Myosin cross-bridge dynamics in patients with hypertension and concentric left ventricular remodeling

Pathophysiological insights into HFpEF from studies of human cardiac tissue

Heart failure with preserved ejection fraction (HFpEF) is a major, worldwide health-care problem. Few therapies for HFpEF exist because the pathophysiology of this condition is poorly defined and, increasingly, postulated to be diverse. Although perturbations in other organs contribute to the clinical profile in HFpEF, altered cardiac structure, function or both are the primary causes of this heart failure syndrome. Therefore, studying myocardial tissue is fundamental to improve pathophysiological insights and therapeutic discovery in HFpEF. Most studies of myocardial changes in HFpEF have relied on cardiac tissue from animal models without (or with limited) confirmatory studies in human cardiac tissue. Animal models of HFpEF have evolved based on theoretical HFpEF aetiologies, but these models might not reflect the complex pathophysiology of human HFpEF. The focus of this Review is the pathophysiological insights gained from studies of human HFpEF myocardium. We outline the rationale for these studies, the challenges and opportunities in obtaining myocardial tissue from patients with HFpEF and relevant comparator groups, the analytical approaches, the pathophysiological insights gained to date and the remaining knowledge gaps. Our objective is to provide a roadmap for future studies of cardiac tissue from diverse cohorts of patients with HFpEF, coupling discovery biology with measures to account for pathophysiological diversity.

Mavacamten facilitates myosin head ON-to-OFF transitions and shortens thin filament length in relaxed skeletal muscle

The first-in-its-class cardiac drug mavacamten reduces the proportion of so-called ON-state myosin heads in relaxed sarcomeres, altering contraction performance. However, mavacamten is not completely specific to cardiac myosin and can also affect skeletal muscle myosin, an important consideration since mavacamten is administered orally and so will also be present in skeletal tissue. Here, we studied the effect of mavacamten on skeletal muscle structure using small-angle X-ray diffraction. Mavacamten treatment reduced the proportion of ON myosin heads but did not eliminate the molecular underpinnings of length-dependent activation, demonstrating similar effects to those observed in cardiac muscle. These findings provide valuable insights for the potential use of mavacamten as a tool to study muscle contraction across striated muscle.

Computational investigation of the role of ventricular remodelling in HFpEF: The key to phenotype dissection

Recent clinical studies have reported that heart failure with preserved ejection fraction (HFpEF) can be divided into two phenotypes based on the range of ejection fraction (EF), namely HFpEF with higher EF and HFpEF with lower EF. These phenotypes exhibit distinct left ventricle (LV) remodelling patterns and dynamics. However, the influence of LV remodelling on various LV functional indices and the underlying mechanics for these two phenotypes are not well understood. To address these issues, this study employs a coupled finite element analysis (FEA) framework to analyse the impact of various ventricular remodelling patterns, specifically concentric remodelling (CR), concentric hypertrophy (CH), and eccentric hypertrophy (EH), with and without LV wall thickening on LV functional indices. Further, the geometries with a moderate level of remodelling from each pattern are subjected to fibre stiffening and contractile impairment to examine their effect in replicating the different features of HFpEF. The results show that with severe CR, LV could exhibit the characteristics of HFpEF with higher EF, as observed in recent clinical studies. Controlled fibre stiffening can simultaneously increase the end-diastolic pressure (EDP) and reduce the peak longitudinal strain (ell) without significant reduction in EF, facilitating the moderate CR geometries to fit into this phenotype. Similarly, fibre stiffening can assist the CH and 'EH with wall thickening' cases to replicate HFpEF with lower EF. These findings suggest that potential treatment for these two phenotypes should target the bio-origins of their distinct ventricular remodelling patterns and the extent of myocardial stiffening.

Fast myosin binding protein C knockout in skeletal muscle alters length-dependent activation and myofilament structure

In striated muscle, the sarcomeric protein myosin-binding protein-C (MyBP-C) is bound to the myosin thick filament and is predicted to stabilize myosin heads in a docked position against the thick filament, which limits crossbridge formation. Here, we use the homozygous Mybpc2 knockout (C2-/-) mouse line to remove the fast-isoform MyBP-C from fast skeletal muscle and then conduct mechanical functional studies in parallel with small-angle X-ray diffraction to evaluate the myofilament structure. We report that C2-/- fibers present deficits in force production and calcium sensitivity. Structurally, passive C2-/- fibers present altered sarcomere length-independent and -dependent regulation of myosin head conformations, with a shift of myosin heads towards actin. At shorter sarcomere lengths, the thin filament is axially extended in C2-/-, which we hypothesize is due to increased numbers of low-level crossbridges. These findings provide testable mechanisms to explain the etiology of debilitating diseases associated with MyBP-C.

Myosin-binding protein C regulates the sarcomere lattice and stabilizes the OFF states of myosin heads

Muscle contraction is produced via the interaction of myofilaments and is regulated so that muscle performance matches demand. Myosin-binding protein C (MyBP-C) is a long and flexible protein that is tightly bound to the thick filament at its C-terminal end (MyBP-CC8C10), but may be loosely bound at its middle- and N-terminal end (MyBP-CC1C7) to myosin heads and/or the thin filament. MyBP-C is thought to control muscle contraction via the regulation of myosin motors, as mutations lead to debilitating disease. We use a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-CC1C7 domains of fast MyBP-C in permeabilized skeletal muscle. We show that cleavage leads to alterations in crossbridge kinetics and passive structural signatures of myofilaments that are indicative of a shift of myosin heads towards the ON state, highlighting the importance of MyBP-CC1C7 to myofilament force production and regulation.

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

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

Increased Chamber Resting Tone Is a Key Determinant of Left Ventricular Diastolic Dysfunction

BACKGROUND

Twitch-independent tension has been demonstrated in cardiomyocytes, but its role in heart failure (HF) is unclear. We aimed to address twitch-independent tension as a source of diastolic dysfunction by isolating the effects of chamber resting tone (RT) from impaired relaxation and stiffness.

METHODS

We invasively monitored pressure-volume data during cardiopulmonary exercise in 20 patients with hypertrophic cardiomyopathy, 17 control subjects, and 35 patients with HF with preserved ejection fraction. To measure RT, we developed a new method to fit continuous pressure-volume measurements, and first validated it in a computational model of loss of cMyBP-C (myosin binding protein-C).

RESULTS

In hypertrophic cardiomyopathy, RT (estimated marginal mean [95% CI]) was 3.4 (0.4-6.4) mm Hg, increasing to 18.5 (15.5-21.5) mm Hg with exercise (P<0.001). At peak exercise, RT was responsible for 64% (53%-76%) of end-diastolic pressure, whereas incomplete relaxation and stiffness accounted for the rest. RT correlated with the levels of NT-proBNP (N-terminal pro-B-type natriuretic peptide; R=0.57; P=0.02) and with pulmonary wedge pressure but following different slopes at rest and during exercise (R2=0.49; P<0.001). In controls, RT was 0.0 mm Hg and 1.2 (0.3-2.8) mm Hg in HF with preserved ejection fraction patients and was also exacerbated by exercise. In silico, RT increased in parallel to the loss of cMyBP-C function and correlated with twitch-independent myofilament tension (R=0.997).

CONCLUSIONS

Augmented RT is the major cause of LV diastolic chamber dysfunction in hypertrophic cardiomyopathy and HF with preserved ejection fraction. RT transients determine diastolic pressures, pulmonary pressures, and functional capacity to a greater extent than relaxation and stiffness abnormalities. These findings support antimyosin agents for treating HF.

Titin-based force regulates cardiac myofilament structures mediating length-dependent activation

The Frank-Starling law states that the heart's stroke volume increases with greater preload due to increased venous return, allowing the heart to adapt to varying circulatory demands. Molecularly, increasing preload increases sarcomere length (SL), which alters sarcomere structures that are correlated to increased calcium sensitivity upon activation. The titin protein, spanning the half-sarcomere, acts as a spring in the I-band, applying a SL-dependent force suggested to pull against and alter myofilaments in a way that supports the Frank-Starling effect. To evaluate this, we employed the titin cleavage (TC) model, where a tobacco-etch virus protease recognition site is inserted into distal I-band titin and allows for rapid, specific cleavage of titin in an otherwise-healthy sarcomere. Here, we evaluated the atomic-level structures of amyopathic cardiac myofilaments following 50% titin cleavage under passive stretch conditions using small-angle X-ray diffraction, which measures these structures under near-physiological (functional) conditions. We report that titin-based forces in permeabilized papillary muscle regulate both thick and thin myofilament structures clearly supporting titin's role in the Frank-Starling mechanism.

Fast myosin binding protein C knockout in skeletal muscle alters length-dependent activation and myofilament structure

In striated muscle, some sarcomere proteins regulate crossbridge cycling by varying the propensity of myosin heads to interact with actin. Myosin-binding protein C (MyBP-C) is bound to the myosin thick filament and is predicted to interact and stabilize myosin heads in a docked position against the thick filament and limit crossbridge formation, the so-called OFF state. Via an unknown mechanism, MyBP-C is thought to release heads into the so-called ON state, where they are more likely to form crossbridges. To study this proposed mechanism, we used the C2-/- mouse line to knock down fast-isoform MyBP-C completely and total MyBP-C by ~24%, and conducted mechanical functional studies in parallel with small-angle X-ray diffraction to evaluate the myofilament structure. We report that C2-/- fibers presented deficits in force production and reduced calcium sensitivity. Structurally, passive C2-/- fibers presented altered SL-independent and SL-dependent regulation of myosin head ON/OFF states, with a shift of myosin heads towards the ON state. Unexpectedly, at shorter sarcomere lengths, the thin filament was axially extended in C2-/- vs. non-transgenic controls, which we postulate is due to increased low-level crossbridge formation arising from relatively more ON myosins in the passive muscle that elongates the thin filament. The downstream effect of increasing crossbridge formation in a passive muscle on contraction performance is not known. Such widespread structural changes to sarcomere proteins provide testable mechanisms to explain the etiology of debilitating MyBP-C-associated diseases.

Sarcomere length affects Ca2+ sensitivity of contraction in ischemic but not non-ischemic myocardium

In healthy hearts, myofilaments become more sensitive to Ca2+ as the myocardium is stretched. This effect is known as length-dependent activation and is an important cellular-level component of the Frank-Starling mechanism. Few studies have measured length-dependent activation in the myocardium from failing human hearts. We investigated whether ischemic and non-ischemic heart failure results in different length-dependent activation responses at physiological temperature (37°C). Myocardial strips from the left ventricular free wall were chemically permeabilized and Ca2+-activated at sarcomere lengths (SLs) of 1.9 and 2.3 µm. Data were acquired from 12 hearts that were explanted from patients receiving cardiac transplants; 6 had ischemic heart failure and 6 had non-ischemic heart failure. Another 6 hearts were obtained from organ donors. Maximal Ca2+-activated force increased at longer SL for all groups. Ca2+ sensitivity increased with SL in samples from donors (P < 0.001) and patients with ischemic heart failure (P = 0.003) but did not change with SL in samples from patients with non-ischemic heart failure. Compared with donors, troponin I phosphorylation decreased in ischemic samples and even more so in non-ischemic samples; cardiac myosin binding protein-C (cMyBP-C) phosphorylation also decreased with heart failure. These findings support the idea that troponin I and cMyBP-C phosphorylation promote length-dependent activation and show that length-dependent activation of contraction is blunted, yet extant, in the myocardium from patients with ischemic heart failure and further reduced in the myocardium from patients with non-ischemic heart failure. Patients who have a non-ischemic disease may exhibit a diminished contractile response to increased ventricular filling.

Inertial artifact in viscoelastic measurements of striated muscle: Modeling and experimental results

Viscoelastic properties of striated muscle are often measured using length perturbation analysis and quantified as a complex modulus, whose elastic and viscous components reflect the energy-storage and energy-absorbing properties of the tissue, respectively. The energy stored as inertia is commonly ignored due to the small size of samples examined, typically <1 mm. Considering recent advances in tissue engineering to generate muscle tissues of larger sizes, we questioned whether ignoring the inertial artifact was still reasonable in these samples. To answer this question, we derived and solved the one-dimensional wave equation that describes the propagation of strain along the length of a sample. The inertial artifact was predicted to contaminate the elastic modulus with (2πf)2L02ρ/6, where f is perturbation frequency, L0 is muscle length, and ρ is muscle density. We then measured viscoelastic properties up to 500 Hz in mouse skeletal muscle fibers at long (4.8 mm) and short (<1 mm) lengths and up to 100 Hz in rat cardiac slices at long (10-12 mm) and short (<2 mm) lengths. We found the elastic modulus of long preparations was elevated as frequency increased and was about half the magnitude of that predicted by the model. While the prediction tended to overestimate the measured inertial artifact, these results provided some validity to the model. We used the predicted artifact as an overly conservative estimate of error that might arise in a mechanics assay of mammalian striated muscle, whose nominal resting stiffness is on the order 100 kN m-2. We found that muscle lengths of <1 mm resulted in negligible inertial artifact (<0.5% error) for perturbation frequencies under 250 Hz. Muscle samples longer than 5 mm, on the other hand, would result in >5% error at frequencies of 200 Hz and higher.

Enhancing diastolic function by strain-dependent detachment of cardiac myosin crossbridges

The force response of cardiac muscle undergoing a quick stretch is conventionally interpreted to represent stretching of attached myosin crossbridges (phase 1) and detachment of these stretched crossbridges at an exponential rate (phase 2), followed by crossbridges reattaching in increased numbers due to an enhanced activation of the thin filament (phases 3 and 4). We propose that, at least in mammalian cardiac muscle, phase 2 instead represents an enhanced detachment rate of myosin crossbridges due to stretch, phase 3 represents the reattachment of those same crossbridges, and phase 4 is a passive-like viscoelastic response with power-law relaxation. To test this idea, we developed a two-state model of crossbridge attachment and detachment. Unitary force was assigned when a crossbridge was attached, and an elastic force was generated when an attached crossbridge was displaced. Attachment rate, f(x), was spatially distributed with a total magnitude f₀. Detachment rate was modeled as g(x) = g₀+ g₁x, where g₀ is a constant and g₁ indicates sensitivity to displacement. The analytical solution suggested that the exponential decay rate of phase 2 represents (f₀ + g₀) and the exponential rise rate of phase 3 represents g₀. The depth of the nadir between phases 2 and 3 is proportional to g₁. We prepared skinned mouse myocardium and applied a 1% stretch under varying concentrations of inorganic phosphate (Pi). The resulting force responses fitted the analytical solution well. The interpretations of phases 2 and 3 were consistent with lower f₀ and higher g₀ with increasing Pi. This novel scheme of interpreting the force response to a quick stretch does not require enhanced thin-filament activation and suggests that the myosin detachment rate is sensitive to stretch. Furthermore, the enhanced detachment rate is likely not due to the typical detachment mechanism following MgATP binding, but rather before MgADP release, and may involve reversal of the myosin power stroke.

Computational Modeling Studies of the Roles of Left Ventricular Geometry, Afterload, and Muscle Contractility on Myocardial Strains in Heart Failure with Preserved Ejection Fraction

Global longitudinal strain and circumferential strain are found to be reduced in HFpEF, which some have interpreted that the global left ventricular (LV) contractility is impaired. This finding is, however, contradicted by a preserved ejection fraction (EF) and confounded by changes in LV geometry and afterload resistance that may also affect the global strains. To reconcile these issues, we used a validated computational framework consisting of a finite element LV model to isolate the effects of HFpEF features in affecting systolic function metrics. Simulations were performed to quantify the effects on myocardial strains due to changes in LV geometry, active tension developed by the tissue, and afterload. We found that only a reduction in myocardial contractility and an increase in afterload can simultaneously reproduce the blood pressures, EF and strains measured in HFpEF patients. This finding suggests that it is likely that the myocardial contractility is reduced in HFpEF patients. Graphical abstract.

Suppression of myofilament cross-bridge kinetic in the heart of orchidectomized rats

AIMS

The increased incidence of heart failure with reduced ejection fraction in men compared with women suggests that male sex hormones significantly impact myocardial contractile activation. This study aims to examine associations among molecular alterations, cellular modulations and in vivo cardiac contractile function upon deprivation of testicular hormones.

MAIN METHODS

Myocardial structure and functions were compared among sham-operated control and twelve-week orchidectomized (ORX) male rats with and without testosterone supplementation.

KEY FINDINGS

Echocardiography and pressure-volume relationships demonstrated a decreased left ventricular ejection fraction compared with sham-operated controls. The percentage of contractility reduction was generally similar to the decrease in tension development detected in both right ventricular trabeculae and skinned isolated left ventricular cardiomyocytes of ORX rats. Reductions in tension cost and the rate constant of tension redevelopment (k_tr) in ORX samples suggested a decrease in the rate of cross-bridge formation, reflecting a reduced number of cross-bridges. Slow cross-bridge detachment in ORX rat hearts could result from a shift of myosin heavy chain isoforms towards a slower ATPase activity β-isoform and reductions in the phosphorylation levels of cardiac troponin I and myosin binding protein-C. All the changes in the ORX rat heart, including ejection fractions and myofilament protein expression and phosphorylation, were completed attenuated by a physiological dose of testosterone.

SIGNIFICANCE

Testosterone plays a critical role in regulating the mechanical and contractile dynamics of the heart. Deprivation of male sex hormones cause the loss of normal preserved cardiac contractile function leading to a high risk of severe cardiomyopathy progression.

The prevalent I686T human variant and loss-of-function mutations in the cardiomyocyte-specific kinase gene TNNI3K cause adverse contractility and concentric remodeling in mice

TNNI3K expression worsens disease progression in several mouse heart pathology models. TNNI3K expression also reduces the number of diploid cardiomyocytes, which may be detrimental to adult heart regeneration. However, the gene is evolutionarily conserved, suggesting a beneficial function that has remained obscure. Here, we show that C57BL/6J-inbred Tnni3k mutant mice develop concentric remodeling, characterized by ventricular wall thickening and substantial reduction of cardiomyocyte aspect ratio. This pathology occurs in mice carrying a Tnni3k null allele, a K489R point mutation rendering the protein kinase-dead, or an allele corresponding to human I686T, the most common human non-synonymous TNNI3K variant, which is hypomorphic for kinase activity. Mutant mice develop these conditions in the absence of fibrosis or hypertension, implying a primary cardiomyocyte etiology. In culture, mutant cardiomyocytes were impaired in contractility and calcium dynamics and in protein kinase A signaling in response to isoproterenol, indicating diminished contractile reserve. These results demonstrate a beneficial function of TNNI3K in the adult heart that might explain its evolutionary conservation and imply that human TNNI3K variants, in particular the widespread I686T allele, may convey elevated risk for altered heart geometry and hypertrophy.

Heart Rate-Induced Myocardial Ca2+ Retention and Left Ventricular Volume Loss in Patients With Heart Failure With Preserved Ejection Fraction

Background

Increases in heart rate are thought to result in incomplete left ventricular (LV) relaxation and elevated filling pressures in patients with heart failure with preserved ejection fraction (HFpEF). Experimental studies in isolated human myocardium have suggested that incomplete relaxation is a result of cellular Ca²⁺ overload caused by increased myocardial Na⁺ levels. We tested these heart rate paradigms in patients with HFpEF and referent controls without hypertension.

Methods and Results

In 22 fully sedated and instrumented patients (12 controls and 10 patients with HFpEF) in sinus rhythm with a preserved ejection fraction (≥50%) we assessed left‐sided filling pressures and volumes in sinus rhythm and with atrial pacing (95 beats per minute and 125 beats per minute) before atrial fibrillation ablation. Coronary sinus blood samples and flow measurements were also obtained. Seven women and 15 men were studied (aged 59±10 years, ejection fraction 61%±4%). Patients with HFpEF had a history of hypertension, dyspnea on exertion, concentric LV remodeling and a dilated left atrium, whereas controls did not. Pacing at 125 beats per minute lowered the mean LV end‐diastolic pressure in both groups (controls −4.3±4.1 mm Hg versus patients with HFpEF −8.5±6.0 mm Hg, P=0.08). Pacing also reduced LV end‐diastolic volumes. The volume loss was about twice as much in the HFpEF group (controls −15%±14% versus patients with HFpEF −32%±11%, P=0.009). Coronary venous [Ca²⁺] increased after pacing at 125 beats per minute in patients with HFpEF but not in controls. [Na⁺] did not change.

Conclusions

Higher resting heart rates are associated with lower filling pressures in patients with and without HFpEF. Incomplete relaxation and LV filling at high heart rates lead to a reduction in LV volumes that is more pronounced in patients with HFpEF and may be associated with myocardial Ca²⁺ retention.

Heart failure with preserved ejection fraction: insights into diagnosis and pathophysiology

Heart failure with preserved ejection fraction (HFpEF) accounts for at least half the cases of heart failure, currently diagnosed. There are several cardiac and non-cardiac manifestations of the syndrome. Structure and function abnormalities can include all four cardiac chambers. The left ventricle has abnormal systolic and diastolic functions which can be examined by invasive and non-invasive measurements. In addition, the left atrium enlarges with abnormal left atrial function, pulmonary hypertension occurs, and the right ventricle can develop hypertrophy, enlargement, and systolic dysfunction. There are a paucity of data on calcium handling in HFpEF patients. Growing literature supports the presence of abnormalities in titin and its phosphorylation, and increased interstitial fibrosis contributing to increased chamber stiffness. A systemic inflammatory state causing reduced myocardial cyclic guanosine monophosphate along with defects in the unfolded protein response have been recently reported. Diagnosis relies on signs and symptoms of heart failure, preserved ejection fraction, and detection of diastolic function abnormalities based on echocardiographic findings and abnormally elevated natriuretic peptide levels or invasive measurements of wedge pressure at rest or with exercise. There are currently two diagnostic algorithms: H2FPEF, and HFA-PEFF with limited data comparing their performance head to head in the same patient population. Despite the growing understanding of the syndrome's pathophysiology, there have been little success in developing specific treatment for patients with HFpEF.

Microtubules Increase Diastolic Stiffness in Failing Human Cardiomyocytes and Myocardium

BACKGROUND

Diastolic dysfunction is a prevalent and therapeutically intractable feature of heart failure (HF). Increasing ventricular compliance can improve diastolic performance, but the viscoelastic forces that resist diastolic filling and become elevated in human HF are poorly defined. Having recently identified posttranslationally detyrosinated microtubules as a source of viscoelasticity in cardiomyocytes, we sought to test whether microtubules contribute meaningful viscoelastic resistance to diastolic stretch in human myocardium.

METHODS

Experiments were conducted in isolated human cardiomyocytes and trabeculae. First, slow and rapid (diastolic) stretch was applied to intact cardiomyocytes from nonfailing and HF hearts and viscoelasticity was characterized after interventions targeting microtubules. Next, intact left ventricular trabeculae from HF patient hearts were incubated with colchicine or vehicle and subject to pre- and posttreatment mechanical testing, which consisted of a staircase protocol and rapid stretches from slack length to increasing strains.

RESULTS

Viscoelasticity was increased during diastolic stretch of HF cardiomyocytes compared with nonfailing counterparts. Reducing either microtubule density or detyrosination reduced myocyte stiffness, particularly at diastolic strain rates, indicating reduced viscous forces. In myocardial tissue, we found microtubule depolymerization reduced myocardial viscoelasticity, with an effect that decreased with increasing strain. Colchicine reduced viscoelasticity at strains below, but not above, 15%, with a 2-fold reduction in energy dissipation upon microtubule depolymerization. Post hoc subgroup analysis revealed that myocardium from patients with HF with reduced ejection fraction were more fibrotic and elastic than myocardium from patients with HF with preserved ejection fraction, which were relatively more viscous. Colchicine reduced viscoelasticity in both HF with preserved ejection fraction and HF with reduced ejection fraction myocardium.

CONCLUSIONS

Failing cardiomyocytes exhibit elevated viscosity and reducing microtubule density or detyrosination lowers viscoelastic resistance to diastolic stretch in human myocytes and myocardium. In failing myocardium, microtubules elevate stiffness over the typical working range of strains and strain rates, but exhibited diminishing effects with increasing length, consistent with an increasing contribution of the extracellular matrix or myofilament proteins at larger excursions. These studies indicate that a stabilized microtubule network provides a viscous impediment to diastolic stretch, particularly in HF.

Cardiac Myosin Binding Protein-C Phosphorylation Mitigates Age-Related Cardiac Dysfunction: Hope for Better Aging?

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation prevents aging-related cardiac dysfunction. We tested this hypothesis by aging genetic mouse models of hypophosphorylated cMyBP-C, wild-type equivalent, and phosphorylated-mimetic cMyBP-C for 18 to 20 months. Phosphorylated-mimetic cMyBP-C mice exhibited better survival, better preservation of systolic and diastolic functions, and unchanging wall thickness. Wild-type equivalent mice showed decreasing cMyBP-C phosphorylation along with worsening cardiac function and hypertrophy approaching those found in hypophosphorylated cMyBP-C mice. Intact papillary muscle experiments suggested that cMyBP-C phosphorylation increased cross-bridge detachment rates as the underlying mechanism. Thus, phosphorylating cMyBP-C is a novel mechanism with potential to treat aging-related cardiac dysfunction.

Histone deacetylase activity governs diastolic dysfunction through a nongenomic mechanism

There are no approved drugs for the treatment of heart failure with preserved ejection fraction (HFpEF), which is characterized by left ventricular (LV) diastolic dysfunction. We demonstrate that ITF2357 (givinostat), a clinical-stage inhibitor of histone deacetylase (HDAC) catalytic activity, is efficacious in two distinct murine models of diastolic dysfunction with preserved EF. ITF2357 blocked LV diastolic dysfunction due to hypertension in Dahl salt-sensitive (DSS) rats and suppressed aging-induced diastolic dysfunction in normotensive mice. HDAC inhibitor-mediated efficacy was not due to lowering blood pressure or inhibiting cellular and molecular events commonly associated with diastolic dysfunction, including cardiac fibrosis, cardiac hypertrophy, or changes in cardiac titin and myosin isoform expression. Instead, ex vivo studies revealed impairment of cardiac myofibril relaxation as a previously unrecognized, myocyte-autonomous mechanism for diastolic dysfunction, which can be ameliorated by HDAC inhibition. Translating these findings to humans, cardiac myofibrils from patients with diastolic dysfunction and preserved EF also exhibited compromised relaxation. These data suggest that agents such as HDAC inhibitors, which potentiate cardiac myofibril relaxation, hold promise for the treatment of HFpEF in humans.

Omecamtiv Mecarbil Slows Myosin Kinetics in Skinned Rat Myocardium at Physiological Temperature

Heart failure is a life-threatening condition that occurs when the heart muscle becomes weakened and cannot adequately circulate blood and nutrients around the body. Omecamtiv mecarbil (OM) is a compound that has been developed to treat systolic heart failure via targeting the cardiac myosin heavy chain to increase myocardial contractility. Biophysical and biochemical studies have found that OM increases calcium (Ca²⁺) sensitivity of contraction by prolonging the myosin working stroke and increasing the actin-myosin cross-bridge duty ratio. Most in vitro studies probing the effects of OM on cross-bridge kinetics and muscle force production have been conducted at subphysiological temperature, even though temperature plays a critical role in enzyme activity and cross-bridge function. Herein, we used skinned, ventricular papillary muscle strips from rats to investigate the effects of [OM] on Ca²⁺-activated force production, cross-bridge kinetics, and myocardial viscoelasticity at physiological temperature (37°C). We find that OM only increases myocardial contractility at submaximal Ca²⁺ activation levels and not maximal Ca²⁺ activation levels. As [OM] increased, the kinetic rate constants for cross-bridge recruitment and detachment slowed for both submaximal and maximal Ca²⁺-activated conditions. These findings support a mechanism by which OM increases cardiac contractility at physiological temperature via increasing cross-bridge contributions to thin-filament activation as cross-bridge kinetics slow and the duration of cross-bridge attachment increases. Thus, force only increases at submaximal Ca²⁺ activation due to cooperative recruitment of neighboring cross-bridges, because thin-filament activation is not already saturated. In contrast, OM does not increase myocardial force production for maximal Ca²⁺-activated conditions at physiological temperature because cooperative activation of thin filaments may already be saturated.

Age- and Body Size-Adjusted Left Ventricular End-Diastolic Dimension in a Japanese Hospital-Based Population

BACKGROUND

Using the normal values for the East Asian population, we evaluated age- and body size-adjusted left ventricular end-diastolic dimension (LVEDD) and its prognostic impact in a hospital-based population in Japan.

METHODS AND RESULTS

We retrospectively analyzed data obtained from 4,444 consecutive patients who had undergone both transthoracic echocardiography and electrocardiography at Kitano Hospital in 2013. Those who presented with a history of previous episodes of myocardial infarction and severe or moderate valvular disease or with low ejection fraction (<50%) were excluded from the analysis. We calculated LVEDD adjusted by age and body surface area. A total of 3,474 patients were categorized into 3 groups: 401 with large adjusted LVEDD, 2,829 with normal adjusted LVEDD, and 244 with small adjusted LVEDD. Mean patient age in the large, normal, and small adjusted LVEDD groups was 66.6±18.4, 65.6±15.7, and 62.1±15.5 years, respectively (P<0.001). After adjusting for confounding factors, the excess adjusted 3-year risk of primary outcome of large adjusted LVEDD relative to normal LVEDD was significant (HR, 1.40; 95% CI: 1.08-1.78). The risk for primary outcomes of small adjusted LVEDD relative to normal adjusted LVEDD was significantly lower (HR, 0.55; 95% CI: 0.34-0.85).

CONCLUSIONS

Adjusted large LVEDD has a deleterious impact on long-term mortality, whereas small LVEDD carried a significantly lower risk.

Abundance, localization, and functional correlates of the advanced glycation end-product carboxymethyl lysine in human myocardium

Advanced glycation end‐products (AGEs) play a role in the pathophysiology of diabetes mellitus (DM) and possibly hypertension (HTN). In experimental DM, AGEs accumulate in myocardium. Little is known about AGEs in human myocardium. We quantified abundance, localization, and functional correlates of the AGE carboxymethyl lysine (CML) in left ventricular (LV) myocardium from patients undergoing coronary bypass grafting (CBG). Immunoelectron microscopy was used to quantify CML in epicardial biopsies from 98 patients (71 M, 27 F) with HTN, HTN + DM or neither (controls), all with normal LV ejection fraction. Myofilament contraction‐relaxation function was measured in demembranated myocardial strips. Echocardiography was used to quantify LV structure and function. We found that CML was abundant within cardiomyocytes, but minimally associated with extracellular collagen. CML counts/μm² were 14.7% higher in mitochondria than the rest of the cytoplasm (P < 0.001). There were no significant sex or diagnostic group differences in CML counts [controls 45.6 ± 3.6/μm² (±SEM), HTN 45.8 ± 3.6/μm², HTN + DM 49.3 ± 6.2/μm²; P = 0.85] and no significant correlations between CML counts and age, HgbA1c or myofilament function indexes. However, left atrial volume was significantly correlated with CML counts (r = 0.41, P = 0.004). We conclude that in CBG patients CML is abundant within cardiomyocytes but minimally associated with collagen, suggesting that AGEs do not directly modify the stiffness of myocardial collagen. Coexistent HTN or HTN + DM do not significantly influence CML abundance. The correlation of CML counts with LAV suggests an influence on diastolic function independent of HTN, DM or sex whose mechanism remains to be determined.

Pro-Inflammatory Biomarkers in Stable Versus Acutely Decompensated Heart Failure With Preserved Ejection Fraction

Background

Underlying inflammation has been increasingly recognized in heart failure with a preserved ejection fraction (HFpEF). In this study we tested the hypothesis that pro‐inflammatory biomarkers are elevated in patients with acutely decompensated HFpEF (AD‐HFpEF) compared with patients with stable HFpEF (S‐HFpEF).

Methods and Results

Using a post hoc analysis the serum biomarkers tumor necrosis factor‐alpha, high‐sensitivity C‐reactive protein interleukin 6 and pentraxin 3 (PTX3) and clinical, demographic, echocardiographic‐Doppler and clinical outcomes data were analyzed in HFpEF patients enrolled in NHLBI Heart Failure Research Network clinical trials which enrolled patients with either AD‐HFpEF or S‐HFpEF. Compared to S‐HFpEF, AD‐HFpEF patients had higher levels of PTX3 (3.08 ng/mL versus 1.27 ng/mL, P<0.0001), interleukin‐6 (4.14 pg/mL versus 1.71 pg/mL, P<0.0001), tumor necrosis factor‐alpha (11.54 pg/mL versus 8.62 pg/mL, P=0.0015), and high‐sensitivity C‐reactive protein (11.90 mg/dL versus 3.42 mg/dL, P<0.0001). Moreover, high‐sensitivity C‐reactive protein, interleukin‐6 and PTX3 levels were significantly higher in AD‐HFpEF compared with S‐HFpEF patients admitted for decompensated HF within the previous year. PTX3 was positively correlated with left atrial volume index (r=0.41, P=0.0017) and left ventricular mass (r=0.26, P=0.0415), while tumor necrosis factor‐alpha was inversely correlated with E/A ratio (r=−0.31, P=0.0395).

Conclusions

Levels of pro‐inflammatory biomarkers are strikingly higher in AD‐HFpEF compared with S‐HFpEF patients. PTX3 and tumor necrosis factor‐alpha are correlated with echocardiographic‐Doppler evidence of diastolic dysfunction. Taken together these data support the concept that a heightened pro‐inflammatory state has a pathophysiologic role in the development of AD‐HFpEF.

Relaxation and the Role of Calcium in Isolated Contracting Myocardium From Patients With Hypertensive Heart Disease and Heart Failure With Preserved Ejection Fraction

BACKGROUND

Relaxation characteristics and Ca²⁺ homeostasis have not been studied in isolated myocardium from patients with hypertensive heart disease (HHD) and heart failure with preserved ejection fraction (HFpEF). Prolonged myocardial relaxation is believed to play an important role in the pathophysiology of these conditions. In this study, we evaluated relaxation parameters, myocardial calcium (Ca²⁺), and sodium (Na⁺) handling, as well as ion transporter expression and tested the effect of Na⁺-influx inhibitors on relaxation in isolated myocardium from patients with HHD and HFpEF.

METHODS AND RESULTS

Relaxation characteristics were studied in myocardial strip preparations under physiological conditions at stimulation rates of 60 and 180 per minute. Intracellular Ca²⁺ and Na⁺ were simultaneously assessed using Fura-2 and AsanteNATRIUMGreen-2, whereas elemental analysis was used to measure total myocardial concentrations of Ca, Na, and other elements. Quantitative polymerase chain reaction was used to measure expression levels of key ion transport proteins. The lusitropic effect of Na⁺-influx inhibitors ranolazine, furosemide, and amiloride was evaluated. Myocardial left ventricular biopsies were obtained from 36 control patients, 29 HHD and 19 HHD+HFpEF. When compared with control patients, half maximal relaxation time (RT₅₀) at 60 per minute was prolonged by 13% in HHD and by 18% in HHD+HFpEF (both P<0.05). Elevated resting Ca²⁺ levels and a tachycardia-induced increase in diastolic Ca²⁺ were associated with incomplete relaxation and an increase in diastolic tension in HHD and HHD+HFpEF. Na⁺ levels were not increased, and expression levels of Ca²⁺- or Na⁺-handling proteins were not altered. Na⁺-influx inhibitors did not improve relaxation or prevent incomplete relaxation at high stimulation rates.

CONCLUSIONS

Contraction and relaxation are prolonged in isolated myocardium from patients with HHD and HHD+HFpEF. This leads to incomplete relaxation at higher rates. Elevated calcium levels in HFpEF are neither a result of an impaired Na⁺ gradient nor expression changes in key ion transporters and regulatory proteins.

Cardiac Myosin Binding Protein-C Phosphorylation Modulates Myofilament Length-Dependent Activation

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is an important regulator of contractile function, however, its contributions to length-dependent changes in cross-bridge (XB) kinetics is unknown. Therefore, we performed mechanical experiments to quantify contractile function in detergent-skinned ventricular preparations isolated from wild-type (WT) hearts, and hearts expressing non-phosphorylatable cMyBP-C [Ser to Ala substitutions at residues Ser273, Ser282, and Ser302 (i.e., 3SA)], at sarcomere length (SL) 1.9 μm or 2.1μm, prior and following protein kinase A (PKA) treatment. Steady-state force generation measurements revealed a blunting in the length-dependent increase in myofilament Ca(2+)-sensitivity of force generation (pCa50) following an increase in SL in 3SA skinned myocardium compared to WT skinned myocardium. Dynamic XB behavior was assessed at submaximal Ca(2+)-activations by imposing an acute rapid stretch of 2% of initial muscle length, and measuring both the magnitudes and rates of resultant phases of force decay due to strain-induced XB detachment and delayed force rise due to recruitment of additional XBs with increased SL (i.e., stretch activation). The magnitude (P2) and rate of XB detachment (k rel) following stretch was significantly reduced in 3SA skinned myocardium compared to WT skinned myocardium at short and long SL, and prior to and following PKA treatment. Furthermore, the length-dependent acceleration of k rel due to decreased SL that was observed in WT skinned myocardium was abolished in 3SA skinned myocardium. PKA treatment accelerated the rate of XB recruitment (k df) following stretch at both SL's in WT but not in 3SA skinned myocardium. The amplitude of the enhancement in force generation above initial pre-stretch steady-state levels (P3) was not different between WT and 3SA skinned myocardium at any condition measured. However, the magnitude of the entire delayed force phase which can dip below initial pre-stretch steady-state levels (Pdf) was significantly lower in 3SA skinned myocardium under all conditions, in part due to a reduced magnitude of XB detachment (P2) in 3SA skinned myocardium compared to WT skinned myocardium. These findings demonstrate that cMyBP-C phospho-ablation regulates SL- and PKA-mediated effects on XB kinetics in the myocardium, which would be expected to contribute to the regulation of the Frank-Starling mechanism.

Right ventricular protein expression profile in end-stage heart failure

Little is known about the right ventricular (RV) proteome in human heart failure (HF), including possible differences compared to the left ventricular (LV) proteome. We used 2-dimensional differential in-gel electrophoresis (pH: 4-7, 10-150 kDa), followed by liquid chromatography tandem mass spectrometry, to compare the RV and LV proteomes in 12 explanted human hearts. We used Western blotting and multiple-reaction monitoring for protein verification and RNA sequencing for messenger RNA and protein expression correlation. In all 12 hearts, the right ventricles (RVs) demonstrated differential expression of 11 proteins relative to the left ventricles (LVs), including lesser expression of CRYM, TPM1, CLU, TXNL1, and COQ9 and greater expression of TNNI3, SAAI, ERP29, ACTN2, HSPB2, and NDUFS3. Principal-components analysis did not suggest RV-versus-LV proteome partitioning. In the nonischemic RVs (n = 6), 7 proteins were differentially expressed relative to the ischemic RVs (n = 6), including increased expression of CRYM, B7Z964, desmin, ANXA5, and MIME and decreased expression of SERPINA1 and ANT3. Principal-components analysis demonstrated partitioning of the nonischemic and ischemic RV proteomes, and gene ontology analysis identified differences in hemostasis and atherosclerosis-associated networks. There were no proteomic differences between RVs with echocardiographic dysfunction (n = 8) and those with normal function (n = 4). Messenger RNA and protein expression did not correlate consistently, suggesting a major role for RV posttranscriptional protein expression regulation. Differences in contractile, cytoskeletal, metabolic, signaling, and survival pathways exist between the RV and the LV in HF and may be related to the underlying HF etiology and differential posttranscriptional regulation.

Phosphorylation of cardiac Myosin-binding protein-C is a critical mediator of diastolic function

BACKGROUND

Heart failure (HF) with preserved ejection fraction (HFpEF) accounts for ≈50% of all cases of HF and currently has no effective treatment. Diastolic dysfunction underlies HFpEF; therefore, elucidation of the mechanisms that mediate relaxation can provide new potential targets for treatment. Cardiac myosin-binding protein-C (cMyBP-C) is a thick filament protein that modulates cross-bridge cycling rates via alterations in its phosphorylation status. Thus, we hypothesize that phosphorylated cMyBP-C accelerates the rate of cross-bridge detachment, thereby enhancing relaxation to mediate diastolic function.

METHODS AND RESULTS

We compared mouse models expressing phosphorylation-deficient cMyBP-C(S273A/S282A/S302A)-cMyBP-C(t3SA), phosphomimetic cMyBP-C(S273D/S282D/S302D)-cMyBP-C(t3SD), and wild-type-control cMyBP-C(tWT) to elucidate the functional effects of cMyBP-C phosphorylation. Decreased voluntary running distances, increased lung/body weight ratios, and increased brain natriuretic peptide levels in cMyBP-C(t3SA) mice demonstrate that phosphorylation deficiency is associated with signs of HF. Echocardiography (ejection fraction and myocardial relaxation velocity) and pressure/volume measurements (-dP/dtmin, pressure decay time constant τ-Glantz, and passive filling stiffness) show that cMyBP-C phosphorylation enhances myocardial relaxation in cMyBP-C(t3SD) mice, whereas deficient cMyBP-C phosphorylation causes diastolic dysfunction with HFpEF in cMyBP-C(t3SA) mice. Simultaneous force and [Ca(2+)]i measurements on intact papillary muscles show that enhancement of relaxation in cMyBP-C(t3SD) mice and impairment of relaxation in cMyBP-C(t3SA) mice are not because of altered [Ca(2+)]i handling, implicating that altered cross-bridge detachment rates mediate these changes in relaxation rates.

CONCLUSIONS

cMyBP-C phosphorylation enhances relaxation, whereas deficient phosphorylation causes diastolic dysfunction and phenotypes resembling HFpEF. Thus, cMyBP-C is a potential target for treatment of HFpEF.

Myocardial stiffness in patients with heart failure and a preserved ejection fraction: contributions of collagen and titin

BACKGROUND

The purpose of this study was to determine whether patients with heart failure and a preserved ejection fraction (HFpEF) have an increase in passive myocardial stiffness and the extent to which discovered changes depend on changes in extracellular matrix fibrillar collagen and cardiomyocyte titin.

METHODS AND RESULTS

Seventy patients undergoing coronary artery bypass grafting underwent an echocardiogram, plasma biomarker determination, and intraoperative left ventricular epicardial anterior wall biopsy. Patients were divided into 3 groups: referent control (n=17, no hypertension or diabetes mellitus), hypertension (HTN) without (-) HFpEF (n=31), and HTN with (+) HFpEF (n=22). One or more of the following studies were performed on the biopsies: passive stiffness measurements to determine total, collagen-dependent and titin-dependent stiffness (differential extraction assay), collagen assays (biochemistry or histology), or titin isoform and phosphorylation assays. In comparison with controls, patients with HTN(-)HFpEF had no change in left ventricular end-diastolic pressure, myocardial passive stiffness, collagen, or titin phosphorylation but had an increase in biomarkers of inflammation (C-reactive protein, soluble ST2, tissue inhibitor of metalloproteinase 1). In comparison with both control and HTN(-)HFpEF, patients with HTN(+)HFpEF had increased left ventricular end-diastolic pressure, left atrial volume, N-terminal propeptide of brain natriuretic peptide, total, collagen-dependent, and titin-dependent stiffness, insoluble collagen, increased titin phosphorylation on PEVK S11878(S26), reduced phosphorylation on N2B S4185(S469), and increased biomarkers of inflammation.

CONCLUSIONS

Hypertension in the absence of HFpEF did not alter passive myocardial stiffness. Patients with HTN(+)HFpEF had a significant increase in passive myocardial stiffness; collagen-dependent and titin-dependent stiffness were increased. These data suggest that the development of HFpEF depends on changes in both collagen and titin homeostasis.