Association of Cardiotoxicity With Doxorubicin and Trastuzumab: A Double-Edged Sword in Chemotherapy

Anticancer drugs play an important role in reducing mortality rates and increasing life expectancy in cancer patients. Treatments include monotherapy and/or a combination of radiation therapy, chemotherapy, hormone therapy, or immunotherapy. Despite great advances in drug development, some of these treatments have been shown to induce cardiotoxicity directly affecting heart function and structure, as well as accelerating the development of cardiovascular disease. Such side effects restrict treatment options and can negatively affect disease management. Consequently, when managing cancer patients, it is vital to understand the mechanisms causing cardiotoxicity to better monitor heart function, develop preventative measures against cardiotoxicity, and treat heart failure when it occurs in this patient population. This review discusses the role and mechanism of major chemotherapy agents with principal cardiovascular complications in cancer patients.

Diurnal Differences in Human Muscle Isometric Force In Vivo Are Associated with Differential Phosphorylation of Sarcomeric M-Band Proteins

We investigated whether diurnal differences in muscle force output are associated with the post-translational state of muscle proteins. Ten physically active men (mean ± SD; age 26.7 ± 3.7 y) performed experimental sessions in the morning (08:00 h) and evening (17:00 h), which were counterbalanced in order of administration and separated by at least 72 h. Knee extensor maximal voluntary isometric contraction (MVIC) force and peak rate of force development (RFD) were measured, and samples of vastus lateralis were collected immediately after exercise. MVIC force was greater in the evening (mean difference of 67 N, 10.2%; p < 0.05). Two-dimensional (2D) gel analysis encompassed 122 proteoforms and discovered 6 significant (p < 0.05; false discovery rate [FDR] = 10%) diurnal differences. Phosphopeptide analysis identified 1693 phosphopeptides and detected 140 phosphopeptides from 104 proteins that were more (p < 0.05, FDR = 22%) phosphorylated in the morning. Myomesin 2, muscle creatine kinase, and the C-terminus of titin exhibited the most robust (FDR < 10%) diurnal differences. Exercise in the morning, compared to the evening, coincided with a greater phosphorylation of M-band-associated proteins in human muscle. These protein modifications may alter the M-band structure and disrupt force transmission, thus potentially explaining the lower force output in the morning.

The molecular mechanisms associated with the physiological responses to inflammation and oxidative stress in cardiovascular diseases

The complex physiological signal transduction networks that respond to the dual challenges of inflammatory and oxidative stress are major factors that promote the development of cardiovascular pathologies. These signaling networks contribute to the development of age-related diseases, suggesting crosstalk between the development of aging and cardiovascular disease. Inhibition and/or attenuation of these signaling networks also delays the onset of disease. Therefore, a concept of targeting the signaling networks that are involved in inflammation and oxidative stress may represent a novel treatment paradigm for many types of heart disease. In this review, we discuss the molecular mechanisms associated with the physiological responses to inflammation and oxidative stress especially in heart failure with preserved ejection fraction and emphasize the nature of the crosstalk of these signaling processes as well as possible therapeutic implications for cardiovascular medicine.

Impact of anesthesia and storage on posttranslational modifications of cardiac myofilament proteins

Although high fidelity measurements of posttranslational modifications (PTMs) of cardiac myofilament proteins exist, important issues remain regarding basic techniques of sample acquisition and storage. We investigated the effects of anesthetic regimen and sample storage conditions on PTMs of major ventricular sarcomeric proteins. Mice were anesthetized with pentobarbital (Nembutal), ketamine/xylazine mixture (Ket/Xyl), or tribromoethanol (Avertin), and the ventricular tissue was prepared and stored for 1, 7, 30, 60, or 90 days at −80°C. Myofilament protein phosphorylation and glutathionylation were analyzed by Pro-Q Diamond stain and Western blotting, respectively. With up to 7 days of storage, phosphorylation of troponin T was stable for samples from mice anesthetized with either Nembutal or Ket/Xyl but not Avertin; while myosin-binding protein C (MyBP-C) phosphorylation was reduced at 7 days with Nembutal and Ket/Xyl, though generally not significant until 90 days. Tropomyosin and regulatory myosin light chain phosphorylation were stable for up to 7 days regardless of the anesthetic regimen employed. In the case of Troponin I, by 7 days of storage there was a significant fall in phosphorylation across all anesthetics. Storage of samples from 30 to 90 days resulted in a general decrease in myofilament phosphorylation independent of the anesthetic. S-glutathionylation of MyBP-C presented a trend in reduced glutathionylation from days 30–90 in all anesthetics, with only day 90 being statistically significant. Our findings suggest that there are alterations in PTMs of major myofilament proteins from both storage and anesthetic selection, and that storage beyond 30 days will likely result in distortion of data.

Cardiomyocyte subdomain contractility arising from microenvironmental stiffness and topography

Cellular structure and function are interdependent. To understand this relationship in beating heart cells, individual neonatal rat ventricular myocytes (NRVMs) were analyzed one and 3 days after plating when cultured on different stiffness (100, 400 kPa) and surface structures (flat or [Formula: see text] high, [Formula: see text] diameter, microposts spaced [Formula: see text] apart) manufactured from polydimethylsiloxane. Myofibril structure seen by immunohistochemistry was organized in three dimensions when NRVMs were attached to microposts. On day three, paxillin distribution near the post serving as cellular anchorage was quantified on both soft posts (12.04 % of total voxel count) and stiff posts (8.16 %). Living NRVMs were analyzed using line scans for sarcomeric shortening and shortening velocity, and traction force microscopy for surface stress and surface tension. One day after plating, NRVMs shortened more on soft posts ([Formula: see text] at [Formula: see text]) compared to either soft flat ([Formula: see text] at [Formula: see text]), stiff posts ([Formula: see text] at [Formula: see text]) or stiff flat ([Formula: see text] at [Formula: see text]). NRVMs have decreased shortening and shortening velocity on soft posts ([Formula: see text] at [Formula: see text]) compared to soft flat ([Formula: see text] at [Formula: see text]) substrates. The surface stress and surface tension increased over time for both soft post ([Formula: see text] and [Formula: see text] to [Formula: see text] and [Formula: see text]) and flat ([Formula: see text] and [Formula: see text] to [Formula: see text] and [Formula: see text]) substrates. Paxillin displacement during contraction on day three was significantly greater in NRVMs attached to soft posts [Formula: see text] compared to flat [Formula: see text] substrates. The volume and time creating four-dimensional data, interpreted by structural engineering theory, demonstrate subdomain structure is maintained by the counterbalance between the external load acting upon and the internal forces generated by the cardiomyocyte. These findings provide further insight into localized regulation of cellular mechanical function.

Essential role of the zinc finger transcription factor Casz1 for mammalian cardiac morphogenesis and development

Chromosome 1p36 deletion syndrome is one of the most common terminal deletions observed in humans and is related to congenital heart disease (CHD). However, the 1p36 genes that contribute to heart disease have not been clearly delineated. Human CASZ1 gene localizes to 1p36 and encodes a zinc finger transcription factor. Casz1 is required for Xenopus heart ventral midline progenitor cell differentiation. Whether Casz1 plays a role during mammalian heart development is unknown. Our aim is to determine 1p36 gene CASZ1 function at regulating heart development in mammals. We generated a Casz1 knock-out mouse using Casz1-trapped embryonic stem cells. Casz1 deletion in mice resulted in abnormal heart development including hypoplasia of myocardium, ventricular septal defect, and disorganized morphology. Hypoplasia of myocardium was caused by decreased cardiomyocyte proliferation. Comparative genome-wide RNA transcriptome analysis of Casz1 depleted embryonic hearts identifies abnormal expression of genes that are critical for muscular system development and function, such as muscle contraction genes TNNI2, TNNT1, and CKM; contractile fiber gene ACTA1; and cardiac arrhythmia associated ion channel coding genes ABCC9 and CACNA1D. The transcriptional regulation of some of these genes by Casz1 was also found in cellular models. Our results showed that loss of Casz1 during mouse development led to heart defect including cardiac noncompaction and ventricular septal defect, which phenocopies 1p36 deletion syndrome related CHD. This suggests that CASZ1 is a novel 1p36 CHD gene and that the abnormal expression of cardiac morphogenesis and contraction genes induced by loss of Casz1 contributes to the heart defect.

RhoA/ROCK signaling and pleiotropic α1A-adrenergic receptor regulation of cardiac contractility

Aims

To determine the mechanisms by which the α_1A-adrenergic receptor (AR) regulates cardiac contractility.

Background

We reported previously that transgenic mice with cardiac-restricted α_1A-AR overexpression (α_1A-TG) exhibit enhanced contractility but not hypertrophy, despite evidence implicating this Gα_q/11-coupled receptor in hypertrophy.

Methods

Contractility, calcium (Ca²⁺) kinetics and sensitivity, and contractile proteins were examined in cardiomyocytes, isolated hearts and skinned fibers from α_1A-TG mice (170-fold overexpression) and their non-TG littermates (NTL) before and after α_1A-AR agonist stimulation and blockade, angiotensin II (AngII), and Rho kinase (ROCK) inhibition.

Results

Hypercontractility without hypertrophy with α_1A-AR overexpression is shown to result from increased intracellular Ca²⁺ release in response to agonist, augmenting the systolic amplitude of the intracellular Ca²⁺ concentration [Ca²⁺]_i transient without changing resting [Ca²⁺]_i. In the absence of agonist, however, α_1A-AR overexpression reduced contractility despite unchanged [Ca²⁺]_i. This hypocontractility is not due to heterologous desensitization: the contractile response to AngII, acting via its Gα_q/11-coupled receptor, was unaltered. Rather, the hypocontractility is a pleiotropic signaling effect of the α_1A-AR in the absence of agonist, inhibiting RhoA/ROCK activity, resulting in hypophosphorylation of both myosin phosphatase targeting subunit 1 (MYPT1) and cardiac myosin light chain 2 (cMLC2), reducing the Ca²⁺ sensitivity of the contractile machinery: all these effects were rapidly reversed by selective α_1A-AR blockade. Critically, ROCK inhibition in normal hearts of NTLs without α_1A-AR overexpression caused hypophosphorylation of both MYPT1 and cMLC2, and rapidly reduced basal contractility.

Conclusions

We report for the first time pleiotropic α_1A-AR signaling and the physiological role of RhoA/ROCK signaling in maintaining contractility in the normal heart.

Sexually dimorphic myofilament function and cardiac troponin I phosphospecies distribution in hypertrophic cardiomyopathy mice

The pathological progression of hypertrophic cardiomyopathy (HCM) is sexually dimorphic such that male HCM mice develop phenotypic indicators of cardiac disease well before female HCM mice. Here, we hypothesized that alterations in myofilament function underlies, in part, this sex dimorphism in HCM disease development. Firstly, 10-12month female HCM (harboring a mutant [R403Q] myosin heavy chain) mice presented with proportionately larger hearts than male HCM mice. Next, we determined Ca(2+)-sensitive tension development in demembranated cardiac trabeculae excised from 10-12month female and male HCM mice. Whereas HCM did not impact Ca(2+)-sensitive tension development in male trabeculae, female HCM trabeculae were more sensitive to Ca(2+) than wild-type (WT) counterparts and both WT and HCM males. We hypothesized that the underlying cause of this sex difference in Ca(2+)-sensitive tension development was due to changes in Ca(2+) handling and sarcomeric proteins, including expression of SR Ca(2+) ATPase (2a) (SERCA2a), β-myosin heavy chain (β-MyHC) and post-translational modifications of myofilament proteins. Female HCM hearts showed an elevation of SERCA2a and β-MyHC protein whereas male HCM hearts showed a similar elevation of β-MyHC protein but a reduced level of cardiac troponin T (cTnT) phosphorylation. We also measured the distribution of cardiac troponin I (cTnI) phosphospecies using phosphate-affinity SDS-PAGE. The distribution of cTnI phosphospecies depended on sex and HCM. In conclusion, female and male HCM mice display sex dimorphic myofilament function that is accompanied by a sex- and HCM-dependent distribution of sarcomeric proteins and cTnI phosphospecies.

DCM-related tropomyosin mutants E40K/E54K over-inhibit the actomyosin interaction and lead to a decrease in the number of cycling cross-bridges

Two DCM mutants (E40K and E54K) of tropomyosin (Tm) were examined using the thin-filament extraction/reconstitu­tion technique. The effects of the Ca²⁺, ATP, phos­phate (Pi), and ADP concentrations on isometric tension and its transients were studied at 25°C, and the results were com­pared to those for the WT protein. Our results indicate that both E40K and E54K have a significantly lower T_HC (high Ca²⁺ ten­sion at pCa 4.66) (E40K: 1.21±0.06 T_a, ±SEM, N = 34; E54K: 1.24±0.07 T_a, N = 28), a significantly lower T_LC (low- Ca²⁺ tension at pCa 7.0) (E40K: 0.07±0.02 T_a, N = 34; E54K: 0.06±0.02 T_a, N = 28), and a significantly lower T_act (Ca²⁺ activatable tension) (T_act = T_HC–T_LC, E40K: 1.15±0.08 T_a, N = 34; E54K: 1.18±0.06 T_a, N = 28) than WT (T_HC = 1.53±0.07 T_a, T_LC = 0.12±0.01 T_a, T_act = 1.40±0.07 T_a, N = 25). All tensions were normalized to T_a ( = 13.9±0.8 kPa, N = 57), the ten­sion of actin-filament reconstituted cardiac fibers (myocardium) under the standard activating conditions. The Ca²⁺ sensitivity (pCa₅₀) of E40K (5.23±0.02, N = 34) and E54K (5.24±0.03, N = 28) was similar to that of the WT protein (5.26±0.03, N = 25). The cooper­a­tivity increased significantly in E54K (3.73±0.25, N = 28) compared to WT (2.80±0.17, N = 25). Seven kinetic constants were deduced using sinusoidal analysis at pCa 4.66. These results enabled us to calculate the cross-bridge distribution in the strongly attached states, and thereby deduce the force/cross-bridge. The results indicate that the force/cross-bridge is ∼15% less in E54K than WT, but remains similar to that of the WT protein in the case of E40K. We conclude that over-inhibition of the actomyosin interaction by E40K and E54K Tm mutants leads to a decreased force-generating ability at systole, which is the main mechanism underlying the early pathogenesis of DCM.

Overexpression of microRNA-1 impairs cardiac contractile function by damaging sarcomere assembly

AIMS

The purpose of the present study was to evaluate the effects of overexpression of microRNA-1 (miR-1) on cardiac contractile function and the potential molecular mechanisms.

METHODS AND RESULTS

Transgenic (Tg) mice (C57BL/6) for cardiac-specific overexpression of miR-1 driven by the α-myosin heavy chain promoter were generated and identified by real-time reverse-transcription polymerase chain reaction with left ventricular samples. We found an age-dependent decrease in the heart function in Tg mice by pressure-volume loop analysis. Histological analysis and electron microscopy displayed short sarcomeres with the loss of the clear zone and H-zone as well as myofibril fragmentation and deliquescence in Tg mice. Further studies demonstrated miR-1 post-transcriptionally down-regulated the expression of calmodulin (CaM) and cardiac myosin light chain kinase (cMLCK) proteins by targeting the 3'UTRs of MYLK3, CALM1, and CALM2 genes, leading to decreased phosphorylations of myosin light chain 2v (MLC2v) and cardiac myosin binding protein-C (cMyBP-C). Knockdown of miR-1 by locked nucleic acid-modified anti-miR-1 antisense (LNA-antimiR-1) mitigated the adverse changes of cardiac function associated with overexpression of miR-1.

CONCLUSION

miR-1 induces adverse structural remodelling to impair cardiac contractile function. Targeting cMLCK and CaM likely underlies the detrimental effects of miR-1 on structural components of muscles related to the contractile machinery. Our study provides the first evidence that miRNAs cause adverse structural remodelling of the heart.

A-kinase anchoring proteins that regulate cardiac remodeling

In response to injury or stress, the adult heart undergoes maladaptive changes, collectively defined as pathological cardiac remodeling. Here, we focus on the role of A-kinase anchoring proteins (AKAPs) in 3 main areas associated with cardiac remodeling and the progression of heart failure: excitation-contraction coupling, sarcomeric regulation, and induction of pathological hypertrophy. AKAPs are a diverse family of scaffold proteins that form multiprotein complexes, integrating cAMP signaling with protein kinases, phosphatases, and other effector proteins. Many AKAPs have been characterized in the heart, where they play a critical role in modulating cardiac function.

The sarcomeric protein nebulin: another multifunctional giant in charge of muscle strength optimization

The sliding filament model of the sarcomere was developed more than half a century ago. This model, consisting only of thin and thick filaments, has been successful in explaining many, but not all, features of skeletal muscle. Work during the 1980s revealed the existence of two additional filaments: the giant filamentous proteins titin and nebulin. Whereas the role of titin rapidly progressed, nebulin’s role in muscle structure and function remained long nebulous. An important feature of muscle structure and function that has remained relatively obscure concerns the mechanisms that are involved in regulating thin filament length. Filament length is an important aspect of muscle function as force production is proportional to the amount of overlap between thick and thin filaments. Recent advances, due in part to the generation of nebulin KO models, reveal that nebulin plays an important role in the regulation of thin filament length, most likely by stabilizing F-actin assemblies. Another structural feature of skeletal muscle that has been incompletely understood concerns the mechanisms involved in maintaining Z-disk structure and the regular lateral alignment of adjacent sarcomeres during contraction. Recent studies indicate that nebulin is part of a protein complex that mechanically links adjacent myofibrils. In addition to these structural roles in support of myofibrillar force generation, nebulin has been also shown to regulate directly muscle contraction at the level of individual crossbridges: cycling kinetics and the calcium sensitivity of force producing crossbridges is enhanced in the presence of nebulin. Thus, these recent data all point to nebulin being important for muscle force optimization. Consequently, muscle weakness as the lead symptom develops in the case of patients with nemaline myopathy that have mutations in the nebulin gene. Here, we discuss these important novel insights into the role of nebulin in skeletal muscle function.

Attenuated fatigue in slow twitch skeletal muscle during isotonic exercise in rats with chronic heart failure

During isometric contractions, slow twitch soleus muscles (SOL) from rats with chronic heart failure (chf) are more fatigable than those of sham animals. However, a muscle normally shortens during activity and fatigue development is highly task dependent. Therefore, we examined the development of skeletal muscle fatigue during shortening (isotonic) contractions in chf and sham-operated rats. Six weeks following coronary artery ligation, infarcted animals were classified as failing (chf) if left ventricle end diastolic pressure was >15 mmHg. During isoflurane anaesthesia, SOL with intact blood supply was stimulated (1s on 1s off) at 30 Hz for 15 min and allowed to shorten isotonically against a constant afterload. Muscle temperature was maintained at 37°C. In resting muscle, maximum isometric force (F(max)) and the concentrations of ATP and CrP were not different in the two groups. During stimulation, F(max) and the concentrations declined in parallel sham and chf. Fatigue, which was evident as reduced shortening during stimulation, was also not different in the two groups. The isometric force decline was fitted to a bi-exponential decay equation. Both time constants increased transiently and returned to initial values after approximately 200 s of the fatigue protocol. This resulted in a transient rise in baseline tension between stimulations, although this effect which was less prominent in chf than sham. Myosin light chain 2s phosphorylation declined in both groups after 100 s of isotonic contractions, and remained at this level throughout 15 min of stimulation. In spite of higher energy demand during isotonic than isometric contractions, both shortening capacity and rate of isometric force decline were as well or better preserved in fatigued SOL from chf rats than in sham. This observation is in striking contrast to previous reports which have employed isometric contractions to induce fatigue.

The role of protein kinase C-mediated phosphorylation of sarcomeric proteins in the heart-detrimental or beneficial?

Protein kinase C (PKC) is a family of serine/threonine protein kinases, and alterations have been found in PKC isoform expression and localization in the failing heart. These alterations in PKC activation levels influence the PKC-mediated phosphorylation status of cellular target proteins involved in Ca²⁺-handling and sarcomeric contraction. The differences observed in the effects due to PKC-mediated phosphorylation may underlie part of the contractile dysfunction observed in the failing heart. It is therefore important to establish the beneficial and detrimental effects of this kinase in the healthy and failing heart. The function of PKC has been studied intensively; however, the complexity of the regulation of this kinase makes the interpretation of the different effects difficult. The main focus of this review is the (patho)physiological impact of phosphorylation of sarcomeric proteins, myosin light chain-2, troponin I and T, desmin, myosin binding protein-C, and titin by PKC.

Enhanced active cross-bridges during diastole: molecular pathogenesis of tropomyosin's HCM mutations

Three HCM-causing tropomyosin (Tm) mutants (V95A, D175N, and E180G) were examined using the thin-filament extraction and reconstitution technique. The effects of Ca(2+), ATP, phosphate, and ADP concentrations on cross-bridge kinetics in myocardium reconstituted with each of these mutants were studied at 25°C, and compared to wild-type (WT) Tm at physiological ionic strength (200 mM). All three mutants showed significantly higher (2-3.5 fold) low Ca(2+) tension (T(LC)) and stiffness than WT at pCa 8.0. High Ca(2+) tension (T(HC)) was significantly higher for E180G than that for WT, whereas T(HC) of V95A and D175N was similar to WT; high Ca(2+) stiffness (Y(HC)) had the same trend. The Ca(2+) sensitivity of isometric force was significantly greater for V95A and E180G than for WT, whereas that of D175N remained the same as for WT; for all mutants, cooperativity was lower than for WT. Nine kinetic constants and the cross-bridge distribution were deduced using sinusoidal analysis. The number of force-generating cross bridges was similar among the D175N, E180G, and WT Tm forms, but it was significantly larger in the case of V95A than WT. We conclude that the increased number of actively cycling cross bridges at pCa 8 is the major cause of Tm mutation-related HCM pathogenesis, which may result in diastolic dysfunction. Decreased contractility (T(act)) in V95A and D175N may further contribute to the severity of myocyte hypertrophy and related prognosis of the disease.

Lifting the Nebula: Novel Insights into Skeletal Muscle Contractility

Nebulin is a giant protein and a constituent of the skeletal muscle sarcomere. The name of this protein refers to its unknown (i.e., nebulous) function. However, recent rapid advances reveal that nebulin plays important roles in the regulation of muscle contraction. When these functions of nebulin are compromised, muscle weakness ensues, as is the case in patients with nemaline myoptahy.

[Molecular mechanisms of genetic damages of the myocardium in cardiomyopathy]

The review highlighted problems of reorganization of myocardical contractile and cytoskeletal proteins in cardiomyopathy (CM). The role of the genetic factors coding contractile proteins, proteins of thin and thick filaments, and also extracellular matrix proteins in processes of formation and development of hypertrophic (HCM) and dilated (DCM) cardiomyopathy are analyzed. The mechanisms responsible for the changes in cardiac proteins on regulation involved into force generation, its transfer, recycling ATP, impairments in transmembranal signals, that finally lead to cardiac cell dysfunction determining various manifestations of CM are considered.

Roles of titin in the structure and elasticity of the sarcomere

The giant protein titin is thought to play major roles in the assembly and function of muscle sarcomeres. Structural details, such as widths of Z- and M-lines and periodicities in the thick filaments, correlate with the substructure in the respective regions of the titin molecule. Sarcomere rest length, its operating range of lengths, and passive elastic properties are also directly controlled by the properties of titin. Here we review some recent titin data and discuss its implications for sarcomere architecture and elasticity.

Sarcomere control mechanisms and the dynamics of the cardiac cycle

This review focuses on recent developments in the molecular mechanisms by which Ca activates cardiac sarcomeres and how these mechanisms play out in the cardiac cycle. I emphasize the role of mechanisms intrinsic to the sarcomeres as significant determinants of systolic elastance and ventricular stiffening during ejection. Data are presented supporting the idea that processes intrinsic to the thin filaments may promote cooperative activation of the sarcomeres and be an important factor in maintaining and modifying systolic elastance. Application of these ideas to translational medicine and rationale drug design forms an important rationale for detailed understanding of these processes.

Mechanisms underlying hypothermia-induced cardiac contractile dysfunction

Rewarming patients after profound hypothermia may result in acute heart failure and high mortality (50-80%). However, the underlying pathophysiological mechanisms are largely unknown. We characterized cardiac contractile function in the temperature range of 15-30 degrees C by measuring the intracellular Ca(2+) concentration ([Ca(2+)](i)) and twitch force in intact left ventricular rat papillary muscles. Muscle preparations were loaded with fura-2 AM and electrically stimulated during cooling at 15 degrees C for 1.5 h before being rewarmed to the baseline temperature of 30 degrees C. After hypothermia/rewarming, peak twitch force decreased by 30-40%, but [Ca(2+)](i) was not significantly altered. In addition, we assessed the maximal Ca(2+)-activated force (F(max)) and Ca(2+) sensitivity of force in skinned papillary muscle fibers. F(max) was decreased by approximately 30%, whereas the pCa required for 50% of F(max) was reduced by approximately 0.14. In rewarmed papillary muscle, both total cardiac troponin I (cTnI) phosphorylation and PKA-mediated cTnI phosphorylation at Ser23/24 were significantly increased compared with controls. We conclude that after hypothermia/rewarming, myocardial contractility is significantly reduced, as evidenced by reduced twitch force and F(max). The reduced myocardial contractility is attributed to decreased Ca(2+) sensitivity of force rather than [Ca(2+)](i) itself, resulting from increased cTnI phosphorylation.

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

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

A novel approach to improve cardiac performance: cardiac myosin activators

Decreased systolic function is a central factor in the pathogenesis of heart failure, yet there are no safe medical therapies to improve cardiac function in patients. Currently available inotropes, such as dobutamine and milrinone, increase cardiac contractility at the expense of increased intracellular concentrations of calcium and cAMP, contributing to increased heart rate, hypotension, arrhythmias, and mortality. These adverse effects are inextricably linked to their inotropic mechanism of action. A new class of pharmacologic agents, cardiac myosin activators, directly targets the kinetics of the myosin head. In vitro studies have demonstrated that these agents increase the rate of effective myosin cross-bridge formation, increasing the duration and amount of myocyte contraction, and inhibit non-productive consumption of ATP, potentially improving myocyte energy utilization, with no effect on intracellular calcium or cAMP. Animal models have shown that this novel mechanism increases the systolic ejection time, resulting in improved stroke volume, fractional shortening, and hemodynamics with no effect on myocardial oxygen demand, culminating in significant increases in cardiac efficiency. A first-in-human study in healthy volunteers with the lead cardiac myosin activator, CK-1827452, as well as preliminary results from a study in patients with stable chronic heart failure, have extended these findings to humans, demonstrating significant increases in systolic ejection time, fractional shortening, stroke volume, and cardiac output. These studies suggest that cardiac myosin activators offer the promise of a safe and effective treatment for heart failure. A program of clinical studies are being planned to test whether CK-1827452 will fulfill that promise.

Myofilament length dependent activation

The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca(2+) ions at a longer sarcomere length, commonly referred to as myofilament length-dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development.

Short-term mechanical unloading and reverse remodeling of failing hearts in children

BACKGROUND

Mechanical support using a left ventricular assist device (LVAD) can lead to functional recovery of the myocardium in patients with end-stage heart failure (HF). Molecular remodeling, cytoskeletal disruption, and apoptosis activation are associated with abnormal gene expression in the failing ventricular myocardium of HF subjects and can normalize in response to medium- and long-term mechanical unloading in adults. However, there is little knowledge of the changes in gene expression after short-term mechanical support in children with HF.

METHODS

We evaluated left ventricular biopsies from 4 children with HF. The children had implantation of a continuous- or a pulsatile-flow LVAD for 8 to 16 days before undergoing heart transplantation. At the time of LVAD insertion and removal, we performed quantitative real-time polymerase chain reaction (QPCR) to study the expression of 326 genes encoding for structural, transcriptional, and signaling pathways proteins, and immunoblot analysis on dystrophin and apoptotic factors.

RESULTS

Short-term LVAD therapy significantly decreased brain natriuretic peptide (BNP) levels from pre-LVAD (3,584.5 +/- 378.3 pg/ml [95% CI]) to post-LVAD (447.5 +/- 52.7 pg/ml [95% CI]) in 2 patients in whom comparative BNP measurements were available. In addition, short-term LVAD therapy reduced HF and apoptosis markers, whereas it upregulated structural proteins, including dystrophin, as well as pro-hypertrophic and pro-inotropic markers. Furthermore, LVAD therapy normalized expression of genes involved in calcium homeostasis, cell growth, and differentiation.

CONCLUSIONS

Our pilot study suggests that even short-term LVAD therapy in children with severe HF can reverse molecular remodeling. This favorable effect should be taken into consideration in eligible children with significant ventricular dysfunction.

Cardiac thin filament regulation

Myocardial contraction is initiated upon the release of calcium into the cytosol from the sarcoplasmic reticulum following membrane depolarization. The fundamental physiological role of the heart is to pump an amount blood that is determined by the prevailing requirements of the body. The physiological control systems employed to accomplish this task include regulation of heart rate, the amount of calcium release, and the response of the cardiac myofilaments to activator calcium ions. Thin filament activation and relaxation dynamics has emerged as a pivotal regulatory system tuning myofilament function to the beat-to-beat regulation of cardiac output. Maladaptation of thin filament dynamics, in addition to dysfunctional calcium cycling, is now recognized as an important cellular mechanism causing reduced cardiac pump function in a variety of cardiac diseases. Here, we review current knowledge regarding protein-protein interactions involved in the dynamics of thin filament activation and relaxation and the regulation of these processes by protein kinase-mediated phosphorylation.