Altered regulation of cardiac muscle contraction by troponin T mutations that cause familial hypertrophic cardiomyopathy

Molecular Pathways and Animal Models of Cardiomyopathies

Cardiomyopathies are a heterogeneous group of disorders of the heart muscle that ultimately result in congestive heart failure. Rapid progress in genetics, molecular and cellular biology with breakthrough innovative genetic-engineering techniques, such as next-generation sequencing and multiomics platforms, stem cell reprogramming, as well as novel groundbreaking gene-editing systems over the past 25 years has greatly improved the understanding of pathogenic signaling pathways in inherited cardiomyopathies. This chapter will focus on intracellular and intercellular molecular signaling pathways that are activated by a genetic insult in cardiomyocytes to maintain tissue and organ level regulation and resultant cardiac remodeling in certain forms of cardiomyopathies. In addition, animal models of different clinical forms of human cardiomyopathies with their summaries of triggered key molecules and signaling pathways will be described.

Mechanisms of Pathogenicity of Hypertrophic Cardiomyopathy-Associated Troponin T (TNNT2) Variant R278C+/- During Development

Hypertrophic cardiomyopathy (HCM) is one of the most common heritable cardiovascular diseases and variants of TNNT2 (cardiac troponin T) are linked to increased risk of sudden cardiac arrest despite causing limited hypertrophy. In this study, a TNNT2 variant, R278C+/-, was generated in both human cardiac recombinant/reconstituted thin filaments (hcRTF) and human- induced pluripotent stem cells (hiPSCs) to investigate the mechanisms by which the R278C+/- variant affects cardiomyocytes at the proteomic and functional levels. The results of proteomics analysis showed a significant upregulation of markers of cardiac hypertrophy and remodeling in R278C+/- vs. the isogenic control. Functional measurements showed that R278C+/- variant enhances the myofilament sensitivity to Ca2+, increases the kinetics of contraction, and causes arrhythmia at frequencies >75 bpm. This study uniquely shows the profound impact of the TNNT2 R278C+/- variant on the cardiomyocyte proteomic profile, cardiac electrical and contractile function in the early stages of cardiac development.

Cytoskeletal disarray increases arrhythmogenic vulnerability during sympathetic stimulation in a model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) patients are advised to avoid strenuous exercise due to increased risk of arrhythmias. Mice expressing the human FHC-causing mutation R403Q in the myosin heavy chain gene (MYH6) recapitulate the human phenotype, including cytoskeletal disarray and increased arrhythmia susceptibility. Following in vivo administration of isoproterenol, mutant mice exhibited tachyarrhythmias, poor recovery and fatigue. Arrhythmias were attenuated with the β-blocker atenolol and protein kinase A inhibitor PKI. Mutant cardiac myocytes had significantly prolonged action potentials and triggered automaticity due to reduced repolarization reserve and connexin 43 expression. Isoproterenol shortened cycle length, and escalated electrical instability. Surprisingly isoproterenol did not increase CaV1.2 current. We found alterations in CaV1.2-β1 adrenergic receptor colocalization assessed using super-resolution nanoscopy, and increased CaV1.2 phosphorylation in mutant hearts. Our results reveal for the first time that altered ion channel expression, co-localization and β-adrenergic receptor signaling associated with myocyte disarray contribute to electrical instability in the R403Q mutant heart.

Mechanisms of Arrhythmogenicity of Hypertrophic Cardiomyopathy-Associated Troponin T (TNNT2) Variant I79N

Hypertrophic cardiomyopathy (HCM) is the most common heritable cardiovascular disease and often results in cardiac remodeling and an increased incidence of sudden cardiac arrest (SCA) and death, especially in youth and young adults. Among thousands of different variants found in HCM patients, variants of TNNT2 (cardiac troponin T—TNNT2) are linked to increased risk of ventricular arrhythmogenesis and sudden death despite causing little to no cardiac hypertrophy. Therefore, studying the effect of TNNT2 variants on cardiac propensity for arrhythmogenesis can pave the way for characterizing HCM in susceptible patients before sudden cardiac arrest occurs. In this study, a TNNT2 variant, I79N, was generated in human cardiac recombinant/reconstituted thin filaments (hcRTF) to investigate the effect of the mutation on myofilament Ca²⁺ sensitivity and Ca²⁺ dissociation rate using steady-state and stopped-flow fluorescence techniques. The results revealed that the I79N variant significantly increases myofilament Ca²⁺ sensitivity and decreases the Ca²⁺ off-rate constant (k_off). To investigate further, a heterozygous I79N^+/−TNNT2 variant was introduced into human-induced pluripotent stem cells using CRISPR/Cas9 and subsequently differentiated into ventricular cardiomyocytes (hiPSC-CMs). To study the arrhythmogenic properties, monolayers of I79N^+/− hiPSC-CMs were studied in comparison to their isogenic controls. Arrhythmogenesis was investigated by measuring voltage (V_m) and cytosolic Ca²⁺ transients over a range of stimulation frequencies. An increasing stimulation frequency was applied to the cells, from 55 to 75 bpm. The results of this protocol showed that the TnT-I79N cells had reduced intracellular Ca²⁺ transients due to the enhanced cytosolic Ca²⁺ buffering. These changes in Ca²⁺ handling resulted in beat-to-beat instability and triangulation of the cardiac action potential, which are predictors of arrhythmia risk. While wild-type (WT) hiPSC-CMs were accurately entrained to frequencies of at least 150 bpm, the I79N hiPSC-CMs demonstrated clear patterns of alternans for both V_m and Ca²⁺ transients at frequencies >75 bpm. Lastly, a transcriptomic analysis was conducted on WT vs. I79N^+/−TNNT2 hiPSC-CMs using a custom NanoString codeset. The results showed a significant upregulation of NPPA (atrial natriuretic peptide), NPPB (brain natriuretic peptide), Notch signaling pathway components, and other extracellular matrix (ECM) remodeling components in I79N^+/− vs. the isogenic control. This significant shift demonstrates that this missense in the TNNT2 transcript likely causes a biophysical trigger, which initiates this significant alteration in the transcriptome. This TnT-I79N hiPSC-CM model not only reproduces key cellular features of HCM-linked mutations but also suggests that this variant causes uncharted pro-arrhythmic changes to the human action potential and gene expression.

Increased cytosolic calcium buffering contributes to a cellular arrhythmogenic substrate in iPSC-cardiomyocytes from patients with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a major risk factor for heart failure and is associated with the development of life-threatening cardiac arrhythmias. Using a patient-specific induced pluripotent stem cell-derived cardiomyocyte (iPSC-CM) model harbouring a mutation in cardiac troponin T (R173W), we aim to examine the cellular basis of arrhythmogenesis in DCM patients with this mutation. iPSC from control (Ctrl) and DCM-TnT-R173W donors from the same family were differentiated into iPSC-CM and analysed through optical action potential (AP) recordings, simultaneous measurement of cytosolic calcium concentration ([Ca²⁺]_i) and membrane currents and separately assayed using field stimulation to detect the threshold for AP- and [Ca²⁺]_i-alternans development. AP duration was unaltered in TnT-R173W iPSC-CM. Nevertheless, TnT-R173W iPSC-CM showed a strikingly low stimulation threshold for AP- and [Ca²⁺]_i-alternans. Myofilaments are known to play a role as intracellular Ca²⁺ buffers and here we show increased Ca²⁺ affinity of intracellular buffers in TnT-R173W cells, indicating increased myofilament sensitivity to Ca²⁺. Similarly, EMD57033, a myofilament Ca²⁺ sensitiser, replicated the abnormal [Ca²⁺]_i dynamics observed in TnT-R173W samples and lowered the threshold for alternans development. In contrast, application of a Ca²⁺ desensitiser (blebbistatin) to TnT-R173W iPSC-CM was able to phenotypically rescue Ca²⁺ dynamics, normalising Ca²⁺ transient profile and minimising the occurrence of Ca²⁺ alternans at physiological frequencies. This finding suggests that increased Ca²⁺ buffering likely plays a major arrhythmogenic role in patients with DCM, specifically in those with mutations in cardiac troponin T. In addition, we propose that modulation of myofilament Ca²⁺ sensitivity could be an effective anti-arrhythmic target for pharmacological management of this disease.

Sophorae tonkinensis radix et rhizome-induced pulmonary toxicity: A study on the toxic mechanism and material basis based on integrated omics and bioinformatics analyses

The root and rhizome of Sophora tonkinensis Gagnep. (ST) are widely used for the treatment of tonsillitis, sore throats, and heat-evil-induced diseases in traditional Chinese medicine. However, the clinical application of ST is relatively limited due to its toxicity. The mechanism and material basis of ST-induced pulmonary toxicity are still unclear. In the present research, integrated omics and bioinformatics analyses were used to investigate the toxic mechanism and material basis of ST in lung tissue. Proteomics and metabonomics were integrated to analyze the differentially expressed proteins and metabolites. Joint pathway analysis was used to analyze the significantly dysregulated pathways. PubChem and the Comparative Toxicogenomics Database were applied for the screen of toxic targets and compounds. Integrated omics revealed that 323 proteins and 50 metabolites were differentially expressed after treating with ST, out of which 19 proteins and 1 metabolite were significantly enriched in seven pathways. Bioinformatics showed that 15 compounds may indirectly affect the expression of 9 toxic targets of ST. Multiple toxic targets of ST-induced pulmonary injury were found in the study, whose dysregulation may trigger pulmonary cancer, dyspnea, and oxidative stress. Multiple compounds may be the toxic material basis in response to these effects.

Increased Myocardial Oxygen Consumption Precedes Contractile Dysfunction in Hypertrophic Cardiomyopathy Caused by Pathogenic TNNT2 Gene Variants

Background

Hypertrophic cardiomyopathy is caused by pathogenic sarcomere gene variants. Individuals with a thin‐filament variant present with milder hypertrophy than carriers of thick‐filament variants, although prognosis is poorer. Herein, we defined if decreased energetic status of the heart is an early pathomechanism in TNNT2 (troponin T gene) variant carriers.

Methods and Results

Fourteen individuals with TNNT2 variants (genotype positive), without left ventricular hypertrophy (G+/LVH−; n=6) and with LVH (G+/LVH+; n=8) and 14 healthy controls were included. All participants underwent cardiac magnetic resonance and [¹¹C]‐acetate positron emission tomography imaging to assess LV myocardial oxygen consumption, contractile parameters and myocardial external efficiency. Cardiac efficiency was significantly reduced compared with controls in G+/LVH− and G+/LVH+. Lower myocardial external efficiency in G+/LVH− is explained by higher global and regional oxygen consumption compared with controls without changes in contractile parameters. Reduced myocardial external efficiency in G+/LVH+ is explained by the increase in LV mass and higher oxygen consumption. Septal oxygen consumption was significantly lower in G+/LVH+ compared with G+/LVH−. Although LV ejection fraction was higher in G+/LVH+, both systolic and diastolic strain parameters were lower compared with controls, which was most evident in the hypertrophied septal wall.

Conclusions

Using cardiac magnetic resonance and [¹¹C]‐acetate positron emission tomography imaging, we show that G+/LVH− have an initial increase in oxygen consumption preceding contractile dysfunction and cardiac hypertrophy, followed by a decline in oxygen consumption in G+/LVH+. This suggests that high oxygen consumption and reduced myocardial external efficiency characterize the early gene variant–mediated disease mechanisms that may be used for early diagnosis and development of preventive treatments.

MRAS Variants Cause Cardiomyocyte Hypertrophy in Patient-Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes: Additional Evidence for MRAS as a Definitive Noonan Syndrome-Susceptibility Gene

BACKGROUND

MRAS was identified recently as a novel Noonan syndrome (NS)-susceptibility gene. Phenotypically, both patients with NS, harboring pathogenic MRAS variants, displayed severe cardiac hypertrophy. This study aimed to demonstrate both the necessity and sufficiency of a patient-specific variant (p.Gly23Val-MRAS) to cause NS through the generation and characterization of patient-specific, isogenic control, and disease modeled induced pluripotent stem cell (iPSC) lines.

METHODS

iPSCs were derived from a patient with a p.Gly23Val-MRAS variant to assess the effect of MRAS variants on pathogenesis of NS-associated cardiac hypertrophy. CRISPR/Cas9 gene editing was used to correct the pathogenic p.Gly23Val-MRAS variant in patient cells (isogenic control) and to introduce the pathogenic variant into unrelated control cells (disease modeled) to determine the necessity and sufficiency of the p.Gly23Val-MRAS variant to elicit the disease phenotype in iPSC-derived cardiomyocytes (iPSC-CMs). iPSC-CMs were analyzed by microscopy and immunofluroesence, single-cell RNAseq, Western blot, room temperature-quantitative polymerase chain reaction, and live-cell calcium imaging to define an in vitro phenotype of MRAS-mediated cardiac hypertrophy.

RESULTS

Compared with controls, both patient and disease modeled iPSC-CMs were significantly larger and demonstrated changes in gene expression and intracellular pathway signaling characteristic of cardiac hypertrophy. Additionally, patient and disease modeled iPSC-CMs displayed impaired Ca²⁺ handling, including increased frequency of irregular Ca²⁺ transients and changes in Ca²⁺ handling kinetics.

CONCLUSIONS

p.Gly23Val-MRAS is both necessary and sufficient to elicit a cardiac hypertrophy phenotype in iPSC-CMs that includes increased cell size, changes in cardiac gene expression, and abnormal calcium handling-providing further evidence to establish the monogenetic pathogenicity of p.Gly23Val-MRAS in NS with cardiac hypertrophy.

CaMKII-mediated phosphorylation of RyR2 plays a crucial role in aberrant Ca2+ release as an arrhythmogenic substrate in cardiac troponin T-related familial hypertrophic cardiomyopathy

AIMS

Cardiac Troponin T (TnT) mutation-linked familial hypertrophic cardiomyopathy (FHC) is known to cause sudden cardiac death at a young age. Here, we investigated the role of the Ca²⁺ release channel of the cardiac sarcoplasmic reticulum (SR), ryanodine receptor (RyR2), in the pathogenic mechanism of lethal arrhythmia in FHC-related TnT-mutated transgenic mice (TG; TnT-delta160E).

METHODS AND RESULTS

In TG cardiomyocytes, the Ca²⁺ spark frequency (SpF) was much higher than that in non-TG cardiomyocytes. These differences were more pronounced in the presence of isoproterenol (ISO; 10 nM). This increase in SpF was largely reversed by a CaMKII inhibitor (KN-93), but not by a protein kinase A inhibitor (H89). CaMKII phosphorylation at Ser2814 in RyR2 was increased significantly in TG. Spontaneous Ca²⁺ transients (sCaTs) after cessation of a 1-5 Hz pacing, frequently observed in ISO-treated TG cardiomyocytes, were also attenuated by KN-93, but not by H89. The RyR2 stabilizer dantrolene attenuated Ca²⁺ sparks and sCaTs in ISO-treated TG cardiomyocytes, indicating that the mutation-linked aberrant Ca²⁺ release is mediated by destabilized RyR2.

CONCLUSIONS

In FHC-linked TnT-mutated hearts, RyR2 is susceptible to CaMKII-mediated phosphorylation, presumably because of a mutation-linked increase in diastolic [Ca²⁺]_i, causing aberrant Ca²⁺ release leading to lethal arrhythmia.

Hypertrophic cardiomyopathy-linked mutation in troponin T causes myofibrillar disarray and pro-arrhythmic action potential changes in human iPSC cardiomyocytes

BACKGROUND

Mutations in cardiac troponin T (TnT) are linked to increased risk of ventricular arrhythmia and sudden death despite causing little to no cardiac hypertrophy. Studies in mice suggest that the hypertrophic cardiomyopathy (HCM)-associated TnT-I79N mutation increases myofilament Ca sensitivity and is arrhythmogenic, but whether findings from mice translate to human cardiomyocyte electrophysiology is not known.

OBJECTIVES

To study the effects of the TnT-I79N mutation in human cardiomyocytes.

METHODS

Using CRISPR/Cas9, the TnT-I79N mutation was introduced into human induced pluripotent stem cells (hiPSCs). We then used the matrigel mattress method to generate single rod-shaped cardiomyocytes (CMs) and studied contractility, Ca handling and electrophysiology.

RESULTS

Compared to isogenic control hiPSC-CMs, TnT-I79N hiPSC-CMs exhibited sarcomere disorganization, increased systolic function and impaired relaxation. The Ca-dependence of contractility was leftward shifted in mutation containing cardiomyocytes, demonstrating increased myofilament Ca sensitivity. In voltage-clamped hiPSC-CMs, TnT-I79N reduced intracellular Ca transients by enhancing cytosolic Ca buffering. These changes in Ca handling resulted in beat-to-beat instability and triangulation of the cardiac action potential, which are predictors of arrhythmia risk. The myofilament Ca sensitizer EMD57033 produced similar action potential triangulation in control hiPSC-CMs.

CONCLUSIONS

The TnT-I79N hiPSC-CM model not only reproduces key cellular features of TnT-linked HCM such as myofilament disarray, hypercontractility and diastolic dysfunction, but also suggests that this TnT mutation causes pro-arrhythmic changes of the human ventricular action potential.

Protein Structure-Function Relationship at Work: Learning from Myopathy Mutations of the Slow Skeletal Muscle Isoform of Troponin T

Troponin T (TnT) is the sarcomeric thin filament anchoring subunit of the troponin complex in striated muscles. A nonsense mutation in exon 11 of the slow skeletal muscle isoform of TnT (ssTnT) gene (TNNT1) was found in the Amish populations in Pennsylvania and Ohio. This single nucleotide substitution causes a truncation of the ssTnT protein at Glu¹⁸⁰ and the loss of the C-terminal tropomyosin (Tm)-binding site 2. As a consequence, it abolishes the myofilament integration of ssTnT and the loss of function causes an autosomal recessive nemaline myopathy (NM). More TNNT1 mutations have recently been reported in non-Amish ethnic groups with similar recessive NM phenotypes. A nonsense mutation in exon 9 truncates ssTnT at Ser¹⁰⁸, deleting Tm-binding site 2 and a part of the middle region Tm-binding site 1. Two splicing site mutations result in truncation of ssTnT at Leu²⁰³ or deletion of the exon 14-encoded C-terminal end segment. Another splicing mutation causes an internal deletion of the 39 amino acids encoded by exon 8, partially damaging Tm-binding site 1. The three splicing mutations of TNNT1 all preserve the high affinity Tm-binding site 2 but still present recessive NM phenotypes. The molecular mechanisms for these mutations to cause myopathy provide interesting models to study and understand the structure-function relationship of TnT. This focused review summarizes the current knowledge of TnT isoform regulation, structure-function relationship of TnT and how various ssTnT mutations cause recessive NM, in order to promote in depth studies for further understanding the pathogenesis and pathophysiology of TNNT1 myopathies toward the development of effective treatments.

Mutations in troponin T associated with Hypertrophic Cardiomyopathy increase Ca(2+)-sensitivity and suppress the modulation of Ca(2+)-sensitivity by troponin I phosphorylation

We investigated the effect of 7 Hypertrophic Cardiomyopathy (HCM)-causing mutations in troponin T (TnT) on troponin function in thin filaments reconstituted with actin and human cardiac tropomyosin. We used the quantitative in vitro motility assay to study Ca²⁺-regulation of unloaded movement and its modulation by troponin I phosphorylation. Troponin from a patient with the K280N TnT mutation showed no difference in Ca²⁺-sensitivity when compared with donor heart troponin and the Ca²⁺-sensitivity was also independent of the troponin I phosphorylation level (uncoupled). The recombinant K280N TnT mutation increased Ca²⁺-sensitivity 1.7-fold and was also uncoupled. The R92Q TnT mutation in troponin from transgenic mouse increased Ca²⁺-sensitivity and was also completely uncoupled. Five TnT mutations (Δ14, Δ28 + 7, ΔE160, S179F and K273E) studied in recombinant troponin increased Ca²⁺-sensitivity and were all fully uncoupled. Thus, for HCM-causing mutations in TnT, Ca²⁺-sensitisation together with uncoupling in vitro is the usual response and both factors may contribute to the HCM phenotype. We also found that Epigallocatechin-3-gallate (EGCG) can restore coupling to all uncoupled HCM-causing TnT mutations. In fact the combination of Ca²⁺-desensitisation and re-coupling due to EGCG completely reverses both the abnormalities found in troponin with a TnT HCM mutation suggesting it may have therapeutic potential.

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7 HCM-causing mutations in cardiac TnT were studied using in vitro motility assay.

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All the mutations increased myofilament Ca²⁺-sensitivity (range 1.5–2.7 fold).

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All mutations suppressed the modulation of Ca²⁺-sensitivity by TnI phosphorylation.

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Epigallocatechin-3-gallate (EGCG) restored this modulation to all mutations.

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This suggests a therapeutic potential for EGCG in the treatment of HCM.

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Uncoupling of myofilament Ca2+ sensitivity from troponin I phosphorylation by mutations can be reversed by epigallocatechin-3-gallate

AIMS

Heart muscle contraction is regulated via the β-adrenergic response that leads to phosphorylation of Troponin I (TnI) at Ser22/23, which changes the Ca(2+) sensitivity of the cardiac myofilament. Mutations in thin filament proteins that cause dilated cardiomyopathy (DCM) and some mutations that cause hypertrophic cardiomyopathy (HCM) abolish the relationship between TnI phosphorylation and Ca(2+) sensitivity (uncoupling). Small molecule Ca(2+) sensitizers and Ca(2+) desensitizers that act upon troponin alter the Ca(2+) sensitivity of the thin filament, but their relationship with TnI phosphorylation has never been studied before.

METHODS AND RESULTS

Quantitative in vitro motility assay showed that 30 µM EMD57033 and 100 µM Bepridil increase Ca(2+) sensitivity of phosphorylated cardiac thin filaments by 3.1- and 2.8-fold, respectively. Additionally they uncoupled Ca(2+) sensitivity from TnI phosphorylation, mimicking the effect of HCM mutations. Epigallocatechin-3-gallate (EGCG) decreased Ca(2+) sensitivity of phosphorylated and unphosphorylated wild-type thin filaments equally (by 2.15 ± 0.45- and 2.80 ± 0.48-fold, respectively), retaining the coupling. Moreover, EGCG also reduced Ca(2+) sensitivity of phosphorylated but not unphosphorylated thin filaments containing DCM and HCM-causing mutations; thus, the dependence of Ca(2+) sensitivity upon TnI phosphorylation of uncoupled mutant thin filaments was restored in every case. In single mouse heart myofibrils, EGCG reduced Ca(2+) sensitivity of force and kACT and also preserved coupling. Myofibrils from the ACTC E361G (DCM) mouse were uncoupled; EGCG reduced Ca(2+) sensitivity more for phosphorylated than for unphosphorylated myofibrils, thus restoring coupling.

CONCLUSION

We conclude that it is possible to both mimic and reverse the pathological defects in troponin caused by cardiomyopathy mutations pharmacologically. Re-coupling by EGCG may be of potential therapeutic significance for treating cardiomyopathies.

Effect of myofilament Ca(2+) sensitivity on Ca(2+) wave propagation in rat ventricular muscle

BACKGROUND

The propagation velocity of Ca(2+) waves determines delayed afterdepolarization and affects the occurrence of triggered arrhythmias in cardiac muscle. We focused on myofilament Ca(2+) sensitivity, investigating how the velocity of Ca(2+) waves responds to its increased sensitivity resulting from muscle stretch or the addition of a myofilament Ca(2+) sensitizer, SCH00013. We further investigated whether production of reactive oxygen species (ROS) may be involved in the change in velocity.

METHODS

Trabeculae were obtained from rat hearts. Force, sarcomere length, and [Ca(2+)]i were measured. ROS production was estimated from 2',7'-dichlorofluorescein (DCF) fluorescence. Trabeculae were exposed to a 10 mM Ca(2+) jet for the induction of Ca(2+) leak from the sarcoplasmic reticulum in its exposed region. Ca(2+) waves were induced by 2.5-Hz stimulus trains for 7.5s (24 °C, 2.0 mM [Ca(2+)]o). Muscle stretch of 5, 10, and 15% was applied 300 ms after the last stimulus of the train.

RESULTS

Muscle stretch increased the DCF fluorescence, the amplitude of aftercontractions, and the velocity of Ca(2+) waves depending on the degree of stretch. After preincubation with 3 μM diphenyleneiodonium (DPI), muscle stretch increased only the amplitude of aftercontractions but not the DCF fluorescence nor the velocity of Ca(2+) waves. SCH00013 (30 μM) increased the DCF fluorescence, the amplitude of aftercontractions, and the velocity of Ca(2+) waves. DPI suppressed these increases.

CONCLUSIONS

Muscle stretch increases the velocity of Ca(2+) waves by increasing ROS production, not by increasing myofilament Ca(2+) sensitivity. In the case of SCH00013, ROS production increases myofilament Ca(2+) sensitivity and the velocity of Ca(2+) waves. These results suggest that ROS rather than myofilament Ca(2+) sensitivity plays an important role in the determination of the velocity of Ca(2+) waves, that is, arrhythmogenesis.

Fluorescent-based methods for gene knockdown and functional cardiac imaging in zebrafish

A notable advantage of zebrafish as a model organism is the ease of gene knockdown using morpholino antisense oligonucleotide (MO). However, zebrafish morphants injected with MO for a target protein often show heterogeneous phenotypes, despite controlling the injection volume of the MO solution in all embryos. We developed a method for estimating the quantity of MO injected into each living morphant, based on the co-injection of a control MO labeled with the fluorophore lissamine. By applying this method for knockdown of cardiac troponin T (tnnt2a) in zebrafish, we could efficiently select the partial tnnt2a-depleted zebrafish with a decreased heart rate and impairment of cardiac contraction. To investigate cardiac impairment of the tnnt2a morphant, we performed fluorescent cardiac imaging using Bodipy-ceramide. Cardiac image analysis showed moderate reduction of tnnt2a impaired diastolic distensibility and decreased contraction and relaxation velocities. To the best of our knowledge, this is the first report to analyze the role of tnnt2a in cardiac function in tnnt2a-depleted living animals. Our combinatorial approach can be applied for analyzing the molecular function of any protein associated with human cardiac diseases.

Dose-dependent diastolic dysfunction and early death in a mouse model with cardiac troponin mutations

Our aim was to explore the dose-dependent diastolic dysfunction and the mechanisms of heart failure and early death in transgenic (TG) mice modeling human restrictive cardiomyopathy (RCM). The first RCM mouse model (cTnI(193His) mice) carrying cardiac troponin I (cTnI) R193H mutation (mouse cTnI R193H equals to human cTnI R192H) was generated several years ago in our laboratory. The RCM mice manifested a phenotype similar to that observed in RCM patients carrying the same cTnI mutation, i.e. enlarged atria and restricted ventricles. However, the causes of heart failure and early death observed in RCM mice remain unclear. In this study, we have produced RCM TG mice (cTnI(193His)-L, cTnI(193His)-M and cTnI(193His)-H) that express various levels of mutant cTnI in the heart. Histological examination and echocardiography were performed on these mice to monitor the time course of the disease development and heart failure. Our data demonstrate that cTnI mutation-caused diastolic dysfunction is dose-dependent. The key mechanism is myofibril hypersensitivity to Ca(2+) resulting in an impaired relaxation in the mutant cardiac myocytes. Prolonged relaxation time and delay of Ca(2+) decay observed in the mutant cardiac myocytes are correlated with the level of the mutant protein in the heart. Markedly enlarged atria due to the elevated end-diastolic pressure and myocardial ischemia are observed in the heart of the transgenic mice. In the mice with the highest level of the mutant protein, restricted ventricles and systolic dysfunction occur followed immediately by heart failure and early death. Diastolic dysfunction caused by R193H troponin I mutation is specific, showing a dose-dependent pattern. These mouse models are useful tools for the study of diastolic dysfunction. Impaired diastole can cause myocardial ischemia and fibrosis formation, resulting in the development of systolic dysfunction and heart failure with early death in the RCM mice with a high level of the mutant protein in the heart.

Inherited cardiomyopathies caused by troponin mutations

Genetic investigations of cardiomyopathy in the recent two decades have revealed a large number of mutations in the genes encoding sarcomeric proteins as a cause of inherited hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), or restrictive cardiomyopathy (RCM). Most functional analyses of the effects of mutations on cardiac muscle contraction have revealed significant changes in the Ca(2+)-regulatory mechanism, in which cardiac troponin (cTn) plays important structural and functional roles as a key regulatory protein. Over a hundred mutations have been identified in all three subunits of cTn, i.e., cardiac troponins T, I, and C. Recent studies on cTn mutations have provided plenty of evidence that HCM- and RCM-linked mutations increase cardiac myofilament Ca(2+) sensitivity, while DCM-linked mutations decrease it. This review focuses on the functional consequences of mutations found in cTn in terms of cardiac myofilament Ca(2+) sensitivity, ATPase activity, force generation, and cardiac troponin I phosphorylation, to understand potential molecular and cellular pathogenic mechanisms of the three types of inherited cardiomyopathy.

Focal energy deprivation underlies arrhythmia susceptibility in mice with calcium-sensitized myofilaments

RATIONALE

The Ca(2+) sensitivity of the myofilaments is increased in hypertrophic cardiomyopathy and other heart diseases and may contribute to a higher risk for sudden cardiac death. Ca(2+) sensitization increases susceptibility to reentrant ventricular tachycardia in animal models, but the underlying mechanism is unknown.

OBJECTIVE

To investigate how myofilament Ca(2+) sensitization creates reentrant arrhythmia susceptibility.

METHODS AND RESULTS

Using hypertrophic cardiomyopathy mouse models (troponinT-I79N) and a Ca(2+) sensitizing drug (EMD57033), here we identify focal energy deprivation as a direct consequence of myofilament Ca(2+) sensitization. To detect ATP depletion and thus energy deprivation, we measured accumulation of dephosphorylated Connexin 43 (Cx43) isoform P0 and AMP kinase activation by Western blotting and immunostaining. No differences were detected between groups at baseline, but regional accumulation of Connexin 43 isoform P0 occurred within minutes in all Ca(2+)-sensitized hearts, in vivo after isoproterenol challenge and in isolated hearts after rapid pacing. Lucifer yellow dye spread demonstrated reduced gap junctional coupling in areas with Connexin 43 isoform P0 accumulation. Optical mapping revealed that selectively the transverse conduction velocity was slowed and anisotropy increased. Myofilament Ca(2+) desensitization with blebbistatin prevented focal energy deprivation, transverse conduction velocity slowing, and the reentrant ventricular arrhythmias.

CONCLUSIONS

Myofilament Ca(2+) sensitization rapidly leads to focal energy deprivation and reduced intercellular coupling during conditions that raise arrhythmia susceptibility. This is a novel proarrhythmic mechanism that can increase arrhythmia susceptibility in structurally normal hearts within minutes and may, therefore, contribute to sudden cardiac death in diseases with increased myofilament Ca(2+) sensitivity.

Diastolic dysfunction and thin filament dysregulation resulting from excitation-contraction uncoupling in a mouse model of restrictive cardiomyopathy

Restrictive cardiomyopathy (RCM) has been linked to mutations in the thin filament regulatory protein cardiac troponin I (cTnI). As the pathogenesis of RCM from genotype to clinical phenotype is not fully understood, transgenic (Tg) mice were generated with cardiac specific expression of an RCM-linked missense mutation (R193H) in cTnI. R193H Tg mouse hearts with 15% stoichiometric replacement had smaller hearts and significantly elevated end diastolic pressures (EDP) in vivo. Using a unique carbon microfiber-based assay, membrane intact R193H adult cardiac myocytes generated higher passive tensions across a range of physiologic sarcomere lengths resulting in significant Ca(2+) independent cellular diastolic tone that was manifest in vivo as elevated organ-level EDP. Sarcomere relaxation and Ca(2+) decay was uncoupled in isolated R193H Tg adult myocytes due to the increase in myofilament Ca(2+) sensitivity of tension, decreased passive compliance of the sarcomere, and adaptive in vivo changes in which phospholamban (PLN) content was decreased. Further evidence of Ca(2+) and mechanical uncoupling in R193H Tg myocytes was demonstrated by the biphasic response of relaxation to increased pacing frequency versus the negative staircase seen with Ca(2+) decay. In comparison, non-transgenic myocyte relaxation closely paralleled the accelerated Ca(2+) decay. Ca(2+) transient amplitude was also significantly blunted in R193H Tg myocytes despite normal mechanical shortening resulting in myocyte hypercontractility when compared to non-transgenics. These results identify for the first time that a single point mutation in cTnI, R193H, directly causes elevated EDP due to a myocyte intrinsic loss of compliance independent of Ca(2+) cycling or altered cardiac morphology. The compound influence of impaired relaxation and elevated EDP represents a clinically severe form of diastolic dysfunction similar to the hemodynamic state documented in RCM patients.

Myofilament Ca sensitization increases cytosolic Ca binding affinity, alters intracellular Ca homeostasis, and causes pause-dependent Ca-triggered arrhythmia

RATIONALE

Ca binding to the troponin complex represents a major portion of cytosolic Ca buffering. Troponin mutations that increase myofilament Ca sensitivity are associated with familial hypertrophic cardiomyopathy and confer a high risk for sudden death. In mice, Ca sensitization causes ventricular arrhythmias, but the underlying mechanisms remain unclear.

OBJECTIVE

To test the hypothesis that myofilament Ca sensitization increases cytosolic Ca buffering and to determine the resulting arrhythmogenic changes in Ca homeostasis in the intact mouse heart.

METHODS AND RESULTS

Using cardiomyocytes isolated from mice expressing troponin T (TnT) mutants (TnT-I79N, TnT-F110I, TnT-R278C), we found that increasing myofilament Ca sensitivity produced a proportional increase in cytosolic Ca binding. The underlying cause was an increase in the cytosolic Ca binding affinity, whereas maximal Ca binding capacity was unchanged. The effect was sufficiently large to alter Ca handling in intact mouse hearts at physiological heart rates, resulting in increased end-diastolic [Ca] at fast pacing rates, and enhanced sarcoplasmic reticulum Ca content and release after pauses. Accordingly, action potential (AP) regulation was altered, with postpause action potential prolongation, afterdepolarizations, and triggered activity. Acute Ca sensitization with EMD 57033 mimicked the effects of Ca-sensitizing TnT mutants and produced pause-dependent ventricular ectopy and sustained ventricular tachycardia after acute myocardial infarction.

CONCLUSIONS

Myofilament Ca sensitization increases cytosolic Ca binding affinity. A major proarrhythmic consequence is a pause-dependent potentiation of Ca release, action potential prolongation, and triggered activity. Increased cytosolic Ca binding represents a novel mechanism of pause-dependent arrhythmia that may be relevant for inherited and acquired cardiomyopathies.

Molecular mechanism of the E99K mutation in cardiac actin (ACTC Gene) that causes apical hypertrophy in man and mouse

We generated a transgenic mouse model expressing the apical hypertrophic cardiomyopathy-causing mutation ACTC E99K at 50% of total heart actin and compared it with actin from patients carrying the same mutation. The actin mutation caused a higher Ca(2+) sensitivity in reconstituted thin filaments measured by in vitro motility assay (2.3-fold for mice and 1.3-fold for humans) and in skinned papillary muscle. The mutation also abolished the change in Ca(2+) sensitivity normally linked to troponin I phosphorylation. MyBP-C and troponin I phosphorylation levels were the same as controls in transgenic mice and human carrier heart samples. ACTC E99K mice exhibited a high death rate between 28 and 45 days (48% females and 22% males). At 21 weeks, the hearts of the male survivors had enlarged atria, increased interstitial fibrosis, and sarcomere disarray. MRI showed hypertrophy, predominantly at the apex of the heart. End-diastolic volume and end-diastolic pressure were increased, and relaxation rates were reduced compared with nontransgenic littermates. End-systolic pressures and volumes were unaltered. ECG abnormalities were present, and the contractile response to β-adrenergic stimulation was much reduced. Older mice (29-week-old females and 38-week-old males) developed dilated cardiomyopathy with increased end-systolic volume and continuing increased end-diastolic pressure and slower contraction and relaxation rates. ECG showed atrial flutter and frequent atrial ectopic beats at rest in some ACTC E99K mice. We propose that the ACTC E99K mutation causes higher myofibrillar Ca(2+) sensitivity that is responsible for the sudden cardiac death, apical hypertrophy, and subsequent development of heart failure in humans and mice.

Strong cross-bridges potentiate the Ca(2+) affinity changes produced by hypertrophic cardiomyopathy cardiac troponin C mutants in myofilaments: a fast kinetic approach

This spectroscopic study examined the steady-state and kinetic parameters governing the cross-bridge effect on the increased Ca(2+) affinity of hypertrophic cardiomyopathy-cardiac troponin C (HCM-cTnC) mutants. Previously, we found that incorporation of the A8V and D145E HCM-cTnC mutants, but not E134D into thin filaments (TFs), increased the apparent Ca(2+) affinity relative to TFs containing the WT protein. Here, we show that the addition of myosin subfragment 1 (S1) to TFs reconstituted with these mutants in the absence of MgATP(2-), the condition conducive to rigor cross-bridge formation, further increased the apparent Ca(2+) affinity. Stopped-flow fluorescence techniques were used to determine the kinetics of Ca(2+) dissociation (k(off)) from the cTnC mutants in the presence of TFs and S1. At a high level of complexity (i.e. TF + S1), an increase in the Ca(2+) affinity and decrease in k(off) was achieved for the A8V and D145E mutants when compared with WT. Therefore, it appears that the cTnC Ca(2+) off-rate is most likely to be affected rather than the Ca(2+) on rate. At all levels of TF complexity, the results obtained with the E134D mutant reproduced those seen with the WT protein. We conclude that strong cross-bridges potentiate the Ca(2+)-sensitizing effect of HCM-cTnC mutants on the myofilament. Finally, the slower k(off) from the A8V and D145E mutants can be directly correlated with the diastolic dysfunction seen in these patients.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Animal models of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathies are a heterogeneous group of pathological conditions that give rise to myocardial dysfunction with an increased risk for atrial or ventricular arrhythmias. Inherited defects in cardiomyocyte proteins in the sarcomeric contractile apparatus, the cytoskeleton and desmosomal cell-cell contact junctions are becoming recognized increasingly as major causes of sudden cardiac death in the general population. Animal models have been developed for the systematic dissection of the genetic pathways involved in the pathogenesis of arrhythmogenic cardiomyopathies. This review presents an overview of current animal models for arrhythmogenic right ventricular cardiomyopathy (ARVC), hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) associated with cardiac arrhythmias and sudden cardiac death.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

Functional analysis of a unique troponin c mutation, GLY159ASP, that causes familial dilated cardiomyopathy, studied in explanted heart muscle

BACKGROUND

Familial dilated cardiomyopathy can be caused by mutations in the proteins of the muscle thin filament. In vitro, these mutations decrease Ca(2+) sensitivity and cross-bridge turnover rate, but the mutations have not been investigated in human tissue. We studied the Ca(2+)-regulatory properties of myocytes and troponin extracted from the explanted heart of a patient with inherited dilated cardiomyopathy due to the cTnC G159D mutation.

METHODS AND RESULTS

Mass spectroscopy showed that the mutant cTnC was expressed approximately equimolar with wild-type cTnC. Contraction was compared in skinned ventricular myocytes from the cTnC G159D patient and nonfailing donor heart. Maximal Ca(2+)-activated force was similar in cTnC G159D and donor myocytes, but the Ca(2+) sensitivity of cTnC G159D myocytes was higher (EC(50) G159D/donor=0.60). Thin filaments reconstituted with skeletal muscle actin and human cardiac tropomyosin and troponin were studied by in vitro motility assay. Thin filaments containing the mutation had a higher Ca(2+) sensitivity (EC(50) G159D/donor=0.55 + or - 0.13), whereas the maximally activated sliding speed was unaltered. In addition, the cTnC G159D mutation blunted the change in Ca(2+) sensitivity when TnI was dephosphorylated. With wild-type troponin, Ca(2+) sensitivity was increased (EC(50) P/unP=4.7 + or - 1.9) but not with cTnC G159D troponin (EC(50) P/unP=1.2 + or - 0.1).

CONCLUSIONS

We propose that uncoupling of the relationship between phosphorylation and Ca(2+) sensitivity could be the cause of the dilated cardiomyopathy phenotype. The differences between these data and previous in vitro results show that native phosphorylation of troponin I and troponin T and other posttranslational modifications of sarcomeric proteins strongly influence the functional effects of a mutation.

Temporal and mutation-specific alterations in Ca2+ homeostasis differentially determine the progression of cTnT-related cardiomyopathies in murine models

Naturally occurring mutations in cardiac troponin T (cTnT) result in a clinical subset of familial hypertrophic cardiomyopathy. To determine the mechanistic links between thin-filament mutations and cardiovascular phenotypes, we have generated and characterized several transgenic mouse models carrying cTnT mutations. We address two central questions regarding the previously observed changes in myocellular mechanics and Ca(2+) homeostasis: 1) are they characteristic of all severe cTnT mutations, and 2) are they primary (early) or secondary (late) components of the myocellular response? Adult left ventricular myocytes were isolated from 2- and 6-mo-old transgenic mice carrying missense mutations at residue 92, flanking the TNT1 NH(2)-terminal tail domain. Results from R92L and R92W myocytes showed mutation-specific alterations in contraction and relaxation indexes at 2 mo with improvements by 6 mo. Alterations in Ca(2+) kinetics remained consistent with mechanical data in which R92L and R92W exhibited severe diastolic impairments at the early time point that improved with increasing age. A normal regulation of Ca(2+) kinetics in the context of an altered baseline cTnI phosphorylation suggested a pathogenic mechanism at the myofilament level taking precedence for R92L. The quantitation of Ca(2+)-handling proteins in R92W mice revealed a synergistic compensatory mechanism involving an increased Ser16 and Thr17 phosphorylation of phospholamban, contributing to the temporal onset of improved cellular mechanics and Ca(2+) homeostasis. Therefore, independent cTnT mutations in the TNT1 domain result in primary mutation-specific effects and a differential temporal onset of altered myocellular mechanics, Ca(2+) kinetics, and Ca(2+) homeostasis, complex mechanisms which may contribute to the clinical variability in cTnT-related familial hypertrophic cardiomyopathy mutations.

From genotype to phenotype: a longitudinal study of a patient with hypertrophic cardiomyopathy due to a mutation in the MYBPC3 gene

Many of the links between the genotype and phenotype in hypertrophic cardiomyopathy remain unexplained. In this unique longitudinal study we have investigated a patient with classical clinical phenotypic features of hypertrophic obstructive cardiomyopathy, with a known mutation in MYBPC3, the most commonly affected gene in this disease. By collecting cardiac tissue from the patient at the time of surgical myectomy for relief of left ventricular outflow tract obstruction, we have been able to examine the structure of the myocytes and the functional differences that occur in MyBP-C mutated HCM cardiac tissue from single protein level, onto single cardiomyocyte contractility, through to whole organ function as assessed clinically by echocardiography.

Ile90Met, a novel mutation in the cardiac troponin T gene for familial hypertrophic cardiomyopathy in a Chinese pedigree

To identify the disease-causing gene for a large multi-generational Chinese family affected by familial hypertrophic cardiomyopathy (FHCM), genome-wide screening was carried out in a Chinese family with FHCM using micro-satellite markers, and linkage analysis was performed using the MLINK program. The disease locus was mapped to 1q32 in this family. Screening for a mutation in the cardiac troponin T (cTnT) gene was performed by a PCR and sequencing was done with an ABI Prism 3700 sequencer. A novel C-->G transition located in the ninth exon of the cTnT gene, leading to a predicted amino acid residue change from Ile to Met at codon 90, was identified in all individuals with hypertrophic cardiomyopathy (HCM). The results presented here strongly suggest that Ile90Met, a novel mutation in the cTnT gene, is causative agent of HCM in this family.

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.

The molecular phenotype of human cardiac myosin associated with hypertrophic obstructive cardiomyopathy

AIM

The aim of the study was to compare the functional and structural properties of the motor protein, myosin, and isolated myocyte contractility in heart muscle excised from hypertrophic cardiomyopathy patients by surgical myectomy with explanted failing heart and non-failing donor heart muscle.

METHODS

Myosin was isolated and studied using an in vitro motility assay. The distribution of myosin light chain-1 isoforms was measured by two-dimensional electrophoresis. Myosin light chain-2 phosphorylation was measured by sodium dodecyl sulphate-polyacrylamide gel electrophoresis using Pro-Q Diamond phosphoprotein stain.

RESULTS

The fraction of actin filaments moving when powered by myectomy myosin was 21% less than with donor myosin (P = 0.006), whereas the sliding speed was not different (0.310 +/- 0.034 for myectomy myosin vs. 0.305 +/- 0.019 microm/s for donor myosin in six paired experiments). Failing heart myosin showed 18% reduced motility. One myectomy myosin sample produced a consistently higher sliding speed than donor heart myosin and was identified with a disease-causing heavy chain mutation (V606M). In myectomy myosin, the level of atrial light chain-1 relative to ventricular light chain-1 was 20 +/- 5% compared with 11 +/- 5% in donor heart myosin and the level of myosin light chain-2 phosphorylation was decreased by 30-45%. Isolated cardiomyocytes showed reduced contraction amplitude (1.61 +/- 0.25 vs. 3.58 +/- 0.40%) and reduced relaxation rates compared with donor myocytes (TT(50%) = 0.32 +/- 0.09 vs. 0.17 +/- 0.02 s).

CONCLUSION

Contractility in myectomy samples resembles the hypocontractile phenotype found in end-stage failing heart muscle irrespective of the primary stimulus, and this phenotype is not a direct effect of the hypertrophy-inducing mutation. The presence of a myosin heavy chain mutation causing hypertrophic cardiomyopathy can be predicted from a simple functional assay.

A revised method of troponin exchange in permeabilised cardiac trabeculae using vanadate: functional consequences of a HCM-causing mutation in troponin I

In order to incorporate human cardiac troponin I (TnI) and troponin C (TnC) into guinea pig skinned cardiac trabeculae, fibres were treated with vanadate to extract endogenous TnI and TnC using established protocols. After addition of human TnI and TnC force was inadequately restored and it was found that the vanadate treatment had unexpectedly also removed some troponin T. To recover Ca(2+)-sensitive force, the fibres had to be incubated with all three troponin subunits. Using this revised method, the hypertrophic cardiomyopathy-causing mutation TnI Gly203Ser had no significant effect on Ca(2+)-sensitivity of force production, contrasting with our earlier report of decreased Ca(2+)-sensitivity which was likely caused by the unexpectedly harsh effect of vanadate.

Differential effect of troponin T mutations on the inotropic responsiveness of mouse hearts--role of myofilament Ca2+ sensitivity increase

Troponin T (TnT) mutations that cause familial hypertrophic cardiomyopathy (FHC) and sudden cardiac death frequently increase myofilament Ca2+ sensitivity, suggesting that their Ca2+-sensitizing effect contributes importantly to the FHC pathogenesis. To test this hypothesis, we compared transgenic mice expressing the Ca2+-sensitizing TnT-I79N mutant (I79N), which causes a high rate of sudden cardiac death in patients, with mice expressing the more benign TnT-R278C mutant (R278C) that does not affect myofilament Ca2+ sensitivity. Acutely increasing myofilament Ca2+ sensitivity with EMD57033 served as a positive control. Isovolumically contracting hearts were compared over a range of loading conditions (Frank-Starling curve). Consistent with their increased myofilament Ca2+ sensitivity, I79N-Tg hearts demonstrated significantly higher systolic performance at low perfusate [Ca2+] compared with R278C-Tg hearts, which were not statistically different from control hearts expressing either human wild-type TnT or no transgene (CON). Diastolic function was impaired in both FHC mutants (time to 90% relaxation: I79N 48 +/- 1.0 ms, n = 10 or R278C 47 +/- 0.4 ms, n = 7, versus CON 44 +/- 1.0 ms, n = 20, P < 0.05). In the presence of isoproterenol, almost all contractile parameters of R278C hearts became indistinguishable from control hearts, whereas both systolic and diastolic function of I79N hearts significantly worsened (end-diastolic pressure: I79N 20 +/- 4 mmHg versus CON 13 +/- 2 mmHg or R278C 11 +/- 2 mmHg, P < 0.05). The Ca2+ sensitizer EMD57033 produced an even greater contractile dysfunction than the I79N mutation at fast pacing rates. In vivo, maximal exercise tolerance was significantly impaired only in I79N mice. Pretreatment with beta-adrenergic receptor antagonists abolished differences in exercise tolerance. In conclusion, the Ca2+-sensitizing effects of TnT mutations may reduce the responsiveness of mouse hearts to inotropic stimuli.

Myocardial regulatory proteins and heart failure

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

Diltiazem treatment prevents diastolic heart failure in mice with familial hypertrophic cardiomyopathy

BACKGROUND

The cardiac troponin T I79N mutation, linked to familial hypertrophic cardiomyopathy, carries a high risk of sudden cardiac death even in the absence of significant cardiac hypertrophy. The pathology underlying this mechanism has not yet been identified.

AIMS

To study the underlying mechanism of this phenomenon we characterized the left ventricular (LV) performance of transgenic mice carrying the human troponin T mutation I79N under basal and isoproterenol-induced stress conditions.

METHODS AND RESULTS

LV function was analyzed by recording pressure-volume loops using a microconductance catheter. Despite a hypercontractile systolic function under basal conditions TnT-I79N mice showed a diastolic dysfunction indicated by an increase in end-diastolic pressure-volume relationship (EDPVR), a load-independent factor of LV stiffness (0.06+/-0.01 vs. 0.02+/-0.01; P<0.05), when compared to mice expressing human wild-type troponin T (TnT-WT). TnT-I79N mutants developed severe diastolic heart failure and cardiac sudden death under isoproterenol stress. This was prevented after pretreatment with the L-type Ca2+ channel inhibitor diltiazem.

CONCLUSIONS

Diastolic dysfunction due to increased LV stiffness in TnT-I79N mice leads to severe primary diastolic heart failure and finally to cardiac sudden death, which can be prevented by diltiazem.

Increase in tension-dependent ATP consumption induced by cardiac troponin T mutation

How different mutations in cardiac troponin T (cTnT) lead to distinct secondary downstream cellular remodeling in familial hypertrophic cardiomyopathy (FHC) remains elusive. To explore the molecular basis for the distinct impact of different mutations in cTnT on cardiac myocytes, we studied mechanical activity of detergent-skinned muscle fiber bundles from different lines of transgenic (TG) mouse hearts that express wild-type cTnT (WTTG), R92W cTnT, R92L cTnT, and Delta-160 cTnT (deletion of amino acid 160). The amount of mutant cTnT is approximately 50% of the total myocellular cTnT in both R92W and R92L TG mouse hearts and approximately 35% in Delta-160 TG mouse hearts. Myofilament Ca2+ sensitivity was enhanced in all mutant cTnT TG cardiac muscle fibers. Compared with the WTTG fibers, Ca2+ sensitivity increased significantly at short sarcomere length (SL) of 1.9 microm (P < 0.001) in R92W TG fibers by 2.2-fold, in R92L by 2.0-fold, and in Delta-160 by 1.3-fold. At long SL of 2.3 microm, Ca2+ sensitivity increased significantly (P < 0.01) in a similar manner (R92W, 2.5-fold; R92L, 1.9-fold; Delta-160, 1.3-fold). Ca2+-activated maximal tension remained unaltered in all TG muscle fibers. However, tension-dependent ATP consumption increased significantly in Delta-160 TG muscle fibers at both short SL (23%, P < 0.005) and long SL (37%, P < 0.0001), suggesting a mutation-induced change in cross-bridge detachment rate constant. Chronic stresses on relative cellular ATP level in cardiac myocytes may cause a strain on energy-dependent Ca2+ homeostatic mechanisms. This may result in pathological remodeling that we observed in Delta-160 TG cardiac myocytes where the ratio of sarco(endo)plasmic reticulum Ca2+-ATPase 2/phospholamban decreased significantly. Our results suggest that different types of stresses imposed on cardiac myocytes would trigger distinct cellular signaling, which leads to remodeling that may be unique to some mutants.

Inherited cardiomyopathies as a troponin disease

Troponin, one of the sarcomeric proteins, plays a central role in the Ca(2+) regulation of contraction in vertebrate skeletal and cardiac muscles. It consists of three subunits with distinct structure and function, troponin T, troponin I, and troponin C, and their accurate and complex intermolecular interaction in response to the rapid rise and fall of Ca(2+) in cardiomyocytes plays a key role in maintaining the normal cardiac pump function. More than 200 mutations in the cardiac sarcomeric proteins, including myosin heavy and light chains, actin, troponin, tropomyosin, myosin-binding protein-C, and titin/connectin, have been found to cause various types of cardiomyopathy in human since 1990, and more than 60 mutations in human cardiac troponin subunits have been identified in dilated, hypertrophic, and restrictive forms of cardiomyopathy. In this review, we have focused on the mutations in the genes for human cardiac troponin subunits and discussed their functional consequences that might be involved in the primary mechanisms for the pathogenesis of these different types of cardiomyopathy.

Dilated cardiomyopathy mutations in three thin filament regulatory proteins result in a common functional phenotype

Dilated cardiomyopathy (DCM), characterized by cardiac dilatation and contractile dysfunction, is a major cause of heart failure. Inherited DCM can result from mutations in the genes encoding cardiac troponin T, troponin C, and alpha-tropomyosin; different mutations in the same genes cause hypertrophic cardiomyopathy. To understand how certain mutations lead specifically to DCM, we have investigated their effect on contractile function by comparing wild-type and mutant recombinant proteins. Because initial studies on two troponin T mutations have generated conflicting findings, we analyzed all eight published DCM mutations in troponin T, troponin C, and alpha-tropomyosin in a range of in vitro assays. Thin filaments, reconstituted with a 1:1 ratio of mutant/wild-type proteins (the likely in vivo ratio), all showed reduced Ca(2+) sensitivity of activation in ATPase and motility assays, and except for one alpha-tropomyosin mutant showed lower maximum Ca(2+) activation. Incorporation of either of two troponin T mutants in skinned cardiac trabeculae also decreased Ca(2+) sensitivity of force generation. Structure/function considerations imply that the diverse thin filament DCM mutations affect different aspects of regulatory function yet change contractility in a consistent manner. The DCM mutations depress myofibrillar function, an effect fundamentally opposite to that of hypertrophic cardiomyopathy-causing thin filament mutations, suggesting that decreased contractility may trigger pathways that ultimately lead to the clinical phenotype.

The Delta 14 mutation of human cardiac troponin T enhances ATPase activity and alters the cooperative binding of S1-ADP to regulated actin

The complex of tropomyosin and troponin binds to actin and inhibits activation of myosin ATPase activity and force production of striated muscles at low free Ca(2+) concentrations. Ca(2+) stimulates ATP activity, and at subsaturating actin concentrations, the binding of NEM-modified S1 to actin-tropomyosin-troponin increases the rate of ATP hydrolysis even further. We show here that the Delta14 mutation of troponin T, associated with familial hypertrophic cardiomyopathy, results in an increase in ATPase rate like that seen with wild-type troponin in the presence of NEM-S1. The enhanced ATPase activity was not due to a decreased incorporation of mutant troponin T with troponin I and troponin C to form an active troponin complex. The activating effect was more prominent with a hybrid troponin (skeletal TnI, TnC, and cardiac TnT) than with all cardiac troponin. Thus it appears that changes in the troponin-troponin contacts that result from mutations or from forming hybrids stabilize a more active state of regulated actin. An analysis of the effect of the Delta14 mutation on the equilibrium binding of S1-ADP to actin was consistent with stabilization of an active state of actin. This change in activation may be important in the development of cardiac disease.

Biology of the troponin complex in cardiac myocytes

Troponin is the regulatory complex of the myofibrillar thin filament that plays a critical role in regulating excitation-contraction coupling in the heart. Troponin is composed of three distinct gene products: troponin C (cTnC), the 18-kD Ca(2+)-binding subunit; troponin I (cTnI), the approximately 23-kD inhibitory subunit that prevents contraction in the absence of Ca2+ binding to cTnC; and troponin T (cTnT), the approximately 35-kD subunit that attaches troponin to tropomyosin (Tm) and to the myofibrillar thin filament. Over the past 45 years, extensive biochemical, biophysical, and structural studies have helped to elucidate the molecular basis of troponin function and thin filament activation in the heart. At the onset of systole, Ca2+ binds to the N-terminal Ca2+ binding site of cTnC initiating a conformational change in cTnC, which catalyzes protein-protein associations activating the myofibrillar thin filament. Thin filament activation in turn facilitates crossbridge cycling, myofibrillar activation, and contraction of the heart. The intrinsic length-tension properties of cardiac myocytes as well as the Frank-Starling properties of the intact heart are mediated primarily through Ca(2+)-responsive thin filament activation. cTnC, cTnI, and cTnT are encoded by distinct single-copy genes in the human genome, each of which is expressed in a unique cardiac-restricted developmentally regulated fashion. Elucidation of the transcriptional programs that regulate troponin transcription and gene expression has provided insights into the molecular mechanisms that regulate and coordinate cardiac myocyte differentiation and provided unanticipated insights into the pathogenesis of cardiac hypertrophy. Autosomal dominant mutations in cTnI and cTnT have been identified and are associated with familial hypertrophic and restrictive cardiomyopathies.

Modest actomyosin energy conservation increases myocardial postischemic function

We have proposed that pharmacological preconditioning, leading to PKC-epsilon activation, in hearts improves postischemic functional recovery through a decrease in actomyosin ATPase activity and subsequent ATP conservation. The purpose of the present study was to determine whether moderate PKC-independent decreases in actomyosin ATPase are sufficient to improve myocardial postischemic function. Rats were given propylthiouracil (PTU) for 8 days to induce a 25% increase in beta-myosin heavy chain with a 28% reduction in actomyosin ATPase activity. Recovery of postischemic left ventricular developed pressure (LVDP) was significantly higher in PTU-treated rat hearts subjected to 30 min of global ischemia than in control hearts: 57.9 +/- 6.2 vs. 32.6 +/- 5.1% of preischemic values. In addition, PTU-treated hearts exhibited a delayed onset of rigor contracture during ischemia and a higher global ATP content after ischemia. In the second part of our study, we demonstrated a lower maximal actomyosin ATPase and a higher global ATP content after ischemia in human troponin T (TnT) transgenic mouse hearts. In mouse hearts with and without a point mutation at F110I of human TnT, recovery of postischemic LVDP was 55.4 +/- 5.5 and 62.5 +/- 14.5% compared with 20.0 +/- 2.9% in nontransgenic mouse hearts after 35 min of global ischemia. These results are consistent with the hypothesis that moderate decreases in actomyosin ATPase activity result in net ATP conservation that is sufficient to improve postischemic contractile function.

Familial Hypertrophic Cardiomyopathy Mutations from Different Functional Regions of Troponin T Result in Different Effects on the pH and Ca2+ Sensitivity of Cardiac Muscle Contraction

To understand the molecular function of troponin T (TnT) in the Ca(2+) regulation of muscle contraction as well as the molecular pathogenesis of familial hypertrophic cardiomyopathy (FHC), eight FHC-linked TnT mutations, which are located in different functional regions of human cardiac TnT (HCTnT), were produced, and their structural and functional properties were examined. Circular dichroism spectroscopy demonstrated different secondary structures of these TnT mutants. Each of the recombinant HCTnTs was incorporated into porcine skinned fibers along with human cardiac troponin I (HCTnI) and troponin C (HCTnC), and the Ca(2+) dependent isometric force development of these troponin-replaced fibers was determined at pH 7.0 and 6.5. All eight mutants altered the contractile properties of skinned cardiac fibers. E244D potentiated the maximum force development without changing Ca(2+) sensitivity. In contrast, the other seven mutants increased the Ca(2+) sensitivity of force development but not the maximal force. R92L, R92W, and R94L also decreased the change in Ca(2+) sensitivity of force development observed on lowering the pH from 7 to 6.5, when compared with wild type TnT. The examination of additional mutants, H91Q and a double mutant H91Q/R92W, suggests that mutations in a region including residues 91-94 in HCTnT can perturb the proper response of cardiac contraction to changes in pH. These results suggest that different regions of TnT may contribute to the pathogenesis of TnT-linked FHC through different mechanisms.

Different functional properties of troponin T mutants that cause dilated cardiomyopathy

The effects of Troponin T (TnT) mutants R141W and DeltaK210, the only two currently known mutations in TnT that cause dilated cardiomyopathy(DCM) independent of familial hypertrophic cardiomyopathy (FHC), and TnT-K273E, a mutation that leads to a progression from FHC to DCM, were investigated. Studies on the Ca2+ sensitivity of force development in porcine cardiac fibers demonstrated that TnT-DeltaK210 caused a significant decrease in Ca2+ sensitivity, whereas the TnT-R141W did not result in any change in Ca2+ sensitivity when compared with human cardiac wild-type TnT (HCWTnT). TnT-DeltaK210 also caused a decrease in maximal force when compared with HCWTnT and TnT-R141W. In addition, the TnT-DeltaK210 mutant decreased maximal ATPase activity in the presence of Ca2+. However, the TnT-K273E mutation caused a significant increase in Ca2+ sensitivity but behaved similarly to HCWTnT in actomyosin activation assays. Inhibition of ATPase activity in reconstituted actin-activated myosin ATPase assays was similar for all three TnT mutants and HCWTnT. Additionally, circular dichroism studies suggest that the secondary structure of all three TnT mutants was similar to that of the HCWTnT. These results suggest that a rightward shift in Ca2+ sensitivity is not the only determinant for the phenotype of DCM.

Truncation by Glu180 nonsense mutation results in complete loss of slow skeletal muscle troponin T in a lethal nemaline myopathy

A lethal form of nemaline myopathy, named "Amish Nemaline Myopathy" (ANM), is linked to a nonsense mutation at codon Glu180 in the slow skeletal muscle troponin T (TnT) gene. We found that neither the intact nor the truncated slow TnT protein was present in the muscle of patients with ANM. The complete loss of slow TnT is consistent with the observed recessive pattern of inheritance of the disease and indicates a critical role of the COOH-terminal T2 domain in the integration of TnT into myofibrils. Expression of slow and fast isoforms of TnT is fiber-type specific. The lack of slow TnT results in selective atrophy of type 1 fibers. Slow TnT confers a higher Ca2+ sensitivity than does fast TnT in single fiber contractility assays. Despite the lack of slow TnT, individuals with ANM have normal muscle power at birth. The postnatal onset and infantile progression of ANM correspond to a down-regulation of cardiac and embryonic splice variants of fast TnT in normal developing human skeletal muscle, suggesting that the fetal TnT isoforms complement slow TnT. These results lay the foundation for understanding the molecular pathophysiology and the potential targeted therapy of ANM.

Familial hypertrophic cardiomyopathy mutations in troponin I (K183D, G203S, K206Q) enhance filament sliding

A major cause of familial hypertrophic cardiomyopathy (FHC) is dominant mutations in cardiac sarcomeric genes. Linkage studies identified FHC-related mutations in the COOH terminus of cardiac troponin I (cTnI), a region with unknown function in Ca(2+) regulation of the heart. Using in vitro assays with recombinant rat troponin subunits, we tested the hypothesis that mutations K183Delta, G203S, and K206Q in cTnI affect Ca(2+) regulation. All three mutants enhanced Ca(2+) sensitivity and maximum speed (s(max)) of filament sliding of in vitro motility assays. Enhanced s(max) (pCa 5) was observed with rabbit skeletal and rat cardiac (alpha-MHC or beta-MHC) heavy meromyosin (HMM). We developed a passive exchange method for replacing endogenous cTn in permeabilized rat cardiac trabeculae. Ca(2+) sensitivity and maximum isometric force did not differ between preparations exchanged with cTn(cTnI,K206Q) or wild-type cTn. In both trabeculae and motility assays, there was no loss of inhibition at pCa 9. These results are consistent with COOH terminus of TnI modulating actomyosin kinetics during unloaded sliding, but not during isometric force generation, and implicate enhanced cross-bridge cycling in the cTnI-related pathway(s) to hypertrophy.

Familial hypertrophic cardiomyopathy-linked mutant troponin T causes stress-induced ventricular tachycardia and Ca2+-dependent action potential remodeling

The cardiac troponin T (TnT) I79N mutation has been linked to familial hypertrophic cardiomyopathy and high incidence of sudden death, despite causing little or no cardiac hypertrophy in patients. Transgenic mice expressing mutant human TnT (I79N-Tg) have increased cardiac contractility, but no ventricular hypertrophy or fibrosis. Enhanced cardiac function has been associated with myofilament Ca2+ sensitization, suggesting altered cellular Ca2+ handling. In the present study, we compare cellular Ca2+ transients and electrophysiological parameters of 64 I79N-Tg and 106 control mice in isolated myocytes, isolated perfused hearts, and whole animals. Ventricular action potentials (APs) measured in isolated I79N-Tg hearts and myocytes were significantly shortened only at 70% repolarization. No significant differences were found either in L-type Ca2+ or transient outward K+ currents, but inward rectifier K+ current (IK1) was significantly decreased. More critically, Ca2+ transients of field-stimulated ventricular I79N-Tg myocytes were reduced and had slow decay kinetics, consistent with increased Ca2+ sensitivity of I79N mutant fibers. AP differences were abolished when myocytes were dialyzed with Ca2+ buffers or after the Na+-Ca2+ exchanger was blocked by Li+. At higher pacing rates or in presence of isoproterenol, diastolic Ca2+ became significantly elevated in I79N-Tg compared with control myocytes. Ventricular ectopy could be induced by isoproterenol-challenge in isolated I79N-Tg hearts and anesthetized I79N-Tg mice. Freely moving I79N-Tg mice had a higher incidence of nonsustained ventricular tachycardia (VT) during mental stress (warm air jets). We conclude that the TnT-I79N mutation causes stress-induced VT even in absence of hypertrophy and/or fibrosis, arising possibly from the combination of AP remodeling related to altered Ca2+ transients and suppression of IK1.

Alterations in thin filament regulation induced by a human cardiac troponin T mutant that causes dilated cardiomyopathy are distinct from those induced by troponin T mutants that cause hypertrophic cardiomyopathy

We have compared the in vitro regulatory properties of recombinant human cardiac troponin reconstituted using wild type troponin T with troponin containing the DeltaLys-210 troponin T mutant that causes dilated cardiomyopathy (DCM) and the R92Q troponin T known to cause hypertrophic cardiomyopathy (HCM). Troponin containing DeltaLys-210 troponin T inhibited actin-tropomyosin-activated myosin subfragment-1 ATPase activity to the same extent as wild type at pCa8.5 (>80%) but produced substantially less enhancement of ATPase at pCa4.5. The Ca(2+) sensitivity of ATPase activation was increased (DeltapCa(50) = +0.2 pCa units) and cooperativity of Ca(2+) activation was virtually abolished. Equimolar mixtures of wild type and DeltaLys-210 troponin T gave a lower Ca(2+) sensitivity than with wild type, while maintaining the diminished ATPase activation at pCa4.5 observed with 100% mutant. In contrast, R92Q troponin gave reduced inhibition at pCa8.5 but greater activation than wild type at pCa4.5; Ca(2+) sensitivity was increased but there was no change in cooperativity. In vitro motility assay of reconstituted thin filaments confirmed the ATPase results and moreover indicated that the predominant effect of the DeltaLys-210 mutation was a reduced sliding speed. The functional consequences of this DCM mutation are qualitatively different from the R92Q or any other studied HCM troponin T mutation, suggesting that DCM and HCM may be triggered by distinct primary stimuli.