Navigating the signalling network in mouse cardiac myocytes

Transcription factor CHF1/Hey2 suppresses cardiac hypertrophy through an inhibitory interaction with GATA4

Pathological cardiac hypertrophy is considered a precursor to clinical heart failure. Understanding the transcriptional regulators that suppress the hypertrophic response may have profound implications for the treatment of heart disease. We report the generation of transgenic mice that overexpress the transcription factor CHF1/Hey2 in the myocardium. In response to the alpha-adrenergic agonist phenylephrine, they show marked attenuation in the hypertrophic response compared with wild-type controls, even though blood pressure is similar in both groups. Isolated myocytes from transgenic mice demonstrate a similar resistance to phenylephrine-induced hypertrophy in vitro, providing further evidence that the protective effect of CHF1/Hey2 is mediated at the myocyte level. Induction of the hypertrophy marker genes ANF, BNP, and beta-MHC in the transgenic cells is concurrently suppressed in vivo and in vitro, demonstrating that the induction of hypertrophy-associated genes is repressed by CHF1/Hey2. Transfection of CHF1/Hey2 into neonatal cardiomyocytes suppresses activation of an ANF reporter plasmid by the transcription factor GATA4, which has previously been shown to activate a hypertrophic transcriptional program. Furthermore, CHF1/Hey2 binds GATA4 directly in coimmunoprecipitation assays and inhibits the binding of GATA4 to its recognition sequence within the ANF promoter. Our findings demonstrate that CHF1/Hey2 functions as an antihypertrophic gene, possibly through inhibition of a GATA4-dependent hypertrophic program.

Fructose-1,6-bisphosphate enhances hypothermic preservation of cardiac myocytes

BACKGROUND

Previous studies from our project found that fructose-1,6-bisphosphate (FBP) enhanced the functional recovery of animal hearts after hypothermic preservation, and that rat cardiac myocytes take up FBP at 3 degrees C. In this study we tested the effects of FBP, as well as other compounds related to glycolysis and pyruvate oxidation, on the hypothermic preservation of myocytes.

METHODS

Isolated myocytes were incubated in ischemic suspensions at 3 degrees C, and aliquots examined over 72 hours for retention of rod-shaped morphology. In some experiments adenine nucleotide levels were measured by high-performance liquid chromatography (HPLC).

RESULTS

FBP at 1 to 10 mmol/liter markedly reduced the death rate (65% reduction at 5 mmol/liter). Glucose at 2 to 10 mmol/liter was less beneficial (20% reduction). Insulin increased the death rate by about 25% when present alone, and it did not enhance the beneficial effects of FBP or glucose. Dichloroacetate (DCA), which stimulates pyruvate dehydrogenase, had little effect at 0.5 to 10 mmol/liter. Glucose and DCA did not increase the beneficial effects of FBP. After 6 to 24 hours of hypothermia, FBP- and glucose-treated cells had 25% to 50% higher ATP levels and 10% to 20% higher ATP:ADP ratios than untreated cells. Effects of FBP on preservation of morphology were much greater than effects on ATP levels.

CONCLUSIONS

The results suggest that the effects of FBP and glucose were through glycolytic ATP production rather than through sugar oxidation via pyruvate dehydrogenase. The divergence in effects on preservation and effects on ATP suggests a role for a sub-cellular compartment of ATP in preservation.

Akt1 Is Required for Physiological Cardiac Growth

Background— Postnatal growth of the heart chiefly involves nonproliferative cardiomyocyte enlargement. Cardiac hypertrophy exists in a “physiological” form that is an adaptive response to long-term exercise training and as a “pathological” form that often is a maladaptive response to provocative stimuli such as hypertension and aortic valvular stenosis. A signaling cascade that includes the protein kinase Akt regulates the growth and survival of many cell types, but the precise role of Akt1 in either form of cardiac hypertrophy is unknown.

Methods and Results— To evaluate the role of Akt1 in physiological cardiac growth, akt1 −/− adult murine cardiac myocytes (AMCMs) were treated with IGF-1, and akt1 −/− mice were subjected to exercise training. akt1 −/− AMCMs were resistant to insulin-like growth factor-1–stimulated protein synthesis. The akt1 −/− mice were found to be resistant to swimming training–induced cardiac hypertrophy. To evaluate the role of Akt in pathological cardiac growth, akt1 −/− AMCMs were treated with endothelin-1, and akt1 −/− mice were subjected to pressure overload by transverse aortic constriction. Surprisingly, akt1 −/− AMCMs were sensitized to endothelin-1–induced protein synthesis, and akt1 −/− mice developed an exacerbated form of cardiac hypertrophy in response to transverse aortic constriction.

Conclusions— These results establish Akt1 as a pivotal regulatory switch that promotes physiological cardiac hypertrophy while antagonizing pathological hypertrophy.

Insulin and inositol 1,4,5-trisphosphate trigger abnormal cytosolic Ca2+ transients and reveal mitochondrial Ca2+ handling defects in cardiomyocytes of ob/ob mice

Obesity, insulin resistance, and type 2 diabetes are leading causes of heart failure, and defective cellular Ca2+ handling seems to be a fundamental problem in diabetes. Therefore, we studied the effect of insulin on Ca2+ homeostasis in normal, freshly isolated mouse ventricular cardiomyocytes and whether Ca2+ handling was changed in an animal model of obesity and type 2 diabetes, ob/ob mice. Electrically evoked Ca2+ transients were smaller and slower in ob/ob compared with wild-type cardiomyocytes. Application of insulin (6 or 60 nmol/l) increased the amplitude of Ca2+ transients in wild-type cells by approximately 30%, whereas it broadened the transients and triggered extra Ca2+ transients in ob/ob cells. The effects of insulin in ob/ob cells could be reproduced by application of a membrane-permeant inositol trisphosphate (IP3) analog and blocked by a frequently used IP3 receptor inhibitor, 2-aminoethoxydiphenyl borate. In ob/ob cardiomyocytes, insulin increased the IP3 concentration and mitochondrial Ca2+ handling was impaired. In conclusion, we propose a model where insulin increases IP3 in ob/ob cardiomyocytes, which prolongs the electrically evoked Ca2+ release. This, together with an impaired mitochondrial Ca2+ handling, results in insulin-mediated extra Ca2+ transients in ob/ob cardiomyocytes that may predispose for arrhythmias in vivo.

Caffeine-induced arrhythmias in murine hearts parallel changes in cellular Ca(2+) homeostasis

Heart failure leading to ventricular arrhythmogenesis is a major cause of clinical mortality and has been associated with a leak of sarcoplasmic reticular Ca(2+) into the cytosol due to increased open probabilities in cardiac ryanodine receptor Ca(2+)-release channels. Caffeine similarly increases such open probabilities, and so we explored its arrhythmogenic effects on intact murine hearts. A clinically established programmed electrical stimulation protocol adapted for studies of isolated intact mouse hearts demonstrated that caffeine (1 mM) increased the frequency of ventricular tachycardia from 0 to 100% yet left electrogram duration and latency unchanged during programmed electrical stimulation, thereby excluding slowed conduction as a cause of arrhythmogenesis. We then used fluorescence measurements of intracellular Ca(2+) concentration in isolated mouse ventricular cells to investigate parallel changes in Ca(2+) homeostasis associated with these arrhythmias. Both caffeine (1 mM) and FK506 (30 microM) reduced electrically evoked cytosolic Ca(2+) transients yet increased the frequency of spontaneous Ca(2+)-release events. Diltiazem (1 microM) but not nifedipine (1 microM) pretreatment suppressed these increases in frequency. Identical concentrations of diltiazem but not nifedipine correspondingly suppressed the arrhythmogenic effects of caffeine in whole hearts. These findings thus directly implicate spontaneous Ca(2+) waves in triggered arrhythmogenesis in intact hearts.

The Cytosolic Phospholipase A2 Pathway, a Safeguard of β2-Adrenergic Cardiac Effects in Rat

We have recently demonstrated that in human heart, beta2-adrenergic receptors (beta2-ARs) are biochemically coupled not only to the classical adenylyl cyclase (AC) pathway but also to the cytosolic phospholipase A2 (cPLA2) pathway (Pavoine, C., Behforouz, N., Gauthier, C., Le Gouvello, S., Roudot-Thoraval, F., Martin, C. R., Pawlak, A., Feral, C., Defer, N., Houel, R., Magne, S., Amadou, A., Loisance, D., Duvaldestin, P., and Pecker, F. (2003) Mol. Pharmacol. 64, 1117-1125). In this study, using Fura-2-loaded cardiomyocytes isolated from adult rats, we showed that stimulation of beta2-ARs triggered an increase in the amplitude of electrically stimulated [Ca2+]i transients and contractions. This effect was abolished with the PKA inhibitor, H89, but greatly enhanced upon addition of the selective cPLA2 inhibitor, AACOCF3. The beta2-AR/cPLA2 inhibitory pathway involved G(i) and MSK1. Potentiation of beta2-AR/AC/PKA-induced Ca2+ responses by AACOCF3 did not rely on the enhancement of AC activity but was associated with eNOS phosphorylation (Ser1177) and L-NAME-sensitive NO production. This was correlated with PKA-dependent phosphorylation of PLB (Ser16). The constraint exerted by the beta2-AR/cPLA2 pathway on the beta2-AR/AC/PKA-induced Ca2+ responses required integrity of caveolar structures and was impaired by Filipin III treatment. Immunoblot analyses demonstrated zinterol-induced translocation of cPLA and its cosedimentation with MSK1, eNOS, PLB, and sarcoplasmic reticulum Ca2+ pump (SERCA) 2a in a low density caveolin-3-enriched membrane fraction. This inferred the gathering of beta2-AR signaling effectors around caveolae/sarcoplasmic reticulum (SR) functional platforms. Taken together, these data highlight cPLA as a cardiac beta2-AR signaling pathway that limits beta2-AR/AC/PKA-induced Ca2+ responses in adult rat cardiomyocytes through the impairment of eNOS activation and PLB phosphorylation.

Analyzing the biology on the system level

Although various genome projects have provided us enormous static sequence information, understanding of the sophisticated biology continues to require integrating the computational modeling, system analysis, technology development for experiments, and quantitative experiments all together to analyze the biology architecture on various levels, which is just the origin of systems biology subject. This review discusses the object, its characteristics, and research attentions in systems biology, and summarizes the analysis methods, experimental technologies, research developments, and so on in the four key fields of systems biology—systemic structures, dynamics, control methods, and design principles.

Abnormal Ca(2+) release and catecholamine-induced arrhythmias in mitochondrial cardiomyopathy

Mitochondrial dysfunction is implicated in numerous cardiac disorders. It has been assumed that the functional defects are directly related to a decreased rate of mitochondrial ATP production, but recent studies have challenged this idea. Here, we used mice with tissue-specific knockout of mitochondrial transcription factor A (Tfam) that leads to progressive cardiomyopathy. The role of changes in the excitation-contraction (E-C) coupling in cardiomyocytes of these mice was studied by measuring the free cytosolic Ca(2+) concentration and by analyzing the expression of genes encoding E-C coupling proteins. Action potential-mediated Ca(2+) transients, measured with the fluorescent indicator fluo-3 in isolated cardiomyocytes, were smaller and faster in Tfam knockout cardiomyocytes when compared with controls. The total sarcoplasmic reticulum (SR) Ca(2+) content was decreased in Tfam knockout cells. The gene for the SR Ca(2+) binding protein calsequestrin-2 (CASQ2), as well as other genes encoding proteins involved in SR Ca(2+) handling, showed decreased expression in Tfam knockout hearts. Decreased CASQ2 levels have been linked to severe arrhythmias triggered by beta-adrenergic stimulation. In line with this, application of the beta-adrenergic agonist isoproterenol resulted in frequent doublet Ca(2+) transients in Tfam knockout cardiomyocytes. In conclusion, our results show that mitochondrial dysfunction in the heart induces specific down-regulation of the expression of genes encoding proteins involved in E-C coupling. These changes predispose to cardiac arrhythmias and terminal heart failure and are thus important in the pathogenesis of mitochondrial cardiomyopathy.

Reconstruction of cellular signalling networks and analysis of their properties

The study of cellular signalling over the past 20 years and the advent of high-throughput technologies are enabling the reconstruction of large-scale signalling networks. After careful reconstruction of signalling networks, their properties must be described within an integrative framework that accounts for the complexity of the cellular signalling network and that is amenable to quantitative modelling.

Nav1.5 E1053K mutation causing Brugada syndrome blocks binding to ankyrin-G and expression of Nav1.5 on the surface of cardiomyocytes

We identify a human mutation (E1053K) in the ankyrin-binding motif of Na(v)1.5 that is associated with Brugada syndrome, a fatal cardiac arrhythmia caused by altered function of Na(v)1.5. The E1053K mutation abolishes binding of Na(v)1.5 to ankyrin-G, and also prevents accumulation of Na(v)1.5 at cell surface sites in ventricular cardiomyocytes. Ankyrin-G and Na(v)1.5 are both localized at intercalated disc and T-tubule membranes in cardiomyocytes, and Na(v)1.5 coimmunoprecipitates with 190-kDa ankyrin-G from detergent-soluble lysates from rat heart. These data suggest that Na(v)1.5 associates with ankyrin-G and that ankyrin-G is required for Na(v)1.5 localization at excitable membranes in cardiomyocytes. Together with previous work in neurons, these results in cardiomyocytes suggest that ankyrin-G participates in a common pathway for localization of voltage-gated Na(v) channels at sites of function in multiple excitable cell types.

Natriuretic Peptide Gene Expression after β-Adrenergic Stimulation in Adult Mouse Cardiac Myocytes

The role of A- and B-type natriuretic peptides (ANP and BNP) in cardiac pathophysiology are of increasing interest. Isolated neonatal mouse cardiac myocytes express increased levels of ANP mRNA in the absence of growth factors in culture. Expression of ANP and BNP mRNA has not been studied in isolated adult mouse cardiac myocytes (AMCM). We examined expression of ANP and BNP mRNA in isolated AMCM with and without stimulation with beta-adrenergic receptor agonists and antagonists. AMCM were isolated and maintained in culture for 24-48 h with and without stimulation with the beta-adrenergic receptor agonist isoproterenol (Iso), the beta1-antagonist CGP20712A (CGP), or the beta2-antagonist ICI-118,551 (ICI). Northern blot analysis was performed using probes for mouse ANP and BNP mRNA. TUNEL assay was performed after beta-adrenergic receptor stimulation of AMCM. BNP mRNA expression was increased fivefold (P < 0.001) after 48 h in culture without adrenergic stimulation. BNP mRNA expression was reduced (P < 0.0001) after stimulation with Iso while ANP expression remained similar to unstimulated cells. CGP prevented the Iso reduction in BNP mRNA. Iso stimulation at doses that reduced BNP mRNA expression increased TUNEL positive nuclei, an effect blocked by the beta1-antagonist CGP. In conclusion, we have demonstrated differential gene expression of ANP and BNP in AMCM in culture. Expression of BNP mRNA increases in AMCM in culture and beta1-adrenergic receptor stimulation attenuates increased BNP gene expression and results in apoptosis.

Nifedipine and diltiazem suppress ventricular arrhythmogenesis and calcium release in mouse hearts

Ventricular arrhythmogenesis leading to sudden cardiac death remains responsible for significant mortality in conditions such as cardiac failure and the long-QT syndrome (LQTS). Arrhythmias may be accentuated by beta-adrenergic stimulation and, accordingly, the present study explored the possible effects of beta-adrenergic stimulation and L-type Ca(2+) channel blockade on ventricular arrhythmogenesis and Ca(2+) handling using the mouse heart as an experimental system. Studies in whole, Langendorff-perfused hearts using programmed electrical stimulation protocols adapted from clinical practice demonstrated sustained ventricular tachycardia following addition of 0.1 microM isoprenaline (n=15), whilst no arrhythmias were observed in the absence of the drug (n=15). Arrhythmias were suppressed by nifedipine or diltiazem pre-treatment (both 1 microM) (n=8 and 4 respectively) and were also induced by elevating external [Ca(2+)] (n=3). At the cellular level, 0.1 microM isoprenaline significantly increased normalized fluorescence (F/F(0)) in field-stimulated fluo-3-loaded mouse ventricular myocytes imaged using confocal microscopy, reflecting increases in sarcoplasmic reticulum Ca(2+) release (n=8). Elevated external [Ca(2+)] also increased F/F(0) (n=4) whilst 0.1 microM nifedipine or 0.1 microM diltiazem significantly decreased F/F(0) (n=13 and 6 respectively). Pre-treatment with 0.1 microM nifedipine or 0.1 microM diltiazem suppressed the increases in F/F(0) induced by 0.1 microM isoprenaline alone (n=14 and 6 respectively). The findings thus paralleled suppression of isoprenaline-induced arrhythmias seen with nifedipine or diltiazem at the whole-heart level. Taken together, the findings may have implications for the use of L-type Ca(2+) channel blockade in conditions associated with beta-adrenergically driven ventricular arrhythmias such as cardiac failure and LQTS.

Direct inhibition of type 5 adenylyl cyclase prevents myocardial apoptosis without functional deterioration

Adenylyl cyclase, a major target enzyme of beta-adrenergic receptor signals, is potently and directly inhibited by P-site inhibitors, classic inhibitors of this enzyme, when the enzyme catalytic activity is high. Unlike beta-adrenergic receptor antagonists, this is a non- or uncompetitive inhibition with respect to ATP. We have examined whether we can utilize this enzymatic property to regulate the effects of beta-adrenergic receptor stimulation differentially. After screening multiple new and classic compounds, we found that some compounds, including 1R,4R-3-(6-aminopurin-9-yl)-cyclopentanecarboxylic acid hydroxyamide, potently inhibited type 5 adenylyl cyclase, the major cardiac isoform, but not other isoforms. In normal mouse cardiac myocytes, contraction induced by low beta-adrenergic receptor stimulation was poorly inhibited with this compound, but the induction of cardiac myocyte apoptosis by high beta-adrenergic receptor stimulation was effectively prevented by type 5 adenylyl cyclase inhibitors. In contrast, when cardiac myocytes from type 5 adenylyl cyclase knock-out mice were examined, beta-adrenergic stimulation poorly induced apoptosis. Our data suggest that the inhibition of beta-adrenergic signaling at the level of the type 5 adenylyl cyclase isoform by P-site inhibitors may serve as an effective method to prevent cardiac myocyte apoptosis induced by excessive beta-adrenergic stimulation without deleterious effect on cardiac myocyte contraction.

Mechanical stress activates angiotensin II type 1 receptor without the involvement of angiotensin II

The angiotensin II type 1 (AT1) receptor has a crucial role in load-induced cardiac hypertrophy. Here we show that the AT1 receptor can be activated by mechanical stress through an angiotensin-II-independent mechanism. Without the involvement of angiotensin II, mechanical stress not only activates extracellular-signal-regulated kinases and increases phosphoinositide production in vitro, but also induces cardiac hypertrophy in vivo. Mechanical stretch induces association of the AT1 receptor with Janus kinase 2, and translocation of G proteins into the cytosol. All of these events are inhibited by the AT1 receptor blocker candesartan. Thus, mechanical stress activates AT1 receptor independently of angiotensin II, and this activation can be inhibited by an inverse agonist of the AT1 receptor.

N -Acetylcysteine Prevents the Deleterious Effect of Tumor Necrosis Factor-α on Calcium Transients and Contraction in Adult Rat Cardiomyocytes

Background— The negative effect of tumor necrosis factor-α (TNF-α) on heart contraction, which is mediated by sphingosine, is a major component in heart failure. Because the cellular level of glutathione may limit sphingosine production via the inhibition of the Mg-dependent neutral sphingomyelinase (N-SMase), we hypothesized that cardiac glutathione status might determine the negative contractile response to TNF-α.

Methods and Results— We examined the effects of TNF-α in isolated cardiomyocytes obtained from control rats or rats that were given the glutathione precursor N -acetylcysteine (NAC, 100 mg IP per animal). In cardiomyocytes obtained from control rats, 25 ng/mL TNF-α increased reactive oxygen species generation and N-SMase activity (500% and 34% over basal, respectively) and decreased the amplitude of [Ca 2+ ] i in response to electrical stimulation (22% below basal). NAC treatment increased cardiac glutathione content by 42%. In cardiomyocytes obtained from NAC-treated rats, 25 ng/mL TNF-α had no effect on reactive oxygen species production or N-SMase activity but increased the amplitude of [Ca 2+ ] i transients and contraction in response to electrical stimulation by 40% to 50% over basal after 20 minutes. This was associated with a hastened relaxation (20% reduction in t 1/2 compared with basal) and an increased phosphorylation of both Ser 16 - and Thr 17 -phospholamban residues (260% and 115% of maximal isoproterenol effect, respectively).

Conclusions— It is concluded that cardiac glutathione status, by controlling N-SMase activation, determines the severity of the adverse effects of TNF-α on heart contraction. Glutathione supplementation may therefore provide therapeutic benefits for vulnerable hearts.