Phosphodiesterases and cardiac cGMP: evolving roles and controversies

Phosphodiesterase type-5 inhibitor use in type 2 diabetes is associated with a reduction in all-cause mortality

Objective

Experimental evidence has shown potential cardioprotective actions of phosphodiesterase type-5 inhibitors (PDE5is). We investigated whether PDE5i use in patients with type 2 diabetes, with high-attendant cardiovascular risk, was associated with altered mortality in a retrospective cohort study.

Research design and methods

Between January 2007 and May 2015, 5956 men aged 40–89 years diagnosed with type 2 diabetes before 2007 were identified from anonymised electronic health records of 42 general practices in Cheshire, UK, and were followed for 7.5 years. HRs from multivariable survival (accelerated failure time, Weibull) models were used to describe the association between on-demand PDE5i use and all-cause mortality.

Results

Compared with non-users, men who are prescribed PDE5is (n=1359) experienced lower percentage of deaths during follow-up (19.1% vs 23.8%) and lower risk of all-cause mortality (unadjusted HR=0.69 (95% CI: 0.64 to 0.79); p<0.001)). The reduction in risk of mortality (HR=0.54 (0.36 to 0.80); p=0.002) remained after adjusting for age, estimated glomerular filtration rate, smoking status, prior cerebrovascular accident (CVA) hypertension, prior myocardial infarction (MI), systolic blood pressure, use of statin, metformin, aspirin and β-blocker medication. PDE5i users had lower rates of incident MI (incidence rate ratio (0.62 (0.49 to 0.80), p<0.0001) with lower mortality (25.7% vs 40.1% deaths; age-adjusted HR=0.60 (0.54 to 0.69); p=0.001) compared with non-users within this subgroup.

Conclusion

In a population of men with type 2 diabetes, use of PDE5is was associated with lower risk of overall mortality and mortality in those with a history of acute MI.

Chemotherapeutic efficacy of phosphodiesterase inhibitors in chagasic cardiomyopathy

Molecular mechanisms of Trypanosoma cruzi (Tc)-induced Chagasic cardiomyopathy (CCM) are not well understood. The NO-cGMP-PKG1α pathway maintains cardiac homeostasis and inotropy and may be disturbed due to phosphodiesterase (PDE5)-mediated cGMP catabolism in CCM. To test this, C57BL/6 mice were infected with T. cruzi, and after the control of acute parasitemia (∼45 days post-infection), given sildenafil (SIL) (1 mg/kg) treatment for 3 weeks that ended long before the chronic disease phase (∼150 days post-infection). The PDE5 was increased and cGMP/PKG activity was decreased in chagasic myocardium. Transthoracic echocardiography revealed left ventricular (LV) systolic function, that is, stroke volume, cardiac output, and ejection fraction, was significantly decreased in chagasic mice. SIL treatment resulted in normal levels of PDE5 and cGMP/PKG activity and preserved the LV function. The cardioprotective effects of SIL were provided through inhibition of cardiac collagenosis and chronic inflammation that otherwise were pronounced in CCM. Further, SIL treatment restored the mitochondrial DNA–encoded gene expression, complex I–dependent (but not complex II–dependent) ADP-coupled respiration, and oxidant/antioxidant balance in chagasic myocardium. In vitro studies in cardiomyocytes verified that SIL conserved the redox metabolic state and cellular health via maintaining the antioxidant status that otherwise was compromised in response to T. cruzi infection. We conclude that SIL therapy was useful in controlling the LV dysfunction and chronic pathology in CCM.

•

Mice infected with T. cruzi control acute parasitemia but develop chronic chagasic cardiomyopathy.

•

Treatment with SIL (a phosphodiesterase inhibitor) during a therapeutic window of indeterminate phase provided powerful cardioprotective effects against chronic development of cardiomyopathy and LV dysfunction.

•

SIL normalized the cGMP-dependent protein kinase activity and mitochondrial oxidative metabolism, and established the oxidant/antioxidant balance in chagasic myocardium.

•

SIL prevented the oxidative/inflammatory adducts that precipitate cardiomyocytes death and cardiac remodeling in CCM.

Non-nitric oxide based metallovasodilators: synthesis, reactivity and biological studies

There is an increasing number of compounds developed to target one or more pathways involved in vasodilation. Some studies conducted with azaindole and indazole derivatives showed cardiovascular activity associated with these compounds. Fast and easy structural modification of these organic molecules can be achieved using metal complexes promoting a much larger spatial change than organic strategies, potentially leading to novel drugs. Here, we have prepared a series of complexes with a formula cis-[RuCl(L)(bpy)(2)]PF(6), where L = 7-azaindole (ain), 5-azaindole (5-ain), 4-azaindole (4-ain), indazole (indz), benzimidazole (bzim) or quinoline (qui), which were characterized by spectroscopic and electrochemical techniques (CV, DPV). These compounds showed reasonable stability exhibiting photoreactivity only at low wavelength along with superoxide scavenger activity. Cytotoxicity assays indicated their low activity preliminarily supporting in vivo application. Interestingly, vasodilation assays conducted in rat aorta exhibited great activity that largely improved compared to free ligands and even better than the well-studied organic compound (BAY 41-42272), with IC(50) reaching 55 nM. These results have validated this strategy opening new opportunities to further develop cardiovascular agents based on metallo-bicyclic rings.

Saxagliptin and Tadalafil Differentially Alter Cyclic Guanosine Monophosphate (cGMP) Signaling and Left Ventricular Function in Aortic-Banded Mini-Swine

BACKGROUND

Cyclic guanosine monophosphate-protein kinase G-phosphodiesterase 5 signaling may be disturbed in heart failure (HF) with preserved ejection fraction, contributing to cardiac remodeling and dysfunction. The purpose of this study was to manipulate cyclic guanosine monophosphate signaling using the dipeptidyl-peptidase 4 inhibitor saxagliptin and phosphodiesterase 5 inhibitor tadalafil. We hypothesized that preservation of cyclic guanosine monophosphate cGMP signaling would attenuate pathological cardiac remodeling and improve left ventricular (LV) function.

METHODS AND RESULTS

We assessed LV hypertrophy and function at the organ and cellular level in aortic-banded pigs. Concentric hypertrophy was equal in all groups, but LV collagen deposition was increased in only HF animals. Prevention of fibrotic remodeling by saxagliptin and tadalafil was correlated with neuropeptide Y plasma levels. Saxagliptin better preserved integrated LV systolic and diastolic function by maintaining normal LV chamber volumes and contractility (end-systolic pressure-volume relationship, preload recruitable SW) while preventing changes to early/late diastolic longitudinal strain rate. Function was similar to the HF group in tadalafil-treated animals including increased LV contractility, reduced chamber volume, and decreased longitudinal, circumferential, and radial mechanics. Saxagliptin and tadalafil prevented a negative cardiomyocyte shortening-frequency relationship observed in HF animals. Saxagliptin increased phosphodiesterase 5 activity while tadalafil increased cyclic guanosine monophosphate levels; however, neither drug increased downstream PKG activity. Early mitochondrial dysfunction, evident as decreased calcium-retention capacity and Complex II-dependent respiratory control, was present in both HF and tadalafil-treated animals.

CONCLUSIONS

Both saxagliptin and tadalafil prevented increased LV collagen deposition in a manner related to the attenuation of increased plasma neuropeptide Y levels. Saxagliptin appears superior for treating heart failure with preserved ejection fraction, considering its comprehensive effects on integrated LV systolic and diastolic function.

Whole Genome DNA Methylation Analysis of Obstructive Sleep Apnea: IL1R2, NPR2, AR, SP140 Methylation and Clinical Phenotype

STUDY OBJECTIVES

We hypothesized that DNA methylation patterns may contribute to disease severity or the development of hypertension and excessive daytime sleepiness (EDS) in patients with obstructive sleep apnea (OSA).

METHODS

Illumina's (San Diego, CA, USA) DNA methylation 27-K assay was used to identify differentially methylated loci (DML). DNA methylation levels were validated by pyrosequencing. A discovery cohort of 15 patients with OSA and 6 healthy subjects, and a validation cohort of 72 patients with sleep disordered breathing (SDB).

RESULTS

Microarray analysis identified 636 DMLs in patients with OSA versus healthy subjects, and 327 DMLs in patients with OSA and hypertension versus those without hypertension. In the validation cohort, no significant difference in DNA methylation levels of six selected genes was found between the primary snoring subjects and OSA patients (primary outcome). However, a secondary outcome analysis showed that interleukin-1 receptor 2 (IL1R2) promoter methylation (-114 cytosine followed by guanine dinucleotide sequence [CpG] site) was decreased and IL1R2 protein levels were increased in the patients with SDB with an oxygen desaturation index > 30. Androgen receptor (AR) promoter methylation (-531 CpG site) and AR protein levels were both increased in the patients with SDB with an oxygen desaturation index > 30. Natriuretic peptide receptor 2 (NPR2) promoter methylation (-608/-618 CpG sites) were decreased, whereas levels of both NPR2 and serum C type natriuretic peptide protein were increased in the SDB patients with EDS. Speckled protein 140 (SP140) promoter methylation (-194 CpG site) was increased, and SP140 protein levels were decreased in the patients with SDB and EDS.

CONCLUSIONS

IL1R2 hypomethylation and AR hypermethylation may constitute an important determinant of disease severity, whereas NPR2 hypomethylation and SP140 hypermethylation may provide a biomarker for vulnerability to EDS in OSA.

COMMENTARY

A commentary on this article appears in this issue on page 723.

Scutellarin's Cardiovascular Endothelium Protective Mechanism: Important Role of PKG-Iα

Scutellarin (SCU), a flavonoid glycoside compound, has been successfully used in clinic for treatment of ischemic diseases in China. In this report, we checked the effects of SCU on endothelium dysfunction (ED) of coronary artery (CA) against myocardial ischemia reperfusion (MIR) injury in vivo. The involvement of PKG-Iα was further studied using cultured endothelial cells subjected to hypoxia reoxygenation (HR) injury in vitro. In rat MIR model, SCU (45 and 90 mg/kg, iv) significantly reduced ischemic size and restored the endothelium-dependent vasodilation of isolated CA rings. PKG inhibitor Rp-8-Br-cGMP (50 μg/kg, iv) could ameliorate the protective effects of SCU. Increase in phosphorylation of vasodilator-stimulated phosphoprotein (VASP), a main substrate of PKG, at Ser 239 was observed in both heart tissue and serum of SCU-treated animals. In cultured human cardiac microvascular endothelial cells (HCMECs), SCU (1 and 10 μM) dose-dependently protected cell viability and increased the mRNA and protein level of PKG-Iα against HR injury. The activity of PKG was also increased by SCU treatment. The activation of PKG–1α was then studied using targeted proteomic analysis (MRM-MS) checking the phosphorylation state of the autophosphorylation domain (aa42-94). Significant decrease in phosphorylation of PKG-Iα at Ser50, Ser72, Ser89 was induced by HR injury while SCU treatment significantly increased the phosphorylation of PKG-Iα, not only at Ser50, Ser72 and Ser89, but also at Ser44 and Thr58 (two novel phosphorylation domains). Our results demonstrate PKG-Iα might play an important role in the protective effects of SCU on ED against MIR injury.

Soluble guanylate cyclase modulators blunt hyperoxia effects on calcium responses of developing human airway smooth muscle

Exposure to moderate hyperoxia in prematurity contributes to subsequent airway dysfunction and increases the risk of developing recurrent wheeze and asthma. The nitric oxide (NO)-soluble guanylate cyclase (sGC)-cyclic GMP (cGMP) axis modulates airway tone by regulating airway smooth muscle (ASM) intracellular Ca(2+) ([Ca(2+)]i) and contractility. However, the effects of hyperoxia on this axis in the context of Ca(2+)/contractility are not known. In developing human ASM, we explored the effects of novel drugs that activate sGC independent of NO on alleviating hyperoxia (50% oxygen)-induced enhancement of Ca(2+) responses to bronchoconstrictor agonists. Treatment with BAY 41-2272 (sGC stimulator) and BAY 60-2770 (sGC activator) increased cGMP levels during exposure to 50% O2. Although 50% O2 did not alter sGCα1 or sGCβ1 expression, BAY 60-2770 did increase sGCβ1 expression. BAY 41-2272 and BAY 60-2770 blunted Ca(2+) responses to histamine in cells exposed to 50% O2. The effects of BAY 41-2272 and BAY 60-2770 were reversed by protein kinase G inhibition. These novel data demonstrate that BAY 41-2272 and BAY 60-2770 stimulate production of cGMP and blunt hyperoxia-induced increases in Ca(2+) responses in developing ASM. Accordingly, sGC stimulators/activators may be a useful therapeutic strategy in improving bronchodilation in preterm infants.

Phosphodiesterase 1 regulation is a key mechanism in vascular aging

Reduced nitric oxide (NO)/cGMP signalling is observed in age-related vascular disease. We hypothesize that this disturbed signalling involves effects of genomic instability, a primary causal factor in aging, on vascular smooth muscle cells (VSMCs) and that the underlying mechanism plays a role in human age-related vascular disease. To test our hypothesis, we combined experiments in mice with genomic instability resulting from the defective nucleotide excision repair gene ERCC1 (Ercc1(d/-) mice), human VSMC cultures and population genome-wide association studies (GWAS). Aortic rings of Ercc1(d/-) mice showed 43% reduced responses to the soluble guanylate cyclase (sGC) stimulator sodium nitroprusside (SNP). Inhibition of phosphodiesterase (PDE) 1 and 5 normalized SNP-relaxing effects in Ercc1(d/-) to wild-type (WT) levels. PDE1C levels were increased in lung and aorta. cGMP hydrolysis by PDE in lungs was higher in Ercc1(d/-) mice. No differences in activity or levels of cGMP-dependent protein kinase 1 or sGC were observed in Ercc1(d/-) mice compared with WT. Senescent human VSMC showed elevated PDE1A and PDE1C and PDE5 mRNA levels (11.6-, 9- and 2.3-fold respectively), which associated with markers of cellular senescence. Conversely, PDE1 inhibition lowered expression of these markers. Human genetic studies revealed significant associations of PDE1A single nucleotide polymorphisms with diastolic blood pressure (DBP; β=0.28, P=2.47×10(-5)) and carotid intima-media thickness (cIMT; β=-0.0061, P=2.89×10(-5)). In summary, these results show that genomic instability and cellular senescence in VSMCs increase PDE1 expression. This might play a role in aging-related loss of vasodilator function, VSMC senescence, increased blood pressure and vascular hypertrophy.

Cell- and molecular-level mechanisms contributing to diastolic dysfunction in HFpEF

Heart failure with preserved ejection fraction (HFpEF) is the default diagnosis for patients who have symptoms of heart failure, an ejection fraction >0.5, and evidence of diastolic dysfunction. The clinical condition, which was largely unrecognized 30 years ago, is now a major health problem and currently accounts for 50% of all patients with heart failure. Clinical studies show that patients with HFpEF exhibit increased passive stiffness of the ventricles and a slower rate of pressure decline during diastole. This review discusses some of the cell- and molecular-level mechanisms that contribute to these effects and focuses on data obtained using human samples. Collagen cross linking, modulation of protein kinase G-related pathways, Ca(2+) handling, and strain-dependent detachment of cross bridges are highlighted as potential factors that could be modulated to improve ventricular function in patients with HFpEF.

Alterations in the interactome of serine/threonine protein phosphatase type-1 in atrial fibrillation patients

BACKGROUND

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, yet current pharmacological treatments are limited. Serine/threonine protein phosphatase type-1 (PP1), a major phosphatase in the heart, consists of a catalytic subunit (PP1c) and a large set of regulatory (R)-subunits that confer localization and substrate specificity to the holoenzyme. Previous studies suggest that PP1 is dysregulated in AF, but the mechanisms are unknown.

OBJECTIVES

The purpose of this study was to test the hypothesis that PP1 is dysregulated in paroxysmal atrial fibrillation (PAF) at the level of its R-subunits.

METHODS

Cardiac lysates were coimmunoprecipitated with anti-PP1c antibody followed by mass spectrometry-based, quantitative profiling of associated R-subunits. Subsequently, label-free quantification (LFQ) was used to evaluate altered R-subunit-PP1c interactions in PAF patients. R-subunits with altered binding to PP1c in PAF were further studied using bioinformatics, Western blotting (WB), immunocytochemistry, and coimmunoprecipitation.

RESULTS

A total of 135 and 78 putative PP1c interactors were captured from mouse and human cardiac lysates, respectively, including many previously unreported interactors with conserved PP1c docking motifs. Increases in binding were found between PP1c and PPP1R7, cold-shock domain protein A (CSDA), and phosphodiesterase type-5A (PDE5A) in PAF patients, with CSDA and PDE5A being novel interactors validated by bioinformatics, immunocytochemistry, and coimmunoprecipitation. WB confirmed that these increases in binding cannot be ascribed to their changes in global protein expression alone.

CONCLUSIONS

Subcellular heterogeneity in PP1 activity and downstream protein phosphorylation in AF may be attributed to alterations in PP1c-R-subunit interactions, which impair PP1 targeting to proteins involved in electrical and Ca(2+) remodeling. This represents a novel concept in AF pathogenesis and may provide more specific drug targets for treating AF.

The cGMP/PKG pathway as a common mediator of cardioprotection: translatability and mechanism

Cardiomyocyte cell death occurring during myocardial reperfusion (reperfusion injury) contributes to final infarct size after transient coronary occlusion. Different interrelated mechanisms of reperfusion injury have been identified, including alterations in cytosolic Ca(2+) handling, sarcoplasmic reticulum-mediated Ca(2+) oscillations and hypercontracture, proteolysis secondary to calpain activation and mitochondrial permeability transition. All these mechanisms occur during the initial minutes of reperfusion and are inhibited by intracellular acidosis. The cGMP/PKG pathway modulates the rate of recovery of intracellular pH, but has also direct effect on Ca(2+) oscillations and mitochondrial permeability transition. The cGMP/PKG pathway is depressed in cardiomyocytes by ischaemia/reperfusion and preserved by ischaemic postconditioning, which importantly contributes to postconditioning protection. The present article reviews the mechanisms and consequences of the effect of ischaemic postconditioning on the cGMP/PKG pathway, the different pharmacological strategies aimed to stimulate it during myocardial reperfusion and the evidence, limitations and promise of translation of these strategies to the clinical practice. Overall, the preclinical and clinical evidence suggests that modulation of the cGMP/PKG pathway may be a therapeutic target in the context of myocardial infarction.

Extending the translational potential of targeting NO/cGMP-regulated pathways in the CVS

The discovery of NO as both an endogenous signalling molecule and as a mediator of the cardiovascular effects of organic nitrates was acknowledged in 1998 by the Nobel Prize in Physiology/Medicine. The characterization of its downstream signalling, mediated through stimulation of soluble GC (sGC) and cGMP generation, initiated significant translational interest, but until recently this was almost exclusively embodied by the use of PDE5 inhibitors in erectile dysfunction. Since then, research progress in two areas has contributed to an impressive expansion of the therapeutic targeting of the NO-sGC-cGMP axis: first, an increased understanding of the molecular events operating within this complex pathway and second, a better insight into its dys-regulation and uncoupling in human disease. Already-approved PDE5 inhibitors and novel, first-in-class molecules, which up-regulate the activity of sGC independently of NO and/or of the enzyme's haem prosthetic group, are undergoing clinical evaluation to treat pulmonary hypertension and myocardial failure. These molecules, as well as combinations or second-generation compounds, are also being assessed in additional experimental disease models and in patients in a wide spectrum of novel indications, such as endotoxic shock, diabetic cardiomyopathy and Becker's muscular dystrophy. There is well-founded optimism that the modulation of the NO-sGC-cGMP pathway will sustain the development of an increasing number of successful clinical candidates for years to come.

PDE5 inhibitors as therapeutics for heart disease, diabetes and cancer

The phosphodiesterase 5 (PDE5) inhibitors, including sildenafil (Viagra™), vardenafil (Levitra™), and tadalafil (Cialis™) have been developed for treatment of erectile dysfunction. Moreover, sildenafil and tadalafil are used for the management of pulmonary arterial hypertension in patients. Since our first report showing the cardioprotective effect of sildenafil in 2002, there has been tremendous growth of preclinical and clinical studies on the use of PDE5 inhibitors for cardiovascular diseases and cancer. Numerous animal studies have demonstrated that PDE5 inhibitors have powerful protective effect against myocardial ischemia/reperfusion (I/R) injury, doxorubicin cardiotoxicity, ischemic and diabetic cardiomyopathy, cardiac hypertrophy, Duchenne muscular dystrophy and the improvement of stem cell efficacy for myocardial repair. Mechanistically, PDE5 inhibitors protect the heart against I/R injury through increased expression of nitric oxide synthases, activation of protein kinase G (PKG), PKG-dependent hydrogen sulfide generation, and phosphorylation of glycogen synthase kinase-3β - a master switch immediately proximal to mitochondrial permeability transition pore and the end effector of cardioprotection. In addition, PDE5 inhibitors enhance the sensitivity of certain types of cancer to standard chemotherapeutic drugs, including doxorubicin. Many clinical trials with PDE5 inhibitors have focused on the potential cardiovascular and anti-cancer benefits. Despite mixed results of these clinical trials, there is a continuing strong interest by basic scientists and clinical investigators in exploring their new clinical uses. It is our hope that future new mechanistic investigations and carefully designed clinical trials would help in reaping additional benefits of PDE5 inhibitors for cardiovascular disease and cancer in patients.

Left ventricular diastolic dysfunction and myocardial stiffness in diabetic mice is attenuated by inhibition of dipeptidyl peptidase 4

AIMS

Obesity and Type 2 diabetes mellitus (DM) induce left ventricular (LV) diastolic dysfunction, which contributes to an increasing prevalence of heart failure with a preserved LV ejection fraction. We investigated the effects of sitagliptin (SITA), an inhibitor of dipeptidylpeptidase-4 (DPP-4) and anti-diabetic drug, on LV structure and function of obese mice with Type 2 DM.

METHODS AND RESULTS

Obese Type 2 diabetic mice (Lepr(db/db), BKS.Cg-Dock7(m)+/+ Lepr(db)/J), displaying increased cardiomyocyte and LV stiffness at the age of 16 weeks, were treated with SITA (300 mg/kg/day) or vehicle for 8 weeks. SITA severely impaired serum DPP-4 activity, but had no effect on glycaemia. Invasive haemodynamic recordings showed that SITA reduced LV passive stiffness and increased LV stroke volume; LV end-systolic elastance remained unchanged. In addition, SITA reduced resting tension of isolated single cardiomyocytes and intensified phosphorylation of the sarcomeric protein titin. SITA also increased LV concentrations of cGMP and increased activity of protein kinase G (PKG). In vitro activation of PKG decreased resting tension of cardiomyocytes from vehicle-treated mice, but had no effect on resting tension of cardiomyocytes from SITA-treated mice.

CONCLUSIONS

In obese Type 2 diabetic mice, in the absence of hypoglycaemic effects, inhibition of DPP-4 decreases LV passive stiffness and improves global LV performance. These effects seem at least partially mediated by stimulatory effects on the myocardial cGMP-PKG pathway and, hence, on the phosphorylation status of titin and the hereto coupled cardiomyocyte stiffness modulus.

Expression analysis of mouse Rhobtb3 using a LacZ reporter and preliminary characterization of a knockout strain

RhoBTB3 is an atypical member of the Rho family of small GTPases. It localizes at the Golgi apparatus and endosomes and is involved in vesicle trafficking and in targeting proteins for degradation in the proteasome. Previous studies using Northern blot analysis showed that Rhobtb3 is ubiquitously expressed in adult mice, but expression is particularly high in brain, heart and uterus. The gene is also expressed between embryonic days 11.5 and 17.5. To investigate the specific cell types that express this gene across tissues, both in the embryo and in the adult organism, we have made use of a gene trap mouse strain that expresses the LacZ gene under the transcriptional control of the endogenous Rhobtb3 promoter. Histochemical detection of β-galactosidase expression revealed a profile characterized by nearly ubiquitous expression of Rhobtb3 in the embryo, but with particularly high levels in bone, cartilage, all types of muscle, testis and restricted areas of the nervous system. In the adult, expression persists at much lower levels in cardiac muscle, the tunica media of blood vessels and cartilage and at high levels in the seminiferous tubules. A general preliminary characterization of this gene trap mouse strain revealed reduced viability, a postnatal growth defect and reduced testis size. Our results should pave the way for future studies aimed at investigating the roles of RhoBTB3 in tissue development and in cardiac, vascular and testicular function.

Gigantic business: titin properties and function through thick and thin

The giant protein titin forms a unique filament network in cardiomyocytes, which engages in both mechanical and signaling functions of the heart. TTN, which encodes titin, is also a major human disease gene. In this review, we cover the roles of cardiac titin in normal and failing hearts, with a special emphasis on the contribution of titin to diastolic stiffness. We provide an update on disease-associated titin mutations in cardiac and skeletal muscles and summarize what is known about the impact of protein-protein interactions on titin properties and functions. We discuss the importance of titin-isoform shifts and titin phosphorylation, as well as titin modifications related to oxidative stress, in adjusting the diastolic stiffness of the healthy and the failing heart. Along the way we distinguish among titin alterations in systolic and in diastolic heart failure and ponder the evidence for titin stiffness as a potential target for pharmacological intervention in heart disease.

Exercise training in adverse cardiac remodeling

Cardiac remodeling in response to a myocardial infarction or chronic pressure-overload is an independent risk factor for the development of heart failure. In contrast, cardiac remodeling produced by regular physical exercise is associated with a decreased risk for heart failure. There is evidence that exercise training has a beneficial effect on disease progression and survival in patients with cardiac remodeling and dysfunction, but concern has also been expressed that exercise training may aggravate pathological remodeling and dysfunction. Here we present studies from our laboratory into the effects of exercise training on pathological cardiac remodeling and dysfunction in mice. The results indicate that even in the presence of a large infarct, exercise training exerts beneficial effects on the heart. These effects were mimicked in part by endothelial nitric oxide synthase (eNOS) overexpression and abrogated by eNOS deficiency, demonstrating the importance of nitric oxide signaling in mediating the cardiac effects of exercise. Exercise prior to a myocardial infarction was also cardioprotective. In contrast, exercise tended to aggravate pathological cardiac remodeling and dysfunction in the setting of pressure-overload produced by an aortic stenosis. These observations emphasize the critical importance of the underlying pathological stimulus for cardiac hypertrophy and remodeling, in determining the effects of exercise training. Future studies are needed to define the influence of exercise type, intensity and duration in different models and severities of pathological cardiac remodeling. Together such studies will aid in optimizing the therapy of exercise training in the setting of cardiovascular disease.

PDE2-mediated cAMP hydrolysis accelerates cardiac fibroblast to myofibroblast conversion and is antagonized by exogenous activation of cGMP signaling pathways

Recent studies suggest that the signal molecules cAMP and cGMP have antifibrotic effects by negatively regulating pathways associated with fibroblast to myofibroblast (MyoCF) conversion. The phosphodiesterase 2 (PDE2) has the unique property to be stimulated by cGMP, which leads to a remarkable increase in cAMP hydrolysis and thus mediates a negative cross-talk between both pathways. PDE2 has been recently investigated in cardiomyocytes; here we specifically addressed its role in fibroblast conversion and cardiac fibrosis. PDE2 is abundantly expressed in both neonatal rat cardiac fibroblasts (CFs) and cardiomyocytes. The overexpression of PDE2 in CFs strongly reduced basal and isoprenaline-induced cAMP synthesis, and this decrease was sufficient to induce MyoCF conversion even in the absence of exogenous profibrotic stimuli. Functional stress-strain experiments with fibroblast-derived engineered connective tissue (ECT) demonstrated higher stiffness in ECTs overexpressing PDE2. In regard to cGMP, neither basal nor atrial natriuretic peptide-induced cGMP levels were affected by PDE2, whereas the response to nitric oxide donor sodium nitroprusside was slightly but significantly reduced. Interestingly, despite persistently depressed cAMP levels, both cGMP-elevating stimuli were able to completely prevent the PDE2-induced MyoCF phenotype, arguing for a double-tracked mechanism. In conclusion, PDE2 accelerates CF to MyoCF conversion, which leads to greater stiffness in ECTs. Atrial natriuretic peptide- and sodium nitroprusside-mediated cGMP synthesis completely reverses PDE2-induced fibroblast conversion. Thus PDE2 may augment cardiac remodeling, but this effect can also be overcome by enhanced cGMP. The redundant role of cAMP and cGMP as antifibrotic meditators may be viewed as a protective mechanism in heart failure.

Left Ventricular Radial Function Associated With Genetic Variation in the cGMP-Dependent Protein Kinase

cGMP-dependent protein kinase type I is a major mediator of cGMP signaling in the cardiovascular system. Recent studies on cardiac-specific PRKG1 knockout mice demonstrated that cGMP-dependent protein kinase type I mediates the negative inotropic effect of cGMP in the myocardium. We therefore investigated the association between left ventricular (LV) function and common polymorphisms in the PRKG1 gene in a general population. In 609 randomly selected participants (51.2% women; mean age, 48.8 years; 36.6% hypertensive) who were free from overt cardiac disease, we performed echocardiography and genotyped intronic tag single-nucleotide polymorphisms (SNPs) rs1904694, rs7897633, and rs7905063 in PRKG1. On the basis of color Doppler myocardial motion data, we calculated end-systolic longitudinal and radial deformation (strain) of the LV inferolateral wall. In multivariable-adjusted analyses accounting for confounders and relatedness, systolic radial strain was significantly ( P ≤0.005) higher in homozygotes for rs1904694 (GG), rs7897633 (AA), and rs7905063 (TT) compared with heterozygotes or noncarriers. Haplotype analysis confirmed that LV radial strain was significantly higher in GAT homozygotes than in noncarriers (62.3% versus 56.0%; P =0.0005). Transmission of the PRKG1 GAT haplotype to informative offspring was associated with higher LV radial strain (effect size, 6.11%; P =0.017). For other LV phenotypes, none of the phenotype–genotype associations reached statistical significance. In conclusion, LV systolic radial function was associated with common polymorphisms in PRKG1 . If experimental studies and longitudinal follow-up of LV function confirm the causality of this association, interference with cGMP-dependent protein kinase type I function might be a target for pharmacological intervention.

Role of phosphodiesterase-5 inhibitors in heart failure: emerging data and concepts

Novel treatment of congestive heart failure (HF) involves utilizing unique pathways to improve upon contemporary therapies. Increasing the availability of cyclic guanosine monophosphate (cGMP) by inhibition of phosphodiesterase-5 (PDE5) is a relatively new, but promising therapeutic strategy. Preclinical studies suggest a favorable myocardial effect of PDE5 inhibitors by blocking adrenergic, hypertrophic and pro-apoptotic signaling, thereby supporting their use in HF. The clinical benefits of acute and chronic PDE5 inhibition on lung diffusion capacity, exercise performance and ejection fraction in humans are emerging and appear promising. Larger, controlled trials are now on-going to assess the safety, efficacy and tolerability of PDE5 inhibitors on morbidity and mortality in patients with both systolic and diastolic heart failure. If the results of these trials are positive, a new avenue for the treatment of HF will open, which will help curtail the societal effects of this costly and morbid disease.

Protein kinase-dependent oxidative regulation of the cardiac Na+-K+ pump: evidence from in vivo and in vitro modulation of cell signalling

The widely reported stimulation of the cardiac Na(+)-K(+) pump by protein kinase A (PKA) should oppose other effects of PKA to increase contractility of the normal heart. It should also reduce harmful raised myocyte Na(+) levels in heart failure, yet blockade of the β1 adrenergic receptor (AR), coupled to PKA signalling, is beneficial. We treated rabbits with the β1 AR antagonist metoprolol to modulate PKA activity and studied cardiac myocytes ex vivo. Metoprolol increased electrogenic pump current (Ip) in voltage clamped myocytes and reduced glutathionylation of the β1 pump subunit, an oxidative modification causally related to pump inhibition. Activation of adenylyl cyclase with forskolin to enhance cAMP synthesis or inclusion of the catalytic subunit of PKA in patch pipette solutions abolished the increase in Ip in voltage clamped myocytes induced by treatment with metoprolol, supporting cAMP/PKA-mediated pump inhibition. Metoprolol reduced myocardial PKA and protein kinase C (PKC) activities, reduced coimmunoprecipitation of cytosolic p47(phox) and membranous p22(phox) NADPH oxidase subunits and reduced myocardial O2(•-)-sensitive dihydroethidium fluorescence. Treatment also enhanced coimmunoprecipitation of the β1 pump subunit with glutaredoxin 1 that catalyses de-glutathionylation. Since angiotensin II induces PKC-dependent activation of NADPH oxidase, we examined the effects of angiotensin-converting enzyme inhibition with captopril. This treatment had no effect on PKA activity but reduced the activity of PKC, reduced β1 subunit glutathionylation and increased Ip. The PKA-induced Na(+)-K(+) pump inhibition we report should act with other mechanisms that enhance contractility of the normal heart but accentuate the harmful effects of raised cytosolic Na(+) in the failing heart. This scheme is consistent with the efficacy of β1 AR blockade in the treatment of heart failure.

Phosphodiesterases and cyclic GMP regulation in heart muscle

The cyclic nucleotide cGMP and its corresponding activated kinase cGK-1 serve as a counterbalance to acute and chronic myocardial stress. cGMP hydrolysis by several members of the phosphodiesterase (PDE) superfamily, PDE1, PDE2, and PDE5, regulate this signaling in the heart. This review details new insights regarding how these PDEs modulate cGMP and cGK-1 to influence heart function and chronic stress responses, and how their inhibition may provide potential therapeutic benefits.

Cardiac role of cyclic-GMP hydrolyzing phosphodiesterase type 5: from experimental models to clinical trials

Cyclic guanosine monophosphate (cGMP) and its primary signaling kinase, protein kinase G, play an important role in counterbalancing stress remodeling in the heart. Growing evidence supports a positive impact on a variety of cardiac disease conditions from the suppression of cGMP hydrolysis. The latter is regulated by members of the phosphodiesterase (PDE) superfamily, of which cGMP-selective PDE5 has been best studied. Inhibitors such as sildenafil and tadalafil ameliorate cardiac pressure and volume overload, ischemic injury, and cardiotoxicity. Clinical trials have begun exploring their potential to benefit dilated cardiomyopathy and heart failure with a preserved ejection fraction. This review discusses recent developments in the field, highlighting basic science and clinical studies.

The endocrine stress response is linked to one specific locus on chromosome 3 in a mouse model based on extremes in trait anxiety

Background

The hypothalamic-pituitary-adrenal (HPA) axis is essential to control physiological stress responses in mammals. Its dysfunction is related to several mental disorders, including anxiety and depression. The aim of this study was to identify genetic loci underlying the endocrine regulation of the HPA axis.

Method

High (HAB) and low (LAB) anxiety-related behaviour mice were established by selective inbreeding of outbred CD-1 mice to model extremes in trait anxiety. Additionally, HAB vs. LAB mice exhibit comorbid characteristics including a differential corticosterone response upon stress exposure. We crossbred HAB and LAB lines to create F1 and F2 offspring. To identify the contribution of the endocrine phenotypes to the total phenotypic variance, we examined multiple behavioural paradigms together with corticosterone secretion-based phenotypes in F2 mice by principal component analysis. Further, to pinpoint the genomic loci of the quantitative trait of the HPA axis stress response, we conducted genome-wide multipoint oligogenic linkage analyses based on Bayesian Markov chain Monte Carlo approach as well as parametric linkage in three-generation pedigrees, followed by a two-dimensional scan for epistasis and association analysis in freely segregating F2 mice using 267 single-nucleotide polymorphisms (SNPs), which were identified to consistently differ between HAB and LAB mice as genetic markers.

Results

HPA axis reactivity measurements and behavioural phenotypes were represented by independent principal components and demonstrated no correlation. Based on this finding, we identified one single quantitative trait locus (QTL) on chromosome 3 showing a very strong evidence for linkage (2ln (L-score) > 10, LOD > 23) and significant association (lowest Bonferroni adjusted p < 10^-28) to the neuroendocrine stress response. The location of the linkage peak was estimated at 42.3 cM (95% confidence interval: 41.3 - 43.3 cM) and was shown to be in epistasis (p-adjusted < 0.004) with the locus at 35.3 cM on the same chromosome. The QTL harbours genes involved in steroid synthesis and cardiovascular effects.

Conclusion

The very prominent effect on stress-induced corticosterone secretion of the genomic locus on chromosome 3 and its involvement in epistasis highlights the critical role of this specific locus in the regulation of the HPA axis.

Distinct patterns of constitutive phosphodiesterase activity in mouse sinoatrial node and atrial myocardium

Phosphodiesterases (PDEs) are critical regulators of cyclic nucleotides in the heart. In ventricular myocytes, the L-type Ca(2+) current (I(Ca,L)) is a major target of regulation by PDEs, particularly members of the PDE2, PDE3 and PDE4 families. Conversely, much less is known about the roles of PDE2, PDE3 and PDE4 in the regulation of action potential (AP) properties and I(Ca,L) in the sinoatrial node (SAN) and the atrial myocardium, especially in mice. Thus, the purpose of our study was to measure the effects of global PDE inhibition with Isobutyl-1-methylxanthine (IBMX) and selective inhibitors of PDE2, PDE3 and PDE4 on AP properties in isolated mouse SAN and right atrial myocytes. We also measured the effects of these inhibitors on I(Ca,L) in SAN and atrial myocytes in comparison to ventricular myocytes. Our data demonstrate that IBMX markedly increases spontaneous AP frequency in SAN myocytes and AP duration in atrial myocytes. Spontaneous AP firing in SAN myocytes was also increased by the PDE2 inhibitor erythro-9-[2-hydroxy-3-nonyl] adenine (EHNA), the PDE3 inhibitor milrinone (Mil) and the PDE4 inhibitor rolipram (Rol). In contrast, atrial AP duration was increased by EHNA and Rol, but not by Mil. IBMX also potently, and similarly, increased I(Ca,L) in SAN, atrial and ventricular myocytes; however, important differences emerged in terms of which inhibitors could modulate I(Ca,L) in each myocyte type. Consistent with our AP measurements, EHNA, Mil and Rol each increased I(Ca,L) in SAN myocytes. Also, EHNA and Rol, but not Mil, increased atrial I(Ca,L). In complete contrast, no selective PDE inhibitors increased I(Ca,L) in ventricular myocytes when given alone. Thus, our data show that the effects of selective PDE2, PDE3 and PDE4 inhibitors are distinct in the different regions of the myocardium indicating important differences in how each PDE family constitutively regulates ion channel function in the SAN, atrial and ventricular myocardium.

Cyclic guanosine monophosphate signalling pathway in pulmonary arterial hypertension

During the last decade, it emerged that cyclic guanosine monophosphate (cGMP) is a novel drug target for the treatment of pulmonary arterial hypertension (PAH). cGMP regulates many cellular functions, ranging from contractility to growth, of relevance to the disease. Generated from guanylyl cyclases in response to natriuretic peptides or nitric oxide (NO), cGMP transduces its effects through a number of cGMP effectors, including cGMP-regulated phosphodiesterases and protein kinases. Furthermore, the cGMP concentration is modulated by cGMP-degrading phosphodiesterases. Data to date demonstrate that increasing intracellular cGMP through stimulation of GCs, inhibition of PDEs, or both is a valid therapeutic strategy in drug development for PAH. New advances in understanding of cGMP are unravelled, as well as the pathobiology of PAH. cGMP remains an attractive future PAH drug target. This review makes a more detailed examination of cGMP signalling with particular reference to PAH.

Cardiac cyclic nucleotide phosphodiesterases: function, regulation, and therapeutic prospects

The second messengers cAMP and cGMP exist in multiple discrete compartments and regulate a variety of biological processes in the heart. The cyclic nucleotide phosphodiesterases, by catalyzing the hydrolysis of cAMP and cGMP, play crucial roles in controlling the amplitude, duration, and compartmentalization of cyclic nucleotide signaling. Over 60 phosphodiesterase isoforms, grouped into 11 families, have been discovered to date. In the heart, both cAMP- and cGMP-hydrolyzing phosphodiesterases play important roles in physiology and pathology. At least 7 of the 11 phosphodiesterase family members appear to be expressed in the myocardium, and evidence supports phosphodiesterase involvement in regulation of many processes important for normal cardiac function including pacemaking and contractility, as well as many pathological processes including remodeling and myocyte apoptosis. Pharmacological inhibitors for a number of phosphodiesterase families have also been used clinically or preclinically to treat several types of cardiovascular disease. In addition, phosphodiesterase inhibitors are also being considered for treatment of many forms of disease outside the cardiovascular system, raising the possibility of cardiovascular side effects of such agents. This review will discuss the roles of phosphodiesterases in the heart, in terms of expression patterns, regulation, and involvement in physiological and pathological functions. Additionally, the cardiac effects of various phosphodiesterase inhibitors, both potentially beneficial and detrimental, will be discussed.

Chronic Inhibition of cGMP phosphodiesterase 5A improves diabetic cardiomyopathy: a randomized, controlled clinical trial using magnetic resonance imaging with myocardial tagging

BACKGROUND

cGMP phosphodiesterase type 5 protein is upregulated in myocardial hypertrophy. However, it has never been ascertained whether phosphodiesterase type 5 inhibition exerts an antiremodeling effect in nonischemic heart disease in humans. We explored the cardioreparative properties of a selective phosphodiesterase type 5 inhibitor, sildenafil, in a model of diabetic cardiomyopathy.

METHODS AND RESULTS

Fifty-nine diabetic men (60.3 ± 7.4 years) with cardiac magnetic resonance imaging consistent with nonischemic, nonfailing diabetic cardiomyopathy (reduced circumferential strain [σ], -12.6 ± 3.1; increased left ventricular [LV] torsion [θ], 18.4 ± 4.6°; and increased ratio of LV mass to volume, 2.1 ± 0.5 g/mL) were randomized to receive sildenafil or placebo (100 mg/d). At baseline, the metabolic indices were correlated with torsion, strain, N-terminal pro-B-type natriuretic peptide, vascular endothelial growth factor, monocyte chemotactic protein-1, and blood pressure. After 3 months, sildenafil produced a significant improvement compared with placebo in LV torsion (Δθ: sildenafil, -3.89 ± 3.11° versus placebo, 2.13 ± 2.35°; P<0.001) and strain (Δσ: sildenafil, -3.30 ± 1.86 versus placebo, 1.22 ± 1.84; P<0.001). Sildenafil-induced improvement of LV contraction was accompanied by consistent changes in chamber geometry and performance, with a 6.5 ± 11 improvement in mass-to-volume ratio over placebo (P=0.021). Monocyte chemotactic protein-1 and transforming growth factor-β were the only markers affected by active treatment (Δmonocyte chemotactic protein-1: -75.30 ± 159.28 pg/mL, P=0.032; Δtransforming growth factor-β: 5.26 ± 9.67 ng/mL, P=0.009). No changes were found in endothelial function, afterload, or metabolism.

CONCLUSIONS

The early features of diabetic cardiomyopathy are LV concentric hypertrophy associated with altered myocardial contraction dynamics. Chronic phosphodiesterase type 5 inhibition, at this stage, has an antiremodeling effect, resulting in improved cardiac kinetics and circulating markers. This effect is independent of any other vasodilatory or endothelial effects and is apparently exerted through a direct intramyocardial action.

Study of the regulation of the inotropic response to 5-HT4 receptor activation via phosphodiesterases and its cross-talk with C-type natriuretic peptide in porcine left atrium

We studied how 5-HT(4) receptor-mediated inotropic responses are regulated at the level of cAMP in porcine left atrium. We used selective phosphodiesterase (PDE) inhibitors to assess which PDE subtypes are responsible for the fade with time of inotropic responses to 5-HT(4) receptor activation with 5-HT and the 5-HT(4) receptor agonist prucalopride. A possible cross-talk via PDEs between cGMP and 5-HT(4) receptor-induced cAMP signalling was evaluated. Electrically paced left atrial pectinate muscles from young male pigs (15-25 kg) were studied in vitro. Simultaneous inhibition of PDE3 plus PDE4 subtypes was necessary to increase the amplitude and completely prevent the fade of the inotropic response to 5-HT and prucalopride. When responses to 5-HT or prucalopride had faded 1 h after addition, the nonspecific PDE-inhibitor IBMX still fully recovered inotropic responses. Stimulation of particulate guanylyl cyclase, together with PDE2 and PDE4 inhibition, delayed the fade of the response to 5-HT, while stimulation of soluble guanylyl cyclase independently of PDEs accelerated the fade of the response to 5-HT. In conclusion, both PDE3 and PDE4 subtypes are responsible for the suppression and the fade of the inotropic response to 5-HT and prucalopride. Signalling through the 5-HT(4) receptor remains fully active for at least 90 min with PDEs continuously regulating the response. cGMP levels, elevated by activation of particulate guanylyl cyclase under PDE2 inhibition, can indirectly enhance 5-HT(4) receptor-mediated signalling, at least when also PDE4 is inhibited, presumably through inhibition of PDE3. Elevation of cGMP generated by soluble guanylyl cyclase attenuates responses to 5-HT independently of PDEs.