beat control of cardiac function by neural and hormonal factors and by the Frank-Starling mechanism.

Adenosine attenuates C-terminal but not N-terminal proteolysis of cTnI during cardioplegic arrest

BACKGROUND

Specific site proteolysis and loss of troponin I (TnI) during myocardial ischemic events can contribute to myocardial dysfunction. Adenosine supplementation of cardioplegic solutions results in improved functional preservation of the heart. We investigated the effect of adenosine on N-terminal and C-terminal proteolysis of TnI in the heart.

MATERIALS AND METHODS

Hearts from male Sprague-Dawley rats were isolated and perfused at a constant pressure. Cardioplegic arrest (St. Thomas #2 +/- 100 microm adenosine) was induced and hearts frozen at various times during the arrest. Antibodies directed against specific regions of TnI were used to visualize TnI in whole heart homogentates, as well as from cellular fractions, using western blot analysis.

RESULTS

Cardioplegic arrest alone resulted in early N-terminal proteolysis of TnI, followed by later loss of sequences from the C-terminal end of the molecule. In addition, secondary protein bands that were immunoreactive to amino acid sequences centrally located on the TnI molecule were observed. There was also evidence of dissociation of TnI from the other myofibrillar proteins. The supplementation of cardioplegic solution with adenosine significantly attenuated the late C-terminal proteolytic degradation of TnI and its apparent dissociation from myofibrils proteins but had no effect on the early N-terminal proteolysis associated with cardioplegic arrest.

CONCLUSIONS

These data may provide an explanation for partial protection against postarrest myocardial dysfunction provided by adenosine.

Developing the next generation of cardiac markers: Disease-induced modifications of troponin I

Troponin I (TnI) and Troponin T (TnT) have evolved into arguably the two most important diagnostic markers for acute myocardial injury. Part of their diagnostic utility lies in the uniquely important roles that both TnI and TnT play in the calcium-dependent regulation of cardiac muscle contraction. Both proteins undergo extensive physiologic regulation, principally through phosphorylation, as well as specific disease-induced pathologic modifications, including phosphorylation, oxidation, and proteolysis. Many, if not all, of these protein modifications in some way modulate contractility, and when detected in serum may therefore provide important information about both the disease state and functional status of the heart. However, the complexity of the TnI (and TnT) forms in the serum is large, which leads to difficulty in detecting all of the Tn subunits in serum, and hence interpreting the biologic significance of each modified product. But, as diagnostic tools and modalities improve, our ability to monitor and detect specific disease-induced modified forms of proteins will inevitably lead to better and more specific diagnoses and therapies.

Disruption of Caenorhabditis elegans muscle structure and function caused by mutation of troponin I

Caenorhabditis elegans strains mutant for the unc-27 gene show abnormal locomotion and muscle structure. Experiments revealed that unc-27 is one of four C. elegans troponin I genes and that three mutant alleles truncate the protein: recessive and presumed null allele e155 terminates after nine codons; semidominant su142sd eliminates the inhibitory and C-terminal regions; and semidominant su195sd abbreviates the extreme C-terminus. Assays of in vivo muscular performance at high and low loads indicated that su142sd is most deleterious, with e155 least and su195sd intermediate. Microscopy revealed in mutant muscle a prevalent disorder of dense body positioning and a less well defined sarcomeric structure, with small islands of thin filaments interspersed within the overlap region of A bands and even within the H zone. The mutants' rigid paralysis and sarcomeric disarray are consistent with unregulated contraction of the sarcomeres, in which small portions of each myofibril shorten irregularly and independently of one another, thereby distorting the disposition of filaments. The exacerbated deficits of su142sd worms are compatible with involvement in vivo of the N-terminal portion of troponin I in enhancing force production, and the severe impairment associated with su195sd highlights importance of the extreme C-terminus in the protein's inhibitory function.

Augmented systolic response to the calcium sensitizer EMD-57033 in a transgenic model with troponin I truncation

Myocardial stunning is a form of acute reversible cardiac dysfunction that occurs after brief periods of ischemia and reperfusion. In several animal models, stunning is associated with proteolytic truncation of troponin I (TnI). Mice expressing the same proteolytic TnI fragment [TnI-(1–193)] demonstrate cardiac depression with a decreased maximal calcium-activated tension. We therefore hypothesized preferential improvement in mice expressing TnI-(1–193) treated with the calcium-sensitizing drug EMD-57033. TnI-(1–193) and nontransgenic myofibrils exhibited significant sensitization to calcium in Mg-ATPase assays after EMD-57033 exposure. However, only transgenic myofibrils exhibited an increase in maximal activity ( P = 0.023). EMD-57033 also increased maximal calcium-activated force in TnI-(1–193) muscle, such that it was comparable to nontransgenic cardiac muscle. EMD-57033 enhanced in vivo systolic function modestly in controls but had a marked effect in transgenic mice, with an almost threefold greater leftward shift of the end-systolic pressure-volume relation ( P = 0.0005). These data indicate a targeted efficacy of EMD-57033 in offsetting the contractile defect in TnI-(1–193) mice, and this may have therapeutic implications in models displaying this myofilament defect.