Phosphorylation of titin modulates passive stiffness of cardiac muscle in a titin isoform-dependent manner

Discovery of Titin and Its Role in Heart Function and Disease

This review examines the giant elastic protein titin and its critical roles in heart function, both in health and disease, as discovered since its identification nearly 50 years ago. Encoded by the TTN (titin gene), titin has emerged as a major disease locus for cardiac disorders. Functionally, titin acts as a third myofilament type, connecting sarcomeric Z-disks and M-bands, and regulating myocardial passive stiffness and stretch sensing. Its I-band segment, which includes the N2B element and the PEVK (proline, glutamate, valine, and lysine-rich regions), serves as a viscoelastic spring, adjusting sarcomere length and force in response to cardiac stretch. The review details how alternative splicing of titin pre-mRNA produces different isoforms that greatly impact passive tension and cardiac function, under physiological and pathological conditions. Key posttranslational modifications, especially phosphorylation, play crucial roles in adjusting titin's stiffness, allowing for rapid adaptation to changing hemodynamic demands. Abnormal titin modifications and dysregulation of isoforms are linked to cardiac diseases such as heart failure with preserved ejection fraction, where increased stiffness impairs diastolic function. In addition, the review discusses the importance of the A-band region of titin in setting thick filament length and enhancing Ca²+ sensitivity, contributing to the Frank-Starling Mechanism of the heart. TTN truncating variants are frequently associated with dilated cardiomyopathy, and the review outlines potential disease mechanisms, including haploinsufficiency, sarcomere disarray, and altered thick filament regulation. Variants in TTN have also been linked to conditions such as peripartum cardiomyopathy and chemotherapy-induced cardiomyopathy. Therapeutic avenues are explored, including targeting splicing factors such as RBM20 (RNA binding motif protein 20) to adjust isoform ratios or using engineered heart tissues to study disease mechanisms. Advances in genetic engineering, including CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats), offer promise for modifying TTN to treat titin-related cardiomyopathies. This comprehensive review highlights titin's structural, mechanical, and signaling roles in heart function and the impact of TTN mutations on cardiac diseases.

Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin

Of the ions involved in myocardial function, Ca2+ is the most important. Ca2+ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation-contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca2+ concentration is regulated to elicit excitation-contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca2+ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca2+ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation-contraction coupling.

Fatiguing exercise reduces cellular passive Young's modulus in human vastus lateralis muscle

Previous studies demonstrated that acute fatiguing exercise transiently reduces whole-muscle stiffness, which might contribute to increased risk of injury and impaired contractile performance. We sought to elucidate potential intracellular mechanisms underlying these reductions. To that end, the cellular passive Young's modulus was measured in muscle fibres from healthy, young males and females. Eight volunteers (four male and four female) completed unilateral, repeated maximal voluntary knee extensions until task failure, immediately followed by bilateral percutaneous needle muscle biopsy of the post-fatigued followed by the non-fatigued control vastus lateralis. Muscle samples were processed for mechanical assessment and separately for imaging and phosphoproteomics. Fibres were passively (pCa 8.0) stretched incrementally to 156% of initial sarcomere length to assess Young's modulus, calculated as the slope of the resulting stress-strain curve at short (sarcomere length = 2.4-3.0 µm) and long (sarcomere length = 3.2-3.8 µm) lengths. Titin phosphorylation was assessed by liquid chromatography followed by high-resolution mass spectrometry. The passive modulus was significantly reduced in post-fatigued versus control fibres from male, but not female, participants. Post-fatigued samples showed altered phosphorylation of five serine residues (four located within the elastic region of titin) but did not exhibit altered active tension or sarcomere ultrastructure. Collectively, these results suggest that acute fatigue is sufficient to alter phosphorylation of skeletal titin in multiple locations. We also found reductions in the passive modulus, consistent with prior reports in the literature investigating striated muscle stiffness. These results provide mechanistic insight contributing to the understanding of dynamic regulation of whole-muscle tissue mechanics in vivo. HIGHLIGHTS: What is the central question of this study? Previous studies have shown that skeletal muscle stiffness is reduced following a single bout of fatiguing exercise in whole muscle, but it is not known whether these changes manifest at the cellular level, and their potential mechanisms remain unexplored. What is the main finding and its importance? Fatiguing exercise reduces cellular stiffness in skeletal muscle from males but not females, suggesting that fatigue alters tissue compliance in a sex-dependent manner. The phosphorylation status of titin, a potential mediator of skeletal muscle cellular stiffness, is modified by fatiguing exercise. Previous studies have shown that passive skeletal muscle stiffness is reduced following a single bout of fatiguing exercise. Lower muscle passive stiffness following fatiguing exercise might increase risk for soft-tissue injury; however, the underlying mechanisms of this change are unclear. Our findings show that fatiguing exercise reduces the passive Young's modulus in skeletal muscle cells from males but not females, suggesting that intracellular proteins contribute to reduced muscle stiffness following repeated loading to task failure in a sex-dependent manner. The phosphorylation status of the intracellular protein titin is modified by fatiguing exercise in a way that might contribute to altered muscle stiffness after fatiguing exercise. These results provide important mechanistic insight that might help to explain why biological sex impacts the risk for soft-tissue injury with repeated or high-intensity mechanical loading in athletes and the risk of falls in older adults.

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.

Heart Failure With Preserved Ejection Fraction: An Evolving Understanding

Heart failure (HF) with preserved ejection fraction (HFpEF) is a clinical syndrome in which patients have signs and symptoms of HF due to high left ventricular (LV) filling pressure despite normal or near normal LV ejection fraction. It is more common than HF with reduced ejection fraction (HFrEF), and its diagnosis and treatment are more challenging than HFrEF. Although hypertension is the primary risk factor, coronary artery disease and other comorbidities, such as atrial fibrillation (AF), diabetes, chronic kidney disease (CKD), and obesity, also play an essential role in its formation. This review summarizes current knowledge about HFpEF, its pathophysiology, clinical presentation, diagnostic challenges, current treatments, and promising novel treatments. It is essential to continue to be updated on the latest treatments for HFpEF so that patients always receive the most therapeutic treatments. The use of GnRH agonists in the management of HFpEF, infusion of Apo a-I nanoparticle, low-level transcutaneous vagal stimulation (LLTS), and estrogen only in post-menopausal women are promising strategies to prevent diastolic dysfunction and HFpEF; however, there is still no proven curative treatment for HFpEF yet.

Late-life Rapamycin Treatment Enhances Cardiomyocyte Relaxation Kinetics and Reduces Myocardial Stiffness

Diastolic dysfunction is a key feature of the aging heart. We have shown that late-life treatment with mTOR inhibitor, rapamycin, reverses age-related diastolic dysfunction in mice but the molecular mechanisms of the reversal remain unclear. To dissect the mechanisms by which rapamycin improves diastolic function in old mice, we examined the effects of rapamycin treatment at the levels of single cardiomyocyte, myofibril and multicellular cardiac muscle. Compared to young cardiomyocytes, isolated cardiomyocytes from old control mice exhibited prolonged time to 90% relaxation (RT 90 ) and time to 90% Ca 2+ transient decay (DT 90 ), indicating slower relaxation kinetics and calcium reuptake with age. Late-life rapamycin treatment for 10 weeks completely normalized RT 90 and partially normalized DT 90 , suggesting improved Ca 2+ handling contributes partially to the rapamycin-induced improved cardiomyocyte relaxation. In addition, rapamycin treatment in old mice enhanced the kinetics of sarcomere shortening and Ca 2+ transient increase in old control cardiomyocytes. Myofibrils from old rapamycin-treated mice displayed increased rate of the fast, exponential decay phase of relaxation compared to old controls. The improved myofibrillar kinetics were accompanied by an increase in MyBP-C phosphorylation at S282 following rapamycin treatment. We also showed that late-life rapamycin treatment normalized the age-related increase in passive stiffness of demembranated cardiac trabeculae through a mechanism independent of titin isoform shift. In summary, our results showed that rapamycin treatment normalizes the age-related impairments in cardiomyocyte relaxation, which works conjointly with reduced myocardial stiffness to reverse age-related diastolic dysfunction.

Heart Failure with Preserved Ejection Fraction in Children

Diastolic dysfunction (DD) refers to abnormalities in the mechanical function of the left ventricle (LV) during diastole. Severe LVDD can cause symptoms and the signs of heart failure (HF) in the setting of normal or near normal LV systolic function and is referred to as diastolic HF or HF with preserved ejection fraction (HFpEF). Pediatric cardiologists have long speculated HFpEF in children with congenital heart disease and cardiomyopathy. However, understanding the risk factors, clinical course, and validated biomarkers predictive of the outcome of HFpEF in children is challenging due to heterogeneous etiologies and overlapping pathophysiological mechanisms. The natural history of HFpEF varies depending upon the patient's age, sex, race, geographic location, nutritional status, biochemical risk factors, underlying heart disease, and genetic-environmental interaction, among other factors. Pediatric onset HFpEF is often not the same disease as in adults. Advances in the noninvasive evaluation of the LV diastolic function by strain, and strain rate analysis with speckle-tracking echocardiography, tissue Doppler imaging, and cardiac magnetic resonance imaging have increased our understanding of the HFpEF in children. This review addresses HFpEF in children and identifies knowledge gaps in the underlying etiologies, pathogenesis, diagnosis, and management, especially compared to adults with HFpEF.

Regulation of cardiac function by cAMP nanodomains

Cyclic adenosine monophosphate (cAMP) is a diffusible intracellular second messenger that plays a key role in the regulation of cardiac function. In response to the release of catecholamines from sympathetic terminals, cAMP modulates heart rate and the strength of contraction and ease of relaxation of each heartbeat. At the same time, cAMP is involved in the response to a multitude of other hormones and neurotransmitters. A sophisticated network of regulatory mechanisms controls the temporal and spatial propagation of cAMP, resulting in the generation of signaling nanodomains that enable the second messenger to match each extracellular stimulus with the appropriate cellular response. Multiple proteins contribute to this spatiotemporal regulation, including the cAMP-hydrolyzing phosphodiesterases (PDEs). By breaking down cAMP to a different extent at different locations, these enzymes generate subcellular cAMP gradients. As a result, only a subset of the downstream effectors is activated and a specific response is executed. Dysregulation of cAMP compartmentalization has been observed in cardiovascular diseases, highlighting the importance of appropriate control of local cAMP signaling. Current research is unveiling the molecular organization underpinning cAMP compartmentalization, providing original insight into the physiology of cardiac myocytes and the alteration associated with disease, with the potential to uncover novel therapeutic targets. Here, we present an overview of the mechanisms that are currently understood to be involved in generating cAMP nanodomains and we highlight the questions that remain to be answered.

Molecular regulation of stretch activation

Stretch activation is defined as a delayed increase in force after rapid stretches. Although there is considerable evidence for stretch activation in isolated cardiac myofibrillar preparations, few studies have measured mechanisms of stretch activation in mammalian skeletal muscle fibers. We measured stretch activation following rapid step stretches [∼1%-4% sarcomere length (SL)] during submaximal Ca2+ activations of rat permeabilized slow-twitch skeletal muscle fibers before and after protein kinase A (PKA), which phosphorylates slow myosin binding protein-C. PKA significantly increased stretch activation during low (∼25%) Ca2+ activation and accelerated rates of delayed force development (kef) during both low and half-maximal Ca2+ activation. Following the step stretches and subsequent force development, fibers were rapidly shortened to original sarcomere length, which often elicited a shortening-induced transient force overshoot. After PKA, step shortening-induced transient force overshoot increased ∼10-fold following an ∼4% SL shortening during low Ca2+ activation levels. kdf following step shortening also increased after PKA during low and half-maximal Ca2+ activations. We next investigated thin filament regulation of stretch activation. We tested the interplay between cardiac troponin I (cTnI) phosphorylation at the canonical PKA and novel tyrosine kinase sites on stretch activation. Native slow-skeletal Tn complexes were exchanged with recombinant human cTn complex with different human cTnI N-terminal pseudo-phosphorylation molecules: 1) nonphosphorylated wild type (WT), 2) the canonical S22/23D PKA sites, 3) the tyrosine kinase Y26E site, and 4) the combinatorial S22/23D + Y26E cTnI. All three pseudo-phosphorylated cTnIs elicited greater stretch activation than WT. Following stretch activation, a new, elevated stretch-induced steady-state force was reached with pseudo-phosphorylated cTnI. Combinatorial S22/23D + Y26E pseudo-phosphorylated cTnI increased kdf. These results suggest that slow-skeletal myosin binding protein-C (sMyBP-C) phosphorylation modulates stretch activation by a combination of cross-bridge recruitment and faster cycling kinetics, whereas cTnI phosphorylation regulates stretch activation by both redundant and synergistic mechanisms; and, taken together, these sarcomere phosphoproteins offer precision targets for enhanced contractility.

Shared and unique phosphoproteomics responses in skeletal muscle from exercise models and in hyperammonemic myotubes

Skeletal muscle generation of ammonia, an endogenous cytotoxin, is increased during exercise. Perturbations in ammonia metabolism consistently occur in chronic diseases, and may blunt beneficial skeletal muscle molecular responses and protein homeostasis with exercise. Phosphorylation of skeletal muscle proteins mediates cellular signaling responses to hyperammonemia and exercise. Comparative bioinformatics and machine learning-based analyses of published and experimentally derived phosphoproteomics data identified differentially expressed phosphoproteins that were unique and shared between hyperammonemic murine myotubes and skeletal muscle from exercise models. Enriched processes identified in both hyperammonemic myotubes and muscle from exercise models with selected experimental validation included protein kinase A (PKA), calcium signaling, mitogen-activated protein kinase (MAPK) signaling, and protein homeostasis. Our approach of feature extraction from comparative untargeted "omics" data allows for selection of preclinical models that recapitulate specific human exercise responses and potentially optimize functional capacity and skeletal muscle protein homeostasis with exercise in chronic diseases.

Effects of omecamtiv mecarbil on the contractile properties of skinned porcine left atrial and ventricular muscles

Omecamtiv mecarbil (OM) is a novel inotropic agent for heart failure with systolic dysfunction. OM prolongs the actomyosin attachment duration, which enhances thin filament cooperative activation and accordingly promotes the binding of neighboring myosin to actin. In the present study, we investigated the effects of OM on the steady-state contractile properties in skinned porcine left ventricular (PLV) and atrial (PLA) muscles. OM increased Ca²⁺ sensitivity in a concentration-dependent manner in PLV, by left shifting the mid-point (pCa₅₀) of the force-pCa curve (ΔpCa₅₀) by ∼0.16 and ∼0.33 pCa units at 0.5 and 1.0 μM, respectively. The Ca²⁺-sensitizing effect was likewise observed in PLA, but less pronounced with ΔpCa₅₀ values of ∼0.08 and ∼0.22 pCa units at 0.5 and 1.0 μM, respectively. The Ca²⁺-sensitizing effect of OM (1.0 μM) was attenuated under enhanced thin filament cooperative activation in both PLV and PLA; this attenuation occurred directly via treatment with fast skeletal troponin (ΔpCa₅₀: ∼0.16 and ∼0.10 pCa units in PLV and PLA, respectively) and indirectly by increasing the number of strongly bound cross-bridges in the presence of 3 mM MgADP (ΔpCa₅₀: ∼0.21 and ∼0.08 pCa units in PLV and PLA, respectively). It is likely that this attenuation of the Ca²⁺-sensitizing effect of OM is due to a decrease in the number of “recruitable” cross-bridges that can potentially produce active force. When cross-bridge detachment was accelerated in the presence of 20 mM inorganic phosphate, the Ca²⁺-sensitizing effect of OM (1.0 μM) was markedly decreased in both types of preparations (ΔpCa₅₀: ∼0.09 and ∼0.03 pCa units in PLV and PLA, respectively). The present findings suggest that the positive inotropy of OM is more markedly exerted in the ventricle than in the atrium, which results from the strongly bound cross-bridge-dependent allosteric activation of thin filaments.

Nitric oxide and skeletal muscle contractile function

Nitric oxide (NO) is complex modulator of skeletal muscle contractile function, capable of increasing or decreasing force and power output depending on multiple factors. This review explores the effects and potential mechanisms for modulation of skeletal muscle contractile function by NO, from pharmacological agents in isolated muscle preparations to dietary nitrate supplementation in humans and animals. Pharmacological manipulation in vitro suggests that NO signaling diminishes submaximal isometric force, whereas dietary manipulation in vivo suggest that NO enhances submaximal force. The bases for these different responses are unknown but could reflect dose-dependent effects. Maximal isometric force is unaffected by physiologically relevant levels of NO, which do not induce overt protein oxidation. Pharmacological and dietary manipulation of NO signaling enhances the maximal rate of isometric force development, unloaded shortening velocity, and peak power. We hypothesize that these effects are mediated by post-translational modifications of myofibrillar proteins that modulate thick filament regulation of contraction (e.g., mechanosensing and strain-dependence of cross-bridge kinetics). NO effects on contractile function appear to have some level of fiber type and sex-specificity. The mechanisms behind NO-mediated changes in skeletal muscle function need to be explored through proteomics analysis and advanced biophysical assays to advance the development of small molecules and open intriguing therapeutic and ergogenic possibilities for aging, disease, and athletic performance.

Functional and structural differences between skinned and intact muscle preparations

Myofilaments and their associated proteins, which together constitute the sarcomeres, provide the molecular-level basis for contractile function in all muscle types. In intact muscle, sarcomere-level contraction is strongly coupled to other cellular subsystems, in particular the sarcolemmal membrane. Skinned muscle preparations (where the sarcolemma has been removed or permeabilized) are an experimental system designed to probe contractile mechanisms independently of the sarcolemma. Over the last few decades, experiments performed using permeabilized preparations have been invaluable for clarifying the understanding of contractile mechanisms in both skeletal and cardiac muscle. Today, the technique is increasingly harnessed for preclinical and/or pharmacological studies that seek to understand how interventions will impact intact muscle contraction. In this context, intrinsic functional and structural differences between skinned and intact muscle pose a major interpretational challenge. This review first surveys measurements that highlight these differences in terms of the sarcomere structure, passive and active tension generation, and calcium dependence. We then highlight the main practical challenges and caveats faced by experimentalists seeking to emulate the physiological conditions of intact muscle. Gaining an awareness of these complexities is essential for putting experiments in due perspective.

A guide for assessment of myocardial stiffness in health and disease

Myocardial stiffness is an intrinsic property of the myocardium that influences both diastolic and systolic cardiac function. Myocardial stiffness represents the resistance of this tissue to being deformed and depends on intracellular components of the cardiomyocyte, particularly the cytoskeleton, and on extracellular components, such as collagen fibers. Myocardial disease is associated with changes in myocardial stiffness, and its assessment is a key diagnostic marker of acute or chronic pathological myocardial disease with the potential to guide therapeutic decision-making. In this Review, we appraise the different techniques that can be used to estimate myocardial stiffness, evaluate their advantages and disadvantages, and discuss potential clinical applications.

The titin N2B and N2A regions: biomechanical and metabolic signaling hubs in cross-striated muscles

Muscle specific signaling has been shown to originate from myofilaments and their associated cellular structures, including the sarcomeres, costameres or the cardiac intercalated disc. Two signaling hubs that play important biomechanical roles for cardiac and/or skeletal muscle physiology are the N2B and N2A regions in the giant protein titin. Prominent proteins associated with these regions in titin are chaperones Hsp90 and αB-crystallin, members of the four-and-a-half LIM (FHL) and muscle ankyrin repeat protein (Ankrd) families, as well as thin filament-associated proteins, such as myopalladin. This review highlights biological roles and properties of the titin N2B and N2A regions in health and disease. Special emphasis is placed on functions of Ankrd and FHL proteins as mechanosensors that modulate muscle-specific signaling and muscle growth. This region of the sarcomere also emerged as a hotspot for the modulation of passive muscle mechanics through altered titin phosphorylation and splicing, as well as tethering mechanisms that link titin to the thin filament system.

Synchrony of sarcomeric movement regulates left ventricular pump function in the in vivo beating mouse heart

Sarcomeric contraction in cardiomyocytes serves as the basis for the heart's pump functions. It has generally been considered that in cardiac muscle as well as in skeletal muscle, sarcomeres equally contribute to myofibrillar dynamics in myocytes at varying loads by producing similar levels of active and passive force. In the present study, we expressed α-actinin-AcGFP in Z-disks to analyze dynamic behaviors of sequentially connected individual sarcomeres along a myofibril in a left ventricular (LV) myocyte of the in vivo beating mouse heart. To quantify the magnitude of the contribution of individual sarcomeres to myofibrillar dynamics, we introduced the novel parameter "contribution index" (CI) to measure the synchrony in movements between a sarcomere and a myofibril (from -1 [complete asynchrony] to 1 [complete synchrony]). First, CI varied markedly between sarcomeres, with an average value of ∼0.3 during normal systole. Second, when the movements between adjacent sarcomeres were asynchronous (CI < 0), a sarcomere and the ones next to the adjacent sarcomeres and farther away moved in synchrony (CI > 0) along a myofibril. Third, when difference in LV pressure in diastole and systole (ΔLVP) was lowered to <10 mm Hg, diastolic sarcomere length increased. Under depressed conditions, the movements between adjacent sarcomeres were in marked asynchrony (CI, -0.3 to -0.4), and, as a result, average CI was linearly decreased in association with a decrease in ΔLVP. These findings suggest that in the left ventricle of the in vivo beating mouse heart, (1) sarcomeres heterogeneously contribute to myofibrillar dynamics due to an imbalance of active and passive force between neighboring sarcomeres, (2) the force imbalance is pronounced under depressed conditions coupled with a marked increase in passive force and the ensuing tug-of-war between sarcomeres, and (3) sarcomere synchrony via the distal intersarcomere interaction regulates the heart's pump function in coordination with myofibrillar contractility.

Increased Expression of N2BA Titin Corresponds to More Compliant Myofibrils in Athlete's Heart

Long-term exercise induces physiological cardiac adaptation, a condition referred to as athlete’s heart. Exercise tolerance is known to be associated with decreased cardiac passive stiffness. Passive stiffness of the heart muscle is determined by the giant elastic protein titin. The adult cardiac muscle contains two titin isoforms: the more compliant N2BA and the stiffer N2B. Titin-based passive stiffness may be controlled by altering the expression of the different isoforms or via post-translational modifications such as phosphorylation. Currently, there is very limited knowledge about titin’s role in cardiac adaptation during long-term exercise. Our aim was to determine the N2BA/N2B ratio and post-translational phosphorylation of titin in the left ventricle and to correlate the changes with the structure and transverse stiffness of cardiac sarcomeres in a rat model of an athlete’s heart. The athlete’s heart was induced by a 12-week-long swim-based training. In the exercised myocardium the N2BA/N2B ratio was significantly increased, Ser11878 of the PEVK domain was hypophosphorlyated, and the sarcomeric transverse elastic modulus was reduced. Thus, the reduced passive stiffness in the athlete’s heart is likely caused by a shift towards the expression of the longer cardiac titin isoform and a phosphorylation-induced softening of the PEVK domain which is manifested in a mechanical rearrangement locally, within the cardiac sarcomere.

Need for Speed: The Importance of Physiological Strain Rates in Determining Myocardial Stiffness

The heart is viscoelastic, meaning its compliance is inversely proportional to the speed at which it stretches. During diastolic filling, the left ventricle rapidly expands at rates where viscoelastic forces impact ventricular compliance. In heart disease, myocardial viscoelasticity is often increased and can directly impede diastolic filling to reduce cardiac output. Thus, treatments that reduce myocardial viscoelasticity may provide benefit in heart failure, particularly for patients with diastolic heart failure. Yet, many experimental techniques either cannot or do not characterize myocardial viscoelasticity, and our understanding of the molecular regulators of viscoelasticity and its impact on cardiac performance is lacking. Much of this may stem from a reliance on techniques that either do not interrogate viscoelasticity (i.e., use non-physiological rates of strain) or techniques that compromise elements that contribute to viscoelasticity (i.e., skinned or permeabilized muscle preparations that compromise cytoskeletal integrity). Clinically, cardiac viscoelastic characterization is challenging, requiring the addition of strain-rate modulation during invasive hemodynamics. Despite these challenges, data continues to emerge demonstrating a meaningful contribution of viscoelasticity to cardiac physiology and pathology, and thus innovative approaches to characterize viscoelasticity stand to illuminate fundamental properties of myocardial mechanics and facilitate the development of novel therapeutic strategies.

Muscle ankyrin repeat protein 1 (MARP1) locks titin to the sarcomeric thin filament and is a passive force regulator

Muscle ankyrin repeat protein 1 (MARP1) is frequently up-regulated in stressed muscle, but its effect on skeletal muscle function is poorly understood. Here, we focused on its interaction with the titin–N2A element, found in titin’s molecular spring region. We show that MARP1 binds to F-actin, and that this interaction is stronger when MARP1 forms a complex with titin–N2A. Mechanics and super-resolution microscopy revealed that MARP1 “locks” titin–N2A to the sarcomeric thin filament, causing increased extension of titin’s elastic PEVK element and, importantly, increased passive force. In support of this mechanism, removal of thin filaments abolished the effect of MARP1 on passive force. The clinical relevance of this mechanism was established in diaphragm myofibers of mechanically ventilated rats and of critically ill patients. Thus, MARP1 regulates passive force by locking titin to the thin filament. We propose that in stressed muscle, this mechanism protects the sarcomere from mechanical damage.

Mechanisms of Frank-Starling law of the heart and stretch activation in striated muscles may have a common molecular origin

Vertebrate cardiac muscle generates progressively larger systolic force when the end diastolic chamber volume is increased, a property called the "Frank-Starling Law", or "length dependent activation (LDA)". In this mechanism a larger force develops when the sarcomere length (SL) increased, and the overlap between thick and thin filament decreases, indicating increased production of force per unit length of the overlap. To account for this phenomenon at the molecular level, we examined several hypotheses: as the muscle length is increased, (1) lattice spacing decreases, (2) Ca2+ sensitivity increases, (3) titin mediated rearrangement of myosin heads to facilitate actomyosin interaction, (4) increased SL activates cross-bridges (CBs) in the super relaxed state, (5) increased series stiffness at longer SL promotes larger elementary force/CB to account for LDA, and (6) stretch activation (SA) observed in insect muscles and LDA in vertebrate muscles may have similar mechanisms. SA is also known as delayed tension or oscillatory work, and universally observed among insect flight muscles, as well as in vertebrate skeletal and cardiac muscles. The sarcomere stiffness observed in relaxed muscles may significantly contributes to the mechanisms of LDA. In vertebrate striated muscles, the sarcomere stiffness is mainly caused by titin, a single filamentary protein spanning from Z-line to M-line and tightly associated with the myosin thick filament. In insect flight muscles, kettin connects Z-line and the thick filament to stabilize the sarcomere structure. In vertebrate cardiac muscles, titin plays a similar role, and may account for LDA and may constitute a molecular mechanism of Frank-Starling response.

Targeting cardiac fibrosis in heart failure with preserved ejection fraction: mirage or miracle?

Cardiac fibrosis is central to the pathology of heart failure, particularly heart failure with preserved ejection fraction (HFpEF). Irrespective of the underlying profibrotic condition (e.g. ageing, diabetes, hypertension), maladaptive cardiac fibrosis is defined by the transformation of resident fibroblasts to matrix-secreting myofibroblasts. Numerous profibrotic factors have been identified at the molecular level (e.g. TGFβ, IL11, AngII), which activate gene expression programs for myofibroblast activation. A number of existing HF therapies indirectly target fibrotic pathways; however, despite multiple clinical trials in HFpEF, a specific clinically effective antifibrotic therapy remains elusive. Therapeutic inhibition of TGFβ, the master-regulator of fibrosis, has unfortunately proven toxic and ineffective in clinical trials to date, and new approaches are needed. In this review, we discuss the pathophysiology and clinical implications of interstitial fibrosis in HFpEF. We provide an overview of trials targeting fibrosis in HFpEF to date and discuss the promise of potential new therapeutic approaches and targets in the context of underlying molecular mechanisms.

Is Cardiac Diastolic Dysfunction a Part of Post-Menopausal Syndrome?

Post-menopausal women exhibit an exponential increase in the incidence of heart failure with preserved ejection fraction compared with men of the same age, which indicates a potential role of hormonal changes in subclinical and clinical diastolic dysfunction. This paper reviews the preclinical evidence that demonstrates the involvement of estrogen in many regulatory molecular pathways of cardiac diastolic function and the clinical data that investigates the effect of estrogen on diastolic function in post-menopausal women. Published reports show that estrogen deficiency influences both early diastolic relaxation via calcium homeostasis and the late diastolic compliance associated with cardiac hypertrophy and fibrosis. Because of the high risk of diastolic dysfunction and heart failure with preserved ejection fraction in post-menopausal women and the positive effects of estrogen on preserving cardiac function, further clinical studies are needed to clarify the role of endogenous estrogen or hormone replacement in mitigating the onset and progression of heart failure with preserved ejection fraction in women.

Enhanced Cardiomyocyte Function in Hypertensive Rats With Diastolic Dysfunction and Human Heart Failure Patients After Acute Treatment With Soluble Guanylyl Cyclase (sGC) Activator

Aims

Our aim was to investigate the effect of nitric oxide (NO)-independent activation of soluble guanylyl cyclase (sGC) on cardiomyocyte function in a hypertensive animal model with diastolic dysfunction and in biopsies from human heart failure with preserved ejection fraction (HFpEF).

Methods

Dahl salt-sensitive (DSS) rats and control rats were fed a high-salt diet for 10 weeks and then acutely treated in vivo with the sGC activator BAY 58-2667 (cinaciguat) for 30 min. Single skinned cardiomyocyte passive stiffness (F_passive) was determined in rats and human myocardium biopsies before and after acute treatment. Titin phosphorylation, activation of the NO/sGC/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) cascade, as well as hypertrophic pathways including NO/sGC/cGMP/PKG, PKA, calcium–calmodulin kinase II (CaMKII), extracellular signal-regulated kinase 2 (ERK2), and PKC were assessed. In addition, we explored the contribution of pro-inflammatory cytokines and oxidative stress levels to the modulation of cardiomyocyte function. Immunohistochemistry and electron microscopy were used to assess the translocation of sGC and connexin 43 proteins in the rat model before and after treatment.

Results

High cardiomyocyte F_passive was found in rats and human myocardial biopsies compared to control groups, which was attributed to hypophosphorylation of total titin and to deranged site-specific phosphorylation of elastic titin regions. This was accompanied by lower levels of PKG and PKA activity, along with dysregulation of hypertrophic pathway markers such as CaMKII, PKC, and ERK2. Furthermore, DSS rats and human myocardium biopsies showed higher pro-inflammatory cytokines and oxidative stress compared to controls. DSS animals benefited from treatment with the sGC activator, as F_passive, titin phosphorylation, PKG and the hypertrophic pathway kinases, pro-inflammatory cytokines, and oxidative stress markers all significantly improved to the level observed in controls. Immunohistochemistry and electron microscopy revealed a translocation of sGC protein toward the intercalated disc and t-tubuli following treatment in both control and DSS samples. This translocation was confirmed by staining for the gap junction protein connexin 43 at the intercalated disk. DSS rats showed a disrupted connexin 43 pattern, and sGC activator was able to partially reduce disruption and increase expression of connexin 43. In human HFpEF biopsies, the high F_passive, reduced titin phosphorylation, dysregulation of the NO–sGC–cGMP–PKG pathway and PKA activity level, and activity of kinases involved in hypertrophic pathways CaMKII, PKC, and ERK2 were all significantly improved by sGC treatment and accompanied by a reduction in pro-inflammatory cytokines and oxidative stress markers.

Conclusion

Our data show that sGC activator improves cardiomyocyte function, reduces inflammation and oxidative stress, improves sGC–PKG signaling, and normalizes hypertrophic kinases, indicating that it is a potential treatment option for HFpEF patients and perhaps also for cases with increased hypertrophic signaling.

Modulation of Titin-Based Stiffness in Hypertrophic Cardiomyopathy via Protein Kinase D

The giant protein titin performs structure-preserving functions in the sarcomere and is important for the passive stiffness (F_passive) of cardiomyocytes. Protein kinase D (PKD) enzymes play crucial roles in regulating myocardial contraction, hypertrophy, and remodeling. PKD phosphorylates myofilament proteins, but it is not known whether the giant protein titin is also a PKD substrate. Here, we aimed to determine whether PKD phosphorylates titin and thereby modulates cardiomyocyte F_passive in normal and failing myocardium. The phosphorylation of titin was assessed in cardiomyocyte-specific PKD knock-out mice (cKO) and human hearts using immunoblotting with a phosphoserine/threonine and a phosphosite-specific titin antibody. PKD-dependent site-specific titin phosphorylation in vivo was quantified by mass spectrometry using stable isotope labeling by amino acids in cell culture (SILAC) of SILAC-labeled mouse heart protein lysates that were mixed with lysates isolated from hearts of either wild-type control (WT) or cKO mice. F_passive of single permeabilized cardiomyocytes was recorded before and after PKD and HSP27 administration. All-titin phosphorylation was reduced in cKO compared to WT hearts. Multiple conserved PKD-dependent phosphosites were identified within the Z-disk, A-band and M-band regions of titin by quantitative mass spectrometry, and many PKD-dependent phosphosites detected in the elastic titin I-band region were significantly decreased in cKO. Analysis of titin site-specific phosphorylation showed unaltered or upregulated phosphorylation in cKO compared to matched WT hearts. F_passive was elevated in cKO compared to WT cardiomyocytes and PKD administration lowered F_passive of WT and cKO cardiomyocytes. Cardiomyocytes from hypertrophic cardiomyopathy (HCM) patients showed higher F_passive compared to control hearts and significantly lower F_passive after PKD treatment. In addition, we found higher phosphorylation at CaMKII-dependent titin sites in HCM compared to control hearts. Expression and phosphorylation of HSP27, a substrate of PKD, were elevated in HCM hearts, which was associated with increased PKD expression and phosphorylation. The relocalization of HSP27 in HCM away from the sarcomeric Z-disk and I-band suggested that HSP27 failed to exert its protective action on titin extensibility. This protection could, however, be restored by administration of HSP27, which significantly reduced F_passive in HCM cardiomyocytes. These findings establish a previously unknown role for PKDin regulating diastolic passive properties of healthy and diseased hearts.

Single-Molecule Force Spectroscopy on the N2A Element of Titin: Effects of Phosphorylation and CARP

Titin is a large filamentous protein that forms a sarcomeric myofilament with a molecular spring region that develops force in stretched sarcomeres. The molecular spring has a complex make-up that includes the N2A element. This element largely consists of a 104-residue unique sequence (N2A-Us) flanked by immunoglobulin domains (I80 and I81). The N2A element is of interest because it assembles a signalosome with CARP (Cardiac Ankyrin Repeat Protein) as an important component; CARP both interacts with the N2A-Us and I81 and is highly upregulated in response to mechanical stress. The mechanical properties of the N2A element were studied using single-molecule force spectroscopy, including how these properties are affected by CARP and phosphorylation. Three protein constructs were made that consisted of 0, 1, or 2 N2A-Us elements with flanking I80 and I81 domains and with specific handles at their ends for study by atomic force microscopy (AFM). The N2A-Us behaved as an entropic spring with a persistence length (Lp) of ∼0.35 nm and contour length (Lc) of ∼39 nm. CARP increased the Lp of the N2A-Us and the unfolding force of the Ig domains; force clamp experiments showed that CARP reduced the Ig domain unfolding kinetics. These findings suggest that CARP might function as a molecular chaperone that protects I81 from unfolding when mechanical stress is high. The N2A-Us was found to be a PKA substrate, and phosphorylation was blocked by CARP. Mass spectrometry revealed a PKA phosphosite (Ser-9895 in NP_001254479.2) located at the border between the N2A-Us and I81. AFM studies showed that phosphorylation affected neither the Lp of the N2A-Us nor the Ig domain unfolding force (Funfold). Simulating the force-sarcomere length relation of a single titin molecule containing all spring elements showed that the compliance of the N2A-Us only slightly reduces passive force (1.4%) with an additional small reduction by CARP (0.3%). Thus, it is improbable that the compliance of the N2A element has a mechanical function per se. Instead, it is likely that this compliance has local effects on binding of signaling molecules and that it contributes thereby to strain- and phosphorylation- dependent mechano-signaling.

Changes in rat myocardium contractility under subchronic intoxication with lead and cadmium salts administered alone or in combination

Subchronic intoxications induced in male rats by repeated intraperitoneal injections of lead acetate and cadmium chloride, administered either alone or in combination, are shown to affect the biochemical, cytological and morphometric parameters of blood, liver, heart and kidneys. The single twitch parameters of myocardial trabecular and papillary muscle preparations were measured in the isometric regime to identify changes in the heterometric (length-force) and chronoinotropic (frequency-force) contractility regulation systems. Differences in the responses of these systems in trabecules and papillary muscles to the above intoxications are shown. A number of myocardium mechanical characteristics changing in rats under the effect of a combined lead-cadmium intoxication and increased proportion of α-myosin heavy chains were observed to normalize fully or partially if such intoxication was induced against background administration of a proposed bioprotective complex. Based on the experimental results and literature data, some assumptions are suggested concerning the mechanisms of the cardiotoxic effects produced by lead and cadmium.

Compliant Titin Isoform Content Is Reduced in Left Ventricles of Sedentary Versus Active Rats

A sedentary lifestyle is associated with increased cardiovascular risk factors and reduced cardiac compliance when compared to a lifestyle that includes exercise training. Exercise training increases cardiac compliance in humans, but the mechanisms underlying this improvement are unknown. A major determinant of cardiac compliance is the compliance of the giant elastic protein titin. Experimentally reducing titin compliance in animal models reduces exercise tolerance, but it is not known whether sedentary versus chronic exercise conditions cause differences in titin isoform content. We hypothesized that sedentary conditions would be associated with a reduction in the content of the longer, more compliant N2BA isoform relative to the stiffer N2B isoform (yielding a reduced N2BA:N2B ratio) compared to age-matched exercising controls. We obtained left ventricles from 16-week old rats housed for 12 weeks in standard (sedentary) or voluntary running wheel (exercised) housing. The N2BA:N2B ratio was decreased in the hearts of sedentary versus active rats (p = 0.041). Gene expression of a titin mRNA splicing factor, RNA Binding Motif 20 protein (RBM20), correlated negatively with N2BA:N2B ratios (p = 0.006, r = -0.449), but was not different between groups, suggesting that RBM20 may be regulated post-transcriptionally. Total phosphorylation of cardiac titin was not different between the active and sedentary groups. This study is the first to demonstrate that sedentary rats exhibit reduced cardiac titin N2BA:N2B isoform ratios, which implies reduced cardiac compliance. These data suggest that a lack of exercise (running wheel) reduces cardiac compliance and that exercise itself increases cardiac compliance.

[Why Myocardial Relaxation Always Slows at Cardiac Pathology?]

Chronic heart failure (CHF) in most cases is due to a decrease in myocardial contractility. In particular, this results in a reduction in the maximum rate of the pressure development in the left ventricle. At the same time the maximal rate of pressure fall at relaxation is also reduced. This is not surprising, since both depend on Ca ++ myoplasmic concentration. But most of cardiac pathologies have been associated with the impairement of myocardial relaxation to a greater extent than the contraction. In the review a new view has been proposed according to which this phenomenon is attributable to restructuring of titin, the sarcomeric protein that connects the ends of myosin filaments with the sarcomeric board, lines Z. A spring-like molecule of titin shrinks at sarcomeric contraction and straightens in parallel with removing of Ca ++ from myofibrils. A reduction of its stiffness, facilitating the filling of the left ventricle, can reduce restoring force of titin and thereby slow relaxation. The survey provides information about the functions of the calcium transport system and titin in the normal heart and in CHF observed both in experimental models and in patients.

Sarcomeric Auto-Oscillations in Single Myofibrils From the Heart of Patients With Dilated Cardiomyopathy

BACKGROUND

Left ventricular wall motion is depressed in patients with dilated cardiomyopathy (DCM). However, whether or not the depressed left ventricular wall motion is caused by impairment of sarcomere dynamics remains to be fully clarified.

METHODS AND RESULTS

We analyzed the mechanical properties of single sarcomere dynamics during sarcomeric auto-oscillations (calcium spontaneous oscillatory contractions [Ca-SPOC]) that occurred at partial activation under the isometric condition in myofibrils from donor hearts and from patients with severe DCM (New York Heart Association classification III-IV). Ca-SPOC reproducibly occurred in the presence of 1 μmol/L free Ca²⁺ in both nonfailing and DCM myofibrils, and sarcomeres exhibited a saw-tooth waveform along single myofibrils composed of quick lengthening and slow shortening. The period of Ca-SPOC was longer in DCM myofibrils than in nonfailing myofibrils, in association with prolonged shortening time. Lengthening time was similar in both groups. Then, we performed Tn (troponin) exchange in myofibrils with a DCM-causing homozygous mutation (K36Q) in cTnI (cardiac TnI). On exchange with the Tn complex from healthy porcine ventricles, period, shortening time, and shortening velocity in cTnI-K36Q myofibrils became similar to those in Tn-reconstituted nonfailing myofibrils. Protein kinase A abbreviated period in both Tn-reconstituted nonfailing and cTnI-K36Q myofibrils, demonstrating acceleration of cross-bridge kinetics.

CONCLUSIONS

Sarcomere dynamics was found to be depressed under loaded conditions in DCM myofibrils because of impairment of thick-thin filament sliding. Thus, microscopic analysis of Ca-SPOC in human cardiac myofibrils is beneficial to systematically unveil the kinetic properties of single sarcomeres in various types of heart disease.

Acetylome profiling reveals extensive involvement of lysine acetylation in the conversion of muscle to meat

Protein lysine acetylation is an post-translational modification that regulates gene expression, metabolism, cell signaling, and diseases, but its implication in the postmortem (PM) meat quality development is basically unclear. In the present study, a quantitative proteomic analysis was conducted to profile acetylome in porcine muscle within 24 h PM. In total 595 acetylation sites assigned to 163 proteins were identified in porcine muscle, of which 460 sites distributing to 110 proteins significantly changed in acetylation levels in the conversion of muscle to meat. The dynamic acetylation/deacetylaion of muscle proteins was closely associated with critical chemical-biophysical changes in PM muscle. Bioinformatic analysis revealed that protein lysine acetylation likely regulated postmortem meat quality development by regulating glycolysis and muscle pH, cell stress reponse and apoptosis, muscle contraction and rigor mortis, calcium signaling and proteolysis, IMP synthesis and meat flavor development, and even the stability of pigment proteins and meat color. This study provided the first overview of protein lysine acetylation in PM muscle and revealed its significance in the conversion of muscle to meat. Future exploration of the exact role of protein lysine acetylation at specific sites will further our understanding regarding the underlying mechanisms and be helpful for meat quality control. SIGNIFICANCE: This is the first analysis of acetylome in farm animal and postmortem muscle. Our data showed that the dynamic acetylation/deacetylation of muscle proteins was closely related to the postmortem changes of muscle that affect the final quality of raw meat. Proteins related to glucose metabolism and muscle contraction were the two largest clusters of acetylproteins identified in postmortem porcine muscle. Networks of acetylproteins involved in apoptosis, calcium signaling and IMP synthesis were identified in postmortem porcine muscle at the same time. Our results revealed that protein lysine acetylation regulated the conversion of muscle to meat. It likely regulated meat quality development by regulating postmortem glycolysis, mitochondrion initiated cell apoptosis, calcium signaling, rigor mortis, meat flavor compound sysnthesis and meat tenderization. Our study broadened our understanding of the biochemistry regulating the postmortem conversion of muscle to meat and final meat quality development, which may be helpful for future meat quality control.

Early myocardial changes induced by doxorubicin in the nonfailing dilated ventricle

Several studies have demonstrated that administration of doxorubicin (DOXO) results in cardiotoxicity, which eventually progresses to dilated cardiomyopathy. The present work aimed to evaluate the early myocardial changes of DOXO-induced cardiotoxicity. Male New Zealand White rabbits were injected intravenously with DOXO twice weekly for 8 wk [DOXO-induced heart failure (DOXO-HF)] or with an equivolumetric dose of saline (control). Echocardiographic evaluation was performed, and myocardial samples were collected to evaluate myocardial cellular and molecular modifications. The DOXO-HF group presented cardiac hypertrophy and higher left ventricular cavity diameters, showing a dilated phenotype but preserved ejection fraction. Concerning cardiomyocyte function, the DOXO-HF group presented a trend toward increased active tension without significant differences in passive tension. The myocardial GSSG-to-GSH ratio and interstitial fibrosis were increased and Bax-to- Bcl-2 ratio presented a trend toward an increase, suggesting the activation of apoptosis signaling pathways. The macromolecule titin shifted toward the more compliant isoform (N2BA), whereas the stiffer one (N2B) was shown to be hypophosphorylated. Differential protein analysis from the aggregate-enriched fraction through gel liquid chromatography-tandem mass spectrometry revealed an increase in the histidine-rich glycoprotein fragment in DOXO-HF animals. This work describes novel and early myocardial effects of DOXO-induced cardiotoxicity. Thus, tracking these changes appears to be of extreme relevance for the early detection of cardiac damage (as soon as ventricular dilation becomes evident) before irreversible cardiac function deterioration occurs (reduced ejection fraction). Moreover, it allows for the adjustment of the therapeutic approach and thus the prevention of cardiomyopathy progression.

NEW & NOTEWORTHY Identification of early myocardial effects of doxorubicin in the heart is essential to hinder the development of cardiac complications and adjust the therapeutic approach. This study describes doxorubicin-induced cellular and molecular modifications before the onset of dilated cardiomyopathy. Myocardial samples from doxorubicin-treated rabbits showed a tendency for higher cardiomyocyte active tension, titin isoform shift from N2B to N2BA, hypophosphorylation of N2B, increased apoptotic genes, left ventricular interstitial fibrosis, and increased aggregation of histidine-rich glycoprotein.

Cardiac contractility modulation: mechanisms of action in heart failure with reduced ejection fraction and beyond

Heart failure (HF) is responsible for substantial morbidity and mortality and is increasing in prevalence. Although there has been remarkable progress in the treatment of HF with reduced ejection fraction (HFrEF), morbidity and mortality are still substantial. Cardiac contractility modulation (CCM) signals, consisting of biphasic high-voltage bipolar signals delivered to the right ventricular septum during the absolute refractory period, have been shown to improve symptoms, exercise tolerance and quality of life and reduce the rate of HF hospitalizations in patients with ejection fractions (EF) between 25% and 45%. CCM therapy is currently approved in the European Union, China, India, Australia and Brazil for use in symptomatic HFrEF patients with normal or slightly prolonged QRS duration. CCM is particularly beneficial in patients with baseline EF between 35% and 45%, which includes half the range of HF patients with mid-range EFs (HFmrEF). At the cellular level, CCM has been shown in HFrEF patients to improve calcium handling, to reverse the foetal myocyte gene programme associated with HF, and to facilitate reverse remodelling. This review highlights the preclinical and clinical literature related to CCM in HFrEF and HFmrEF and outlines the potential of CCM for HF with preserved EF, concluding that CCM may fill an important unmet need in the therapeutic approach to HF across the range of EFs.

Metformin improves diastolic function in an HFpEF-like mouse model by increasing titin compliance

Heart failure with preserved ejection fraction (HFpEF) is a syndrome characterized by increased diastolic stiffness, for which effective therapies are lacking. Slater et al. show that metformin lowers titin-based passive stiffness in an HFpEF mouse model and may therefore be of therapeutic benefit.

Heart failure with preserved ejection fraction (HFpEF) is a complex syndrome characterized by a preserved ejection fraction but increased diastolic stiffness and abnormalities of filling. Although the prevalence of HFpEF is high and continues to rise, no effective therapies exist; however, the diabetic drug metformin has been associated with improved diastolic function in diabetic patients. Here we determine the therapeutic potential of metformin for improving diastolic function in a mouse model with HFpEF-like symptoms. We combine transverse aortic constriction (TAC) surgery with deoxycorticosterone acetate (DOCA) supplementation to obtain a mouse model with increased diastolic stiffness and exercise intolerance. Echocardiography and pressure–volume analysis reveal that providing metformin to TAC/DOCA mice improves diastolic function in the left ventricular (LV) chamber. Muscle mechanics show that metformin lowers passive stiffness of the LV wall muscle. Concomitant with this improvement in diastolic function, metformin-treated TAC/DOCA mice also demonstrate preserved exercise capacity. No metformin effects are seen in sham operated mice. Extraction experiments on skinned ventricular muscle strips show that the metformin-induced reduction of passive stiffness in TAC/DOCA mice is due to an increase in titin compliance. Using phospho-site-specific antibodies, we assay the phosphorylation of titin’s PEVK and N2B spring elements. Metformin-treated mice have unaltered PEVK phosphorylation but increased phosphorylation of PKA sites in the N2B element, a change which has previously been shown to lower titin’s stiffness. Consistent with this result, experiments with a mouse model deficient in the N2B element reveal that the beneficial effect of metformin on LV chamber and muscle stiffness requires the presence of the N2B element. We conclude that metformin offers therapeutic benefit during HFpEF by lowering titin-based passive stiffness.

Regulatory light chain phosphorylation augments length-dependent contraction in PTU-treated rats

Contraction of cardiac muscle is regulated by sarcomere length and proteins that comprise the sarcomeric filaments. Breithaupt et al. find that phosphorylation of myosin regulatory light chain augments length-dependent activation of contraction when β-cardiac myosin heavy chain predominates.

Force production by actin–myosin cross-bridges in cardiac muscle is regulated by thin-filament proteins and sarcomere length (SL) throughout the heartbeat. Prior work has shown that myosin regulatory light chain (RLC), which binds to the neck of myosin heavy chain, increases cardiac contractility when phosphorylated. We recently showed that cross-bridge kinetics slow with increasing SLs, and that RLC phosphorylation amplifies this effect, using skinned rat myocardial strips predominantly composed of the faster α-cardiac myosin heavy chain isoform. In the present study, to assess how RLC phosphorylation influences length-dependent myosin function as myosin motor speed varies, we used a propylthiouracil (PTU) diet to induce >95% expression of the slower β-myosin heavy chain isoform in rat cardiac ventricles. We measured the effect of RLC phosphorylation on Ca²⁺-activated isometric contraction and myosin cross-bridge kinetics (via stochastic length perturbation analysis) in skinned rat papillary muscle strips at 1.9- and 2.2-µm SL. Maximum tension and Ca²⁺ sensitivity increased with SL, and RLC phosphorylation augmented this response at 2.2-µm SL. Subtle increases in viscoelastic myocardial stiffness occurred with RLC phosphorylation at 2.2-µm SL, but not at 1.9-µm SL, thereby suggesting that RLC phosphorylation increases β-myosin heavy chain binding or stiffness at longer SLs. The cross-bridge detachment rate slowed as SL increased, providing a potential mechanism for prolonged cross-bridge attachment to augment length-dependent activation of contraction at longer SLs. Length-dependent slowing of β-myosin heavy chain detachment rate was not affected by RLC phosphorylation. Together with our previous studies, these data suggest that both α- and β-myosin heavy chain isoforms show a length-dependent activation response and prolonged myosin attachment as SL increases in rat myocardial strips, and that RLC phosphorylation augments length-dependent activation at longer SLs. In comparing cardiac isoforms, however, we found that β-myosin heavy chain consistently showed greater length-dependent sensitivity than α-myosin heavy chain. Our work suggests that RLC phosphorylation is a vital contributor to the regulation of myocardial contractility in both cardiac myosin heavy chain isoforms.

Cardiac-specific knockout of Lmod2 results in a severe reduction in myofilament force production and rapid cardiac failure

Leiomodin-2 (Lmod2) is a striated muscle-specific actin binding protein that is implicated in assembly of thin filaments. The necessity of Lmod2 in the adult mouse and role it plays in the mechanics of contraction are unknown. To answer these questions, we generated cardiac-specific conditional Lmod2 knockout mice (cKO). These mice die within a week of induction of the knockout with severe left ventricular systolic dysfunction and little change in cardiac morphology. Cardiac trabeculae isolated from cKO mice have a significant decrease in maximum force production and a blunting of myofilament length-dependent activation. Thin filaments are non-uniform and substantially reduced in length in cKO hearts, affecting the functional overlap of the thick and thin filaments. Remarkably, we also found that Lmod2 levels are directly linked to thin filament length and cardiac function in vivo, with a low amount (<20%) of Lmod2 necessary to maintain cardiac function. Thus, Lmod2 plays an essential role in maintaining proper cardiac thin filament length in adult mice, which in turn is necessary for proper generation of contractile force. Dysregulation of thin filament length in the absence of Lmod2 contributes to heart failure.

Force properties of skinned cardiac muscle following increasing volumes of aerobic exercise in rats

The positive effects of chronic endurance exercise training on health and performance have been well documented. These positive effects have been evaluated primarily at the structural level, and work has begun to evaluate mechanical adaptations of the myocardium. However, it remains poorly understood how the volume of exercise training affects cardiac adaptation. To gain some understanding, we subjected 3-mo-old Sprague-Dawley rats ( n = 23) to treadmill running for 11 wk at one of three exercise volumes (moderate, high, and extra high). Following training, hearts were excised and mechanical testing was completed on skinned trabecular fiber bundles. Performance on a maximal fitness test was dose dependent on training volume, where greater levels of training led to greater performance. No differences were observed between animals from any group for maximal active stress and passive stress at a sarcomere length of 2.2 µm. Heart mass and passive stress at sarcomere lengths beyond 2.4 µm increased in a dose-dependent manner for animals in the control and moderate- and high-duration groups. However, hearts from animals in the extra high-duration group presented with inhibited responses for heart mass and passive stress, despite performing greatest on a graded treadmill fitness test. These results suggest that heart mass and passive stress adapt in a dose-dependent manner, until exercise becomes excessive and adaptation is inhibited. Our findings are in agreement with the beneficial role exercise has in cardiac adaptation. However, excessive exercise comes with risks of maladaptation, which must be weighed against the desire to increase performance. NEW & NOTEWORTHY For the first time, we present findings on cardiac trabecular muscle passive stiffness and show the effect of excessive exercise on the heart. We demonstrated that heart mass increases with exercise until a maximum, after which greater exercise volume results in inhibited adaptation. At paraphysiological lengths, passive stiffness increases with exercise but to a lesser degree with excessive training. Despite greater performance on graded exercise tests, animals in the highest trained group exhibited possible maladaptation.

Increased cross-bridge recruitment contributes to transient increase in force generation beyond maximal capacity in human myocardium

Cross-bridge attachment allows force generation to occur, and rate of tension redevelopment (k_tr) is a commonly used index of cross-bridge cycling rate. Tension overshoots have been observed briefly after a slack-restretch k_tr maneuver in various species of animal models and humans. In this study, we set out to determine the properties of these overshoots and their possible underlying mechanism. Utilizing human cardiac trabeculae, we have found that tension overshoots are temperature-dependent and that they do not occur at resting states. In addition, we have found that myosin cross-bridge cycle is vital to these overshoots as inhibition of the cycle results in the blunting of the overshoots and the magnitude of the overshoots are dependent on the level of myofilament activation. Lastly, we show that the number of cross-bridges transiently increase during tension overshoots. These findings lead us to conclude that tension overshoots are likely due to a transient enhancement of the recruitment of myosin heads into the cross-bridge cycling, regulated by the myocardium, and with potential physiological significance in determining cardiac output.

NEWS AND NOTEWORTHY

We show that isolated human myocardium is capable of transiently increasing its maximal force generation capability by increasing cross-bridge recruitment following slack-restretch maneuver. This process can potentially have important implications and significance in cardiac contraction in vivo.

Biological Phenotypes of Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) involves multiple pathophysiological mechanisms, which result in the heterogeneous phenotypes that are evident clinically, and which have potentially confounded previous HFpEF trials. A greater understanding of the in vivo human processes involved, and in particular, which are the causes and which are the downstream effects, may allow the syndrome of HFpEF to be distilled into distinct diagnoses based on the underlying biology. From this, specific interventions can follow, targeting individuals identified on the basis of their biological phenotype. This review describes the biological phenotypes of HFpEF and therapeutic interventions aimed at targeting these phenotypes.

Targets for Heart Failure With Preserved Ejection Fraction

Heart failure (HF) with preserved ejection fraction (HFPEF) is responsible for half of all HF cases and will be the most common form of HF within the next 5 years. Previous studies of pharmacological agents in HFPEF have proved neutral or negative, in part due to phenotypic heterogeneity and complex underlying mechanisms. This review summarizes the key molecular and cellular pathways characterized in HFPEF as well as current and future therapies that target these mechanisms.

α-B Crystallin Reverses High Diastolic Stiffness of Failing Human Cardiomyocytes

BACKGROUND

Cardiomyocytes with a less distensible titin and interstitial collagen contribute to the high diastolic stiffness of failing myocardium. Their relative contributions and mechanisms underlying loss of titin distensibility were assessed in failing human hearts.

METHODS AND RESULTS

Left ventricular tissue was procured in patients with aortic stenosis (AS, n=9) and dilated cardiomyopathy (DCM, n=6). Explanted donor hearts (n=8) served as controls. Stretches were performed in myocardial strips, and an extraction protocol differentiated between passive tension (F_passive) attributable to cardiomyocytes or to collagen. F_passive-cardiomyocytes was higher in AS and DCM at shorter muscle lengths, whereas F_passive-collagen was higher in AS at longer muscle lengths and in DCM at shorter and longer muscle lengths. Cardiomyocytes were stretched to investigate titin distensibility. Cardiomyocytes were incubated with alkaline phosphatase, subsequently reassessed after a period of prestretch and finally treated with the heat shock protein α-B crystallin. Alkaline phosphatase shifted the F_passive-sarcomere length relation upward only in donor. Prestretch shifted the F_passive-sarcomere length relation further upward in donor and upward in AS and DCM. α-B crystallin shifted the F_passive-sarcomere length relation downward to baseline in donor and to lower than baseline in AS and DCM. In failing myocardium, confocal laser microscopy revealed α-B crystallin in subsarcolemmal aggresomes.

CONCLUSIONS

High cardiomyocyte stiffness contributed to stiffness of failing human myocardium because of reduced titin distensibility. The latter resulted from an absent stiffness-lowering effect of baseline phosphorylation and from titin aggregation. High cardiomyocyte stiffness was corrected by α-B crystallin probably through relief of titin aggregation.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Effect of exercise on passive myocardial stiffness in mice with diastolic dysfunction

Heart failure with preserved ejection fraction (HFpEF) is a complex syndrome, characterized by increased diastolic stiffness and a preserved ejection fraction, with no effective treatment options. Here we studied the therapeutic potential of exercise for improving diastolic function in a mouse model with HFpEF-like symptoms, the TtnΔIAjxn mouse model. TtnΔIAjxn mice have increased diastolic stiffness and reduced exercise tolerance, mimicking aspects of HFpEF observed in patients. We investigated the effect of free-wheel running exercise on diastolic function. Mechanical studies on cardiac muscle strips from the LV free wall revealed that both TtnΔIAjxn and wildtype (WT) exercised mice had a reduction in passive stiffness, relative to sedentary controls. In both genotypes, this reduction is due to an increase in the compliance of titin whereas ECM-based stiffness was unaffected. Phosphorylation of titin's PEVK and N2B spring elements were assayed with phospho-site specific antibodies. Exercised mice had decreased PEVK phosphorylation and increased N2B phosphorylation both of which are predicted to contribute to the increased compliance of titin. Since exercise lowers the heart rate we examined whether reduction in heart rate per se can improve passive stiffness by administering the heart-rate-lowering drug ivabradine. Ivabradine lowered heart rate in our study but it did not affect passive tension, in neither WT nor TtnΔIAjxn mice. We conclude that exercise is beneficial for decreasing passive stiffness and that it involves beneficial alterations in titin phosphorylation.

Tampering with springs: phosphorylation of titin affecting the mechanical function of cardiomyocytes

Reversible post-translational modifications of various cardiac proteins regulate the mechanical properties of the cardiomyocytes and thus modulate the contractile performance of the heart. The giant protein titin forms a continuous filament network in the sarcomeres of striated muscle cells, where it determines passive tension development and modulates active contraction. These mechanical properties of titin are altered through post-translational modifications, particularly phosphorylation. Titin contains hundreds of potential phosphorylation sites, the functional relevance of which is only beginning to emerge. Here, we provide a state-of-the-art summary of the phosphorylation sites in titin, with a particular focus on the elastic titin spring segment. We discuss how phosphorylation at specific amino acids can reduce or increase the stretch-induced spring force of titin, depending on where the spring region is phosphorylated. We also review which protein kinases phosphorylate titin and how this phosphorylation affects titin-based passive tension in cardiomyocytes. A comprehensive overview is provided of studies that have measured altered titin phosphorylation and titin-based passive tension in myocardial samples from human heart failure patients and animal models of heart disease. As our understanding of the broader implications of phosphorylation in titin progresses, this knowledge could be used to design targeted interventions aimed at reducing pathologically increased titin stiffness in patients with stiff hearts.

Global comparison of phosphoproteins in human and rodent hearts: implications for translational studies of myosin light chain and troponin phosphorylations

Cardiac remodeling and failure are regulated by a myriad of cardiac protein phosphorylations. In the present study, cardiac phosphoprotein patterns were examined in rodent and human hearts Left ventricular tissue samples were obtained from human systolic failing (n = 5) and control (n = 5) hearts and from two rat models of hypertensive heart failure, i.e., spontaneously hypertensive heart failure and Dahl salt-sensitive rats and corresponding controls. Phosphoproteins were separated by 2D-DIGE with Cydye staining, phosphoprotein patterns were analyzed using pixel intensity in rectified images. Specific phosphoproteins which were different in human versus rodent hearts were identified by MALDI-TOF/TOF Mass Spectrometry. Targeted pair-wise analyses showed differences (p < 0.05) in 26 % of the pixels, which included pixels containing phosphorylated troponin T, myosin light chain, peroxiredoxin, and haptoglobin. These results show differences in rodent versus human cardiac remodeling which will influence the translation rodent studies to humans in this area.

The contributions of cardiac myosin binding protein C and troponin I phosphorylation to β-adrenergic enhancement of in vivo cardiac function

KEY POINTS

β-adrenergic stimulation increases cardiac myosin binding protein C (MyBP-C) and troponin I phosphorylation to accelerate pressure development and relaxation in vivo, although their relative contributions remain unknown. Using a novel mouse model lacking protein kinase A-phosphorylatable troponin I (TnI) and MyBP-C, we examined in vivo haemodynamic function before and after infusion of the β-agonist dobutamine. Mice expressing phospho-ablated MyBP-C displayed cardiac hypertrophy and prevented full acceleration of pressure development and relaxation in response to dobutamine, whereas expression of phosphor-ablated TnI alone had little effect on the acceleration of contractile function in response to dobutamine. Our data demonstrate that MyBP-C phosphorylation is the principal mediator of the contractile response to increased β-agonist stimulation in vivo. These results help us understand why MyBP-C dephosphorylation in the failing heart contributes to contractile dysfunction and decreased adrenergic reserve in response to acute stress. β-adrenergic stimulation plays a critical role in accelerating ventricular contraction and speeding relaxation to match cardiac output to changing circulatory demands. Two key myofilaments proteins, troponin I (TnI) and myosin binding protein-C (MyBP-C), are phosphorylated following β-adrenergic stimulation; however, their relative contributions to the enhancement of in vivo cardiac contractility are unknown. To examine the roles of TnI and MyBP-C phosphorylation in β-adrenergic-mediated enhancement of cardiac function, transgenic (TG) mice expressing non-phosphorylatable TnI protein kinase A (PKA) residues (i.e. serine to alanine substitution at Ser23/24; TnI(PKA-)) were bred with mice expressing non-phosphorylatable MyBP-C PKA residues (i.e. serine to alanine substitution at Ser273, Ser282 and Ser302; MyBPC(PKA-)) to generate a novel mouse model expressing non-phosphorylatable PKA residues in TnI and MyBP-C (DBL(PKA-)). MyBP-C dephosphorylation produced cardiac hypertrophy and increased wall thickness in MyBPC(PKA-) and DBL(PKA-) mice, and in vivo echocardiography and pressure-volume catheterization studies revealed impaired systolic function and prolonged diastolic relaxation compared to wild-type and TnI(PKA-) mice. Infusion of the β-agonist dobutamine resulted in accelerated rates of pressure development and relaxation in all mice; however, MyBPC(PKA-) and DBL(PKA-) mice displayed a blunted contractile response compared to wild-type and TnI(PKA-) mice. Furthermore, unanaesthesized MyBPC(PKA-) and DBL(PKA-) mice displayed depressed maximum systolic pressure in response to dobutamine as measured using implantable telemetry devices. Taken together, our data show that MyBP-C phosphorylation is a critical modulator of the in vivo acceleration of pressure development and relaxation as a result of enhanced β-adrenergic stimulation, and reduced MyBP-C phosphorylation may underlie depressed adrenergic reserve in heart failure.

Phosphorylating Titin's Cardiac N2B Element by ERK2 or CaMKIIδ Lowers the Single Molecule and Cardiac Muscle Force

Titin is a large filamentous protein that is responsible for the passive force of the cardiac sarcomere. Titin's force is generated by its I-band region, which includes the cardiac-specific N2B element. The N2B element consists of three immunoglobulin domains, two small unique sequence insertions, and a large 575-residue unique sequence, the N2B-Us. Posttranslational modifications of the N2B element are thought to regulate passive force, but the underlying mechanisms are unknown. Increased passive-force levels characterize diastolic stiffening in heart-failure patients, and it is critical to understand the underlying molecular mechanisms and identify therapeutic targets. Here, we used single-molecule force spectroscopy to study the mechanical effects of the kinases calcium/calmodulin-dependent protein kinase II delta (CaMKIIδ) and extracellular signal-regulated kinase 2 (ERK2) on the single-molecule mechanics of the N2B element. Both CaMKIIδ and ERK2 were found to phosphorylate the N2B element, and single-molecule force spectroscopy revealed an increase in the persistence length (Lp) of the molecule, indicating that the bending rigidity of the molecule was increased. Experiments performed under oxidizing conditions and with a recombinant N2B element that had a simplified domain composition provided evidence that the Lp increase requires the N2B-Us of the N2B element. Mechanical experiments were also performed on skinned myocardium before and after phosphorylation. The results revealed a large (∼30%) passive force reduction caused by CaMKIIδ and a much smaller (∼6%) reduction caused by ERK2. These findings support the notion that the important kinases ERK2 and CaMKIIδ can alter the passive force of myocytes in the heart (although CaMKIIδ appears to be more potent) during physiological and pathophysiological states.

Toward Understanding the Molecular Bases of Stretch Activation: A STRUCTURAL COMPARISON OF THE TWO TROPONIN C ISOFORMS OF LETHOCERUS

Muscles are usually activated by calcium binding to the calcium sensory protein troponin-C, which is one of the three components of the troponin complex. However, in cardiac and insect flight muscle activation is also produced by mechanical stress. Little is known about the molecular bases of this calcium-independent activation. In Lethocerus, a giant water bug often used as a model system because of its large muscle fibers, there are two troponin-C isoforms, called F1 and F2, that have distinct roles in activating the muscle. It has been suggested that this can be explained either by differences in structural features or by differences in the interactions with other proteins. Here we have compared the structural and dynamic properties of the two proteins and shown how they differ. We have also mapped the interactions of the F2 isoform with peptides spanning the sequence of its natural partner, troponin-I. Our data have allowed us to build a model of the troponin complex and may eventually help in understanding the specialized function of the F1 and F2 isoforms and the molecular mechanism of stretch activation.