PKA accelerates rate of force development in murine skinned myocardium expressing alpha- or beta-tropomyosin

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.

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.

Mutations in myosin S2 alter cardiac myosin-binding protein-C interaction in hypertrophic cardiomyopathy in a phosphorylation-dependent manner

Hypertrophic cardiomyopathy (HCM) is an inherited cardiovascular disorder primarily caused by mutations in the β-myosin heavy-chain gene. The proximal subfragment 2 region (S2), 126 amino acids of myosin, binds with the C0-C2 region of cardiac myosin-binding protein-C to regulate cardiac muscle contractility in a manner dependent on PKA-mediated phosphorylation. However, it is unknown if HCM-associated mutations within S2 dysregulate actomyosin dynamics by disrupting its interaction with C0-C2, ultimately leading to HCM. Herein, we study three S2 mutations known to cause HCM: R870H, E924K, and E930Δ. First, experiments using recombinant proteins, solid-phase binding, and isothermal titrating calorimetry assays independently revealed that mutant S2 proteins displayed significantly reduced binding with C0-C2. In addition, CD revealed greater instability of the coiled-coil structure in mutant S2 proteins compared with S2^Wt proteins. Second, mutant S2 exhibited 5-fold greater affinity for PKA-treated C0-C2 proteins. Third, skinned papillary muscle fibers treated with mutant S2 proteins showed no change in the rate of force redevelopment as a measure of actin–myosin cross-bridge kinetics, whereas S2^Wt showed increased the rate of force redevelopment. In summary, S2 and C0-C2 interaction mediated by phosphorylation is altered by mutations in S2, which augment the speed and force of contraction observed in HCM. Modulating this interaction could be a potential strategy to treat HCM in the future.

High efficiency preparation of skinned mouse cardiac muscle strips from cryosections for contractility studies

NEW FINDINGS

What is the central question of this study? Can frozen cardiac papillary muscles and cryosectioning be used to reliably obtain uniform cardiac muscle strips with high yields? What is the main finding and its importance? A new method was developed using frozen cardiac papillary muscles and cryosectioning to reliably obtain uniform cardiac muscle strips with high yields. Experimental results demonstrate that this new methodology significantly increases the efficiency and application of quantitative biomechanical studies using skinned muscle fibres with an additional advantage of no need for transferring live animals.

ABSTRACT

Skinned cardiac muscle preparations are widely used to study contractile function of myofilament proteins and pathophysiological changes. The current methods applied in these biomechanical studies include detergent permeabilization of freshly isolated papillary muscle, ventricular trabeculae, surgically dissected ventricular muscle strips, mechanically blended cardiac muscle bundles or myocytes, and enzymatically isolated single cardiomyocytes. To facilitate and expand the skinned cardiac muscle approach, we have developed an efficient and readily practical method for mechanical studies of skinned mouse cardiac papillary muscle strips prepared from cryosections. Longitudinal papillary muscle strips of 120-150 µm width cut from 35-70 µm-thick cryosections are mounted to a force transducer and chemically skinned for the studies of force-pCa and sarcomere length-tension relationship and rate of tension redevelopment. In addition to more effective skinning and perfusion than with whole papillary muscle and much higher yield of useful preparations than that from trabeculae, this new methodology has two more major advantages. One is to allow for the use of frozen cardiac muscle in storage to maximize the value of muscle samples, facilitating resource sharing among research institutions without the need of transferring live animals or fresh biopsies. The other is that the integrity of the muscle strips is well preserved during the preparation and mechanical studies, allowing coupled characterization of myofilament proteins. The combined power of biomechanics and protein biochemistry can provide novel insights into integrative physiological and pathophysiological mechanisms of cardiac muscle contraction while the high yield of high-quality muscle strips also provides an efficient platform for development of therapeutic reagents.

Regulation of myofilament force and loaded shortening by skeletal myosin binding protein C

Myosin binding protein C (MyBP-C) is thought to regulate the contraction of skeletal muscle. Robinett et al. show that phosphorylation of slow skeletal MyBP-C modulates contraction by recruiting cross-bridges, modifying cross-bridge kinetics, and altering internal drag forces in the C-zone.

Myosin binding protein C (MyBP-C) is a 125–140-kD protein located in the C-zone of each half-thick filament. It is thought to be an important regulator of contraction, but its precise role is unclear. Here we investigate mechanisms by which skeletal MyBP-C regulates myofilament function using rat permeabilized skeletal muscle fibers. We mount either slow-twitch or fast-twitch skeletal muscle fibers between a force transducer and motor, use Ca²⁺ to activate a range of forces, and measure contractile properties including transient force overshoot, rate of force development, and loaded sarcomere shortening. The transient force overshoot is greater in slow-twitch than fast-twitch fibers at all Ca²⁺ activation levels. In slow-twitch fibers, protein kinase A (PKA) treatment (a) augments phosphorylation of slow skeletal MyBP-C (sMyBP-C), (b) doubles the magnitude of the relative transient force overshoot at low Ca²⁺ activation levels, and (c) increases force development rates at all Ca²⁺ activation levels. We also investigate the role that phosphorylated and dephosphorylated sMyBP-C plays in loaded sarcomere shortening. We test the hypothesis that MyBP-C acts as a brake to filament sliding within the myofilament lattice by measuring sarcomere shortening as thin filaments traverse into the C-zone during lightly loaded slow-twitch fiber contractions. Before PKA treatment, shortening velocity decelerates as sarcomeres traverse from ∼3.10 to ∼3.00 µm. After PKA treatment, sarcomeres shorten a greater distance and exhibit less deceleration during similar force clamps. After sMyBP-C dephosphorylation, sarcomere length traces display a brief recoil (i.e., “bump”) that initiates at ∼3.06 µm during loaded shortening. Interestingly, the timing of the bump shifts with changes in load but manifests at the same sarcomere length. Our results suggest that sMyBP-C and its phosphorylation state regulate sarcomere contraction by a combination of cross-bridge recruitment, modification of cross-bridge cycling kinetics, and alteration of drag forces that originate in the C-zone.

Deletion of Enigma Homologue from the Z-disc slows tension development kinetics in mouse myocardium

Enigma Homologue (ENH) is a component of the Z-disc, a structure that anchors actin filaments in the contractile unit of muscle, the sarcomere. Cardiac-specific ablation of ENH protein expression causes contractile dysfunction that ultimately culminates in dilated cardiomyopathy. However, whether ENH is involved in the regulation of myocardial contractility is unknown. To determine if ENH is required for the mechanical activity of cardiac muscle, we analyze muscle mechanics of isolated trabeculae from the hearts of ENH ^+/+ and ENH ^-/- mice. We detected no differences in steady-state mechanical properties but show that when muscle fibers are allowed to relax and then are restretched, the rate at which tension redevelops is depressed in ENH ^-/- mouse myocardium relative to that in ENH ^+/+ myocardium. SDS-PAGE analysis demonstrated that the expression of β-myosin heavy chain is increased in ENH ^-/- mouse myocardium, which could partially, but not completely, account for the depression in tension redevelopment kinetics. Using top-down proteomics analysis, we found that the expression of other thin/thick filament regulatory proteins is unaltered, although the phosphorylation of a cardiac troponin T isoform, cardiac troponin I, and myosin regulatory light chain is decreased in ENH ^-/- mouse myocardium. Nevertheless, these alterations are very small and thus insufficient to explain slowed tension redevelopment kinetics in ENH ^-/- mouse myocardium. These data suggest that the ENH protein influences tension redevelopment kinetics in mouse myocardium, possibly by affecting cross-bridge cycling kinetics. Previous studies also indicate that ablation of specific Z-disc proteins in myocardium slows contraction kinetics, which could also be a contributing factor in this study.

Monophosphorylation of cardiac troponin-I at Ser-23/24 is sufficient to regulate cardiac myofibrillar Ca2+ sensitivity and calpain-induced proteolysis

The acceleration of myocardial relaxation produced by β-adrenoreceptor stimulation is mediated in part by protein kinase A (PKA)-mediated phosphorylation of cardiac troponin-I (cTnI), which decreases myofibrillar Ca²⁺ sensitivity. Previous evidence suggests that phosphorylation of both Ser-23 and Ser-24 in cTnI is required for this Ca²⁺ desensitization. PKA-mediated phosphorylation also partially protects cTnI from proteolysis by calpain. Here we report that protein kinase D (PKD) phosphorylates only one serine of cTnI Ser-23/24. To explore the functional consequences of this monophosphorylation, we examined the Ca²⁺ sensitivity of force production and susceptibility of cTnI to calpain-mediated proteolysis when Ser-23/24 of cTnI in mouse cardiac myofibrils was nonphosphorylated, mono-phosphorylated, or bisphosphorylated (using sequential incubations in λ-phosphatase, PKD, and PKA, respectively). Phos-tag gels, Western blotting, and high-resolution MS revealed that PKD produced >90% monophosphorylation of cTnI, primarily at Ser-24, whereas PKA led to cTnI bisphosphorylation exclusively. PKD markedly decreased the Ca²⁺ sensitivity of force production in detergent-permeabilized ventricular trabeculae, whereas subsequent incubation with PKA produced only a small further fall of Ca²⁺ sensitivity. Unlike PKD, PKA also substantially phosphorylated myosin-binding protein-C and significantly accelerated cross-bridge kinetics (k_tr). After phosphorylation by PKD or PKA, cTnI in isolated myofibrils was partially protected from calpain-mediated degradation. We conclude that cTnI monophosphorylation at Ser-23/24 decreases myofibrillar Ca²⁺ sensitivity and partially protects cTnI from calpain-induced proteolysis. In healthy cardiomyocytes, the basal monophosphorylation of cTnI may help tonically regulate myofibrillar Ca²⁺ sensitivity.

Altered Right Ventricular Mechanical Properties Are Afterload Dependent in a Rodent Model of Bronchopulmonary Dysplasia

Infants born premature are at increased risk for development of bronchopulmonary dysplasia (BPD), pulmonary hypertension (PH), and ultimately right ventricular (RV) dysfunction, which together carry a high risk of neonatal mortality. However, the role alveolar simplification and abnormal pulmonary microvascular development in BPD affects RV contractile properties is unknown. We used a rat model of BPD to examine the effect of hyperoxia-induced PH on RV contractile properties. We measured in vivo RV pressure as well as passive force, maximum Ca²⁺ activated force, calcium sensitivity of force (pCa₅₀) and rate of force redevelopment (k_tr) in RV skinned trabeculae isolated from hearts of 21-and 35-day old rats pre-exposed to 21% oxygen (normoxia) or 85% oxygen (hyperoxia) for 14 days after birth. Systolic and diastolic RV pressure were significantly higher at day 21 in hyperoxia exposed rats compared to normoxia control rats, but normalized by 35 days of age. Passive force, maximum Ca²⁺ activated force, and calcium sensitivity of force were elevated and cross-bridge cycling kinetics depressed in 21-day old hyperoxic trabeculae, whereas no differences between normoxic and hyperoxic trabeculae were seen at 35 days. Myofibrillar protein analysis revealed that 21-day old hyperoxic trabeculae had increased levels of beta-myosin heavy chain (β-MHC), atrial myosin light chain 1 (aMLC1; often referred to as essential light chain), and slow skeletal troponin I (ssTnI) compared to age matched normoxic trabeculae. On the other hand, 35-day old normoxic and hyperoxic trabeculae expressed similar level of α- and β-MHC, ventricular MLC1 and predominantly cTnI. These results suggest that neonatal exposure to hyperoxia increases RV afterload and affect both the steady state and dynamic contractile properties of the RV, likely as a result of hyperoxia-induced expression of β-MHC, delayed transition of slow skeletal TnI to cardiac TnI, and expression of atrial MLC1. These hyperoxia-induced changes in contractile properties are reversible and accompany the resolution of PH with further developmental age, underscoring the importance of reducing RV afterload to allow for normalization of RV function in both animal models and humans with BPD.

Cardiac myofibrillar contractile properties during the progression from hypertension to decompensated heart failure

Heart failure arises, in part, from a constellation of changes in cardiac myocytes including remodeling, energetics, Ca2+ handling, and myofibrillar function. However, little is known about the changes in myofibrillar contractile properties during the progression from hypertension to decompensated heart failure. The aim of the present study was to provide a comprehensive assessment of myofibrillar functional properties from health to heart disease. A rodent model of uncontrolled hypertension was used to test the hypothesis that myocytes in compensated hearts exhibit increased force, higher rates of force development, faster loaded shortening, and greater power output; however, with progression to overt heart failure, we predicted marked depression in these contractile properties. We assessed contractile properties in skinned cardiac myocyte preparations from left ventricles of Wistar-Kyoto control rats and spontaneous hypertensive heart failure (SHHF) rats at ~3, ~12, and >20 mo of age to evaluate the time course of myofilament properties associated with normal aging processes compared with myofilaments from rats with a predisposition to heart failure. In control rats, the myofilament contractile properties were virtually unchanged throughout the aging process. Conversely, in SHHF rats, the rate of force development, loaded shortening velocity, and power all increased at ~12 mo and then significantly fell at the >20-mo time point, which coincided with a decrease in left ventricular fractional shortening. Furthermore, these changes occurred independent of changes in β-myosin heavy chain but were associated with depressed phosphorylation of myofibrillar proteins, and the fall in loaded shortening and peak power output corresponded with the onset of clinical signs of heart failure.NEW & NOTEWORTHY This novel study systematically examined the power-generating capacity of cardiac myofilaments during the progression from hypertension to heart disease. Previously undiscovered changes in myofibrillar power output were found and were associated with alterations in myofilament proteins, providing potential new targets to exploit for improved ventricular pump function in heart failure.

Peroxisome proliferator-activated receptor-α expression induces alterations in cardiac myofilaments in a pressure-overload model of hypertrophy

Although alterations in fatty acid (FA) metabolism have been shown to have a negative impact on contractility of the hypertrophied heart, the targets of action remain elusive. In this study we compared the function of skinned fiber bundles from transgenic (Tg) mice that overexpress a relatively low level of the peroxisome proliferator-activated receptor α (PPARα), and nontransgenic (NTg) littermates. The mice (NTg-T and Tg-T) were stressed by transverse aortic constriction (TAC) and compared with shams (NTg-S and Tg-S). There was an approximate 4-fold increase in PPARα expression in Tg-S compared with NTg-S, but Tg-T hearts showed the same PPARα expression as NTg-T. Expression of PPARα did not alter the hypertrophic response to TAC but did reduce ejection fraction (EF) in Tg-T hearts compared with other groups. The rate of actomyosin ATP hydrolysis was significantly higher in Tg-S skinned fiber bundles compared with all other groups. Tg-T hearts showed an increase in phosphorylation of specific sites on cardiac myosin binding protein-C (cMyBP-C) and β-myosin heavy chain isoform. These results advance our understanding of potential signaling to the myofilaments induced by altered FA metabolism under normal and pathological states. We demonstrate that chronic and transient PPARα activation during pathological stress alters myofilament response to Ca²⁺ through a mechanism that is possibly mediated by MyBP-C phosphorylation and myosin heavy chain isoforms.NEW & NOTEWORTHY Data presented here demonstrate novel signaling to sarcomeric proteins by chronic alterations in fatty acid metabolism induced by PPARα. The mechanism involves modifications of key myofilament regulatory proteins modifying cross-bridge dynamics with differential effects in controls and hearts stressed by pressure overload.

Sex dimorphisms of crossbridge cycling kinetics in transgenic hypertrophic cardiomyopathy mice

Familial hypertrophic cardiomyopathy (HCM) is a disease of the sarcomere and may lead to hypertrophic, dilated, restrictive, and/or arrhythmogenic cardiomyopathy, congestive heart failure, or sudden cardiac death. We hypothesized that hearts from transgenic HCM mice harboring a mutant myosin heavy chain increase the energetic cost of contraction in a sex-specific manner. To do this, we assessed Ca(2+) sensitivity of tension and crossbridge kinetics in demembranated cardiac trabeculas from male and female wild-type (WT) and HCM hearts at an early time point (2 mo of age). We found a significant effect of sex on Ca(2+) sensitivity such that male, but not female, HCM mice displayed a decrease in Ca(2+) sensitivity compared with WT counterparts. The HCM transgene and sex significantly impacted the rate of force redevelopment by a rapid release-restretch protocol and tension cost by the ATPase-tension relationship. In each of these measures, HCM male trabeculas displayed a gain-of-function when compared with WT counterparts. In addition, cardiac remodeling measured by echocardiography, histology, morphometry, and posttranslational modifications demonstrated sex- and HCM-specific effects. In conclusion, female and male HCM mice display sex dimorphic crossbridge kinetics accompanied by sex- and HCM-dependent cardiac remodeling at the morphometric, histological, and cellular level.

LKB1/Mo25/STRAD uniquely impacts sarcomeric contractile function and posttranslational modification

The myocardium undergoes extensive metabolic and energetic remodeling during the progression of cardiac disease. Central to remodeling are changes in the adenine nucleotide pool. Fluctuations in these pools can activate AMP-activated protein kinase (AMPK), the central regulator of cellular energetics. Binding of AMP to AMPK not only allosterically activates AMPK but also promotes phosphorylation of AMPK by an upstream kinase complex, LKB1/Mo25/STRAD (liver kinase B 1, mouse protein 25, STE-related adaptor protein). AMPK phosphorylation by the LKB1 complex results in a substantial increase in AMPK activity. Molecular targeting by the LKB1 complex depends on subcellular localization and transcriptional expression. Yet, little is known about the ability of the LKB1 complex to modulate targeting of AMPK after activation. Accordingly, we hypothesized that differing stoichiometric ratios of LKB1 activator complex to AMPK would uniquely impact myofilament function. Demembranated rat cardiac trabeculae were incubated with varying ratios of the LKB1 complex to AMPK or the LKB1 complex alone. After incubation, we measured the Ca(2+) sensitivity of tension, rate constant for tension redevelopment, maximum tension generation, length-dependent activation, cooperativity, and sarcomeric protein phosphorylation status. We found that the Ca(2+) sensitivity of tension and cross-bridge dynamics were dependent on the LKB1 complex/AMPK ratio. We also found that the LKB1 complex desensitizes and suppresses myofilament function independently of AMPK. A phospho-proteomic analysis of myofilament proteins revealed site-specific changes in cardiac Troponin I (cTnI) phosphorylation, as well as a unique distribution of cTnI phosphospecies that were dependent on the LKB1 complex/ AMPK ratio. Fibers treated with the LKB1 complex alone did not alter cTnI phosphorylation or phosphospecies distribution. However, LKB1 complex treatment independent of AMPK increased phosphorylation of myosin-binding protein C. Therefore, we conclude that the LKB1/AMPK signaling axis is able to alter muscle function through multiple mechanisms.

A Spatially Detailed Model of Isometric Contraction Based on Competitive Binding of Troponin I Explains Cooperative Interactions between Tropomyosin and Crossbridges

Biophysical models of cardiac tension development provide a succinct representation of our understanding of force generation in the heart. The link between protein kinetics and interactions that gives rise to high cooperativity is not yet fully explained from experiments or previous biophysical models. We propose a biophysical ODE-based representation of cross-bridge (XB), tropomyosin and troponin within a contractile regulatory unit (RU) to investigate the mechanisms behind cooperative activation, as well as the role of cooperativity in dynamic tension generation across different species. The model includes cooperative interactions between regulatory units (RU-RU), between crossbridges (XB-XB), as well more complex interactions between crossbridges and regulatory units (XB-RU interactions). For the steady-state force-calcium relationship, our framework predicts that: (1) XB-RU effects are key in shifting the half-activation value of the force-calcium relationship towards lower [Ca²⁺], but have only small effects on cooperativity. (2) XB-XB effects approximately double the duty ratio of myosin, but do not significantly affect cooperativity. (3) RU-RU effects derived from the long-range action of tropomyosin are a major factor in cooperative activation, with each additional unblocked RU increasing the rate of additional RU’s unblocking. (4) Myosin affinity for short (1–4 RU) unblocked stretches of actin of is very low, and the resulting suppression of force at low [Ca²⁺] is a major contributor in the biphasic force-calcium relationship. We also reproduce isometric tension development across mouse, rat and human at physiological temperature and pacing rate, and conclude that species differences require only changes in myosin affinity and troponin I/troponin C affinity. Furthermore, we show that the calcium dependence of the rate of tension redevelopment k_tr is explained by transient blocking of RU’s by a temporary decrease in XB-RU effects.

Force generation in cardiac muscle cells is driven by changes in calcium concentration. Relatively small changes in the calcium concentration over the course of a heart beat lead to the large changes in force required to fully contract and relax the heart. This is known as ‘cooperative activation’, and involves a complex interaction of several proteins involved in contraction. Current computer models which reproduce force generation often do not represent these processes explicitly, and stochastic approaches that do tend to require large amounts of computational power to solve, which limit the range of investigations in which they can be used. We have created an new computational model that captures the underlying physiological processes in more detail, and is more efficient than stochastic approaches, while still being able to run a large range of simulations. The model is able to explain the biological processes leading to the cooperative activation of muscle. In addition, the model reproduces how this cooperative activation translates to normal muscle function to generate force from changes in calcium across three different species.

β-adrenergic effects on cardiac myofilaments and contraction in an integrated rabbit ventricular myocyte model

A five-state model of myofilament contraction was integrated into a well-established rabbit ventricular myocyte model of ion channels, Ca(2+) transporters and kinase signaling to analyze the relative contribution of different phosphorylation targets to the overall mechanical response driven by β-adrenergic stimulation (β-AS). β-AS effect on sarcoplasmic reticulum Ca(2+) handling, Ca(2+), K(+) and Cl(-) currents, and Na(+)/K(+)-ATPase properties was included based on experimental data. The inotropic effect on the myofilaments was represented as reduced myofilament Ca(2+) sensitivity (XBCa) and titin stiffness, and increased cross-bridge (XB) cycling rate (XBcy). Assuming independent roles of XBCa and XBcy, the model reproduced experimental β-AS responses on action potentials and Ca(2+) transient amplitude and kinetics. It also replicated the behavior of force-Ca(2+), release-restretch, length-step, stiffness-frequency and force-velocity relationships, and increased force and shortening in isometric and isotonic twitch contractions. The β-AS effect was then switched off from individual targets to analyze their relative impact on contractility. Preventing β-AS effects on L-type Ca(2+) channels or phospholamban limited Ca(2+) transients and contractile responses in parallel, while blocking phospholemman and K(+) channel (IKs) effects enhanced Ca(2+) and inotropy. Removal of β-AS effects from XBCa enhanced contractile force while decreasing peak Ca(2+) (due to greater Ca(2+) buffering), but had less effect on shortening. Conversely, preventing β-AS effects on XBcy preserved Ca(2+) transient effects, but blunted inotropy (both isometric force and especially shortening). Removal of titin effects had little impact on contraction. Finally, exclusion of β-AS from XBCa and XBcy while preserving effects on other targets resulted in preserved peak isometric force response (with slower kinetics) but nearly abolished enhanced shortening. β-AS effects on XBCa and XBcy have greater impact on isometric and isotonic contraction, respectively.

Cardiac tissue structure, properties, and performance: a materials science perspective

From an engineering perspective, many forms of heart disease can be thought of as a reduction in biomaterial performance, in which the biomaterial is the tissue comprising the ventricular wall. In materials science, the structure and properties of a material are recognized to be interconnected with performance. In addition, for most measurements of structure, properties, and performance, some processing is required. Here, we review the current state of knowledge regarding cardiac tissue structure, properties, and performance as well as the processing steps taken to acquire those measurements. Understanding the impact of these factors and their interactions may enhance our understanding of heart function and heart failure. We also review design considerations for cardiac tissue property and performance measurements because, to date, most data on cardiac tissue has been obtained under non-physiological loading conditions. Novel measurement systems that account for these design considerations may improve future experiments and lead to greater insight into cardiac tissue structure, properties, and ultimately performance.

Instability in the central region of tropomyosin modulates the function of its overlapping ends

The causal link between disparate tropomyosin (Tm) functions and the structural instability in Tm is unknown. To test the hypothesis that the structural instability in the central region of Tm modulates the function of the overlapping ends of contiguous Tm dimers, we used transgenic mice (Tm(DM)) that expressed a mutant α-Tm in the heart; S229E and H276N substitutions induce structural instability in the central region and the overlapping ends of Tm, respectively. In addition, two mouse cardiac troponin T mutants (TnT(1-44Δ) and TnT(45-74Δ)) that have a divergent effect on the overlapping ends of Tm were employed. The S229E-induced instability in the central region of Tm(DM) altered the overlapping ends of Tm(DM), thereby it negated the attenuating effect of H276N on Ca(2+)-activated maximal tension. The rate of cross-bridge detachment (g) decreased in Tm(DM)+TnT(WT) and Tm(H276N)+TnT(WT) fibers but increased in Tm(DM)+TnT(45-74Δ) fibers; however, TnT(45-74Δ) did not alter g, demonstrating that S229E in Tm(DM) had divergent effects on g. The S229E substitution in Tm(DM) ablated the H276N-induced desensitization of myofilament Ca(2+) sensitivity in Tm(DM)+TnT(1-44Δ) fibers. To our knowledge, novel findings from this study show that the structural instability in the central region of Tm modifies cardiac contractile function via its effect on the overlapping ends of contiguous Tm.

Depressed Frank-Starling mechanism in the left ventricular muscle of the knock-in mouse model of dilated cardiomyopathy with troponin T deletion mutation ΔK210

It has been reported that the Frank-Starling mechanism is coordinately regulated in cardiac muscle via thin filament "on-off" equilibrium and titin-based lattice spacing changes. In the present study, we tested the hypothesis that the deletion mutation ΔK210 in the cardiac troponin T gene shifts the equilibrium toward the "off" state and accordingly attenuate the sarcomere length (SL) dependence of active force production, via reduced cross-bridge formation. Confocal imaging in isolated hearts revealed that the cardiomyocytes were enlarged, especially in the longitudinal direction, in ΔK210 hearts, with striation patterns similar to those in wild type (WT) hearts, suggesting that the number of sarcomeres is increased in cardiomyocytes but the sarcomere length remains unaltered. For analysis of the SL dependence of active force, skinned muscle preparations were obtained from the left ventricle of WT and knock-in (ΔK210) mice. An increase in SL from 1.90 to 2.20μm shifted the mid-point (pCa50) of the force-pCa curve leftward by ~0.21pCa units in WT preparations. In ΔK210 muscles, Ca(2+) sensitivity was lower by ~0.37pCa units, and the SL-dependent shift of pCa50, i.e., ΔpCa50, was less pronounced (~0.11pCa units), with and without protein kinase A treatment. The rate of active force redevelopment was lower in ΔK210 preparations than in WT preparations, showing blunted thin filament cooperative activation. An increase in thin filament cooperative activation upon an increase in the fraction of strongly bound cross-bridges by MgADP increased ΔpCa50 to ~0.21pCa units. The depressed Frank-Starling mechanism in ΔK210 hearts is the result of a reduction in thin filament cooperative activation.

Post-translational modifications of myofilament proteins involved in length-dependent prolongation of relaxation in rabbit right ventricular myocardium

The phosphorylation state of several cardiac myofilament proteins changes with the level of stretch in intact, twitch-contracting cardiac muscles. It remains unclear which kinases are involved in the length-dependent phosphorylation of these proteins. We set out to investigate which kinases are involved after a step-wise change in cardiac muscle length. We hypothesize that myofilament protein phosphorylation by PKCβII and PKA alters contractile kinetics during length-dependent activation. Right ventricular intact trabeculae were isolated from New Zealand White rabbit hearts and stimulated to contract at 1Hz. Twitch force recordings where taken at taut and optimal muscle lengths before and after administration of kinase inhibitors at 37°C. PKCβII inhibition significantly decreased time from stimulation to peak force (TTP), time from peak force to 50% relaxation (RT50), and 90% relaxation (RT90) at optimal muscle length. This led to a loss in the length-dependent increase of RT50 and RT90 in the presence of the PKCβII inhibitor, whereas the length-dependent increase in RT50 and RT90 was seen in the controls. PKA inhibition using H-89 significantly decreased TTP at both taut and optimal muscle lengths. Detection of Ser/Thr phosphorylation with ProQ-diamond staining indicates a role for PKCβII in the phosphorylation of tropomyosin and myosin light chain-2 (MLC2) and PKA for tropomyosin, troponin-I, MLC2, myosin binding protein-C, troponin-T (TnT) 3 and TnT4. Our data provide evidence for two signaling kinases acting upon myofilament proteins during length-dependent activation, and provide further insight for length-dependent myofilament function.

Ablation of the cardiac-specific gene leucine-rich repeat containing 10 (Lrrc10) results in dilated cardiomyopathy

Leucine-rich repeat containing 10 (LRRC10) is a cardiac-specific protein exclusively expressed in embryonic and adult cardiomyocytes. However, the role of LRRC10 in mammalian cardiac physiology remains unknown. To determine if LRRC10 is critical for cardiac function, Lrrc10-null (Lrrc10^−/−) mice were analyzed. Lrrc10^−/− mice exhibit prenatal systolic dysfunction and dilated cardiomyopathy in postnatal life. Importantly, Lrrc10^−/− mice have diminished cardiac performance in utero, prior to ventricular dilation observed in young adults. We demonstrate that LRRC10 endogenously interacts with α-actinin and α-actin in the heart and all actin isoforms in vitro. Gene expression profiling of embryonic Lrrc10^−/− hearts identified pathways and transcripts involved in regulation of the actin cytoskeleton to be significantly upregulated, implicating dysregulation of the actin cytoskeleton as an early defective molecular signal in the absence of LRRC10. In contrast, microarray analyses of adult Lrrc10^−/− hearts identified upregulation of oxidative phosphorylation and cardiac muscle contraction pathways during the progression of dilated cardiomyopathy. Analyses of hypertrophic signal transduction pathways indicate increased active forms of Akt and PKCε in adult Lrrc10^−/− hearts. Taken together, our data demonstrate that LRRC10 is essential for proper mammalian cardiac function. We identify Lrrc10 as a novel dilated cardiomyopathy candidate gene and the Lrrc10^−/− mouse model as a unique system to investigate pediatric cardiomyopathy.

New insights into the functional significance of the acidic region of the unique N-terminal extension of cardiac troponin I

Previous structural studies indicated a special functional role for an acidic region composed of residues 1-10 in the unique N-terminal peptide of cardiac troponin I (cTnI). Employing LC-MS/MS, we determined the presence of phosphorylation sites at S5/S6 in cTnI from wild type mouse hearts as well as in hearts of mice chronically expressing active protein kinase C-ε (PKCε) and exhibiting severe dilated cardiomyopathy (DCM). To determine the functional significance of these phosphorylations, we cloned and expressed wild-type cTnI, (Wt), and cTnI variants expressing pseudo-phosphorylation cTnI-(S5D), cTnI(S6D), as well as cTnI(S5A) and cTnI(S6A). We exchanged native Tn of detergent-extracted (skinned) fiber bundles with Tn reconstituted with the variant cTnIs and measured tension and cross-bridge dynamics. Compared to controls, myofilaments controlled by cTnI with pseudo-phosphorylation (S6D) or Ala substitution (S6A) demonstrated a significant depression in maximum tension, ATPase rate, and ktr, but no change in half-maximally activating Ca(2+). In contrast, pseudo-phosphorylation at position 5 (S5D) had no effects, although S5A induced an increase in Ca(2+)-sensitivity with no change in maximum tension or ktr. We further tested the impact of acidic domain modifications on myofilament function in studies examining the effects of cTnI(A2V), a mutation linked to DCM. This mutation significantly altered the inhibitory activity of cTnI as well as cooperativity of activation of myofilament tension, but not when S23/S24 were pseudo-phosphorylated. Our data indicate a new functional and pathological role of amino acid modifications in the N-terminal acidic domain of cTnI that is modified by phosphorylations at cTnI(S23/S24). This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Magnitude of length-dependent changes in contractile properties varies with titin isoform in rat ventricles

The effects of differential expression of titin isoforms on sarcomere length (SL)-dependent changes in passive force, maximum Ca(2+)-activated force, apparent cooperativity in activation of force (n(H)), Ca(2+) sensitivity of force (pCa(50)), and rate of force redevelopment (k(tr)) were investigated in rat cardiac muscle. Skinned right ventricular trabeculae were isolated from wild-type (WT) and mutant homozygote (Ho) hearts expressing predominantly a smaller N2B isoform (2,970 kDa) and a giant N2BA-G isoform (3,830 kDa), respectively. Stretching WT and Ho trabeculae from SL 2.0 to 2.35 μm increased passive force, maximum Ca(2+)-activated force, and pCa(50), and it decreased n(H) and k(tr). Compared with WT trabeculae, the magnitude of SL-dependent changes in passive force, maximum Ca(2+)-activated force, pCa(50), and n(H) was significantly smaller in Ho trabeculae. These results suggests that, at least in rat ventricle, the magnitude of SL-dependent changes in passive force, maximum Ca(2+)-activated force, pCa(50), n(H), and k(tr) is defined by the titin isoform.

Myomegalin is a novel A-kinase anchoring protein involved in the phosphorylation of cardiac myosin binding protein C

Background

Cardiac contractility is regulated by dynamic phosphorylation of sarcomeric proteins by kinases such as cAMP-activated protein kinase A (PKA). Efficient phosphorylation requires that PKA be anchored close to its targets by A-kinase anchoring proteins (AKAPs). Cardiac Myosin Binding Protein-C (cMyBPC) and cardiac troponin I (cTNI) are hypertrophic cardiomyopathy (HCM)-causing sarcomeric proteins which regulate contractility in response to PKA phosphorylation.

Results

During a yeast 2-hybrid (Y2H) library screen using a trisphosphorylation mimic of the C1-C2 region of cMyBPC, we identified isoform 4 of myomegalin (MMGL) as an interactor of this N-terminal cMyBPC region. As MMGL has previously been shown to interact with phosphodiesterase 4D, we speculated that it may be a PKA-anchoring protein (AKAP).

To investigate this possibility, we assessed the ability of MMGL isoform 4 to interact with PKA regulatory subunits R1A and R2A using Y2H-based direct protein-protein interaction assays. Additionally, to further elucidate the function of MMGL, we used it as bait to screen a cardiac cDNA library. Other PKA targets, viz. CARP, COMMD4, ENO1, ENO3 and cTNI were identified as putative interactors, with cTNI being the most frequent interactor.

We further assessed and confirmed these interactions by fluorescent 3D-co-localization in differentiated H9C2 cells as well as by in vivo co-immunoprecipitation. We also showed that quantitatively more interaction occurs between MMGL and cTNI under β-adrenergic stress. Moreover, siRNA-mediated knockdown of MMGL leads to reduction of cMyBPC levels under conditions of adrenergic stress, indicating that MMGL-assisted phosphorylation is requisite for protection of cMyBPC against proteolytic cleavage.

Conclusions

This study ascribes a novel function to MMGL isoform 4: it meets all criteria for classification as an AKAP, and we show that is involved in the phosphorylation of cMyBPC as well as cTNI, hence MMGL is an important regulator of cardiac contractility. This has further implications for understanding the patho-aetiology of HCM-causing mutations in the genes encoding cMyBPC and cTNI, and raises the question of whether MMGL might itself be considered a candidate HCM-causing or modifying factor.

Protein kinase A-induced myofilament desensitization to Ca(2+) as a result of phosphorylation of cardiac myosin-binding protein C

In skinned myocardium, cyclic AMP–dependent protein kinase A (PKA)-catalyzed phosphorylation of cardiac myosin–binding protein C (cMyBP-C) and cardiac troponin I (cTnI) is associated with a reduction in the Ca²⁺ responsiveness of myofilaments and an acceleration in the kinetics of cross-bridge cycling, although the respective contribution of these two proteins remains controversial. To further examine the relative roles that cTnI and cMyBP-C phosphorylation play in altering myocardial function, we determined the Ca²⁺ sensitivity of force (pCa₅₀) and the activation dependence of the rate of force redevelopment (k_tr) in control and PKA-treated mouse myocardium (isolated in the presence of 2,3-butanedione monoxime) expressing: (a) phosphorylatable cTnI and cMyBP-C (wild type [WT]), (b) phosphorylatable cTnI on a cMyBP-C–null background (cMyBP-C^−/−), (c) nonphosphorylatable cTnI with serines^23/24/43/45 and threonine¹⁴⁴ mutated to alanines (cTnI_Ala5), and (d) nonphosphorylatable cTnI on a cMyBP-C–null background (cTnI_Ala5/cMyBP-C^−/−). Here, PKA treatment decreased pCa₅₀ in WT, cTnI_Ala5, and cMyBP-C^−/− myocardium by 0.13, 0.08, and 0.09 pCa units, respectively, but had no effect in cTnI_Ala5/cMyBP-C^−/− myocardium. In WT and cTnI_Ala5 myocardium, PKA treatment also increased k_tr at submaximal levels of activation; however, PKA treatment did not have an effect on k_tr in cMyBP-C^−/− or cTnI_Ala5/cMyBP-C^−/− myocardium. In addition, reconstitution of cTnI_Ala5/cMyBP-C^−/− myocardium with recombinant cMyBP-C restored the effects of PKA treatment on pCa₅₀ and k_tr reported in cTnI_Ala5 myocardium. Collectively, these results indicate that the attenuation in myofilament force response to PKA occurs as a result of both cTnI and cMyBP-C phosphorylation, and that the reduction in pCa₅₀ mediated by cMyBP-C phosphorylation most likely arises from an accelerated cross-bridge cycling kinetics partly as a result of an increased rate constant of cross-bridge detachment.

Effects of low-level α-myosin heavy chain expression on contractile kinetics in porcine myocardium

Myosin heavy chain (MHC) isoforms are principal determinants of work capacity in mammalian ventricular myocardium. The ventricles of large mammals including humans normally express ∼10% α-MHC on a predominantly β-MHC background, while in failing human ventricles α-MHC is virtually eliminated, suggesting that low-level α-MHC expression in normal myocardium can accelerate the kinetics of contraction and augment systolic function. To test this hypothesis in a model similar to human myocardium we determined composite rate constants of cross-bridge attachment (f(app)) and detachment (g(app)) in porcine myocardium expressing either 100% α-MHC or 100% β-MHC in order to predict the MHC isoform-specific effect on twitch kinetics. Right atrial (∼100% α-MHC) and left ventricular (∼100% β-MHC) tissue was used to measure myosin ATPase activity, isometric force, and the rate constant of force redevelopment (k(tr)) in solutions of varying Ca(2+) concentration. The rate of ATP utilization and k(tr) were approximately ninefold higher in atrial compared with ventricular myocardium, while tension cost was approximately eightfold greater in atrial myocardium. From these values, we calculated f(app) to be ∼10-fold higher in α- compared with β-MHC, while g(app) was 8-fold higher in α-MHC. Mathematical modeling of an isometric twitch using these rate constants predicts that the expression of 10% α-MHC increases the maximal rate of rise of force (dF/dt(max)) by 92% compared with 0% α-MHC. These results suggest that low-level expression of α-MHC significantly accelerates myocardial twitch kinetics, thereby enhancing systolic function in large mammalian myocardium.

Striated muscle tropomyosin isoforms differentially regulate cardiac performance and myofilament calcium sensitivity

Tropomyosin (TM) plays a central role in calcium mediated striated muscle contraction. There are three muscle TM isoforms: alpha-TM, beta-TM, and gamma-TM. alpha-TM is the predominant cardiac and skeletal muscle isoform. beta-TM is expressed in skeletal and embryonic cardiac muscle. gamma-TM is expressed in slow-twitch musculature, but is not found in the heart. Our previous work established that muscle TM isoforms confer different physiological properties to the cardiac sarcomere. To determine whether one of these isoforms is dominant in dictating its functional properties, we generated single and double transgenic mice expressing beta-TM and/or gamma-TM in the heart, in addition to the endogenously expressed alpha-TM. Results show significant TM protein expression in the betagamma-DTG hearts: alpha-TM: 36%, beta-TM: 32%, and gamma-TM: 32%. These betagamma-DTG mice do not develop pathological abnormalities; however, they exhibit a hyper contractile phenotype with decreased myofilament calcium sensitivity, similar to gamma-TM transgenic hearts. Biophysical studies indicate that gamma-TM is more rigid than either alpha-TM or beta-TM. This is the first report showing that with approximately equivalent levels of expression within the same tissue, there is a functional dominance of gamma-TM over alpha-TM or beta-TM in regulating physiological performance of the striated muscle sarcomere. In addition to the effect expression of gamma-TM has on Ca(2+) activation of the cardiac myofilaments, our data demonstrates an effect on cooperative activation of the thin filament by strongly bound rigor cross-bridges. This is significant in relation to current ideas on the control mechanism of the steep relation between Ca(2+) and tension.

Distinct sarcomeric substrates are responsible for protein kinase D-mediated regulation of cardiac myofilament Ca2+ sensitivity and cross-bridge cycling

Protein kinase D (PKD), a serine/threonine kinase with emerging cardiovascular functions, phosphorylates cardiac troponin I (cTnI) at Ser(22)/Ser(23), reduces myofilament Ca(2+) sensitivity, and accelerates cross-bridge cycle kinetics. Whether PKD regulates cardiac myofilament function entirely through cTnI phosphorylation at Ser(22)/Ser(23) remains to be established. To determine the role of cTnI phosphorylation at Ser(22)/Ser(23) in PKD-mediated regulation of cardiac myofilament function, we used transgenic mice that express cTnI in which Ser(22)/Ser(23) are substituted by nonphosphorylatable Ala (cTnI-Ala(2)). In skinned myocardium from wild-type (WT) mice, PKD increased cTnI phosphorylation at Ser(22)/Ser(23) and decreased the Ca(2+) sensitivity of force. In contrast, PKD had no effect on the Ca(2+) sensitivity of force in myocardium from cTnI-Ala(2) mice, in which Ser(22)/Ser(23) were unavailable for phosphorylation. Surprisingly, PKD accelerated cross-bridge cycle kinetics similarly in myocardium from WT and cTnI-Ala(2) mice. Because cardiac myosin-binding protein C (cMyBP-C) phosphorylation underlies cAMP-dependent protein kinase (PKA)-mediated acceleration of cross-bridge cycle kinetics, we explored whether PKD phosphorylates cMyBP-C at its PKA sites, using recombinant C1C2 fragments with or without site-specific Ser/Ala substitutions. Kinase assays confirmed that PKA phosphorylates Ser(273), Ser(282), and Ser(302), and revealed that PKD phosphorylates only Ser(302). Furthermore, PKD phosphorylated Ser(302) selectively and to a similar extent in native cMyBP-C of skinned myocardium from WT and cTnI-Ala(2) mice, and this phosphorylation occurred throughout the C-zones of sarcomeric A-bands. In conclusion, PKD reduces myofilament Ca(2+) sensitivity through cTnI phosphorylation at Ser(22)/Ser(23) but accelerates cross-bridge cycle kinetics by a distinct mechanism. PKD phosphorylates cMyBP-C at Ser(302), which may mediate the latter effect.

Differential roles of regulatory light chain and myosin binding protein-C phosphorylations in the modulation of cardiac force development

Phosphorylation of myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) and myosin binding protein-C (cMyBP-C) by protein kinase A (PKA) independently accelerate the kinetics of force development in ventricular myocardium. However, while MLCK treatment has been shown to increase the Ca(2+) sensitivity of force (pCa(50)), PKA treatment has been shown to decrease pCa(50), presumably due to cardiac troponin I phosphorylation. Further, MLCK treatment increases Ca(2+)-independent force and maximum Ca(2+)-activated force, whereas PKA treatment has no effect on either force. To investigate the structural basis underlying the kinase-specific differential effects on steady-state force, we used synchrotron low-angle X-ray diffraction to compare equatorial intensity ratios (I(1,1)/I(1,0)) to assess the proximity of myosin cross-bridge mass relative to actin and to compare lattice spacings (d(1,0)) to assess the inter-thick filament spacing in skinned myocardium following treatment with either MLCK or PKA. As we showed previously, PKA phosphorylation of cMyBP-C increases I(1,1)/I(1,0) and, as hypothesized, treatment with MLCK also increased I(1,1)/I(1,0), which can explain the accelerated rates of force development during activation. Importantly, interfilament spacing was reduced by 2 nm (3.5%) with MLCK treatment, but did not change with PKA treatment. Thus, RLC or cMyBP-C phosphorylation increases the proximity of cross-bridges to actin, but only RLC phosphorylation affects lattice spacing, which suggests that RLC and cMyBP-C modulate the kinetics of force development by similar structural mechanisms; however, the effect of RLC phosphorylation to increase the Ca(2+) sensitivity of force is mediated by a distinct mechanism, most probably involving changes in interfilament spacing.

Protein kinase A-mediated phosphorylation of cMyBP-C increases proximity of myosin heads to actin in resting myocardium

Protein kinase A-mediated (PKA) phosphorylation of cardiac myosin binding protein C (cMyBP-C) accelerates the kinetics of cross-bridge cycling and may relieve the tether-like constraint of myosin heads imposed by cMyBP-C. We favor a mechanism in which cMyBP-C modulates cross-bridge cycling kinetics by regulating the proximity and interaction of myosin and actin. To test this idea, we used synchrotron low-angle x-ray diffraction to measure interthick filament lattice spacing and the equatorial intensity ratio, I(11)/I(10), in skinned trabeculae isolated from wild-type and cMyBP-C null (cMyBP-C(-/-)) mice. In wild-type myocardium, PKA treatment appeared to result in radial or azimuthal displacement of cross-bridges away from the thick filaments as indicated by an increase (approximately 50%) in I(11)/I(10) (0.22+/-0.03 versus 0.33+/-0.03). Conversely, PKA treatment did not affect cross-bridge disposition in mice lacking cMyBP-C, because there was no difference in I(11)/I(10) between untreated and PKA-treated cMyBP-C(-/-) myocardium (0.40+/-0.06 versus 0.42+/-0.05). Although lattice spacing did not change after treatment in wild-type (45.68+/-0.84 nm versus 45.64+/-0.64 nm), treatment of cMyBP-C(-/-) myocardium increased lattice spacing (46.80+/-0.92 nm versus 49.61+/-0.59 nm). This result is consistent with the idea that the myofilament lattice expands after PKA phosphorylation of cardiac troponin I, and when present, cMyBP-C, may stabilize the lattice. These data support our hypothesis that tethering of cross-bridges by cMyBP-C is relieved by phosphorylation of PKA sites in cMyBP-C, thereby increasing the proximity of cross-bridges to actin and increasing the probability of interaction with actin on contraction.

Tyrosine phosphorylation modifies protein kinase C delta-dependent phosphorylation of cardiac troponin I

Our study identifies tyrosine phosphorylation as a novel protein kinase Cdelta (PKCdelta) activation mechanism that modifies PKCdelta-dependent phosphorylation of cardiac troponin I (cTnI), a myofilament regulatory protein. PKCdelta phosphorylates cTnI at Ser23/Ser24 when activated by lipid cofactors; Src phosphorylates PKCdelta at Tyr311 and Tyr332 leading to enhanced PKCdelta autophosphorylation at Thr505 (its activation loop) and PKCdelta-dependent cTnI phosphorylation at both Ser23/Ser24 and Thr144. The Src-dependent acquisition of cTnI-Thr144 kinase activity is abrogated by Y311F or T505A substitutions. Treatment of detergent-extracted single cardiomyocytes with lipid-activated PKCdelta induces depressed tension at submaximum but not maximum [Ca2+] as expected for cTnI-Ser23/Ser24 phosphorylation. Treatment of myocytes with Src-activated PKCdelta leads to depressed maximum tension and cross-bridge kinetics, attributable to a dominant effect of cTnI-Thr144 phosphorylation. Our data implicate PKCdelta-Tyr311/Thr505 phosphorylation as dynamically regulated modifications that alter PKCdelta enzymology and allow for stimulus-specific control of cardiac mechanics during growth factor stimulation and oxidative stress.

Modulation of the rate of cardiac muscle contraction by troponin C constructs with various calcium binding affinities

We investigated whether changing thin filament Ca2+sensitivity alters the rate of contraction, either during normal cross-bridge cycling or when cross-bridge cycling is increased by inorganic phosphate (Pi). We increased or decreased Ca2+sensitivity of force production by incorporating into rat skinned cardiac trabeculae the troponin C (TnC) mutants V44QTnCF27Wand F20QTnCF27W. The rate of isometric contraction was assessed as the rate of force redevelopment ( ktr) after a rapid release and restretch to the original length of the muscle. Both in the absence of added Piand in the presence of 2.5 mM added Pi1) Ca2+sensitivity of ktrwas increased by V44QTnCF27Wand decreased by F20QTnCF27Wcompared with control TnCF27W; 2) ktrat submaximal Ca2+activation was significantly faster for V44QTnCF27Wand slower for F20QTnCF27Wcompared with control TnCF27W; 3) at maximum Ca2+activation, ktrvalues were similar for control TnCF27W, V44QTnCF27W, and F20QTnCF27W; and 4) ktrexhibited a linear dependence on force that was indistinguishable for all TnCs. In the presence of 2.5 mM Pi, ktrwas faster at all pCa values compared with the values for no added Pifor TnCF27W, V44QTnCF27W, and F20QTnCF27W. This study suggests that TnC Ca2+binding properties modulate the rate of cardiac muscle contraction at submaximal levels of Ca2+activation. This result has physiological relevance considering that, on a beat-to-beat basis, the heart contracts at submaximal Ca2+activation.

Role of myosin heavy chain composition in the stretch activation response of rat myocardium

The speed and force of myocardial contraction during systolic ejection is largely dependent on the intrinsic contractile properties of cardiac myocytes. As the myosin heavy chain (MHC) isoform of cardiac muscle is an important determinant of the contractile properties of individual myocytes, we studied the effects of altered MHC isoform expression in rat myocardium on the mechanical properties of skinned ventricular preparations. Skinned myocardium from thyroidectomized rats expressing only the beta MHC isoform displayed rates of force redevelopment that were about 2.5-fold slower than in myocardium from hyperthyroid rats expressing only the alpha MHC isoform, but the amount of force generated at a given level of Ca2+ activation was not different. Because recent studies suggest that the stretch activation response in myocardium has an important role in systolic function, we also examined the effect of MHC isoform expression on the stretch activation response by applying a rapid stretch (1% of muscle length) to an otherwise isometrically contracting muscle fibre. Sudden stretch of myocardium resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force (i.e. stretch activation) to levels greater than prestretch force. beta MHC expression dramatically slowed the overall rate of the stretch activation response compared to expression of alpha MHC isoform; specifically, the rate of force decay was approximately 2-fold slower and the rate of delayed force development was approximately 2.5-fold slower. In contrast, MHC isoform had no effect on the amplitude of the stretch activation response. Collectively, these data show that expression of beta MHC in myocardium dramatically slows rates of cross-bridge recruitment and detachment which would be expected to decrease power output and contribute to depressed systolic function in end-stage heart failure.

In vivo left ventricular functional capacity is compromised in cMyBP-C null mice

Cardiac myosin binding protein-C (cMyBP-C) is a thick filament-associated protein that binds tightly to myosin and has a potential role for modulating myocardial contraction. We tested the hypothesis that cMyBP-C 1) contributes to the enhanced in vivo contractile state following beta-adrenergic stimulation and 2) is necessary for myocardial adaptation to chronic increases in afterload. In vivo pressure-volume relations demonstrated that left ventricular (LV) systolic and diastolic function were compromised under basal conditions in cMyBP-C(-/-) compared with WT mice. Moreover, whereas beta-adrenergic treatment significantly improved ejection fraction, peak elastance, and the time to peak elastance in WT mice, these functional indexes remained unchanged in cMyBP-C(-/-) mice. Morphological and functional changes were measured through echocardiography in anesthetized mice following 5 wk of aortic banding. Adaptation to pressure overload was diminished in cMyBP-C(-/-) mice as characterized by a lack of an increase in posterior wall thickness, increased LV diameter, deterioration of fractional shortening, and prolonged isovolumic relaxation time. These results suggest that the absence of cMyBP-C significantly diminishes in vivo LV function and markedly attenuates the increase in LV contractility following beta-adrenergic stimulation or adaptation to pressure overload.

Rate of tension redevelopment is not modulated by sarcomere length in permeabilized human, murine, and porcine cardiomyocytes

The increase in Ca(2+) sensitivity of isometric force development along with sarcomere length (SL) is considered as the basis of the Frank-Starling law of the heart, possibly involving the regulation of cross-bridge turnover kinetics. Therefore, the Ca(2+) dependencies of isometric force production and of the cross-bridge-sensitive rate constant of force redevelopment (k(tr)) were determined at different SLs (1.9 and 2.3 mum) in isolated human, murine, and porcine permeabilized cardiomyocytes. k(tr) was also determined in the presence of 10 mM inorganic phosphate (P(i)), which interfered with the force-generating cross-bridge transitions. The increases in Ca(2+) sensitivities of force with SL were very similar in human, murine, and porcine cardiomyocytes (DeltapCa(50): approximately 0.11). k(tr) was higher (P < 0.05) in mice than in humans or pigs at all Ca(2+) concentrations ([Ca(2+)]) [maximum k(tr) (k(tr,max)) at a SL of 1.9 mum and pCa 4.75: 1.33 +/- 0.11, 7.44 +/- 0.15, and 1.02 +/- 0.05 s(-1), in humans, mice, and pigs, respectively] but k(tr) did not depend on SL in any species. Moreover, when the k(tr) values for each species were expressed relative to their respective maxima, similar Ca(2+) dependencies were obtained. Ten millimolar P(i) decreased force to approximately 60-65% and left DeltapCa(50) unaltered in all three species. P(i) increased k(tr,max) by a factor of approximately 1.6 in humans and pigs and by a factor of approximately 3 in mice, independent of SL. In conclusion, species differences exert a major influence on k(tr), but SL does not appear to modulate the cross-bridge turnover rates in human, murine, and porcine hearts.

Protein kinase-A phosphorylates titin in human heart muscle and reduces myofibrillar passive tension

Protein kinase-A (PKA) is activated during beta-adrenergic stimulation of the heart and is known to phosphorylate several sarcomeric proteins including the giant polypeptide titin. A PKA phosphorylation site on titin is located within the N2B-unique sequence, which is present in the elastic segment of the two major isoforms of cardiac titin, N2B and N2BA, but not in the skeletal-muscle isoforms of the N2A-type. In bovine and rat cardiomyocytes, PKA-mediated phosphorylation decreases passive tension (PT), an effect ascribed to titin phosphorylation. Whether titin is phosphorylated by PKA upon beta-adrenergic stimulation in human heart has not been shown to date. Here we report that PKA induces phosphorylation of N2B and N2BA titin isoforms, as well as a characteristic proteolytic fragment of titin, T2, in human donor hearts. The PKA-induced phosphorylation signals were stronger when myofilaments were first de-phosphorylated by protein phosphatase-1, suggesting inherent phosphorylation of titin in human heart. Titin phosphorylation was associated with a reduction in PT of skinned human cardiac strips; the relative decrease was higher at shorter than at longer physiological sarcomere lengths. The PKA-dependent PT drop was substantially larger when fibers were pre-treated with protein phosphatase-1, indicating that inherent phosphorylation of titin is important for the basal myocardial PT level. Mechanical measurements on isolated myofibrils from rat heart confirmed the PKA effect on passive stiffness and also showed a more pronounced effect in the presence of reducing agent, DTT. In contrast, PKA did not alter the PT of single skinned rat diaphragm muscle fibers; however, the kinase was still able to phosphorylate the skeletal N2A-titin isoform, which lacks the N2B-unique sequence. Thus, an additional phosphorylation site in titin may exist outside the cardiac N2B-unique sequence. We conclude that PKA mediates phosphorylation of titin in normal human myocardium. Titin phosphorylation lowers titin-based passive stiffness in heart but not in skeletal muscle.

Activation dependence of stretch activation in mouse skinned myocardium: implications for ventricular function

Recent evidence suggests that ventricular ejection is partly powered by a delayed development of force, i.e., stretch activation, in regions of the ventricular wall due to stretch resulting from torsional twist of the ventricle around the apex-to-base axis. Given the potential importance of stretch activation in cardiac function, we characterized the stretch activation response and its Ca²⁺ dependence in murine skinned myocardium at 22°C in solutions of varying Ca²⁺ concentrations. Stretch activation was induced by suddenly imposing a stretch of 0.5–2.5% of initial length to the isometrically contracting muscle and then holding the muscle at the new length. The force response to stretch was multiphasic: force initially increased in proportion to the amount of stretch, reached a peak, and then declined to a minimum before redeveloping to a new steady level. This last phase of the response is the delayed force characteristic of myocardial stretch activation and is presumably due to increased attachment of cross-bridges as a consequence of stretch. The amplitude and rate of stretch activation varied with Ca²⁺ concentration and more specifically with the level of isometric force prior to the stretch. Since myocardial force is regulated both by Ca²⁺ binding to troponin-C and cross-bridge binding to thin filaments, we explored the role of cross-bridge binding in the stretch activation response using NEM-S1, a strong-binding, non-force–generating derivative of myosin subfragment 1. NEM-S1 treatment at submaximal Ca²⁺-activated isometric forces significantly accelerated the rate of the stretch activation response and reduced its amplitude. These data show that the rate and amplitude of myocardial stretch activation vary with the level of activation and that stretch activation involves cooperative binding of cross-bridges to the thin filament. Such a mechanism would contribute to increased systolic ejection in response to increased delivery of activator Ca²⁺ during excitation–contraction coupling.

Ablation of cardiac myosin-binding protein-C accelerates stretch activation in murine skinned myocardium

Cardiac myosin binding protein-C (cMyBP-C) is a thick filament accessory protein that binds tightly to myosin, but despite evidence that mutations in the cMyBP-C gene comprise a frequent cause of hypertrophic cardiomyopathy, relatively little is known about the role(s) of cMyBP-C in myocardium. Based on earlier studies demonstrating the potential importance of stretch activation in cardiac contraction, we examined the effects of cMyBP-C on the stretch activation responses of skinned ventricular preparations from wild-type (WT) and homozygous cMyBP-C knockout mice (cMyBP-C(-/-)) previously developed in our laboratory. Sudden stretch of skinned myocardium during maximal or submaximal Ca2+ activations resulted in an instantaneous increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force (ie, stretch activation) to levels greater than prestretch force. Ablation of cMyBP-C dramatically altered the stretch activation response, ie, the rates of force decay and delayed force transient were accelerated compared with WT myocardium. These results suggest that cMyBP-C normally constrains the spatial position of myosin cross-bridges, which, in turn, limits both the rate and extent of interaction of cross-bridges with actin. We propose that ablation of cMyBP-C removes this constraint, increases the likelihood of cross-bridge binding to actin, and speeds the rate of delayed force development following stretch. Regardless of the specific mechanism, acceleration of cross-bridge cycling in cMyBP-C(-/-) myocardium could account for the abbreviation of systolic ejection in this mouse as a direct consequence of premature stretch activation of ventricular myocardium.

Ablation of myosin-binding protein-C accelerates force development in mouse myocardium

Myosin-binding protein-C (MyBP-C) is a thick filament-associated protein that binds tightly to myosin. Given that cMyBP-C may act to modulate cooperative activation of the thin filament by constraining the availability of myosin cross-bridges for binding to actin, we investigated the role of MyBP-C in the regulation of cardiac muscle contraction. We assessed the Ca(2+) sensitivity of force (pCa(50)) and the activation dependence of the rate of force redevelopment (k(tr)) in skinned myocardium isolated from wild-type (WT) and cMyBP-C null (cMyBP-C(-/-)) mice. Mechanical measurements were performed at 22 degrees C in the absence and presence of a strong-binding, nonforce-generating analog of myosin subfragment-1 (NEM-S1). In the absence of NEM-S1, maximal force and k(tr) and the pCa(50) of isometric force did not differ between WT and cMyBP-C(-/-) myocardium; however, ablation of cMyBP-C-accelerated k(tr) at each submaximal force. Treatment of WT and cMyBP-C(-/-) myocardium with 3 muM NEM-S1 elicited similar increases in pCa(50,) but the effects of NEM-S1 to increase k(tr) at submaximal forces and thereby markedly reduce the activation dependence of k(tr) occurred to a greater degree in cMyBP-C(-/-) myocardium. Together, these results support the idea that cMyBP-C normally acts to constrain the interaction between myosin and actin, which in turn limits steady-state force development and the kinetics of cross-bridge interaction.

Calcium, thin filaments, and the integrative biology of cardiac contractility

Although well known as the location of the mechanism by which the cardiac sarcomere is activated by Ca2+ to generate force and shortening, the thin filament is now also recognized as a vital component determining the dynamics of contraction and relaxation. Molecular signaling in the thin filament involves steric, allosteric, and cooperative mechanisms that are modified by protein phosphorylation, sarcomere length and load, the chemical environment, and isoform composition. Approaches employing transgenesis and mutagenesis now permit investigation of these processes at the level of the systems biology of the heart. These studies reveal that the thin filaments are not merely slaves to the levels of Ca2+ determined by membrane channels, transporters and exchangers, but are actively involved in beat to beat control of cardiac function by neural and hormonal factors and by the Frank-Starling mechanism.

Cross-bridge versus thin filament contributions to the level and rate of force development in cardiac muscle

In striated muscle thin filament activation is initiated by Ca(2+) binding to troponin C and augmented by strong myosin binding to actin (cross-bridge formation). Several lines of evidence have led us to hypothesize that thin filament properties may limit the level and rate of force development in cardiac muscle at all levels of Ca(2+) activation. As a test of this hypothesis we varied the cross-bridge contribution to thin filament activation by substituting 2 deoxy-ATP (dATP; a strong cross-bridge augmenter) for ATP as the contractile substrate and compared steady-state force and stiffness, and the rate of force redevelopment (k(tr)) in demembranated rat cardiac trabeculae as [Ca(2+)] was varied. We also tested whether thin filament dynamics limits force development kinetics during maximal Ca(2+) activation by comparing the rate of force development (k(Ca)) after a step increase in [Ca(2+)] with photorelease of Ca(2+) from NP-EGTA to maximal k(tr), where Ca(2+) binding to thin filaments should be in (near) equilibrium during force redevelopment. dATP enhanced steady-state force and stiffness at all levels of Ca(2+) activation. At similar submaximal levels of steady-state force there was no increase in k(tr) with dATP, but k(tr) was enhanced at higher Ca(2+) concentrations, resulting in an extension (not elevation) of the k(tr)-force relationship. Interestingly, we found that maximal k(tr) was faster than k(Ca), and that dATP increased both by a similar amount. Our data suggest the dynamics of Ca(2+)-mediated thin filament activation limits the rate that force develops in rat cardiac muscle, even at saturating levels of Ca(2+).

Protein kinase D is a novel mediator of cardiac troponin I phosphorylation and regulates myofilament function

Protein kinase D (PKD) is a serine kinase whose myocardial substrates are unknown. Yeast 2-hybrid screening of a human cardiac library, using the PKD catalytic domain as bait, identified cardiac troponin I (cTnI), myosin-binding protein C (cMyBP-C), and telethonin as PKD-interacting proteins. In vitro phosphorylation assays revealed PKD-mediated phosphorylation of cTnI, cMyBP-C, and telethonin, as well as myomesin. Peptide mass fingerprint analysis of cTnI by liquid chromatography-coupled mass spectrometry indicated PKD-mediated phosphorylation of a peptide containing Ser22 and Ser23, the protein kinase A (PKA) targets. Ser22 and Ser23 were replaced by Ala, either singly (Ser22Ala or Ser23Ala) or jointly (Ser22/23Ala), and the troponin complex reconstituted in vitro, using wild-type or mutated cTnI together with wild-type cardiac troponin C and troponin T. PKD-mediated cTnI phosphorylation was reduced in complexes containing Ser22Ala or Ser23Ala cTnI and completely abolished in the complex containing Ser22/23Ala cTnI, indicating that Ser22 and Ser23 are both targeted by PKD. Furthermore, troponin complex containing wild-type cTnI was phosphorylated with similar kinetics and stoichiometry (approximately 2 mol phosphate/mol cTnI) by both PKD and PKA. To determine the functional impact of PKD-mediated phosphorylation, Ca2+ sensitivity of tension development was studied in a rat skinned ventricular myocyte preparation. PKD-mediated phosphorylation did not affect maximal tension but produced a significant rightward shift of the tension-pCa relationship, indicating reduced myofilament Ca2+ sensitivity. At submaximal Ca2+ activation, PKD-mediated phosphorylation also accelerated isometric crossbridge cycling kinetics. Our data suggest that PKD is a novel mediator of cTnI phosphorylation at the PKA sites and may contribute to the regulation of myofilament function.

Binding of myosin binding protein-C to myosin subfragment S2 affects contractility independent of a tether mechanism

Mutations in the cardiac myosin binding protein-C gene (cMyBP-C) are among the most prevalent causes of inherited hypertrophic cardiomyopathy. Although most cMyBP-C mutations cause reading frameshifts that are predicted to encode truncated peptides, it is not known if or how expression of these peptides causes disease. One possibility is that because the N-terminus contains a unique binding site for the S2 subfragment of myosin, shortened cMyBP-C peptides could directly affect myosin contraction by binding to S2. To test this hypothesis, we compared the effects of a C1C2 protein containing the myosin S2 binding site on contractile properties in permeabilized myocytes from wild-type and cMyBP-C knockout mice. In wild-type myocytes, the C1C2 protein reversibly increased myofilament Ca2+ sensitivity of tension, but had no effect on resting tension. Identical results were observed in cMyBP-C knockout myocytes where C1C2 increased Ca2+ sensitivity of tension with the half-maximal response elicited at approximately 5 micromol/L C1C2. Maximum force was not affected by C1C2. However, phosphorylation of C1C2 by cAMP-dependent protein kinase reduced its ability to increase Ca2+ sensitivity. These results demonstrate that binding of the C1C2 peptide to S2 alone is sufficient to affect myosin contractile function and suggest that regulated binding of cMyBP-C to myosin S2 by phosphorylation directly influences myofilament Ca2+ sensitivity.

Basal myosin light chain phosphorylation is a determinant of Ca2+ sensitivity of force and activation dependence of the kinetics of myocardial force development

It is generally recognized that ventricular myosin regulatory light chains (RLC) are approximately 40% phosphorylated under basal conditions, and there is little change in RLC phosphorylation with agonist stimulation of myocardium or altered stimulation frequency. To establish the functional consequences of basal RLC phosphorylation in the heart, we measured mechanical properties of rat skinned trabeculae in which approximately 7% or approximately 58% of total RLC was phosphorylated. The protocol for achieving approximately 7% phosphorylation of RLC involved isolating trabeculae in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC from its baseline level. Subsequent phosphorylation to approximately 58% of total was achieved by incubating BDM-treated trabeculae in solution containing smooth muscle myosin light chain kinase, calmodulin, and Ca2+ (i.e., MLCK treatment). After MLCK treatment, Ca2+ sensitivity of force increased by 0.06 pCa units and maximum force increased by 5%. The rate constant of force development (ktr) increased as a function of Ca2+ concentration in the range between pCa 5.8 and pCa 4.5. When expressed versus pCa, the activation dependence of ktr appeared to be unaffected by MLCK treatment; however, when activation was expressed in terms of isometric force-generating capability (as a fraction of maximum), MLCK treatment slowed ktr at submaximal activations. These results suggest that basal phosphorylation of RLC plays a role in setting the kinetics of force development and Ca2+ sensitivity of force in cardiac muscle. Our results also argue that changes in RLC phosphorylation in the range examined here influence actin-myosin interaction kinetics differently in heart muscle than was previously reported for skeletal muscle.

Expression of cardiac troponin T with COOH-terminal truncation accelerates cross-bridge interaction kinetics in mouse myocardium

Transgenic mice expressing an allele of cardiac troponin T (cTnT) with a COOH-terminal truncation (cTnT(trunc)) exhibit severe diastolic and mild systolic dysfunction. We tested the hypothesis that contractile dysfunction in myocardium expressing low levels of cTnT(trunc) (i.e., <5%) is due to slowed cross-bridge kinetics and reduced thin filament activation as a consequence of reduced cross-bridge binding. We measured the Ca(2+) sensitivity of force development [pCa for half-maximal tension generation (pCa(50))] and the rate constant of force redevelopment (k(tr)) in cTnT(trunc) and wild-type (WT) skinned myocardium both in the absence and in the presence of a strong-binding, non-force-generating derivative of myosin subfragment-1 (NEM-S1). Compared with WT mice, cTnT(trunc) mice exhibited greater pCa(50), reduced steepness of the force-pCa relationship [Hill coefficient (n(H))], and faster k(tr) at submaximal Ca(2+) concentration ([Ca(2+)]), i.e., reduced activation dependence of k(tr). Treatment with NEM-S1 elicited similar increases in pCa(50) and similar reductions in n(H) in WT and cTnT(trunc) myocardium but elicited greater increases in k(tr) at submaximal activation in cTnT(trunc) myocardium. Contrary to our initial hypothesis, cTnT(trunc) appears to enhance thin filament activation in myocardium, which is manifested as significant increases in Ca(2+)-activated force and the rate of cross-bridge attachment at submaximal [Ca(2+)]. Although these mechanisms would not be expected to depress systolic function per se in cTnT(trunc) hearts, they would account for slowed rates of myocardial relaxation during early diastole.

SLControl: PC-based data acquisition and analysis for muscle mechanics

SLControl is a computerized data acquisition and analysis system that was developed in our laboratory to help perform mechanical experiments using striated muscle preparations. It consists of a computer program (Windows 2000 or later) and a commercially available data acquisition board (16-bit resolution, DAP5216a, Microstar Laboratories, Bellevue, WA). Signals from the user's existing equipment representing force, fiber length (FL), and (if desired) sarcomere length (SL) are connected to the system through standard Bayonet Neill Concelman cables and saved to data files for later analysis. Output signals from the board control FL and trigger additional equipment, e.g., flash lamps. Windows dialogs drive several different experimental protocols, including slack tests and rate of tension recovery measurements. Precise measurements of muscle stiffness and force velocity/power characteristics can also be accomplished using SL and tension control, respectively. In these situations, the FL command signal is updated in real time (at rates > or =2.5 kHz) in response to changes in the measured SL or force signals. Data files can be exported as raw text or analyzed within SLControl with the use of built-in tools for cursor analysis, digital filtering, curve fitting, etc. The software is available for free download at http://www.slcontrol.com.

Protein kinase C and A sites on troponin I regulate myofilament Ca2+ sensitivity and ATPase activity in the mouse myocardium

Cardiac troponin I (cTnI) is a phosphoprotein subunit of the troponin-tropomyosin complex that is thought to inhibit cardiac muscle contraction during diastole. To investigate the contributions of cTnI phosphorylation to cardiac regulation, transgenic mice were created with the phosphorylation sites of cTnI mutated to alanine. Activation of protein kinase C (PKC) by perfusion of hearts with phorbol-12-myristate-13-acetate (PMA) or endothelin-1 (ET-1) inhibited the maximum ATPase rate by up to 25 % and increased the Ca2+ sensitivity of ATPase activity and of isometric tension by up to 0.15 pCa units. PKC activation no longer altered cTnI phosphorylation, depressed ATPase rates or enhanced myofilament Ca2+ sensitivity in transgenic mice expressing cTnI that could not be phosphorylated on serines43/45 and threonine144 (PKC sites). Modest changes in myosin regulatory light chain phosphorylation occurred in all mouse lines, but increases in myofilament Ca2+ sensitivity required the presence of phosphorylatable cTnI. For comparison, the beta-adrenergic agonist isoproterenol caused a 38 % increase in maximum ATPase rate and a 0.12 pCa unit decrease in myofilament Ca2+ sensitivity. These beta-adrenergic effects were absent in transgenic mice expressing cTnI that could not be phosphorylated on serines23/24 (protein kinase A, PKA, sites). Overall, the results indicate that PKC and PKA exert opposing effects on actomyosin function by phosphorylating cTnI on distinct sites. A primary role of PKC phosphorylation of cTnI may be to reduce the requirements of the contractile apparatus for both Ca2+ and ATP, thereby promoting efficient ATP utilisation during contraction.

Loaded shortening, power output, and rate of force redevelopment are increased with knockout of cardiac myosin binding protein-C

Myosin binding protein-C (MyBP-C) is localized to the thick filaments of striated muscle where it appears to have both structural and regulatory functions. Importantly, mutations in the cardiac MyBP-C gene are associated with familial hypertrophic cardiomyopathy. The purpose of this study was to examine the role that MyBP-C plays in regulating force, power output, and force development rates in cardiac myocytes. Skinned cardiac myocytes from wild-type (WT) and MyBP-C knockout (MyBP-C-/-) mice were attached between a force transducer and position motor. Force, loaded shortening velocities, and rates of force redevelopment were measured during both maximal and half-maximal Ca2+ activations. Isometric force was not different between the two groups with force being 17.0+/-7.2 and 20.5+/-3.1 kN/m2 in wild-type and MyBP-C-/- myocytes, respectively. Peak normalized power output was significantly increased by 26% in MyBP-C-/- myocytes (0.15+/-0.01 versus 0.19+/-0.03 P/Po x ML/sec) during maximal Ca2+ activations. Interestingly, peak power output in MyBP-C-/- myocytes was increased to an even greater extent (46%, 0.09+/-0.03 versus 0.14+/-0.02 P/Po x ML/sec) during half-maximal Ca2+ activations. There was also an effect on the rate constant of force redevelopment (ktr) during half-maximal Ca2+ activations, with ktr being significantly greater in MyBP-C-/- myocytes (WT=5.8+/-0.9 s(-1) versus MyBP-C-/-=7.7+/-1.7 s(-1)). These results suggest that cMyBP-C is an important regulator of myocardial work capacity whereby MyBP-C acts to limit power output.

Cycling cross-bridges increase myocardial stiffness at submaximal levels of Ca2+ activation

Permeabilized multicellular preparations of canine myocardium were subjected to controlled length changes to investigate the extent to which cross-bridges augment passive stiffness components in myocardium at low levels of Ca(2+) activation. When the preparations were immersed in pCa 9.0 solution (negligible free [Ca(2+)]) they behaved as simple elastic systems (i.e., tension increased proportionately with length). In contrast, when the muscles were stretched in Ca(2+) activating solutions, tension rose much more rapidly during the initial phase of the movement than thereafter. Several lines of evidence suggest that the nonlinear response represents the displacement of populations of cycling cross-bridges that are perturbed by interfilamentary movement and take some time to recover. 1), The stiffness of the initial phase increased proportionately with the level of Ca(2+) activation. 2), The magnitude of the short-range response increased with stretch velocity. 3), The initial response was reversibly reduced by 5-mM 2,3-butanedione monoxime, a known cross-bridge inhibitor. The initial stiffness of the passive elastic (pCa 9.0) response was equivalent to the Ca(2+) dependent component at 2% (pCa approximately 6.2) of the maximal (pCa 4.5) level. These results suggest that cross-bridges may significantly affect diastolic chamber stiffness.

It takes "heart" to win: what makes the heart powerful?

The pumping action of the heart varies considerably on a beat-to-beat basis and is ultimately determined by the extent of ventricular myocyte shortening during systole. The use of isolated myocardial preparations has provided new insights about the subcellular factors that modulate power output of the ventricles.

The inotropic effect of nitric oxide on mammalian papillary muscle is dependent on the level of beta1-adrenergic stimulation

We tested the hypothesis that nitric oxide has a positive inotropic effect on mammalian cardiac muscle contractility and that this effect sums with the positive inotropic effect of beta1-adrenergic agonists when both are present. Feline right ventricular papillary muscles were stimulated to contract isometrically at 0.2 Hz in Krebs-Henseleit bicarbonate buffer (KREBS) gassed with 95% O2 and 5% CO2 (26 degrees C; pH 7.34). The nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP, 10(-5) M), and the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclophosphate sodium (Br-cGMP, 10(-5) M), significantly increased developed force by 13.3+/-1.5% (n = 11) and 7.8+/-2.8% (n = 7), respectively. SNAP, at 10(-5) M, significantly increased the force developed by papillary muscle treated with 10(-11) M or 10(-9) M dobutamine hydrochloride (a beta1-adrenergic agonist) (n = 25, 11.3+/-2.9% and 10.0+/-3.6%, respectively) when compared with the addition of KREBS (n = 27, 2.6+/-0.9% and 5.5+/-0.9%), but the increase was less than predicted by the sum of inotropic effects of SNAP and dobutamine. SNAP at 10(-5) M did not change developed force in muscles treated with 10(-7) M dobutamine but it significantly decreased developed force in muscles challenged with 10(-5) M dobutamine (n = 18, 29.3+/-5.0%) when compared with KREBS (n = 10, 41.5+/-6.8%). Similarly, 10(-4) M 8-bromo-adenosine cyclic 3',5'-hydrogen phosphate monosodium (a membrane permeable cAMP analog) increased developed force 14.9+/-3.3% and the addition of 10(-5) M Br-cGMP to those muscles significantly reduced developed force by 3.5%+/-1.1% (n = 7). Thus, the positive inotropic effect of NO decreased and ultimately became an attenuation as the level of beta1-adrenergic stimulation increased due at least in part, to an interaction between the cAMP and cGMP second messenger pathways.