D. Brian Foster

CAPON modulates cardiac repolarization via neuronal nitric oxide synthase signaling in the heart

Congenital long- or short-QT syndrome may lead to life-threatening ventricular tachycardia and sudden cardiac death. Apart from the rare disease-causing mutations, common genetic variants in CAPON, a neuronal nitric oxide synthase (NOS1) regulator, have recently been associated with QT interval variations in a human whole-genome association study. CAPON had been unsuspected of playing a role in cardiac repolarization; indeed, its physiological role in the heart (if any) is unknown. To define the biological effects of CAPON in the heart, we investigated endogenous CAPON protein expression and protein–protein interactions in the heart and performed electrophysiological studies in isolated ventricular myocytes with and without CAPON overexpression. We find that CAPON protein is expressed in the heart and interacts with NOS1 to accelerate cardiac repolarization by inhibition of L-type calcium channel. Our findings provide a rationale for the association of CAPON gene variants with extremes of the QT interval in human populations.

Mitochondrial ROMK Channel Is a Molecular Component of MitoKATP

Rationale: Activation of the mitochondrial ATP-sensitive potassium channel (mitoKATP) has been implicated in the mechanism of cardiac ischemic preconditioning, yet its molecular composition is unknown. Objective: To use an unbiased proteomic analysis of the mitochondrial inner membrane to identify the mitochondrial K+ channel underlying mitoKATP. Methods and Results: Mass spectrometric analysis was used to identify KCNJ1(ROMK) in purified bovine heart mitochondrial inner membrane and ROMK mRNA was confirmed to be present in neonatal rat ventricular myocytes and adult hearts. ROMK2, a short form of the channel, is shown to contain an N-terminal mitochondrial targeting signal, and a full-length epitope-tagged ROMK2 colocalizes with mitochondrial ATP synthase &bgr;. The high-affinity ROMK toxin, tertiapin Q, inhibits mitoKATP activity in isolated mitochondria and in digitonin-permeabilized cells. Moreover, short hairpin RNA—mediated knockdown of ROMK inhibits the ATP-sensitive, diazoxide-activated component of mitochondrial thallium uptake. Finally, the heart-derived cell line, H9C2, is protected from cell death stimuli by stable ROMK2 overexpression, whereas knockdown of the native ROMK exacerbates cell death. Conclusions: The findings support ROMK as the pore-forming subunit of the cytoprotective mitoKATP channel.

Perturbed Length-Dependent Activation in Human Hypertrophic Cardiomyopathy With Missense Sarcomeric Gene Mutations

Rationale: High-myofilament Ca2+ sensitivity has been proposed as a trigger of disease pathogenesis in familial hypertrophic cardiomyopathy (HCM) on the basis of in vitro and transgenic mice studies. However, myofilament Ca2+ sensitivity depends on protein phosphorylation and muscle length, and at present, data in humans are scarce. Objective: To investigate whether high myofilament Ca2+ sensitivity and perturbed length-dependent activation are characteristics for human HCM with mutations in thick and thin filament proteins. Methods and Results: Cardiac samples from patients with HCM harboring mutations in genes encoding thick (MYH7, MYBPC3) and thin (TNNT2, TNNI3, TPM1) filament proteins were compared with sarcomere mutation-negative HCM and nonfailing donors. Cardiomyocyte force measurements showed higher myofilament Ca2+ sensitivity in all HCM samples and low phosphorylation of protein kinase A (PKA) targets compared with donors. After exogenous PKA treatment, myofilament Ca2+ sensitivity was similar (MYBPC3mut, TPM1mut, sarcomere mutation-negative HCM), higher (MYH7mut, TNNT2mut), or even significantly lower (TNNI3mut) compared with donors. Length-dependent activation was significantly smaller in all HCM than in donor samples. PKA treatment increased phosphorylation of PKA-targets in HCM myocardium and normalized length-dependent activation to donor values in sarcomere mutation-negative HCM and HCM with truncating MYBPC3 mutations but not in HCM with missense mutations. Replacement of mutant by wild-type troponin in TNNT2mut and TNNI3mut corrected length-dependent activation to donor values. Conclusions: High-myofilament Ca2+ sensitivity is a common characteristic of human HCM and partly reflects hypophosphorylation of PKA targets compared with donors. Length-dependent sarcomere activation is perturbed by missense mutations, possibly via posttranslational modifications other than PKA hypophosphorylation or altered protein–protein interactions, and represents a common pathomechanism in HCM.

Redox Regulation of Mitochondrial ATP Synthase Implications for Cardiac Resynchronization Therapy

Rationale: Cardiac resynchronization therapy (CRT) is an effective clinical treatment for heart failure patients with conduction delay, impaired contraction, and energetics. Our recent studies have revealed that mitochondrial posttranslational modifications (PTM) may contribute to its benefits, motivating the present study of the oxidative regulation of mitochondrial ATP synthase. Objectives: We tested whether CRT alteration of ATP synthase function is linked to cysteine (Cys) oxidative PTM (Ox-PTM) of specific ATP synthase subunits. Methods and Results: Canine left ventricular myocardium was collected under conditions to preserve Ox-PTM from control, dyssynchronous heart failure (DHF), or hearts that had undergone CRT. In-gel ATPase activity showed that CRT increased ATPase activity by ≈2-fold (P<0.05) over DHF, approaching control levels, and this effect was recapitulated with a reducing agent. ATP synthase function and 3 Ox-PTM: disulfide bond, S-glutathionylation and S-nitrosation were assessed. ATP synthase from DHF hearts contained 2 novel disulfide bonds, between ATP synthase &agr; subunits themselves and between &agr; and &ggr; subunits, both of which were decreased in CRT hearts (4.38±0.13- and 4.23±0.36-fold, respectively, P<0.01). S-glutathionylation of ATP synthase &agr; subunit occurred in DHF hearts and was decreased by CRT (1.56±0.16-fold, P<0.04). In contrast, S-nitrosation of ATP synthase &agr; subunit in DHF hearts was lower than in CRT hearts (1.53±0.19-fold, P<0.05). All modifications occurred at ATP synthase &agr; subunit Cys294 and Cys to Ser mutation indicated that this residue is critical for ATP synthase function. Conclusions: A selective Cys in ATP synthase &agr; subunit is targeted by multiple Ox-PTM suggesting that this Cys residue may act as a redox sensor modulating ATP synthase function.Rationale: Cardiac resynchronization therapy (CRT) is an effective clinical treatment for heart failure patients with conduction delay, impaired contraction, and energetics. Our recent studies have revealed that mitochondrial posttranslational modifications (PTM) may contribute to its benefits, motivating the present study of the oxidative regulation of mitochondrial ATP synthase. Objectives: We tested whether CRT alteration of ATP synthase function is linked to cysteine (Cys) oxidative PTM (Ox-PTM) of specific ATP synthase subunits. Methods and Results: Canine left ventricular myocardium was collected under conditions to preserve Ox-PTM from control, dyssynchronous heart failure (DHF), or hearts that had undergone CRT. In-gel ATPase activity showed that CRT increased ATPase activity by ≈2-fold ( P <0.05) over DHF, approaching control levels, and this effect was recapitulated with a reducing agent. ATP synthase function and 3 Ox-PTM: disulfide bond, S-glutathionylation and S-nitrosation were assessed. ATP synthase from DHF hearts contained 2 novel disulfide bonds, between ATP synthase α subunits themselves and between α and γ subunits, both of which were decreased in CRT hearts (4.38±0.13- and 4.23±0.36-fold, respectively, P <0.01). S-glutathionylation of ATP synthase α subunit occurred in DHF hearts and was decreased by CRT (1.56±0.16-fold, P <0.04). In contrast, S-nitrosation of ATP synthase α subunit in DHF hearts was lower than in CRT hearts (1.53±0.19-fold, P <0.05). All modifications occurred at ATP synthase α subunit Cys294 and Cys to Ser mutation indicated that this residue is critical for ATP synthase function. Conclusions: A selective Cys in ATP synthase α subunit is targeted by multiple Ox-PTM suggesting that this Cys residue may act as a redox sensor modulating ATP synthase function. # Novelty and Significance {#article-title-35}

Nitroxyl-Mediated Disulfide Bond Formation Between Cardiac Myofilament Cysteines Enhances Contractile Function

Rationale: In the myocardium, redox/cysteine modification of proteins regulating Ca2+ cycling can affect contraction and may have therapeutic value. Nitroxyl (HNO), the one-electron-reduced form of nitric oxide, enhances cardiac function in a manner that suggests reversible cysteine modifications of the contractile machinery. Objective: To determine the effects of HNO modification in cardiac myofilament proteins. Methods and Results: The HNO-donor, 1-nitrosocyclohexyl acetate, was found to act directly on the myofilament proteins, increasing maximum force (Fmax) and reducing the concentration of Ca2+ for 50% activation (Ca50) in intact and skinned cardiac muscles. The effects of 1-nitrosocyclohexyl acetate are reversible by reducing agents and distinct from those of another HNO donor, Angeli salt, which was previously reported to increase Fmax without affecting Ca50. Using a new mass spectrometry capture technique based on the biotin switch assay, we identified and characterized the formation by HNO of a disulfide-linked actin–tropomyosin and myosin heavy chain–myosin light chain 1. Comparison of the 1-nitrosocyclohexyl acetate and Angeli salt effects with the modifications induced by each donor indicated the actin–tropomyosin and myosin heavy chain–myosin light chain 1 interactions independently correlated with increased Ca2+ sensitivity and force generation, respectively. Conclusions: HNO exerts a direct effect on cardiac myofilament proteins increasing myofilament Ca2+ responsiveness by promoting disulfide bond formation between critical cysteine residues. These findings indicate a novel, redox-based modulation of the contractile apparatus, which positively impacts myocardial function, providing further mechanistic insight for HNO as a therapeutic agent.

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 (cTnI1–192) 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 &mgr;m, and after treatment with the catalytic subunit of protein kinase A (PKA) to mimic &bgr;-adrenergic stimulation. One-dimensional gel electrophoresis, Western immunoblotting, and 3D imaging revealed that approximately 50% of endogenous cTnI had been homogeneously replaced by cTnI1–192 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. Ca2+ sensitivity of the contractile apparatus was increased in preparations containing cTnI1–192 (pCa50: 5.73±0.03 versus 5.52±0.03 for cTnI1–192 and full-length cTnI, respectively). The sarcomere length dependency of force development and the desensitizing effect of PKA were preserved in cTnI1–192-exchanged cardiomyocytes. These results indicate that degradation of cTnI in human myocardium may impair diastolic function, whereas systolic function is largely preserved.

Redox Regulation of Mitochondrial ATP Synthase

Rationale: Cardiac resynchronization therapy (CRT) is an effective clinical treatment for heart failure patients with conduction delay, impaired contraction, and energetics. Our recent studies have revealed that mitochondrial posttranslational modifications (PTM) may contribute to its benefits, motivating the present study of the oxidative regulation of mitochondrial ATP synthase. Objectives: We tested whether CRT alteration of ATP synthase function is linked to cysteine (Cys) oxidative PTM (Ox-PTM) of specific ATP synthase subunits. Methods and Results: Canine left ventricular myocardium was collected under conditions to preserve Ox-PTM from control, dyssynchronous heart failure (DHF), or hearts that had undergone CRT. In-gel ATPase activity showed that CRT increased ATPase activity by ≈2-fold (P<0.05) over DHF, approaching control levels, and this effect was recapitulated with a reducing agent. ATP synthase function and 3 Ox-PTM: disulfide bond, S-glutathionylation and S-nitrosation were assessed. ATP synthase from DHF hearts contained 2 novel disulfide bonds, between ATP synthase &agr; subunits themselves and between &agr; and &ggr; subunits, both of which were decreased in CRT hearts (4.38±0.13- and 4.23±0.36-fold, respectively, P<0.01). S-glutathionylation of ATP synthase &agr; subunit occurred in DHF hearts and was decreased by CRT (1.56±0.16-fold, P<0.04). In contrast, S-nitrosation of ATP synthase &agr; subunit in DHF hearts was lower than in CRT hearts (1.53±0.19-fold, P<0.05). All modifications occurred at ATP synthase &agr; subunit Cys294 and Cys to Ser mutation indicated that this residue is critical for ATP synthase function. Conclusions: A selective Cys in ATP synthase &agr; subunit is targeted by multiple Ox-PTM suggesting that this Cys residue may act as a redox sensor modulating ATP synthase function.Rationale: Cardiac resynchronization therapy (CRT) is an effective clinical treatment for heart failure patients with conduction delay, impaired contraction, and energetics. Our recent studies have revealed that mitochondrial posttranslational modifications (PTM) may contribute to its benefits, motivating the present study of the oxidative regulation of mitochondrial ATP synthase. Objectives: We tested whether CRT alteration of ATP synthase function is linked to cysteine (Cys) oxidative PTM (Ox-PTM) of specific ATP synthase subunits. Methods and Results: Canine left ventricular myocardium was collected under conditions to preserve Ox-PTM from control, dyssynchronous heart failure (DHF), or hearts that had undergone CRT. In-gel ATPase activity showed that CRT increased ATPase activity by ≈2-fold ( P <0.05) over DHF, approaching control levels, and this effect was recapitulated with a reducing agent. ATP synthase function and 3 Ox-PTM: disulfide bond, S-glutathionylation and S-nitrosation were assessed. ATP synthase from DHF hearts contained 2 novel disulfide bonds, between ATP synthase α subunits themselves and between α and γ subunits, both of which were decreased in CRT hearts (4.38±0.13- and 4.23±0.36-fold, respectively, P <0.01). S-glutathionylation of ATP synthase α subunit occurred in DHF hearts and was decreased by CRT (1.56±0.16-fold, P <0.04). In contrast, S-nitrosation of ATP synthase α subunit in DHF hearts was lower than in CRT hearts (1.53±0.19-fold, P <0.05). All modifications occurred at ATP synthase α subunit Cys294 and Cys to Ser mutation indicated that this residue is critical for ATP synthase function. Conclusions: A selective Cys in ATP synthase α subunit is targeted by multiple Ox-PTM suggesting that this Cys residue may act as a redox sensor modulating ATP synthase function. # Novelty and Significance {#article-title-35}

A mighty small heart: the cardiac proteome of adult Drosophila melanogaster.

Drosophila melanogaster is emerging as a powerful model system for the study of cardiac disease. Establishing peptide and protein maps of the Drosophila heart is central to implementation of protein network studies that will allow us to assess the hallmarks of Drosophila heart pathogenesis and gauge the degree of conservation with human disease mechanisms on a systems level. Using a gel-LC-MS/MS approach, we identified 1228 protein clusters from 145 dissected adult fly hearts. Contractile, cytostructural and mitochondrial proteins were most abundant consistent with electron micrographs of the Drosophila cardiac tube. Functional/Ontological enrichment analysis further showed that proteins involved in glycolysis, Ca2+-binding, redox, and G-protein signaling, among other processes, are also over-represented. Comparison with a mouse heart proteome revealed conservation at the level of molecular function, biological processes and cellular components. The subsisting peptidome encompassed 5169 distinct heart-associated peptides, of which 1293 (25%) had not been identified in a recent Drosophila peptide compendium. PeptideClassifier analysis was further used to map peptides to specific gene-models. 1872 peptides provide valuable information about protein isoform groups whereas a further 3112 uniquely identify specific protein isoforms and may be used as a heart-associated peptide resource for quantitative proteomic approaches based on multiple-reaction monitoring. In summary, identification of excitation-contraction protein landmarks, orthologues of proteins associated with cardiovascular defects, and conservation of protein ontologies, provides testimony to the heart-like character of the Drosophila cardiac tube and to the utility of proteomics as a complement to the power of genetics in this growing model of human heart disease.

The C Terminus of Cardiac Troponin I Stabilizes the Ca2+-Activated State of Tropomyosin on Actin Filaments

Rationale: Ca2+ control of troponin-tropomyosin position on actin regulates cardiac muscle contraction. The inhibitory subunit of troponin, cardiac troponin (cTn)I is primarily responsible for maintaining a tropomyosin conformation that prevents crossbridge cycling. Despite extensive characterization of cTnI, the precise role of its C-terminal domain (residues 193 to 210) is unclear. Mutations within this region are associated with restrictive cardiomyopathy, and C-terminal deletion of cTnI, in some species, has been associated with myocardial stunning. Objective: We sought to investigate the effect of a cTnI deletion-removal of 17 amino acids from the C terminus- on the structure of troponin-regulated tropomyosin bound to actin. Methods and Results: A truncated form of human cTnI (cTnI1-192) was expressed and reconstituted with troponin C and troponin T to form a mutant troponin. Using electron microscopy and 3D image reconstruction, we show that the mutant troponin perturbs the positional equilibrium dynamics of tropomyosin in the presence of Ca2+. Specifically, it biases tropomyosin position toward an “enhanced C-state” that exposes more of the myosin-binding site on actin than found with wild-type troponin. Conclusions: In addition to its well-established role of promoting the so-called “blocked-state” or “B-state,” cTnI participates in proper stabilization of tropomyosin in the “Ca2+-activated state” or “C-state.” The last 17 amino acids perform this stabilizing role. The data are consistent with a “fly-casting” model in which the mobile C terminus of cTnI ensures proper conformational switching of troponin-tropomyosin. Loss of actin-sensing function within this domain, by pathological proteolysis or cardiomyopathic mutation, may be sufficient to perturb tropomyosin conformation.

Effect of troponin I Ser23/24 phosphorylation on Ca2+-sensitivity in human myocardium depends on the phosphorylation background.

Protein kinase A (PKA)-mediated phosphorylation of Ser23/24 of cardiac troponin I (cTnI) causes a reduction in Ca(2+)-sensitivity of force development. This study aimed to determine whether the PKA-induced modulation of the Ca(2+)-sensitivity is solely due to cTnI phosphorylation or depends on the phosphorylation status of other sarcomeric proteins. Endogenous troponin (cTn) complex in donor cardiomyocytes was partially exchanged (up to 66+/-1%) with recombinant unphosphorylated human cTn and in failing cells similar exchange was achieved using PKA-(bis)phosphorylated cTn complex. Cardiomyocytes immersed in exchange solution without complex added served as controls. Partial exchange of unphosphorylated cTn complex in donor tissue significantly increased Ca(2+)-sensitivity (pCa(50)) to 5.50+/-0.02 relative to the donor control value (pCa(50)=5.43+/-0.04). Exchange in failing tissue with PKA-phosphorylated cTn complex did not change Ca(2+)-sensitivity relative to the failing control (pCa(50)=5.60+/-0.02). Subsequent treatment of the cardiomyocytes with the catalytic subunit of PKA significantly decreased Ca(2+)-sensitivity in donor and failing tissue. Analysis of phosphorylated cTnI species revealed the same distribution of un-, mono- and bis-phosphorylated cTnI in donor control and in failing tissue exchanged with PKA-phosphorylated cTn complex. Phosphorylation of myosin-binding protein-C in failing tissue was significantly lower compared to donor tissue. These differences in Ca(2+)-sensitivity in donor and failing cells, despite similar distribution of cTnI species, could be abolished by subsequent PKA-treatment and indicate that other targets of PKA are involved the reduction of Ca(2+)-sensitivity. Our findings suggest that the sarcomeric phosphorylation background, which is altered in cardiac disease, influences the impact of cTnI Ser23/24 phosphorylation by PKA on Ca(2+)-sensitivity.