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

Myosin-actin crossbridge independent sarcomere length induced Ca2+ sensitivity changes in skinned myocardial fibers: Role of myosin heads

Sarcomere length-dependent activation (LDA) is essential to engaging the Frank-Starling mechanism in the beat-to-beat regulation of cardiac output. Through LDA, the heart increases the Ca2+ sensitivity of myocardial contraction at a longer sarcomere length, leading to an enhanced maximal force at the same level of Ca2+. Despite its importance in both normal and pathological states, the molecular mechanism underlying LDA, especially the origin of the sarcomere length (SL) induced increase in myofilament Ca2+sensitivity, remains elusive. The aim of this study is to interrogate the role of changes in the state of myosin heads during diastole as well as effects of strong force-generating cross-bridges (XB) as determinants of SL-induced Ca2+ sensitivity of troponin in membrane-free (skinned) rat myocardial fibers. Skinned myocardial fibers were reconstituted with troponin complex containing a fluorophore-modified cardiac troponin C, cTnC(13C/51C)AEDANS-DDPM, and recombinant cardiac troponin I (cTnI) mutant, ΔSP-cTnI, in which the switch peptide (Sp) of cTnI was replaced by a non-functional peptide link to partially block the force-generating reaction of myosin with actin. We used the reconstituted myocardial fibers as a platform to investigate how Ca2+ sensitivity of troponin within skinned myocardial fibers responds to sarcomere stretch with variations in the status of myosin-actin XBs. Muscle mechanics and fluorescence measurements clearly showed similar SL-induced increases in troponin Ca2+ sensitivity in either the presence or the absence of strong XBs, suggesting that the SL-induced Ca2+ sensitivity change is independent of reactions of force generating XB with the thin filament. The presence of mavacamten, a selective myosin-motor inhibitor known to promote transition of myosin heads from the weakly actin-bound state (ON or disordered relaxed (DRX) state) to the ordered off state (OFF or super-relaxed (SRX) state), blunted the observed SL-induced increases in Ca2+ sensitivity of troponin regardless of the presence of XBs, suggesting that the presence of the myosin heads in the weakly actin bound state, is essential for Ca2+-troponin to sense the sarcomere stretch. Results from skinned myocardial fibers reconstituted with troponin containing engineered TEV digestible mutant cTnI and cTnT suggest that the observed SL effect on Ca2+ sensitivity may involve potential interactions of weakly bound myosin heads with troponin in the actin/Tm cluster region interacting with cTnT-T1 and residues 182-229 of cTnT-T2. The mechanical stretch effects may then be subsequently transmitted to the N-cTnC via the IT arm of troponin and the N-terminus of cTnI. Our findings strongly indicate that SL-induced potential myosin-troponin interaction in diastole, rather than strong myosin-actin XBs, may be an essential molecular mechanism underlying LDA of myofilament.

Velcro-binding by cardiac troponin-I traps tropomyosin on actin in a low-energy relaxed state

During muscle relaxation at low sarcoplasmic Ca2+-concentration, the 40-nm long tropomyosin coiled coil is attracted by the C-terminal regulatory domain of troponin subunit-I to a "steric-blocking" B-state position on actin subunits of cardiac and skeletal muscle thin filaments. Tropomyosin located in this B-state position obstructs myosin-binding sites on actin, limiting access of myosin-crossbridge heads on actin. In turn, the steric-hindrance imposed on myosin-binding diminishes actomyosin ATPase, crossbridge movement along actin, and contractility, thus causing relaxation. In contrast, during muscle activation, at high sarcoplasmic Ca2+ levels, the troponin-induced tropomyosin interference is relieved, the tropomyosin coiled coil returns to its default C-state position on actin, and contractility proceeds. In the current study, we examined the energetics associated with tropomyosin's shift in position from its C-state to its B-state on actin and the influence of troponin-I on this relaxed state transition. Control studies showed that in the absence of troponin, the free energy difference between B- and C-state positions of tropomyosin on actin is negligible, i.e. neither B- nor C-state is obviously preferred on troponin-free actin. In contrast, widely separated sites along the C-terminal regulatory domain of troponin-I are responsible for a favorable free energy change of about -0.75 kcal/mol, driving the tropomyosin C-state to B-state shift. Corresponding truncation and point mutations along C-terminal region of TnI lead to a less favorable regulatory transition and are linked to cardiac muscle dysfunction.

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

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

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

Striated muscle proteins are regulated both by mechanical deformation and by chemical post-translational modification

All cells sense force and build their cytoskeleton to optimize function. How is this achieved? Two major systems are involved. The first is that load deforms specific protein structures in a proportional and orientation-dependent manner. The second is post-translational modification of proteins as a consequence of signaling pathway activation. These two processes work together in a complex way so that local subcellular assembly as well as overall cell function are controlled. This review discusses many cell types but focuses on striated muscle. Detailed information is provided on how load deforms the structure of proteins in the focal adhesions and filaments, using α-actinin, vinculin, talin, focal adhesion kinase, LIM domain-containing proteins, filamin, myosin, titin, and telethonin as examples. Second messenger signals arising from external triggers are distributed throughout the cell causing post-translational or chemical modifications of protein structures, with the actin capping protein CapZ and troponin as examples. There are numerous unanswered questions of how mechanical and chemical signals are integrated by muscle proteins to regulate sarcomere structure and function yet to be studied. Therefore, more research is needed to see how external triggers are integrated with local tension generated within the cell. Nonetheless, maintenance of tension in the sarcomere is the essential and dominant mechanism, leading to the well-known phrase in exercise physiology: "use it or lose it."

Potential impacts of the cardiac troponin I mobile domain on myofilament activation and relaxation

The cardiac thin filament is regulated in a Ca²⁺-dependent manner through conformational changes of troponin and tropomyosin (Tm). It has been generally understood that under conditions of low Ca²⁺ the inhibitory peptide domain (IP) of troponin I (TnI) binds to actin and holds Tm over the myosin binding sites on actin to prevent crossbridge formation. More recently, evidence that the C-terminal mobile domain (MD) of TnI also binds actin has made for a more complex scenario. This study uses a computational model to investigate the consequences of assuming that TnI regulates Tm movement via two actin-binding domains rather than one. First, a 16-state model of the cardiac thin filament regulatory unit was created with TnI-IP as the sole regulatory domain. Expansion of this to include TnI-MD formed a 24-state model. Comparison of these models showed that assumption of a second actin-binding site allows the individual domains to have a lower affinity for actin than would be required for IP acting alone. Indeed, setting actin affinities of the IP and MD to 25% of that assumed for the IP in the single-site model was sufficient to achieve precisely the same degree of Ca²⁺ regulation. We also tested the 24-state model's ability to represent steady-state experimental data in the case of disruption of either the IP or MD. We were able to capture qualitative changes in several properties that matched what was seen in the experimental data. Lastly, simulations were run to examine the effect of disruption of the IP or MD on twitch dynamics. Our results suggest that both domains are required to keep diastolic cross-bridge activity to a minimum and accelerate myofilament relaxation. Overall, our analyses support a paradigm in which two domains of TnI bind with moderate affinity to actin, working in tandem to complete Ca²⁺-dependent regulation of the thin filament.

Association Between Circulating Troponin Concentrations, Left Ventricular Systolic and Diastolic Functions, and Incident Heart Failure in Older Adults

Importance

Cardiac troponin is associated with incident heart failure and greater left ventricular (LV) mass. Its association with LV systolic and diastolic functions is unclear.

Objectives

To define the association of high-sensitivity cardiac troponin T (hs-cTnT) with LV systolic and diastolic functions in the general population, and to evaluate the extent to which that association accounts for the correlation between hs-cTnT concentration and incident heart failure overall, heart failure with preserved LV ejection fraction (LVEF; HFpEF), and heart failure with LVEF less than 50%.

Design, Setting, and Participants

This analysis of the Atherosclerosis Risk in Communities (ARIC) Study, an ongoing epidemiologic cohort study in US communities, included participants without cardiovascular disease (n = 4111). Available hs-cTnT measurements for participants who attended ARIC Study visits 2 (1990 to 1992), 4 (1996 to 1998), and 5 (2011 to 2013) were assessed cross-sectionally against echocardiographic measurements taken at visit 5 and against incident health failure after visit 5. Changes in hs-cTnT concentrations from visits 2 and 4 were also examined. Data analyses were performed from August 2017 to July 2018.

Main Outcomes and Measures

Cardiac structure and function by echocardiography at visit 5, and incident heart failure during a median 4½ years follow-up after visit 5.

Results

Of the 6538 eligible participants, 4111 (62.9%) without cardiovascular disease were included. Among these participants, 2586 (62.9%) were female, and the mean (SD) age was 75 (5) years. Median (interquartile range) hs-cTnT concentration at visit 5 was 9 (7-14) ng/L and was detectable in 3946 participants (96.0%). After adjustment for demographic and clinical covariates, higher hs-cTnT levels were associated with greater LV mass index (adjusted mean [SE] for group 1: 33.8 [0.5] vs group 5: 40.1 [0.4]; P for trend < .001) and with worse diastolic function, including lower tissue Doppler imaging e' (6.00 [0.07] vs 5.54 [0.06]; P for trend < .001), higher E/e' ratio (11.4 [0.2] vs 12.9 [0.1]; P for trend < .001), and greater left atrial volume index (23.4 [0.4] vs 26.4 [0.3]; P for trend < .001), independent of LV mass index; hs-cTnT level was not associated with measures of LV systolic function. Accounting for diastolic function attenuated the association of hs-cTnT concentration with incident HFpEF by 41% and the association with combined heart failure with midrange and reduced ejection fraction combined (LVEF <50) by 17%. Elevated hs-cTnT concentration and diastolic dysfunction were additive risk factors for incident heart failure. For any value of late-life hs-cTnT levels, longer duration of detectable hs-cTnT from midlife to late life was associated with greater LV mass in late life but not with worse LV systolic or diastolic function.

Conclusions and Relevance

This study shows that higher hs-cTnT concentrations were associated with worse diastolic function, irrespective of LV mass, but not with systolic function; these findings suggest that high levels of hs-cTnT may serve as an early marker of subclinical alterations in diastolic function that may lead to a predisposition to heart failure.

Protein Quality Control Activation and Microtubule Remodeling in Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disorder. It is mainly caused by mutations in genes encoding sarcomere proteins. Mutant forms of these highly abundant proteins likely stress the protein quality control (PQC) system of cardiomyocytes. The PQC system, together with a functional microtubule network, maintains proteostasis. We compared left ventricular (LV) tissue of nine donors (controls) with 38 sarcomere mutation-positive (HCMSMP) and 14 sarcomere mutation-negative (HCMSMN) patients to define HCM and mutation-specific changes in PQC. Mutations in HCMSMP result in poison polypeptides or reduced protein levels (haploinsufficiency, HI). The main findings were 1) several key PQC players were more abundant in HCM compared to controls, 2) after correction for sex and age, stabilizing heat shock protein (HSP)B1, and refolding, HSPD1 and HSPA2 were increased in HCMSMP compared to controls, 3) α-tubulin and acetylated α-tubulin levels were higher in HCM compared to controls, especially in HCMHI, 4) myosin-binding protein-C (cMyBP-C) levels were inversely correlated with α-tubulin, and 5) α-tubulin levels correlated with acetylated α-tubulin and HSPs. Overall, carrying a mutation affects PQC and α-tubulin acetylation. The haploinsufficiency of cMyBP-C may trigger HSPs and α-tubulin acetylation. Our study indicates that proliferation of the microtubular network may represent a novel pathomechanism in cMyBP-C haploinsufficiency-mediated HCM.

Cardiac Disorders and Pathophysiology of Sarcomeric Proteins

The sarcomeric proteins represent the structural building blocks of heart muscle, which are essential for contraction and relaxation. During recent years, it has become evident that posttranslational modifications of sarcomeric proteins, in particular phosphorylation, tune cardiac pump function at rest and during exercise. This delicate, orchestrated interaction is also influenced by mutations, predominantly in sarcomeric proteins, which cause hypertrophic or dilated cardiomyopathy. In this review, we follow a bottom-up approach starting from a description of the basic components of cardiac muscle at the molecular level up to the various forms of cardiac disorders at the organ level. An overview is given of sarcomere changes in acquired and inherited forms of cardiac disease and the underlying disease mechanisms with particular reference to human tissue. A distinction will be made between the primary defect and maladaptive/adaptive secondary changes. Techniques used to unravel functional consequences of disease-induced protein changes are described, and an overview of current and future treatments targeted at sarcomeric proteins is given. The current evidence presented suggests that sarcomeres not only form the basis of cardiac muscle function but also represent a therapeutic target to combat cardiac disease.

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

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

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

Associations of sensitive cardiac troponin-I with left ventricular morphology, function and prognosis in end-stage renal disease patients with preserved ejection fraction

Sensitive cardiac troponin I (cTnI) predicts all-cause and cardiovascular mortality in various clinical settings. However, its clinical significance in hemodialysis (HD) patients with preserved left ventricular ejection fraction (LVEF) has not been fully elucidated. This study investigated the association of cTnI with LV morphology and function, and its long-term outcome in HD patients with preserved LVEF. This prospective study consists of 96 HD patients with preserved LVEF (69 ± 8 years and 63% male) who underwent two-dimensional echocardiographic examination and biomarker tests including cTnI, brain natriuretic peptide, and high-sensitive C-reactive protein. The primary endpoint was all-cause death and secondary endpoint was cardiovascular death. Factors independently associated with cTnI were systolic blood pressure (β = - 0.239, p = 0.011), heart rate (β = 0.216, p = 0.021), LV mass index (β = 0.231, p = 0.020), and E to e' ratio (β = 0.237, p = 0.016). During a mean follow-up of 3.6 years, primary and secondary endpoints were observed in 23 (24%) and 18 (19%) patients, respectively. In the multivariate Cox proportional hazard analysis, the upper cTnI tertile has significantly increased risk of all-cause mortality [hazard ratio (HR), 2.69; 95% confidence interval (CI), 1.139-6.386; p = 0.024] and that of cardiovascular death (HR, 4.56; 95% CI 2.021-16.968; p = 0.006) independent of echocardiographic measures and other serum biomarkers. In HD patients with preserved LVEF, serum cTnI levels were significantly associated with diastolic function and risk of mortality independent of echocardiographic variables and other biomarkers.

Proteomics analysis identified peroxiredoxin 2 involved in early-phase left ventricular impairment in hamsters with cardiomyopathy

Given the hypothesis that inflammation plays a critical role in the progression of cardiovascular diseases, the aim of the present study was to identify new diagnostic and prognostic biomarkers of myocardial proteins involved in early-phase cardiac impairment, using proteomics analysis. Using the two-dimensional fluorescence difference gel electrophoresis (2D-DIGE) combined with MALDI-TOF/TOF tandem mass spectrometry, we compared differences in the expression of proteins in the whole left ventricles between control hamsters, dilated cardiomyopathic hamsters (TO-2), and hypertrophy cardiomyopathic hamsters (Bio14.6) at 6 weeks of age (n = 6, each group). Proteomic analysis identified 10 protein spots with significant alterations, with 7 up-regulated and 3 down-regulated proteins in the left ventricles of both TO-2 and Bio 14.6 hamsters, compared with control hamsters. Of the total alterations, peroxiredoxin 2 (PRDX2) showed significant upregulation in the left ventricles of TO-2 and Bio 14.6 hamsters. Our data suggest that PRDX2, a redox regulating molecule, is involved in early-phase left ventricular impairment in hamsters with cardiomyopathy.

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

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

Heart Failure-Related Hyperphosphorylation in the Cardiac Troponin I C Terminus Has Divergent Effects on Cardiac Function In Vivo

BACKGROUND

In human heart failure, Ser199 (equivalent to Ser200 in mouse) of cTnI (cardiac troponin I) is significantly hyperphosphorylated, and in vitro studies suggest that it enhances myofilament calcium sensitivity and alters calpain-mediated cTnI proteolysis. However, how its hyperphosphorylation affects cardiac function in vivo remains unknown.

METHODS AND RESULTS

To address the question, 2 transgenic mouse models were generated: a phospho-mimetic cTnIS200D and a phospho-silenced cTnIS200A, each driven by the cardiomyocyte-specific α-myosin heavy chain promoter. Cardiac structure assessed by echocardiography and histology was normal in both transgenic models compared with littermate controls (n=5). Baseline in vivo hemodynamics and isolated muscle studies showed that cTnIS200D significantly prolonged relaxation and lowered left ventricular peak filling rate, whereas ejection fraction and force development were normal (n=5). However, with increased heart rate or β-adrenergic stimulation, cTnIS200D mice had less enhanced ejection fraction or force development versus controls, whereas relaxation improved similarly to controls (n=5). By contrast, cTnIS200A was functionally normal both at baseline and under the physiological stresses. To test whether either mutation impacted cardiac response to ischemic stress, isolated hearts were subjected to ischemia/reperfusion. cTnIS200D were protected, recovering 88±8% of contractile function versus 35±15% in littermate controls and 28±8% in cTnIS200A (n=5). This was associated with less cTnI proteolysis in cTnIS200D hearts.

CONCLUSIONS

Hyperphosphorylation of this serine in cTnI C terminus impacts heart function by depressing diastolic function at baseline and limiting systolic reserve under physiological stresses. However, paradoxically, it preserves heart function after ischemia/reperfusion injury, potentially by decreasing proteolysis of cTnI.

The Sydney Heart Bank: improving translational research while eliminating or reducing the use of animal models of human heart disease

The Sydney Heart Bank (SHB) is one of the largest human heart tissue banks in existence. Its mission is to provide high-quality human heart tissue for research into the molecular basis of human heart failure by working collaboratively with experts in this field. We argue that, by comparing tissues from failing human hearts with age-matched non-failing healthy donor hearts, the results will be more relevant than research using animal models, particularly if their physiology is very different from humans. Tissue from heart surgery must generally be used soon after collection or it significantly deteriorates. Freezing is an option but it raises concerns that freezing causes substantial damage at the cellular and molecular level. The SHB contains failing samples from heart transplant patients and others who provided informed consent for the use of their tissue for research. All samples are cryopreserved in liquid nitrogen within 40 min of their removal from the patient, and in less than 5-10 min in the case of coronary arteries and left ventricle samples. To date, the SHB has collected tissue from about 450 failing hearts (>15,000 samples) from patients with a wide range of etiologies as well as increasing numbers of cardiomyectomy samples from patients with hypertrophic cardiomyopathy. The Bank also has hearts from over 120 healthy organ donors whose hearts, for a variety of reasons (mainly tissue-type incompatibility with waiting heart transplant recipients), could not be used for transplantation. Donor hearts were collected by the St Vincent's Hospital Heart and Lung transplantation team from local hospitals or within a 4-h jet flight from Sydney. They were flushed with chilled cardioplegic solution and transported to Sydney where they were quickly cryopreserved in small samples. Failing and/or donor samples have been used by more than 60 research teams around the world, and have resulted in more than 100 research papers. The tissues most commonly requested are from donor left ventricles, but right ventricles, atria, interventricular system, and coronary arteries vessels have also been reported. All tissues are stored for long-term use in liquid N or vapor (170-180 °C), and are shipped under nitrogen vapor to avoid degradation of sensitive molecules such as RNAs and giant proteins. We present evidence that the availability of these human heart samples has contributed to a reduction in the use of animal models of human heart failure.

The Relaxation Properties of Myofibrils Are Compromised by Amino Acids that Stabilize α-Tropomyosin

We investigated the functional impact of α-tropomyosin (Tm) substituted with one (D137L) or two (D137L/G126R) stabilizing amino acid substitutions on the mechanical behavior of rabbit psoas skeletal myofibrils by replacing endogenous Tm and troponin (Tn) with recombinant Tm mutants and purified skeletal Tn. Force recordings from myofibrils (15°C) at saturating [Ca²⁺] showed that Tm-stabilizing substitutions did not significantly affect the maximal isometric tension and the rates of force activation (k_ACT) and redevelopment (k_TR). However, a clear effect was observed on force relaxation: myofibrils with D137L/G126R or D137L Tm showed prolonged durations of the slow phase of relaxation and decreased rates of the fast phase. Both Tm-stabilizing substitutions strongly decreased the slack sarcomere length (SL) at submaximal activating [Ca²⁺] and increased the steepness of the SL-passive tension relation. These effects were reversed by addition of 10 mM 2,3-butanedione 2-monoxime. Myofibrils also showed an apparent increase in Ca²⁺ sensitivity. Measurements of myofibrillar ATPase activity in the absence of Ca²⁺ showed a significant increase in the presence of these Tms, indicating that single and double stabilizing substitutions compromise the full inhibition of contraction in the relaxed state. These data can be understood with the three-state (blocked-closed-open) theory of muscle regulation, according to which the mutations increase the contribution of the active open state in the absence of Ca²⁺ (M^-). Force measurements on myofibrils substituted with C-terminal truncated TnI showed similar compromised relaxation effects, indicating the importance of TnI-Tm interactions in maintaining the blocked state. It appears that reducing the flexibility of native Tm coiled-coil structure decreases the optimum interactions of the central part of Tm with the C-terminal region of TnI. This results in a shift away from the blocked state, allowing myosin binding and activity in the absence of Ca²⁺. This work provides a basis for understanding the effects of disease-producing mutations in muscle proteins.

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

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

The functional significance of the last 5 residues of the C-terminus of cardiac troponin I

The C-terminal region of cardiac troponin I (cTnI) is known to be important in cardiac function, as removal of the last 17 C-terminal residues of human cTnI has been associated with myocardial stunning. To investigate the C-terminal region of cTnI, three C-terminal deletion mutations in human cTnI were generated: Δ1 (deletion of residue 210), Δ3 (deletion of residues 208-210), and Δ5 (deletion of residues 206-210). Mammalian two-hybrid studies showed that the interactions between cTnI mutants and cardiac troponin C (cTnC) or cardiac troponin T (cTnT) were impaired in Δ3 and Δ5 mutants when compared to wild-type cTnI. Troponin complexes containing 2-[4'-(iodoacetamido) anilino] naphthalene-6-sulfonic acid (IAANS) labeled cTnC showed that the troponin complex containing cTnI Δ5 had a small increase in Ca(2+) affinity (P < 0.05); while the cTnI Δ1- and Δ3 troponin complexes showed no difference in Ca(2+) affinity when compared to wild-type troponin. In vitro motility assays showed that all truncation mutants had increased Ca(2+) dependent motility relative to wild-type cTnI. These results suggest that the last 5 C-terminal residues of cTnI influence the binding of cTnI with cTnC and cTnT and affect the Ca(2+) dependence of filament sliding, and demonstrate the importance of this region of cTnI.

TNNI1, TNNI2 and TNNI3: Evolution, regulation, and protein structure-function relationships

Troponin I (TnI) is the inhibitory subunit of the troponin complex in the sarcomeric thin filament of striated muscle and plays a central role in the calcium regulation of contraction and relaxation. Vertebrate TnI has evolved into three isoforms encoded by three homologous genes: TNNI1 for slow skeletal muscle TnI, TNNI2 for fast skeletal muscle TnI and TNNI3 for cardiac TnI, which are expressed under muscle type-specific and developmental regulations. To summarize the current knowledge on the TnI isoform genes and products, this review focuses on the evolution, gene regulation, posttranslational modifications, and structure-function relationship of TnI isoform proteins. Their physiological and medical significances are also discussed.

A novel phosphorylation site, Serine 199, in the C-terminus of cardiac troponin I regulates calcium sensitivity and susceptibility to calpain-induced proteolysis

Phosphorylation of cardiac troponin I (cTnI) by protein kinase C (PKC) is implicated in cardiac dysfunction. Recently, Serine 199 (Ser199) was identified as a target for PKC phosphorylation and increased Ser199 phosphorylation occurs in end-stage failing compared with non-failing human myocardium. The functional consequences of cTnI-Ser199 phosphorylation in the heart are unknown. Therefore, we investigated the impact of phosphorylation of cTnI-Ser199 on myofilament function in human cardiac tissue and the susceptibility of cTnI to proteolysis. cTnI-Ser199 was replaced by aspartic acid (199D) or alanine (199A) to mimic phosphorylation and dephosphorylation, respectively, with recombinant wild-type (Wt) cTn as a negative control. Force development was measured at various [Ca(2+)] and at sarcomere lengths of 1.8 and 2.2 μm in demembranated cardiomyocytes in which endogenous cTn complex was exchanged with the recombinant human cTn complexes. In idiopathic dilated cardiomyopathy samples, myofilament Ca(2+)-sensitivity (pCa50) at 2.2 μm was significantly higher in 199D (pCa50 = 5.79 ± 0.01) compared to 199A (pCa50 = 5.65 ± 0.01) and Wt (pCa50 = 5.66 ± 0.02) at ~63% cTn exchange. Myofilament Ca(2+)-sensitivity was significantly higher even with only 5.9 ± 2.5% 199D exchange compared to 199A, and saturated at 12.3 ± 2.6% 199D exchange. Ser199 pseudo-phosphorylation decreased cTnI binding to both actin and actin-tropomyosin. Moreover, altered susceptibility of cTnI to proteolysis by calpain I was found when Ser199 was pseudo-phosphorylated. Our data demonstrate that low levels of cTnI-Ser199 pseudo-phosphorylation (~6%) increase myofilament Ca(2+)-sensitivity in human cardiomyocytes, most likely by decreasing the binding affinity of cTnI for actin-tropomyosin. In addition, cTnI-Ser199 pseudo-phosphorylation or mutation regulates calpain I mediated proteolysis of cTnI.

Tri-modal regulation of cardiac muscle relaxation; intracellular calcium decline, thin filament deactivation, and cross-bridge cycling kinetics

Cardiac muscle relaxation is an essential step in the cardiac cycle. Even when the contraction of the heart is normal and forceful, a relaxation phase that is too slow will limit proper filling of the ventricles. Relaxation is too often thought of as a mere passive process that follows contraction. However, many decades of advancements in our understanding of cardiac muscle relaxation have shown it is a highly complex and well-regulated process. In this review, we will discuss three distinct events that can limit the rate of cardiac muscle relaxation: the rate of intracellular calcium decline, the rate of thin-filament de-activation, and the rate of cross-bridge cycling. Each of these processes are directly impacted by a plethora of molecular events. In addition, these three processes interact with each other, further complicating our understanding of relaxation. Each of these processes is continuously modulated by the need to couple bodily oxygen demand to cardiac output by the major cardiac physiological regulators. Length-dependent activation, frequency-dependent activation, and beta-adrenergic regulation all directly and indirectly modulate calcium decline, thin-filament deactivation, and cross-bridge kinetics. We hope to convey our conclusion that cardiac muscle relaxation is a process of intricate checks and balances, and should not be thought of as a single rate-limiting step that is regulated at a single protein level. Cardiac muscle relaxation is a system level property that requires fundamental integration of three governing systems: intracellular calcium decline, thin filament deactivation, and cross-bridge cycling kinetics.

Impact of tropomyosin isoform composition on fast skeletal muscle thin filament regulation and force development

Tropomyosin (Tm) plays a central role in the regulation of muscle contraction and is present in three main isoforms in skeletal and cardiac muscles. In the present work we studied the functional role of α- and βTm on force development by modifying the isoform composition of rabbit psoas skeletal muscle myofibrils and of regulated thin filaments for in vitro motility measurements. Skeletal myofibril regulatory proteins were extracted (78%) and replaced (98%) with Tm isoforms as homogenous ααTm or ββTm dimers and the functional effects were measured. Maximal Ca(2+) activated force was the same in ααTm versus ββTm myofibrils, but ββTm myofibrils showed a marked slowing of relaxation and an impairment of regulation under resting conditions compared to ααTm and controls. ββTm myofibrils also showed a significantly shorter slack sarcomere length and a marked increase in resting tension. Both these mechanical features were almost completely abolished by 10 mM 2,3-butanedione 2-monoxime, suggesting the presence of a significant degree of Ca(2+)-independent cross-bridge formation in ββTm myofibrils. Finally, in motility assay experiments in the absence of Ca(2+) (pCa 9.0), complete regulation of thin filaments required greater ββTm versus ααTm concentrations, while at full activation (pCa 5.0) no effect was observed on maximal thin filament motility speed. We infer from these observations that high contents of ββTm in skeletal muscle result in partial Ca(2+)-independent activation of thin filaments at rest, and longer-lasting and less complete tension relaxation following Ca(2+) removal.

Computational studies of the effect of the S23D/S24D troponin I mutation on cardiac troponin structural dynamics

During β-adrenergic stimulation, cardiac troponin I (cTnI) is phosphorylated by protein kinase A (PKA) at sites S23/S24, located at the N-terminus of cTnI. This phosphorylation has been shown to decrease KCa and pCa50, and weaken the cTnC-cTnI (C-I) interaction. We recently reported that phosphorylation results in an increase in the rate of early, slow phase of relaxation (kREL,slow) and a decrease in its duration (tREL,slow), which speeds up the overall relaxation. However, as the N-terminus of cTnI (residues 1-40) has not been resolved in the whole cardiac troponin (cTn) structure, little is known about the molecular-level behavior within the whole cTn complex upon phosphorylation of the S23/S24 residues of cTnI that results in these changes in function. In this study, we built up the cTn complex structure (including residues cTnC 1-161, cTnI 1-172, and cTnT 236-285) with the N-terminus of cTnI. We performed molecular-dynamics (MD) simulations to elucidate the structural basis of PKA phosphorylation-induced changes in cTn structure and Ca(2+) binding. We found that introducing two phosphomimic mutations into sites S23/S24 had no significant effect on the coordinating residues of Ca(2+) binding site II. However, the overall fluctuation of cTn was increased and the C-I interaction was altered relative to the wild-type model. The most significant changes involved interactions with the N-terminus of cTnI. Interestingly, the phosphomimic mutations led to the formation of intrasubunit interactions between the N-terminus and the inhibitory peptide of cTnI. This may result in altered interactions with cTnC and could explain the increased rate and decreased duration of slow-phase relaxation seen in myofibrils.

Phosphorylation of protein kinase C sites Ser42/44 decreases Ca(2+)-sensitivity and blunts enhanced length-dependent activation in response to protein kinase A in human cardiomyocytes

Protein kinase C (PKC)-mediated phosphorylation of troponin I (cTnI) at Ser42/44 is increased in heart failure. While studies in rodents demonstrated that PKC-mediated Ser42/44 phosphorylation decreases maximal force and ATPase activity, PKC incubation of human cardiomyocytes did not affect maximal force. We investigated whether Ser42/44 pseudo-phosphorylation affects force development and ATPase activity using troponin exchange in human myocardium. Additionally, we studied if pseudo-phosphorylated Ser42/44 modulates length-dependent activation of force, which is regulated by protein kinase A (PKA)-mediated cTnI-Ser23/24 phosphorylation. Isometric force was measured in membrane-permeabilized cardiomyocytes exchanged with human recombinant wild-type troponin or troponin mutated at Ser42/44 or Ser23/24 into aspartic acid (D) or alanine (A) to mimic phosphorylation and dephosphorylation, respectively. In troponin-exchanged donor cardiomyocytes experiments were repeated after PKA incubation. ATPase activity was measured in troponin-exchanged cardiac muscle strips. Compared to wild-type, 42D/44D decreased Ca(2+)-sensitivity without affecting maximal force in failing and donor cardiomyocytes. In donor myocardium, 42D/44D did not affect maximal ATPase activity or tension cost. Interestingly, 42D/44D blunted the length-dependent increase in Ca(2+)-sensitivity induced upon PKA-mediated phosphorylation. Since the drop in Ca(2+)-sensitivity at physiological Ca(2+)-concentrations is relatively large phosphorylation of Ser42/44 may result in a decrease of force and associated ATP utilization in the human heart.

Combined troponin I Ser-150 and Ser-23/24 phosphorylation sustains thin filament Ca(2+) sensitivity and accelerates deactivation in an acidic environment

The binding of Ca(2+) to troponin C (TnC) in the troponin complex is a critical step regulating the thin filament, the actin-myosin interaction and cardiac contraction. Phosphorylation of the troponin complex is a key regulatory mechanism to match cardiac contraction to demand. Here we demonstrate that phosphorylation of the troponin I (TnI) subunit is simultaneously increased at Ser-150 and Ser-23/24 during in vivo myocardial ischemia. Myocardial ischemia decreases intracellular pH resulting in depressed binding of Ca(2+) to TnC and impaired contraction. To determine the pathological relevance of these simultaneous TnI phosphorylations we measured individual TnI Ser-150 (S150D), Ser-23/24 (S23/24D) and combined (S23/24/150D) pseudo-phosphorylation effects on thin filament regulation at acidic pH similar to that in myocardial ischemia. Results demonstrate that while acidic pH decreased thin filament Ca(2+) binding to TnC regardless of TnI composition, TnI S150D attenuated this decrease rendering it similar to non-phosphorylated TnI at normal pH. The dissociation of Ca(2+) from TnC was unaltered by pH such that TnI S150D remained slow, S23/24D remained accelerated and the combined S23/24/150D remained accelerated. This effect of the combined TnI Ser-150 and Ser-23/24 pseudo-phosphorylations to maintain Ca(2+) binding while accelerating Ca(2+) dissociation represents the first post-translational modification of troponin by phosphorylation to both accelerate thin filament deactivation and maintain Ca(2+) sensitive activation. These data suggest that TnI Ser-150 phosphorylation induced attenuation of the pH-dependent decrease in Ca(2+) sensitivity and its combination with Ser-23/24 phosphorylation to maintain accelerated thin filament deactivation may impart an adaptive role to preserve contraction during acidic ischemia pH without slowing relaxation.

Gene regulation, alternative splicing, and posttranslational modification of troponin subunits in cardiac development and adaptation: a focused review

Troponin plays a central role in regulating the contraction and relaxation of vertebrate striated muscles. This review focuses on the isoform gene regulation, alternative RNA splicing, and posttranslational modifications of troponin subunits in cardiac development and adaptation. Transcriptional and posttranscriptional regulations such as phosphorylation and proteolysis modifications, and structure-function relationships of troponin subunit proteins are summarized. The physiological and pathophysiological significances are discussed for impacts on cardiac muscle contractility, heart function, and adaptations in health and diseases.

Length-dependent activation is modulated by cardiac troponin I bisphosphorylation at Ser23 and Ser24 but not by Thr143 phosphorylation

Frank-Starling's law reflects the ability of the heart to adjust the force of its contraction to changes in ventricular filling, a property based on length-dependent myofilament activation (LDA). The threonine at amino acid 143 of cardiac troponin I (cTnI) is prerequisite for the length-dependent increase in Ca(2+) sensitivity. Thr143 is a known target of protein kinase C (PKC) whose activity is increased in cardiac disease. Thr143 phosphorylation may modulate length-dependent myofilament activation in failing hearts. Therefore, we investigated if pseudo-phosphorylation at Thr143 modulates length dependence of force using troponin exchange experiments in human cardiomyocytes. In addition, we studied effects of protein kinase A (PKA)-mediated cTnI phosphorylation at Ser23/24, which has been reported to modulate LDA. Isometric force was measured at various Ca(2+) concentrations in membrane-permeabilized cardiomyocytes exchanged with recombinant wild-type (WT) troponin or troponin mutated at the PKC site Thr143 or Ser23/24 into aspartic acid (D) or alanine (A) to mimic phosphorylation and dephosphorylation, respectively. In troponin-exchanged donor cardiomyocytes experiments were repeated after incubation with exogenous PKA. Pseudo-phosphorylation of Thr143 increased myofilament Ca(2+) sensitivity compared with WT without affecting LDA in failing and donor cardiomyocytes. Subsequent PKA treatment enhanced the length-dependent shift in Ca(2+) sensitivity after WT and 143D exchange. Exchange with Ser23/24 variants demonstrated that pseudo-phosphorylation of both Ser23 and Ser24 is needed to enhance the length-dependent increase in Ca(2+) sensitivity. cTnI pseudo-phosphorylation did not alter length-dependent changes in maximal force. Thus phosphorylation at Thr143 enhances myofilament Ca(2+) sensitivity without affecting LDA, while Ser23/24 bisphosphorylation is needed to enhance the length-dependent increase in myofilament Ca(2+) sensitivity.

PKCα-specific phosphorylation of the troponin complex in human myocardium: a functional and proteomics analysis

Aims

Protein kinase Cα (PKCα) is one of the predominant PKC isoforms that phosphorylate cardiac troponin. PKCα is implicated in heart failure and serves as a potential therapeutic target, however, the exact consequences for contractile function in human myocardium are unclear. This study aimed to investigate the effects of PKCα phosphorylation of cardiac troponin (cTn) on myofilament function in human failing cardiomyocytes and to resolve the potential targets involved.

Methods and Results

Endogenous cTn from permeabilized cardiomyocytes from patients with end-stage idiopathic dilated cardiomyopathy was exchanged (∼69%) with PKCα-treated recombinant human cTn (cTn (DD+PKCα)). This complex has Ser23/24 on cTnI mutated into aspartic acids (D) to rule out in vitro cross-phosphorylation of the PKA sites by PKCα. Isometric force was measured at various [Ca²⁺] after exchange. The maximal force (F_max) in the cTn (DD+PKCα) group (17.1±1.9 kN/m²) was significantly reduced compared to the cTn (DD) group (26.1±1.9 kN/m²). Exchange of endogenous cTn with cTn (DD+PKCα) increased Ca²⁺-sensitivity of force (pCa₅₀ = 5.59±0.02) compared to cTn (DD) (pCa₅₀ = 5.51±0.02). In contrast, subsequent PKCα treatment of the cells exchanged with cTn (DD+PKCα) reduced pCa₅₀ to 5.45±0.02. Two PKCα-phosphorylated residues were identified with mass spectrometry: Ser198 on cTnI and Ser179 on cTnT, although phosphorylation of Ser198 is very low. Using mass spectrometry based-multiple reaction monitoring, the extent of phosphorylation of the cTnI sites was quantified before and after treatment with PKCα and showed the highest phosphorylation increase on Thr143.

Conclusion

PKCα-mediated phosphorylation of the cTn complex decreases F_max and increases myofilament Ca²⁺-sensitivity, while subsequent treatment with PKCα in situ decreased myofilament Ca²⁺-sensitivity. The known PKC sites as well as two sites which have not been previously linked to PKCα are phosphorylated in human cTn complex treated with PKCα with a high degree of specificity for Thr143.

Time-dependent degradation pattern of cardiac troponin T following myocardial infarction

BACKGROUND

Cardiac troponin T (cTnT) is widely used for the diagnosis of acute myocardial infarction (AMI). However, it is still unclear whether degraded cTnT forms circulate in the patient's blood. We therefore aimed to elucidate which cTnT forms are detected by the clinical assay.

METHODS

Separation of cTnT forms by gel filtration chromatography (GFC) was performed in sera from 13 AMI patients to examine cTnT degradation. The GFC eluates were subjected to Western blot analysis with the original antibodies from the Roche immunoassay used to mimic the clinical cTnT assay. To investigate the degradation pattern with time, standardized serum samples of 18 AMI patients collected 0-72 h after admission were analyzed by Western blot analysis.

RESULTS

GFC analysis of AMI patients' sera revealed 2 cTnT peaks with retention volumes of 5 and 21 mL. Western blot analysis identified these peaks as cTnT fragments of 29 and 14-18 kDa, respectively. Furthermore, the performance of direct Western blots on standardized serum samples demonstrated a time-dependent degradation pattern of cTnT, with fragments ranging between 14 and 40 kDa. Intact cTnT (40 kDa) was present in only 3 patients within the first 8 h after hospital admission.

CONCLUSIONS

These results demonstrate that the Roche cTnT immunoassay detects intact as well as degraded cTnT forms in AMI patients' sera during the period of diagnostic testing. Moreover, following AMI, cTnT is degraded in a time-dependent pattern.

Impact of site-specific phosphorylation of protein kinase A sites Ser23 and Ser24 of cardiac troponin I in human cardiomyocytes

PKA-mediated phosphorylation of contractile proteins upon β-adrenergic stimulation plays an important role in the regulation of cardiac performance. Phosphorylation of the PKA sites (Ser(23)/Ser(24)) of cardiac troponin (cTn)I results in a decrease in myofilament Ca(2+) sensitivity and an increase in the rate of relaxation. However, the relation between the level of phosphorylation of the sites and the functional effects in the human myocardium is unknown. Therefore, site-directed mutagenesis was used to study the effects of phosphorylation at Ser(23) and Ser(24) of cTnI on myofilament function in human cardiac tissue. Serines were replaced by aspartic acid (D) or alanine (A) to mimic phosphorylation and dephosphorylation, respectively. cTnI-DD mimics both sites phosphorylated, cTnI-AD mimics Ser(23) unphosphorylated and Ser(24) phosphorylated, cTnI-DA mimics Ser(23) phosphorylated and Ser(24) unphosphorylated, and cTnI-AA mimics both sites unphosphorylated. Force development was measured at various Ca(2+) concentrations in permeabilized cardiomyocytes in which the endogenous troponin complex was exchanged with these recombinant human troponin complexes. In donor cardiomyocytes, myofilament Ca(2+) sensitivity (pCa(50)) was significantly lower in cTnI-DD (pCa(50): 5.39 ± 0.01) compared with cTnI-AA (pCa(50): 5.50 ± 0.01), cTnI-AD (pCa(50): 5.48 ± 0.01), and cTnI-DA (pCa(50): 5.51 ± 0.01) at ~70% cTn exchange. No effects were observed on the rate of tension redevelopment. In cardiomyocytes from idiopathic dilated cardiomyopathic tissue, a linear decline in pCa(50) with cTnI-DD content was observed, saturating at ~55% bisphosphorylation. Our data suggest that in the human myocardium, phosphorylation of both PKA sites on cTnI is required to reduce myofilament Ca(2+) sensitivity, which is maximal at ~55% bisphosphorylated cTnI. The implications for in vivo cardiac function in health and disease are detailed in the DISCUSSION in this article.

Cardiac myofilaments: from proteome to pathophysiology

This review addresses the functional consequences of altered post-translational modifications of cardiac myofilament proteins in cardiac diseases such as heart failure and ischemia. The modifications of thick and thin filament proteins as well as titin are addressed. Understanding the functional consequences of altered protein modifications is an essential step in the development of targeted therapies for common cardiac diseases.

Nitroxyl-mediated disulfide bond formation between cardiac myofilament cysteines enhances contractile function

RATIONALE

In the myocardium, redox/cysteine modification of proteins regulating Ca(2+) 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 (F(max)) and reducing the concentration of Ca(2+) for 50% activation (Ca(50)) 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 F(max) 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 Ca(2+) sensitivity and force generation, respectively.

CONCLUSIONS

HNO exerts a direct effect on cardiac myofilament proteins increasing myofilament Ca(2+) 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.

Regulation and physiological roles of the calpain system in muscular disorders

Calpains, a family of Ca(2+)-dependent cytosolic cysteine proteases, can modulate their substrates' structure and function through limited proteolytic activity. In the human genome, there are 15 calpain genes. The most-studied calpains, referred to as conventional calpains, are ubiquitous. While genetic studies in mice have improved our understanding about the conventional calpains' physiological functions, especially those essential for mammalian life as in embryogenesis, many reports have pointed to overactivated conventional calpains as an exacerbating factor in pathophysiological conditions such as cardiovascular diseases and muscular dystrophies. For treatment of these diseases, calpain inhibitors have always been considered as drug targets. Recent studies have introduced another aspect of calpains that calpain activity is required to protect the heart and skeletal muscle against stress. This review summarizes the functions and regulation of calpains, focusing on the relevance of calpains to cardiovascular disease.

Engineered troponin C constructs correct disease-related cardiac myofilament calcium sensitivity

Aberrant myofilament Ca(2+) sensitivity is commonly observed with multiple cardiac diseases, especially familial cardiomyopathies. Although the etiology of the cardiomyopathies remains unclear, improving cardiac muscle Ca(2+) sensitivity through either pharmacological or genetic approaches shows promise of alleviating the disease-related symptoms. Due to its central role as the Ca(2+) sensor for cardiac muscle contraction, troponin C (TnC) stands out as an obvious and versatile target to reset disease-associated myofilament Ca(2+) sensitivity back to normal. To test the hypothesis that aberrant myofilament Ca(2+) sensitivity and its related function can be corrected through rationally engineered TnC constructs, three thin filament protein modifications representing different proteins (troponin I or troponin T), modifications (missense mutation, deletion, or truncation), and disease subtypes (familial or acquired) were studied. A fluorescent TnC was utilized to measure Ca(2+) binding to TnC in the physiologically relevant biochemical model system of reconstituted thin filaments. Consistent with the pathophysiology, the restrictive cardiomyopathy mutation, troponin I R192H, and ischemia-induced truncation of troponin I (residues 1-192) increased the Ca(2+) sensitivity of TnC on the thin filament, whereas the dilated cardiomyopathy mutation, troponin T ΔK210, decreased the Ca(2+) sensitivity of TnC on the thin filament. Rationally engineered TnC constructs corrected the abnormal Ca(2+) sensitivities of the thin filament, reconstituted actomyosin ATPase activity, and force generation in skinned trabeculae. Thus, the present study provides a novel and versatile therapeutic strategy to restore diseased cardiac muscle Ca(2+) sensitivity.

Effects of increased preload on the force-frequency response and contractile kinetics in early stages of cardiac muscle hypertrophy

Numerous studies have aimed to elucidate markers for the onset of decompensatory hypertrophy and heart failure in vivo and in vitro. Alterations in the force-frequency relationship are commonly used as markers for heart failure with a negative staircase being a hallmark of decompensated cardiac function. Here we aim to determine the functional and molecular alterations in the very early stages of compensatory hypertrophy through analysis of the force-frequency relationship, using a novel isolated muscle culture system that allows assessment of force-frequency relationship during the development of hypertrophy. New Zealand white male rabbit trabeculae excised from the right ventricular free wall were utilized for all experiments. Briefly, muscles held at constant preload and contracting isometrically were stimulated to contract in culture for 24 h, and in a subset up to 48 h. We found that, upon an increase in the preload and maintaining the muscles in culture for up to 24 h, there was an increase in baseline force produced by isolated trabeculae over time. This suggests a gradual compensatory response to the impact of increased preload. Temporal analysis of the force-frequency response during this progression revealed a significant blunting (at 12 h) and then reversal of the positive staircase as culture time increased (at 24 h). Phosphorylation analysis revealed a significant decrease in desmin and troponin (Tn)I phosphorylation from 12 to 24 h in culture. These results show that even very early on in the compensatory hypertrophy state, the force-frequency relationship is already affected. This effect on force-frequency relationship may, in addition to protein expression changes, be partially attributed to the alterations in myofilament protein phosphorylation.

Early activation of myocardial matrix metalloproteinases and degradation of cardiac troponin I after experimental subarachnoid hemorrhage

BACKGROUND

Matrix metalloproteinases (MMPs) are involved in acute myocardial dysfunction by degrading several intracellular contractile proteins, including cardiac troponin I (cTnI). Here, we examined the temporal profiles of MMPs and cTnI in plasma and myocardial tissue in the acute stage of subarachnoid hemorrhage (SAH).

MATERIALS AND METHODS

SAH was induced by the endovascular suture method in rats. Intracranial pressure and left ventricular (LV) function were recorded. Plasma cTnI and MMPs were measured at 0, 5, 15, 30, 60, 120, and 180 minutes after SAH. Myocardial cTnI and MMP activities were quantified at 30, 60 and 180 min after SAH from homogenized hearts.

RESULTS

SAH-induced rats showed a marked decline in -LV dP/dt(max) (index of LV diastolic function). Plasma samples revealed a noticeable increase in cTnI and pro-MMP-9 activities over the course of 180 minutes. In myocardial tissue, there was a marked increase in pro-MMP-9, pro-MMP-2 activities and expression of activated MMP-2. Western blot analysis revealed a striking decrease in cTnI content and increase in cTnI degradation in myocardium. Simultaneous cTnI depletion and MMP-2 expression in myocardium was detected by immunohistochemistry as early as 30 minutes after SAH. MMPs correlated with -LV dP/dt(max) (% of baseline) both in plasma and in myocardial tissue. Furthermore, activated MMP-2 activity correlated positively with cTnI degradation in myocardium.

CONCLUSIONS

Early activation of MMPs was observed in myocardium and plasma following SAH. Activated MMP-2 may regulate proteolytic cTnI and contribute to myocardium stunning injury in SAH rats.

Structural dynamics of C-domain of cardiac troponin I protein in reconstituted thin filament

The regulatory function of cardiac troponin I (cTnI) involves three important contiguous regions within its C-domain: the inhibitory region (IR), the regulatory region (RR), and the mobile domain (MD). Within these regions, the dynamics of regional structure and kinetics of transitions in dynamic state are believed to facilitate regulatory signaling. This study was designed to use fluorescence anisotropy techniques to acquire steady-state and kinetic information on the dynamic state of the C-domain of cTnI in the reconstituted thin filament. A series of single cysteine cTnI mutants was generated, labeled with the fluorophore tetramethylrhodamine, and subjected to various anisotropy experiments at the thin filament level. The structure of the IR was found to be less dynamic than that of the RR and the MD, and Ca(2+) binding induced minimal changes in IR dynamics: the flexibility of the RR decreased, whereas the MD became more flexible. Anisotropy stopped-flow experiments showed that the kinetics describing the transition of the MD and RR from the Ca(2+)-bound to the Ca(2+)-free dynamic states were significantly faster (53.2-116.8 s(-1)) than that of the IR (14.1 s(-1)). Our results support the fly casting mechanism, implying that an unstructured MD with rapid dynamics and kinetics plays a critical role to initiate relaxation upon Ca(2+) dissociation by rapidly interacting with actin to promote the dissociation of the RR from the N-domain of cTnC. In contrast, the IR responds to Ca(2+) signals with slow structural dynamics and transition kinetics. The collective findings suggested a fourth state of activation.

In vitro studies of early cardiac remodeling: impact on contraction and calcium handling

Cardiac remodeling, hypertrophy, and alterations in calcium signaling are changes of the heart that often lead to failure. After a hypertrophic stimulus, the heart progresses through a state of compensated hypertrophy which over time leads to decompensated hypertrophy or failure. It is at this point that a cardiac transplant is required for survival making early detection imperative. Current experimental systems used to study the remodeling of the heart include in vivo systems (the whole body), isolated organ and sub-organ tissue, and the individual cardiac muscle cells and organelles.. During pathological remodeling there is a derangement in the intracellular calcium handling processes. These derangements are thought to lead to a dysregulation of contractile output. Hence, understanding the mechanism between remodeling and dysregulation is of great interest in the cardiac field and will ultimately help in the development of future treatment and early detection. This review will center on changes in contraction and calcium handling in early cardiac remodeling, with a specific focus on findings in two different in vitro model systems: multicellular and individual cell preparations.

Protein kinase A changes calcium sensitivity but not crossbridge kinetics in human cardiac myofibrils

We investigated the effect of PKA treatment (1 U/ml) on the mechanical properties of isolated human cardiac myofibrils. PKA treatment was associated with significant incorporation of radiolabeled phosphate into several sarcomeric proteins including troponin I and myosin binding protein C and was also associated with a right shift in the tension-pCa relation (ΔpCa(50) = 0.2 ± 0.1). PKA treatment also caused right shifts in the pCa dependence of the rate of tension development, tension redevelopment, and the linear and exponential phases of myofibril relaxation. However, there was no change in the same measures of crossbridge turnover when expressed as a function of tension. We conclude that the changes in crossbridge kinetics as a function of calcium concentration reflect a reduced tension due to a lower calcium sensitivity and that the relationship between crossbridge kinetics and tension was unchanged, indicating no direct effect of PKA treatment on crossbridge cycling.

Mitochondrial dysfunction and oxidative damage to sarcomeric proteins

Hypertension is an important risk factor for the development of heart failure. Increased production of reactive oxygen species (ROS) contributes to cardiac dysfunction by activating numerous pro-hypertrophic signaling cascades and damaging the mitochondria, thus setting off a vicious cycle of ROS generation. The way in which oxidative stress leads to exacerbation of systolic and diastolic dysfunction is still unclear, however. In skeletal muscle and ischemic myocardium, increased ROS production causes preferential oxidation of myofibrillar proteins and provides a mechanistic link between oxidative damage and impaired contractility through disruption of actin-myosin interactions, enzymatic functions, calcium sensitivity, and efficiency of cross-bridge cycling. In this review, we summarize recent findings in the fields of heart failure and sarcomere biology and speculate that oxidative damage to myofibrils may contribute to the development of heart failure.

Diastolic myofilament dysfunction in the failing human heart

In recent years, it has become evident that heart failure is not solely due to reduced contractile performance of the heart muscle as impaired relaxation is evident in almost all heart failure patients. In more than half of all heart failure patients, diastolic dysfunction is the major cardiac deficit. These heart failure patients have normal (or preserved) left ventricular ejection fraction, but impaired diastolic function evident from increased left ventricular end-diastolic pressure. Perturbations at the cellular level which cause impaired relaxation of the heart muscle involve changes in Ca(2+)-handling proteins, extracellular matrix components, and myofilament properties. The present review discusses the deficits in myofilament function observed in human heart failure and the most likely underlying causal protein changes. Moreover, the consequences of impaired myofilament function for in vivo diastolic dysfunction are discussed taking into account the reported changes in Ca(2+) handling.

Development of a Qualitative Sequential Immunoassay for Characterizing the Intrinsic Properties of Circulating Cardiac Troponin I

BACKGROUND

With myocardial infarction (MI), cardiac troponin is released from the heart into circulation, where it can be detected with immunoassays independently quantifying cardiac troponin I (cTnI) or cTnT. There is, however, no single immunoassay that sequentially probes the posttranslational modification status of cTnI or directly characterizes whether circulating cTnI is bound to cTnC and/or cTnT. Here we describe the development of a qualitative immunoassay to directly probe the primary and ternary structure of circulating cTnI through diffractive optics technology (dotLab® System, Axela).

METHODS

Anti-cTnI antibody 8I-7 was immobilized on a patterned sensor to capture cTnI. One or more detector antibodies were sequentially introduced to probe for amino acid sequence integrity or phosphorylation status of cTnI, or its association with cTnC and/or cTnT. Respective immunocaptures were recorded as real-time diffractive intensities (DIs), and the DI differences were analyzed. Each immunodetection was independent of the others but was done in a single sequential assay.

RESULTS

This diffraction-based immunoassay successfully characterized cTnI. The unamplified assay determined whether cTnI was degraded at N-terminus and/or C-terminus or phosphorylated. Sequential application of multiple detector antibodies without an antibody-stripping step enables real-time interrogation of 5 different epitopes of cTnI, or direct detection of the cTn complex (cTnI–cTnC–cTnT) in a single sequential assay. Finally, this assay was optimized with amplification to directly detect circulating cTnI bound to cTnC and cTnT in serum from an MI patient.

CONCLUSIONS

The dot® Immunoassay is the first qualitative sequential immunoassay to address the direct interactions of the troponin subunits and various modified forms of cTnI.

Overexpression of beta(2)AR improves contractile function and cellular survival in rabbit cardiomyocytes under chronic hypoxia

Chronic hypoxia usually evokes sustained release of endogenous neurohormones, leading to beta(2)-adrenergic receptor (beta(2)AR) desensitization and downregulation of expression, which impacts cellular contractility. We investigated whether exogenous beta(2)AR could compensate for the functional deficiency of beta(2)AR in rabbit cardiomyocytes under chronic hypoxia, and whether this led to improved contractility and cellular survival. A surgical experimental model of cyanotic heart disease was established in rabbits. Adv.hbeta(2)AR was transfected into cardiomyocytes isolated from animals subjected to 6-week systemic hypoxia. The levels of cellular contractile function, protein expression of hbeta(2)AR, p-Akt, p-Erk, and caspase-3, and cellular survival pre- and post-Adv.hbeta(2)AR delivery were determined. In the cyanotic cells, decreased shortening and lengthening of TPC and R50 were evident. Cellular diastolic functioning showed greater deterioration compared to the systolic function (P<0.05). In cyanotic cells, the positive inotropic response to isoproterenol was decreased (P<0.01), low levels of cellular survival were found, protein levels of beta(2)AR, p-Akt, and p-Erk were downregulated, and protein levels of caspase-3 were upregulated. After Adv.hbeta(2)AR delivery, enhanced contractile function was achieved (P<0.01), TPC and R50 levels recovered up to 99% and 81.7% of the normal control levels, respectively (P<0.05), and cellular survival improved (P<0.01). Our results demonstrate that overexpression of the beta(2)AR gene in cardiomyocytes exposed to chronic hypoxia provides significant catecholamine-dependent inotropic support and cellular protection.

Effects of chronic atrial fibrillation on active and passive force generation in human atrial myofibrils

RATIONALE

Chronic atrial fibrillation (cAF) is associated with atrial contractile dysfunction. Sarcomere remodeling may contribute to this contractile disorder.

OBJECTIVE

Here, we use single atrial myofibrils and fast solution switching techniques to directly investigate the impact of cAF on myofilament mechanical function eliminating changes induced by the arrhythmia in atrial myocytes membranes and extracellular components. Remodeling of sarcomere proteins potentially related to the observed mechanical changes is also investigated.

METHODS AND RESULTS

Myofibrils were isolated from atrial samples of 15 patients in sinus rhythm and 16 patients with cAF. Active tension changes following fast increase and decrease in [Ca(2+)] and the sarcomere length-passive tension relation were determined in the 2 groups of myofibrils. Compared to sinus rhythm myofibrils, cAF myofibrils showed (1) a reduction in maximum tension and in the rates of tension activation and relaxation; (2) an increase in myofilament Ca(2+) sensitivity; (3) a reduction in myofibril passive tension. The slow beta-myosin heavy chain isoform and the more compliant titin isoform N2BA were up regulated in cAF myofibrils. Phosphorylation of multiple myofilament proteins was increased in cAF as compared to sinus rhythm atrial myocardium.

CONCLUSIONS

Alterations in active and passive tension generation at the sarcomere level, explained by translational and post-translational changes of multiple myofilament proteins, are part of the contractile dysfunction of human cAF and may contribute to the self-perpetuation of the arrhythmia and the development of atrial dilatation.

Why does troponin I have so many phosphorylation sites? Fact and fancy

We discuss a current controversy regarding the relative role of phosphorylation sites on cardiac troponin I (cTnI) (Fig. 1) in physiological and patho-physiological cardiac function. Studies with mouse models and in vitro studies indicate that multi-site phosphorylations are involved in both control of maximum tension and sarcomeric responsiveness to Ca(2+). Thus one hypothesis is that cardiac function reflects a balance of cTnI phosphorylations and a tilt in this balance may be maladaptive in acquired and genetic disorders of the heart. Studies on human heart samples taken mainly at end-stage heart failure, and in depth proteomic analysis of human and rat heart samples demonstrate that Ser23/Ser24 are the major and perhaps the only sites likely to be relevant to control cardiac function. Thus functional significance of Ser23/Ser24 phosphorylation is taken as fact, whereas the function of some other sites is treated as fancy. Maybe the extremes will meet: in any case we both agree that further work needs to be carried out with relatively large mammals and with determination of the time course of changes in phosphorylation to identify transient modifications that may be relevant at a beat-to-beat basis. Moreover, we agree that the changes and effects of cTnI phosphorylation need to be fully integrated into the effects of other phosphorylations in the cardiac myocyte.

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.

Extraction and replacement of the tropomyosin-troponin complex in isolated myofibrils

Tropomyosin (Tm) is an essential component in the regulation of striated muscle contraction. Questions about Tm functional role have been difficult to study because sarcomere Tm content is not as easily manipulated as Troponin (Tn). Here we describe the method we recently developed to replace Tm-Tn of skeletal and cardiac myofibrils from animals and humans to generate an experimental model of homogeneous Tm composition and giving the possibility to measure a wide range of mechanical parameters of contraction (e.g. maximal force and kinetics of force generation). The success of the exchange was determined by SDS-PAGE and by mechanical measurements of calcium dependent force activation on the reconstituted myofibrils. In skeletal and cardiac myofibrils, the percentage of Tm replacement was higher than 90%. Maximal isometric tension was 30-35% lower in the reconstituted myofibrils than in control myofibrils but the rate of force activation (k(ACT)) and that of force redevelopment (k(TR)) were not significantly changed. Preliminary results show the effectiveness of Tm replacement in human cardiac myofibrils. This approach can be used to test the functional impact of Tm mutations responsible for human cardiomyopathies.

The C terminus of cardiac troponin I stabilizes the Ca2+-activated state of tropomyosin on actin filaments

RATIONALE

Ca(2+) 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 (cTnI(1-192)) 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 Ca(2+). 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 "Ca(2+)-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.

Synchronous in situ ATPase activity, mechanics, and Ca2+ sensitivity of human and porcine myocardium

Flash-frozen myocardium samples provide a valuable means of correlating clinical cardiomyopathies with abnormalities in sarcomeric contractile and biochemical parameters. We examined flash-frozen left-ventricle human cardiomyocyte bundles from healthy donors to determine control parameters for isometric tension (P(o)) development and Ca(2+) sensitivity, while simultaneously measuring actomyosin ATPase activity in situ by a fluorimetric technique. P(o) was 17 kN m(-2) and pCa(50%) was 5.99 (28 degrees C, I = 130 mM). ATPase activity increased linearly with tension to 132 muM s(-1). To determine the influence of flash-freezing, we compared the same parameters in both glycerinated and flash-frozen porcine left-ventricle trabeculae. P(o) in glycerinated porcine myocardium was 25 kN m(-2), and maximum ATPase activity was 183 microM s(-1). In flash-frozen porcine myocardium, P(o) was 16 kN m(-2) and maximum ATPase activity was 207 microM s(-1). pCa(50%) was 5.77 in the glycerinated and 5.83 in the flash-frozen sample. Both passive and active stiffness of flash-frozen porcine myocardium were lower than for glycerinated tissue and similar to the human samples. Although lower stiffness and isometric tension development may indicate flash-freezing impairment of axial force transmission, we cannot exclude variability between samples as the cause. ATPase activity and pCa(50%) were unaffected by flash-freezing. The lower ATPase activity measured in human tissue suggests a slower actomyosin turnover by the contractile proteins.

Protein kinase C alpha and epsilon phosphorylation of troponin and myosin binding protein C reduce Ca2+ sensitivity in human myocardium

Previous studies indicated that the increase in protein kinase C (PKC)-mediated myofilament protein phosphorylation observed in failing myocardium might be detrimental for contractile function. This study was designed to reveal and compare the effects of PKCα- and PKCε-mediated phosphorylation on myofilament function in human myocardium. Isometric force was measured at different [Ca²⁺] in single permeabilized cardiomyocytes from failing human left ventricular tissue. Activated PKCα and PKCε equally reduced Ca²⁺ sensitivity in failing cardiomyocytes (ΔpCa₅₀ = 0.08 ± 0.01). Both PKC isoforms increased phosphorylation of troponin I- (cTnI) and myosin binding protein C (cMyBP-C) in failing cardiomyocytes. Subsequent incubation of failing cardiomyocytes with the catalytic subunit of protein kinase A (PKA) resulted in a further reduction in Ca²⁺ sensitivity, indicating that the effects of both PKC isoforms were not caused by cross-phosphorylation of PKA sites. Both isozymes showed no effects on maximal force and only PKCα resulted in a modest significant reduction in passive force. Effects of PKCα were only minor in donor cardiomyocytes, presumably because of already saturated cTnI and cMyBP-C phosphorylation levels. Donor tissue could therefore be used as a tool to reveal the functional effects of troponin T (cTnT) phosphorylation by PKCα. Massive dephosphorylation of cTnT with alkaline phosphatase increased Ca²⁺ sensitivity. Subsequently, PKCα treatment of donor cardiomyocytes reduced Ca²⁺ sensitivity (ΔpCa₅₀ = 0.08 ± 0.02) and solely increased phosphorylation of cTnT, but did not affect maximal and passive force. PKCα- and PKCε-mediated phosphorylation of cMyBP-C and cTnI as well as cTnT decrease myofilament Ca²⁺ sensitivity and may thereby reduce contractility and enhance relaxation of human myocardium.