cGMP catabolism by phosphodiesterase 5A regulates cardiac adrenergic stimulation by NOS3-dependent mechanism

Effect of phosphodiesterase type 5 inhibitors on surgical outcome of ventricular septal defect and pulmonary hypertension patients

BACKGROUND

Children with ventricular septal defect (VSD) and large systemic-to-pulmonary shunts eventually develop pulmonary hypertension (PH). The perioperative management of patients with VSD and PH is quite troublesome and still debatable, especially in developing countries where the different management options and standardization of treatment is not available. Oral phosphodiesterase type 5 (PDE-5) inhibitors are good treatment options being widely available, cheap, easy administration and do not require extensive monitoring. The aim of our study was to evaluate the effect of the PDE-5 inhibitors when given orally, early preoperative and continued for 3 months postoperative on controlling postoperative PH with its effect on right ventricle (RV) functions. Fifty-one patients were randomly assigned to either sildenafil or tadalafil, 1 week before and continued for 3 months after corrective surgery. The control group received a placebo.

RESULTS

There was no significant difference in the improvement in the right ventricle systolic pressure (RVSP) between both groups, early in the postoperative period (P = 0.255) and in follow-up (P = 0.259). There was also no significant difference in the changes in mean pulmonary artery pressure (mPAP), postoperatively and on follow-up (P = 0.788 and 0.059, respectively). There was a drop in RV functions in both groups postoperatively which improved on follow-up; however, it was not significant between both groups. The length of intensive care unit (ICU) stay was similar between both groups (P = 0.143).

CONCLUSION

Perioperative administration of PDE-5 inhibitors does not have an impact on the clinical course as regards improvement in pulmonary artery (PA) pressure, ventricular functions and ICU stay.

Hsa_circ_0092355 Accelerates Papillary Thyroid Cancer Progression by Regulating the miR-543/PDE5A Pathway

CircRNAs have been found to participate in the progression of various tumors. In the present study, we aimed to clarify the role of hsa_circ_0092355 in papillary thyroid cancer (PTC) cell development. RT-qPCR was used to determine the expression of hsa_circ_0092355, miR-543, and PDE5A. PTC cell proliferation was ascertained via a cell colony formation assay and the CCK-8 test. Western blotting was performed to examine the expression levels of PDE5A and apoptosis-associated proteins (Bcl-2 and Bax) in PTC cells. A scratch wound assay was performed to measure the migration of PTC cells. A mouse xenograft test was performed to assess the effects of hsa_circ_0092355 in vivo. RIP and dual-luciferase reporter assays confirmed the association between miR-543 and hsa_circ_0092355 or PDE5A. Associations between miR-543, hsa_circ_0092355, and PDE5A were evaluated using Pearson's correlation coefficient. Upregulation of hsa_circ_0092355 was observed in PTC tissues. The hsa_circ_0092355 knockdown blocked the proliferation and migration of PTC cells and induced apoptosis. Moreover, hsa_circ_0092355 knockdown blocked PTC xenograft tumor growth in vivo. The miR-543 inhibitor could reverse the changes induced by hsa_circ_0092355 knockdown by hsa_circ_0092355 targeting miR-543. Furthermore, miR-543 suppresses PTC progression by downregulating PDE5A expression. Our findings suggest that the PTC tumor promoter hsa_circ_0092355 may promote carcinogenesis by controlling the miR-543/PDE5A pathway.

Phosphodiesterase in heart and vessels: from physiology to diseases

Phosphodiesterases (PDEs) are a superfamily of enzymes that hydrolyze cyclic nucleotides, including cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Both cyclic nucleotides are critical secondary messengers in the neurohormonal regulation in the cardiovascular system. PDEs precisely control spatiotemporal subcellular distribution of cyclic nucleotides in a cell- and tissue-specific manner, playing critical roles in physiological responses to hormone stimulation in the heart and vessels. Dysregulation of PDEs has been linked to the development of several cardiovascular diseases, such as hypertension, aneurysm, atherosclerosis, arrhythmia, and heart failure. Targeting these enzymes has been proven effective in treating cardiovascular diseases and is an attractive and promising strategy for the development of new drugs. In this review, we discuss the current understanding of the complex regulation of PDE isoforms in cardiovascular function, highlighting the divergent and even opposing roles of PDE isoforms in different pathogenesis.

Abnormal phosphorylation / dephosphorylation and Ca2+ dysfunction in heart failure

Heart failure (HF) can be caused by a variety of causes characterized by abnormal myocardial systole and diastole. Ca2+ current through the L-type calcium channel (LTCC) on the membrane is the initial trigger signal for a cardiac cycle. Declined systole and diastole in HF are associated with dysfunction of myocardial Ca2+ function. This disorder can be correlated with unbalanced levels of phosphorylation / dephosphorylation of LTCC, endoplasmic reticulum (ER), and myofilament. Kinase and phosphatase activity changes along with HF progress, resulting in phased changes in the degree of phosphorylation / dephosphorylation. It is important to realize the phosphorylation / dephosphorylation differences between a normal and a failing heart. This review focuses on phosphorylation / dephosphorylation changes in the progression of HF and summarizes the effects of phosphorylation / dephosphorylation of LTCC, ER function, and myofilament function in normal conditions and HF based on previous experiments and clinical research. Also, we summarize current therapeutic methods based on abnormal phosphorylation / dephosphorylation and clarify potential therapeutic directions.

PDE10A Inactivation Prevents Doxorubicin-Induced Cardiotoxicity and Tumor Growth

BACKGROUND

Cyclic nucleotides play critical roles in cardiovascular biology and disease. PDE10A (phosphodiesterase 10A) is able to hydrolyze both cAMP and cGMP. PDE10A expression is induced in various human tumor cell lines, and PDE10A inhibition suppresses tumor cell growth. Chemotherapy drug such as doxorubicin (DOX) is widely used in chemotherapy. However, cardiotoxicity of DOX remains to be a serious clinical complication. In the current study, we aim to determine the role of PDE10A and the effect of PDE10A inhibition on cancer growth and cardiotoxicity induced by DOX.

METHODS

We used global PDE10A knockout (KO) mice and PDE10A inhibitor TP-10 to block PDE10A function. DOX-induced cardiotoxicity was evaluated in C57Bl/6J mice and nude mice with implanted ovarian cancer xenografts. Isolated adult mouse cardiomyocytes and a human ovarian cancer cell line were used for in vitro functional and mechanistic studies.

RESULTS

We found that PDE10A deficiency or inhibition alleviated DOX-induced myocardial atrophy, apoptosis, and dysfunction in C57Bl/6J mice. RNA sequencing study revealed a number of PDE10A-regulated signaling pathways involved in DOX-induced cardiotoxicity. PDE10A inhibition increased the death, decreased the proliferation, and potentiated the effect of DOX on various human cancer cells. Importantly, in nude mice with implanted ovarian cancer xenografts, PDE10A inhibition attenuated tumor growth while protecting DOX-induced cardiotoxicity. In isolated cardiomyocytes, PDE10A contributed to DOX-induced cardiomyocyte death via increasing Top2β (topoisomerase 2β) expression, mitochondrial dysfunction, and DNA damage by antagonizing cGMP/PKG (protein kinase G) signaling. PDE10A contributed to cardiomyocyte atrophy via potentiating FoxO3 (forkhead box O3) signaling via both cAMP/PKA (protein kinase A)- and cGMP/PKG-dependent signaling.

CONCLUSIONS

Taken together, our study elucidates a novel role for PDE10A in cardiotoxicity induced by DOX and cancer growth. Given that PDE10A has been already proven to be a safe drug target, PDE10A inhibition may represent a novel therapeutic strategy in cancer therapy, with effects preventing DOX-induced cardiotoxicity and simultaneously antagonizing cancer growth.

Inhibition of phosphodiesterase 5A by tadalafil improves SIRT1 expression and activity in insulin-resistant podocytes

A decrease in intracellular levels of 3',5'-cyclic guanosine monophosphate (cGMP) has been implicated in the progression of diabetic nephropathy. Hyperglycemia significantly inhibits cGMP-dependent pathway activity in the kidney, leading to glomerular damage and proteinuria. The enhancement of activity of this pathway that is associated with an elevation of cGMP levels may be achieved by inhibition of the cGMP specific phosphodiesterase 5A (PDE5A) using selective inhibitors, such as tadalafil. Hyperglycemia decreased the insulin responsiveness of podocytes and impaired podocyte function. These effects were associated with lower protein amounts and activity of the protein deacetylase sirtuin 1 (SIRT1) and a decrease in the phosphorylation of adenosine monophosphate-dependent protein kinase (AMPK). We found that PDE5A protein levels increased in hyperglycemia, and PDE5A downregulation improved the insulin responsiveness of podocytes with reestablished SIRT1 expression and activity. PDE5A inhibitors potentiate nitric oxide (NO)/cGMP signaling, and NO modulates the activity and expression of SIRT1. Therefore, we investigated the effects of tadalafil on SIRT1 and AMPK in the context of improving the insulin sensitivity in podocytes and podocyte function in hyperglycemia. Our study revealed that tadalafil restored SIRT1 expression and activity and activated AMPK by increasing its phosphorylation. Tadalafil also restored stimulating effect of insulin on glucose transport in podocytes with high glucose-induced insulin resistance. Additionally, tadalafil improved the function of podocytes that were exposed to high glucose concentrations. Our results display novel mechanisms involved in the pathogenesis of glomerulopathies in diabetes, which may contribute to the development of more effective treatment strategies for diabetic nephropathy.

Cyclic nucleotide phosphodiesterases as therapeutic targets in cardiac hypertrophy and heart failure

Cyclic nucleotide phosphodiesterases (PDEs) modulate the neurohormonal regulation of cardiac function by degrading cAMP and cGMP. In cardiomyocytes, multiple PDE isozymes with different enzymatic properties and subcellular localization regulate local pools of cyclic nucleotides and specific functions. This organization is heavily perturbed during cardiac hypertrophy and heart failure (HF), which can contribute to disease progression. Clinically, PDE inhibition has been considered a promising approach to compensate for the catecholamine desensitization that accompanies HF. Although PDE3 inhibitors, such as milrinone or enoximone, have been used clinically to improve systolic function and alleviate the symptoms of acute HF, their chronic use has proved to be detrimental. Other PDEs, such as PDE1, PDE2, PDE4, PDE5, PDE9 and PDE10, have emerged as new potential targets to treat HF, each having a unique role in local cyclic nucleotide signalling pathways. In this Review, we describe cAMP and cGMP signalling in cardiomyocytes and present the various PDE families expressed in the heart as well as their modifications in pathological cardiac hypertrophy and HF. We also appraise the evidence from preclinical models as well as clinical data pointing to the use of inhibitors or activators of specific PDEs that could have therapeutic potential in HF.

NLRP3 Inflammasome Activation Through Heart-Brain Interaction Initiates Cardiac Inflammation and Hypertrophy During Pressure Overload

BACKGROUND

Mechanical stress on the heart, such as high blood pressure, initiates inflammation and causes hypertrophic heart disease. However, the regulatory mechanism of inflammation and its role in the stressed heart remain unclear. IL-1β (interleukin-1β) is a proinflammatory cytokine that causes cardiac hypertrophy and heart failure. Here, we show that neural signals activate the NLRP3 (nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing 3) inflammasome for IL-1β production to induce adaptive hypertrophy in the stressed heart.

METHODS

C57BL/6 mice, knockout mouse strains for NLRP3 and P2RX7 (P2X purinoceptor 7), and adrenergic neuron-specific knockout mice for SLC17A9, a secretory vesicle protein responsible for the storage and release of ATP, were used for analysis. Pressure overload was induced by transverse aortic constriction. Various animal models were used, including pharmacological treatment with apyrase, lipopolysaccharide, 2'(3')-O-(4-benzoylbenzoyl)-ATP, MCC950, anti-IL-1β antibodies, clonidine, pseudoephedrine, isoproterenol, and bisoprolol, left stellate ganglionectomy, and ablation of cardiac afferent nerves with capsaicin. Cardiac function and morphology, gene expression, myocardial IL-1β and caspase-1 activity, and extracellular ATP level were assessed. In vitro experiments were performed using primary cardiomyocytes and fibroblasts from rat neonates and human microvascular endothelial cell line. Cell surface area and proliferation were assessed.

RESULTS

Genetic disruption of NLRP3 resulted in significant loss of IL-1β production, cardiac hypertrophy, and contractile function during pressure overload. A bone marrow transplantation experiment revealed an essential role of NLRP3 in cardiac nonimmune cells in myocardial IL-1β production and cardiac phenotype. Pharmacological depletion of extracellular ATP or genetic disruption of the P2X7 receptor suppressed myocardial NLRP3 inflammasome activity during pressure overload, indicating an important role of ATP/P2X7 axis in cardiac inflammation and hypertrophy. Extracellular ATP induced hypertrophic changes of cardiac cells in an NLRP3- and IL-1β-dependent manner in vitro. Manipulation of the sympathetic nervous system suggested sympathetic efferent nerves as the main source of extracellular ATP. Depletion of ATP release from sympathetic efferent nerves, ablation of cardiac afferent nerves, or a lipophilic β-blocker reduced cardiac extracellular ATP level, and inhibited NLRP3 inflammasome activation, IL-1β production, and adaptive cardiac hypertrophy during pressure overload.

CONCLUSIONS

Cardiac inflammation and hypertrophy are regulated by heart-brain interaction. Controlling neural signals might be important for the treatment of hypertensive heart disease.

Beyond Erectile Dysfunction: cGMP-Specific Phosphodiesterase 5 Inhibitors for Other Clinical Disorders

Cyclic guanosine monophosphate (cGMP), an important intracellular second messenger, mediates cellular functional responses in all vital organs. Phosphodiesterase 5 (PDE5) is one of the 11 members of the cyclic nucleotide phosphodiesterase (PDE) family that specifically targets cGMP generated by nitric oxide-driven activation of the soluble guanylyl cyclase. PDE5 inhibitors, including sildenafil and tadalafil, are widely used for the treatment of erectile dysfunction, pulmonary arterial hypertension, and certain urological disorders. Preclinical studies have shown promising effects of PDE5 inhibitors in the treatment of myocardial infarction, cardiac hypertrophy, heart failure, cancer and anticancer-drug-associated cardiotoxicity, diabetes, Duchenne muscular dystrophy, Alzheimer's disease, and other aging-related conditions. Many clinical trials with PDE5 inhibitors have focused on the potential cardiovascular, anticancer, and neurological benefits. In this review, we provide an overview of the current state of knowledge on PDE5 inhibitors and their potential therapeutic indications for various clinical disorders beyond erectile dysfunction.

Structural Characterization of Murine Phosphodiesterase 5 Isoforms and Involvement of Cysteine Residues in Supramolecular Assembly

Phosphodiesterases (PDEs) are a superfamily of evolutionarily conserved cyclic nucleotide (cAMP/cGMP)-hydrolyzing enzymes, components of transduction pathways regulating crucial aspects of cell life. Within this family, the cGMP-dependent PDE5 is the major hydrolyzing enzyme in many mammalian tissues, where it regulates a number of cellular and tissular processes. Using Kluyveromyces lactis as a model organism, the murine PDE5A1, A2 and A3 isoforms were successfully expressed and studied, evidencing, for the first time, a distinct role of each isoform in the control, modulation and maintenance of the cellular redox metabolism. Moreover, we demonstrated that the short N-terminal peptide is responsible for the tetrameric assembly of MmPDE5A1 and for the mitochondrial localization of MmPDE5A2. We also analyzed MmPDE5A1, A2 and A3 using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), structural mass spectrometry (MS) and polyacrylamide gel electrophoresis in their native conditions (native-PAGE) and in the presence of redox agents. These analyses pointed towards the role of a few specific cysteines in the isoforms' oligomeric assembly and the loss of enzymatic activity when modified.

Biological Assessment of the NO-Dependent Endothelial Function

Nitric oxide (NO) is implicated in numerous physiological processes, including vascular homeostasis. Reduced NO bioavailability is a hallmark of endothelial dysfunction, a prequel to many cardiovascular diseases. Biomarkers of an early NO-dependent endothelial dysfunction obtained from routine venous blood sampling would be of great interest but are currently lacking. The direct measurement of circulating NO remains a challenge due by its high reactivity and short half-life. The current techniques measure stable products from the NO signaling pathway or metabolic end products of NO that do not accurately represent its bioavailability and, therefore, endothelial function per se. In this review, we will concentrate on an original technique of low temperature electron paramagnetic resonance spectroscopy capable to directly measure the 5-α-coordinated heme nitrosyl-hemoglobin in the T (tense) state (5-α-nitrosyl-hemoglobin or HbNO) obtained from fresh venous human erythrocytes. In humans, HbNO reflects the bioavailability of NO formed in the vasculature from vascular endothelial NOS or exogenous NO donors with minor contribution from erythrocyte NOS. The HbNO signal is directly correlated with the vascular endothelial function and inversely correlated with vascular oxidative stress. Pilot studies support the validity of HbNO measurements both for the detection of endothelial dysfunction in asymptomatic subjects and for the monitoring of such dysfunction in patients with known cardiovascular disease. The impact of therapies or the severity of diseases such as COVID-19 infection involving the endothelium could also be monitored and their incumbent risk of complications better predicted through serial measurements of HbNO.

Myocardial TRPC6-mediated Zn2+ influx induces beneficial positive inotropy through β-adrenoceptors

Baroreflex control of cardiac contraction (positive inotropy) through sympathetic nerve activation is important for cardiocirculatory homeostasis. Transient receptor potential canonical subfamily (TRPC) channels are responsible for α₁-adrenoceptor (α₁AR)-stimulated cation entry and their upregulation is associated with pathological cardiac remodeling. Whether TRPC channels participate in physiological pump functions remains unclear. We demonstrate that TRPC6-specific Zn²⁺ influx potentiates β-adrenoceptor (βAR)-stimulated positive inotropy in rodent cardiomyocytes. Deletion of trpc6 impairs sympathetic nerve-activated positive inotropy but not chronotropy in mice. TRPC6-mediated Zn²⁺ influx boosts α₁AR-stimulated βAR/G_s-dependent signaling in rat cardiomyocytes by inhibiting β-arrestin-mediated βAR internalization. Replacing two TRPC6-specific amino acids in the pore region with TRPC3 residues diminishes the α₁AR-stimulated Zn²⁺ influx and positive inotropic response. Pharmacological enhancement of TRPC6-mediated Zn²⁺ influx prevents chronic heart failure progression in mice. Our data demonstrate that TRPC6-mediated Zn²⁺ influx with α₁AR stimulation enhances baroreflex-induced positive inotropy, which may be a new therapeutic strategy for chronic heart failure.

Phosphodiesterases and Compartmentation of cAMP and cGMP Signaling in Regulation of Cardiac Contractility in Normal and Failing Hearts

Cardiac contractility is regulated by several neural, hormonal, paracrine, and autocrine factors. Amongst these, signaling through β-adrenergic and serotonin receptors generates the second messenger cyclic AMP (cAMP), whereas activation of natriuretic peptide receptors and soluble guanylyl cyclases generates cyclic GMP (cGMP). Both cyclic nucleotides regulate cardiac contractility through several mechanisms. Phosphodiesterases (PDEs) are enzymes that degrade cAMP and cGMP and therefore determine the dynamics of their downstream effects. In addition, the intracellular localization of the different PDEs may contribute to regulation of compartmented signaling of cAMP and cGMP. In this review, we will focus on the role of PDEs in regulating contractility and evaluate changes in heart failure.

Pathogenic Mechanisms Underlying Cirrhotic Cardiomyopathy

Cardiac dysfunction associated with cirrhosis in the absence of preexisting heart disease is a condition known as cirrhotic cardiomyopathy (CCM). Cardiac abnormalities consist of enlargement of cardiac chambers, attenuated systolic and diastolic contractile responses to stress stimuli, and repolarization changes. CCM may contribute to cardiovascular morbidity and mortality after liver transplantation and other major surgeries, and also to the pathogenesis of hepatorenal syndrome. The underlying mechanisms of CCM are poorly understood and as such medical therapy is an area of unmet medical need. The present review focuses on the pathogenic mechanisms responsible for development of CCM. The two major concurrent mechanistic pathways are the inflammatory phenotype due to portal hypertension, and protein/lipid synthetic/metabolic defects due to cirrhosis and liver insufficiency. The inflammatory phenotype arises from intestinal congestion due to portal hypertension, resulting in bacteria/endotoxin translocation into the systemic circulation. The cytokine storm associated with inflammation, particularly TNFα acting via NFκB depresses cardiac function. They also stimulate two evanescent gases, nitric oxide and carbon monoxide which produce cardiodepression by cGMP. Inflammation also stimulates the endocannabinoid CB-1 pathway. These systems inhibit the stimulatory beta-adrenergic contractile pathway. The liver insufficiency of cirrhosis is associated with defective synthesis or metabolism of several substances including proteins and lipids/lipoproteins. The protein defects including titin and collagen contribute to diastolic dysfunction. Other protein abnormalities such as a switch of myosin heavy chain isoforms result in systolic dysfunction. Lipid biochemical changes at the cardiac sarcolemmal plasma membrane result in increased cholesterol:phospholipid ratio and decreased membrane fluidity. Final common pathway changes involve abnormal cardiomyocyte intracellular ion kinetics, particularly calcium. In conclusion, cirrhotic cardiomyopathy is caused by two pathways of cellular and molecular dysfunction/damage due to hepatic insufficiency and portal hypertension.

PDE5 Inhibition Suppresses Ventricular Arrhythmias by Reducing SR Ca2+ Content

[Figure: see text].

Visualising and understanding cGMP signals in the cardiovascular system

cGMP is an important signalling molecule in humans. Fluorescent cGMP biosensors have emerged as powerful tools for the sensitive analysis of cGMP pathways at the single-cell level. Here, we briefly outline cGMP's multifaceted role in (patho)physiology and pharmacotherapy. Then we summarise what new insights cGMP imaging has provided into endogenous cGMP signalling and drug action, with a focus on the cardiovascular system. Indeed, the use of cGMP biosensors has led to several conceptual advances, such as the discovery of local, intercellular and mechanosensitive cGMP signals. Importantly, single-cell imaging can provide valuable information about the heterogeneity of cGMP signals within and between individual cells of an isolated cell population or tissue. We also discuss current challenges and future directions of cGMP imaging, such as the direct visualisation of cGMP microdomains, simultaneous monitoring of cGMP and other signalling molecules and, ultimately, cGMP imaging in tissues and animals under close-to-native conditions.

An update of cyclic nucleotide phosphodiesterase as a target for cardiac diseases

INTRODUCTION

Cyclic nucleotides, cAMP, and cGMP, are important second messengers of intracellular signaling and play crucial roles in cardiovascular biology and diseases. Cyclic nucleotide phosphodiesterases (PDEs) control the duration, magnitude, and compartmentalization of cyclic nucleotide signaling by catalyzing the hydrolysis of cyclic nucleotides. Individual PDEs modulate distinct signaling pathways and biological functions in the cell, making it a potential therapeutic target for the treatment of different cardiovascular disorders. The clinical success of several PDE inhibitors has ignited continued interest in PDE inhibitors and in PDE-target therapeutic strategies.

AREAS COVERED

This review concentrates on recent research advances of different PDE isoforms with regard to their expression patterns and biological functions in the heart. The limitations of current research and future directions are then discussed. The current and future development of PDE inhibitors is also covered.

EXPERT OPINION

Despite the therapeutic success of several marketed PDE inhibitors, the use of PDE inhibitors can be limited by their side effects, lack of efficacy, and lack of isoform selectivity. Advances in our understanding of the mechanisms by which cellular functions are changed through PDEs may enable the development of new approaches to achieve effective and specific PDE inhibition for various cardiac therapies.

Myocardial Phosphodiesterases and Their Role in cGMP Regulation

Cyclic nucleotide phosphodiesterases comprise an 11-member superfamily yielding near 100 isoform variants that hydrolyze cAMP or cGMP to their respective 5'-monophosphate form. Each plays a role in compartmentalized cyclic nucleotide signaling, with varying selectivity for each substrate, and conveying cell and intracellular-specific localized control. This review focuses on the 5 phosphodiesterases (PDEs) expressed in the cardiac myocyte capable of hydrolyzing cGMP and that have been shown to play a role in cardiac physiological and pathological processes. PDE1, PDE2, and PDE3 catabolize cAMP as well, whereas PDE5 and PDE9 are cGMP selective. PDE3 and PDE5 are already in clinical use, the former for heart failure, and PDE1, PDE9, and PDE5 are all being actively studied for this indication in patients. Research in just the past few years has revealed many novel cardiac influences of each isoform, expanding the therapeutic potential from their selective pharmacological blockade or in some instances, activation. PDE1C inhibition was found to confer cell survival protection and enhance cardiac contractility, whereas PDE2 inhibition or activation induces beneficial effects in hypertrophied or failing hearts, respectively. PDE3 inhibition is already clinically used to treat acute decompensated heart failure, although toxicity has precluded its long-term use. However, newer approaches including isoform-specific allosteric modulation may change this. Finally, inhibition of PDE5A and PDE9A counter pathological remodeling of the heart and are both being pursued in clinical trials. Here, we discuss recent research advances in each of these PDEs, their impact on the myocardium, and cardiac therapeutic potential.

Interaction among Hydrogen Sulfide and Other Gasotransmitters in Mammalian Physiology and Pathophysiology

Hydrogen sulfide (H₂S), nitric oxide (NO), carbon monoxide (CO), and sulfur dioxide (SO₂) were previously considered as toxic gases, but now they are found to be members of mammalian gasotransmitters family. Both H₂S and SO₂ are endogenously produced in sulfur-containing amino acid metabolic pathway in vivo. The enzymes catalyzing the formation of H₂S are mainly CBS, CSE, and 3-MST, and the key enzymes for SO₂ production are AAT1 and AAT2. Endogenous NO is produced from L-arginine under catalysis of three isoforms of NOS (eNOS, iNOS, and nNOS). HO-mediated heme catabolism is the main source of endogenous CO. These four gasotransmitters play important physiological and pathophysiological roles in mammalian cardiovascular, nervous, gastrointestinal, respiratory, and immune systems. The similarity among these four gasotransmitters can be seen from the same and/or shared signals. With many studies on the biological effects of gasotransmitters on multiple systems, the interaction among H₂S and other gasotransmitters has been gradually explored. H₂S not only interacts with NO to form nitroxyl (HNO), but also regulates the HO/CO and AAT/SO₂ pathways. Here, we review the biosynthesis and metabolism of the gasotransmitters in mammals, as well as the known complicated interactions among H₂S and other gasotransmitters (NO, CO, and SO₂) and their effects on various aspects of cardiovascular physiology and pathophysiology, such as vascular tension, angiogenesis, heart contractility, and cardiac protection.

Phosphodiesterase 5 (PDE5): Structure-function regulation and therapeutic applications of inhibitors

Phosphodiesterase 5 (PDE5) is one of the most well-studied phosphodiesterases (PDEs) that specifically targets cGMP typically generated by nitric oxide (NO)-mediated activation of the soluble guanylyl cyclase. Given the crucial role of cGMP generated through the activation of this cellular signaling pathway in a variety of physiologically processes, pharmacological inhibition of PDE5 has been demonstrated to have several therapeutic applications including erectile dysfunction and pulmonary arterial hypertension. While they are designed to inhibit PDE5, the inhibitors show different affinities and specificities against all PDE subtypes. Additionally, they have been shown to induce allosteric structural changes in the protein. These are mostly attributed to their chemical structure and, therefore, binding interactions with PDE catalytic domains. Therefore, understanding how these inhibitors interact with PDE5 and the structural basis of their selectivity is critically important for the design of novel, highly selective PDE5 inhibitors. Here, we review the structure of PDE5, how its function is regulated, and discuss the clinically available inhibitors that target phosphodiesterase 5, aiming to better understand the structural bases of their affinity and specificity. We also discuss the therapeutic indications of these inhibitors and the potential of repurposing for a wider range of clinical applications.

Angiotensin II-Induced Cardiovascular Fibrosis Is Attenuated by NO-Sensitive Guanylyl Cyclase1

In the NO/cGMP signaling cascade, relevant in the cardiovascular system, two NO-sensitive guanylyl cyclase (NO-GC) isoforms are responsible for NO-dependent cGMP generation. Here, the impact of the major NO-GC isoform, NO-GC1, on fibrosis development in the cardiovascular system was studied in NO-GC1-deficient mice treated with AngiotensinII (AngII), known to induce vascular and cardiac remodeling. Morphometric analysis of NO-GC1 KO’s aortae demonstrated an enhanced increase of perivascular area after AngII treatment accompanied by a higher aortic collagen1 mRNA content. Increased perivascular fibrosis also occurred in cardiac vessels of AngII-treated NO-GC1 KO mice. In line, AngII-induced interstitial fibrosis was 32% more pronounced in NO-GC1 KO than in WT myocardia associated with a higher cardiac Col1 and other fibrotic marker protein content. In sum, increased perivascular and cardiac interstitial fibrosis together with the enhanced collagen1 mRNA content in AngII-treated NO-GC1-deficient mice represent an exciting manifestation of antifibrotic properties of cGMP formed by NO-GC1, a finding with great pharmaco-therapeutic implications.

Cardiac cGMP Signaling in Health and Disease: Location, Location, Location

3',5'-Cyclic guanosine monophosphate (cGMP) is a ubiquitous second messenger, which critically regulates cardiac pump function and protects from the development of cardiac hypertrophy by acting in various subcellular microdomains. Although clinical studies testing the potential of cGMP elevating drugs in patients suffering from cardiac disease showed promising results, deeper insight into the local actions of these drugs at the subcellular level are indispensable to inspire novel therapeutic strategies. Detailed information on the spatio-temporal dynamics of cGMP production and degradation can be provided by the use of fluorescent biosensors that are capable of monitoring this second messenger at different locations inside the cell with high temporal and spatial resolution. In this review, we will summarize how these emerging new tools have improved our understanding of cardiac cGMP signaling in health and disease, and attempt to anticipate future challenges in the field.

Cardiac Cyclic Nucleotide Phosphodiesterases: Roles and Therapeutic Potential in Heart Failure

The cyclic nucleotides cyclic adenosine-3′,5′-monophosphate (cAMP) and cyclic guanosine-3′,5′-monophosphate (cGMP) maintain physiological cardiac contractility and integrity. Cyclic nucleotide–hydrolysing phosphodiesterases (PDEs) are the prime regulators of cAMP and cGMP signalling in the heart. During heart failure (HF), the expression and activity of multiple PDEs are altered, which disrupt cyclic nucleotide levels and promote cardiac dysfunction. Given that the morbidity and mortality associated with HF are extremely high, novel therapies are urgently needed. Herein, the role of PDEs in HF pathophysiology and their therapeutic potential is reviewed. Attention is given to PDEs 1–5, and other PDEs are briefly considered. After assessing the role of each PDE in cardiac physiology, the evidence from pre-clinical models and patients that altered PDE signalling contributes to the HF phenotype is examined. The potential of pharmacologically harnessing PDEs for therapeutic gain is considered.

Genetic manipulation of cyclic nucleotide signaling during hippocampal neuroplasticity and memory formation

Decades of research have underscored the importance of cyclic nucleotide signaling in memory formation and synaptic plasticity. In recent years, several new genetic techniques have expanded the neuroscience toolbox, allowing researchers to measure and modulate cyclic nucleotide gradients with high spatiotemporal resolution. Here, we will provide an overview of studies using genetic approaches to interrogate the role cyclic nucleotide signaling plays in hippocampus-dependent memory processes and synaptic plasticity. Particular attention is given to genetic techniques that measure real-time changes in cyclic nucleotide levels as well as newly-developed genetic strategies to transiently manipulate cyclic nucleotide signaling in a subcellular compartment-specific manner with high temporal resolution.

β3-Adrenoceptor redistribution impairs NO/cGMP/PDE2 signalling in failing cardiomyocytes

Cardiomyocyte β3-adrenoceptors (β3-ARs) coupled to soluble guanylyl cyclase (sGC)-dependent production of the second messenger 3',5'-cyclic guanosine monophosphate (cGMP) have been shown to protect from heart failure. However, the exact localization of these receptors to fine membrane structures and subcellular compartmentation of β3-AR/cGMP signals underpinning this protection in health and disease remain elusive. Here, we used a Förster Resonance Energy Transfer (FRET)-based cGMP biosensor combined with scanning ion conductance microscopy (SICM) to show that functional β3-ARs are mostly confined to the T-tubules of healthy rat cardiomyocytes. Heart failure, induced via myocardial infarction, causes a decrease of the cGMP levels generated by these receptors and a change of subcellular cGMP compartmentation. Furthermore, attenuated cGMP signals led to impaired phosphodiesterase two dependent negative cGMP-to-cAMP cross-talk. In conclusion, topographic and functional reorganization of the β3-AR/cGMP signalosome happens in heart failure and should be considered when designing new therapies acting via this receptor.

Estrogen Receptor-α Non-Nuclear Signaling Confers Cardioprotection and Is Essential to cGMP-PDE5 Inhibition Efficacy

Using genetically engineered mice lacking estrogen receptor-α non-nuclear signaling, this study demonstrated that estrogen receptor-α non-nuclear signaling activated myocardial cyclic guanosine monophosphate-dependent protein kinase G and conferred protection against cardiac remodeling induced by pressure overload. This pathway was indispensable to the therapeutic efficacy of cyclic guanosine monophosphate-phosphodiesterase 5 inhibition but not to that of soluble guanylate cyclase stimulation. These results might partially explain the equivocal results of phosphodiesterase 5 inhibitor efficacy and also provide the molecular basis for the advantage of using a soluble guanylate cyclase simulator as a new therapeutic option in post-menopausal women. This study also highlighted the need for female-specific therapeutic strategies for heart failure.

Role of cyclic nucleotides and their downstream signaling cascades in memory function: Being at the right time at the right spot

A plethora of studies indicate the important role of cAMP and cGMP cascades in neuronal plasticity and memory function. As a result, altered cyclic nucleotide signaling has been implicated in the pathophysiology of mnemonic dysfunction encountered in several diseases. In the present review we provide a wide overview of studies regarding the involvement of cyclic nucleotides, as well as their upstream and downstream molecules, in physiological and pathological mnemonic processes. Next, we discuss the regulation of the intracellular concentration of cyclic nucleotides via phosphodiesterases, the enzymes that degrade cAMP and/or cGMP, and via A-kinase-anchoring proteins that refine signal compartmentalization of cAMP signaling. We also provide an overview of the available data pointing to the existence of specific time windows in cyclic nucleotide signaling during neuroplasticity and memory formation and the significance to target these specific time phases for improving memory formation. Finally, we highlight the importance of emerging imaging tools like Förster resonance energy transfer imaging and optogenetics in detecting, measuring and manipulating the action of cyclic nucleotide signaling cascades.

Mind the gap (junction): cGMP induced by nitric oxide in cardiac myocytes originates from cardiac fibroblasts

Background and Purpose

The intracellular signalling molecule cGMP, formed by NO‐sensitive GC (NO–GC), has an established function in the vascular system. Despite numerous reports about NO‐induced cGMP effects in the heart, the underlying cGMP signals are poorly characterized.

Experimental Approach

Therefore, we analysed cGMP signals in cardiac myocytes and fibroblasts isolated from knock‐in mice expressing a FRET‐based cGMP indicator.

Key Results

Whereas in cardiac myocytes, none of the known NO–GC‐activating substances (NO, GC activators, and GC stimulators) increased cGMP even in the presence of PDE inhibitors, they induced substantial cGMP increases in cardiac fibroblasts. As cardiac myocytes and fibroblasts are electrically connected via gap junctions, we asked whether cGMP can take the same route. Indeed, in cardiomyocytes co‐cultured on cardiac fibroblasts, NO‐induced cGMP signals were detectable, and two groups of unrelated gap junction inhibitors abolished these signals.

Conclusion and Implication

We conclude that NO‐induced cGMP formed in cardiac fibroblasts enters cardiac myocytes via gap junctions thereby turning cGMP into an intercellular signalling molecule. The findings shed new light on NO/cGMP signalling in the heart and will potentially broaden therapeutic opportunities for cardiac disease.

FRET-based cyclic GMP biosensors measure low cGMP concentrations in cardiomyocytes and neurons

Several FRET (fluorescence resonance energy transfer)-based biosensors for intracellular detection of cyclic nucleotides have been designed in the past decade. However, few such biosensors are available for cGMP, and even fewer that detect low nanomolar cGMP concentrations. Our aim was to develop a FRET-based cGMP biosensor with high affinity for cGMP as a tool for intracellular signaling studies. We used the carboxyl-terminal cyclic nucleotide binding domain of Plasmodium falciparum cGMP-dependent protein kinase (PKG) flanked by different FRET pairs to generate two cGMP biosensors (Yellow PfPKG and Red PfPKG). Here, we report that these cGMP biosensors display high affinity for cGMP (EC50 of 23 ± 3 nM) and detect cGMP produced through soluble guanylyl cyclase and guanylyl cyclase A in stellate ganglion neurons and guanylyl cyclase B in cardiomyocytes. These biosensors are therefore optimal tools for real-time measurements of low concentrations of cGMP in living cells.

The Endocrine Function of the Heart: Physiology and Involvements of Natriuretic Peptides and Cyclic Nucleotide Phosphodiesterases in Heart Failure

Besides pumping, the heart participates in hydro-sodium homeostasis and systemic blood pressure regulation through its endocrine function mainly represented by the large family of natriuretic peptides (NPs), including essentially atrial natriuretic (ANP) and brain natriuretic peptides (BNP). Under normal conditions, these peptides are synthesized in response to atrial cardiomyocyte stretch, increase natriuresis, diuresis, and vascular permeability through binding of the second intracellular messenger’s guanosine 3′,5′-cyclic monophosphate (cGMP) to specific receptors. During heart failure (HF), the beneficial effects of the enhanced cardiac hormones secretion are reduced, in connection with renal resistance to NP. In addition, there is a BNP paradox characterized by a physiological inefficiency of the BNP forms assayed by current methods. In this context, it appears interesting to improve the efficiency of the cardiac natriuretic system by inhibiting cyclic nucleotide phosphodiesterases, responsible for the degradation of cGMP. Recent data support such a therapeutic approach which can improve the quality of life and the prognosis of patients with HF.

Acute Enhancement of Cardiac Function by Phosphodiesterase Type 1 Inhibition

BACKGROUND

Phosphodiesterase type-1 (PDE1) hydrolyzes cAMP and cGMP and is constitutively expressed in the heart, although cardiac effects from its acute inhibition in vivo are largely unknown. Existing data are limited to rodents expressing mostly the cGMP-favoring PDE1A isoform. Human heart predominantly expresses PDE1C with balanced selectivity for cAMP and cGMP. Here, we determined the acute effects of PDE1 inhibition in PDE1C-expressing mammals, dogs, and rabbits, in normal and failing hearts, and explored its regulatory pathways.

METHODS

Conscious dogs chronically instrumented for pressure-volume relations were studied before and after tachypacing-induced heart failure (HF). A selective PDE1 inhibitor (ITI-214) was administered orally or intravenously±dobutamine. Pressure-volume analysis in anesthetized rabbits tested the role of β-adrenergic and adenosine receptor signaling on ITI-214 effects. Sarcomere and calcium dynamics were studied in rabbit left ventricular myocytes.

RESULTS

In normal and HF dogs, ITI-214 increased load-independent contractility, improved relaxation, and reduced systemic arterial resistance, raising cardiac output without altering systolic blood pressure. Heart rate increased, but less so in HF dogs. ITI-214 effects were additive to β-adrenergic receptor agonism (dobutamine). Dobutamine but not ITI-214 increased plasma cAMP. ITI-214 induced similar cardiovascular effects in rabbits, whereas mice displayed only mild vasodilation and no contractility effects. In rabbits, β-adrenergic receptor blockade (esmolol) prevented ITI-214-mediated chronotropy, but inotropy and vasodilation remained unchanged. By contrast, adenosine A_2B-receptor blockade (MRS-1754) suppressed ITI-214 cardiovascular effects. Adding fixed-rate atrial pacing did not alter the findings. ITI-214 alone did not affect sarcomere or whole-cell calcium dynamics, whereas β-adrenergic receptor agonism (isoproterenol) or PDE3 inhibition (cilostamide) increased both. Unlike cilostamide, which further enhanced shortening and peak calcium when combined with isoproterenol, ITI-214 had no impact on these responses. Both PDE1 and PDE3 inhibitors increased shortening and accelerated calcium decay when combined with forskolin, yet only cilostamide increased calcium transients.

CONCLUSIONS

PDE1 inhibition by ITI-214 in vivo confers acute inotropic, lusitropic, and arterial vasodilatory effects in PDE1C-expressing mammals with and without HF. The effects appear related to cAMP signaling that is different from that provided via β-adrenergic receptors or PDE3 modulation. ITI-214, which has completed phase I trials, may provide a novel therapy for HF.

Prevention of PKG-1α Oxidation Suppresses Antihypertrophic/Antifibrotic Effects From PDE5 Inhibition but not sGC Stimulation

BACKGROUND

Stimulation of sGC (soluble guanylate cyclase) or inhibition of PDE5 (phosphodiesterase type 5) activates PKG (protein kinase G)-1α to counteract cardiac hypertrophy and failure. PKG1α acts within localized intracellular domains; however, its oxidation at cysteine 42, linking homomonomers, alters this localization, impairing suppression of pathological cardiac stress. Because PDE5 and sGC reside in separate microdomains, we speculated that PKG1α oxidation might also differentially influence the effects from their pharmacological modulation.

METHODS AND RESULTS

Knock-in mice expressing a redox-dead PKG1α (PKG1α^C42S) or littermate controls (PKG1α^WT) were subjected to transaortic constriction to induce pressure overload and treated with a PDE5 inhibitor (sildenafil), sGC activator (BAY602770 [BAY]), or vehicle. In PKG1α^WT controls, sildenafil and BAY similarly enhanced PKG activity and reduced pathological hypertrophy/fibrosis and cardiac dysfunction after transaortic constriction. However, sildenafil failed to protect the heart in PKG1α^C42S, unlike BAY, which activated PKG and thereby facilitated protective effects. This corresponded with minimal PDE5 activation in PKG1α^C42S exposed to transaortic constriction versus higher activity in controls and little colocalization of PDE5 with PKG1α^C42S (versus colocalization with PKG1α^WT) in stressed myocytes.

CONCLUSIONS

In the stressed heart and myocytes, PKG1α C42-disulfide formation contributes to PDE5 activation. This augments the pathological role of PDE5 and so in turn enhances the therapeutic impact from its inhibition. PKG1α oxidation does not change the benefits from sGC activation. This finding favors the use of sGC activators regardless of PKG1α oxidation and may help guide precision therapy leveraging the cyclic GMP/PKG pathway to treat heart disease.

Sildenafil enhances central hemodynamic responses to exercise, but not V̇o2peak, in people with diabetes mellitus

Exercise capacity is frequently reduced in people with diabetes mellitus (DM), and the contribution of pulmonary microvascular dysfunction remains undefined. We hypothesized that pulmonary microvascular disease, measured by a novel exercise echocardiography technique termed pulmonary transit of agitated contrast (PTAC), would be greater in subjects with DM and that the use of pulmonary vasodilator agent sildenafil would improve exercise performance by reducing right ventricular afterload. Forty subjects with DM and 20 matched controls performed cardiopulmonary exercise testing and semisupine exercise echocardiography 1 h after placebo or sildenafil ingestion in a double-blind randomized crossover design. The primary efficacy end point was exercise capacity (V̇o_2peak) while secondary measures included pulmonary vascular resistance, cardiac output, and change in PTAC. DM subjects were aged 44 ± 13 yr, 73% male, with 16 ± 10 yr DM history. Sildenafil caused marginal improvements in echocardiographic measures of biventricular systolic function in DM subjects. Exercise-induced increases in pulmonary artery systolic pressure and pulmonary vascular resistance were attenuated with sildenafil, while heart rate (+2.4 ±1.2 beats/min, P = 0.04) and cardiac output (+322 ± 21 ml, P = 0.03) improved. However, the degree of PTAC did not change (P = 0.93) and V̇o_2peak did not increase following sildenafil as compared with placebo (V̇o_2peak: 31.8 ± 9.7 vs. 32.1 ± 9.5 ml·min^-1·kg^-1, P = 0.42). We conclude that sildenafil administration causes modest acute improvements in central hemodynamics but does not improve exercise capacity. This may be due to the mismatch in action of sildenafil on the pulmonary arteries rather than the distal pulmonary microvasculature and potential adverse effects on peripheral oxygen extraction. NEW & NOTEWORTHY This is one of the largest and most comprehensive studies of cardiopulmonary exercise performance in people with diabetes mellitus and to our knowledge the first to assess the effect of sildenafil using detailed echocardiographic measures during incremental exercise. Sildenafil attenuated the rise in pulmonary vascular resistance while augmenting cardiac output and intriguingly heart rate, without conferring any improvement in exercise capacity. The enhanced central hemodynamic indexes may have been offset by reduced peripheral O₂ extraction.

MicroRNA-19a/b-3p protect the heart from hypertension-induced pathological cardiac hypertrophy through PDE5A

AIM

PDE5A is a leading factor contributing to cGMP signaling and cardiac hypertrophy. However, microRNA-mediated posttranscriptional regulation of PDE5A has not been reported. The aim of this study is to screen the microRNAs that are able to regulate PDE5A and explore the function of the microRNAs in cardiac hypertrophy and remodeling.

METHODS AND RESULTS

Although miR-19a/b-3p (microRNA-19a-3p and microRNA-19b-3p) have been reported to be differentially expressed during cardiac hypertrophy, the direct targets and the functions of this microRNA family for regulation of cardiac hypertrophy have not yet been investigated. The present study identified some direct targets and the underlying functions of miR-19a/b-3p by using bioinformatics tools and gene manipulations within mouse neonatal cardiomyocytes. Transfection of miR-19a/b-3p down-regulated endogenous expressions of PDE5A at both mRNA and protein levels with real-time PCR and western blot. Luciferase reporter assays showed that PDE5A was a direct target of miR-19a/b-3p. In mouse models of cardiac hypertrophy, we found that miR-19a/b-3p was expressed in cardiomyocytes and that its expression was reduced in pressure overload-induced hypertrophic hearts. miR-19a/b-3p transgenic mice prevented the progress of cardiac hypertrophy and cardiac remodeling in response to angiotensin II infusion with echocardiographic assessment and pressure-volume relation analysis.

CONCLUSION

Our study elucidates that PDE5A is a novel direct target of miR-19a/b-3p, and demonstrates that antihypertrophic roles of the miR-19a/b-3p family in Ang II-induced hypertrophy and cardiac remodeling, suggests that endogenous miR-19a/b-3p might have clinical potential to suppress cardiac hypertrophy and heart failure.

Phosphodiesterase 5 inhibition improves contractile function and restores transverse tubule loss and catecholamine responsiveness in heart failure

Heart failure (HF) is characterized by poor survival, a loss of catecholamine reserve and cellular structural remodeling in the form of disorganization and loss of the transverse tubule network. Indeed, survival rates for HF are worse than many common cancers and have not improved over time. Tadalafil is a clinically relevant drug that blocks phosphodiesterase 5 with high specificity and is used to treat erectile dysfunction. Using a sheep model of advanced HF, we show that tadalafil treatment improves contractile function, reverses transverse tubule loss, restores calcium transient amplitude and the heart's response to catecholamines. Accompanying these effects, tadalafil treatment normalized BNP mRNA and prevented development of subjective signs of HF. These effects were independent of changes in myocardial cGMP content and were associated with upregulation of both monomeric and dimerized forms of protein kinase G and of the cGMP hydrolyzing phosphodiesterases 2 and 3. We propose that the molecular switch for the loss of transverse tubules in HF and their restoration following tadalafil treatment involves the BAR domain protein Amphiphysin II (BIN1) and the restoration of catecholamine sensitivity is through reductions in G-protein receptor kinase 2, protein phosphatase 1 and protein phosphatase 2 A abundance following phosphodiesterase 5 inhibition.

Marked disparity of microRNA modulation by cGMP-selective PDE5 versus PDE9 inhibitors in heart disease

MicroRNAs (miRs) posttranscriptionally regulate mRNA and its translation into protein, and are considered master controllers of genes modulating normal physiology and disease. There is growing interest in how miRs change with drug treatment, and leveraging this for precision guided therapy. Here we contrast 2 closely related therapies, inhibitors of phosphodiesterase type 5 or type 9 (PDE5-I, PDE9-I), given to mice subjected to sustained cardiac pressure overload (PO). Both inhibitors augment cyclic guanosine monophosphate (cGMP) to activate protein kinase G, with PDE5-I regulating nitric oxide (NO) and PDE9-I natriuretic peptide-dependent signaling. While both produced strong phenotypic improvement of PO pathobiology, they surprisingly showed binary differences in miR profiles; PDE5-I broadly reduces more than 120 miRs, including nearly half those increased by PO, whereas PDE9-I has minimal impact on any miR (P < 0.0001). The disparity evolves after pre-miR processing and is organ specific. Lastly, even enhancing NO-coupled cGMP by different methods leads to altered miR regulation. Thus, seemingly similar therapeutic interventions can be barcoded by profound differences in miR signatures, and reversing disease-associated miR changes is not required for therapy success.

Phosphodiesterase 2 inhibition preferentially promotes NO/guanylyl cyclase/cGMP signaling to reverse the development of heart failure

Heart failure (HF) is a shared manifestation of several cardiovascular pathologies, including hypertension and myocardial infarction, and a limited repertoire of treatment modalities entails that the associated morbidity and mortality remain high. Impaired nitric oxide (NO)/guanylyl cyclase (GC)/cyclic guanosine-3',5'-monophosphate (cGMP) signaling, underpinned, in part, by up-regulation of cyclic nucleotide-hydrolyzing phosphodiesterase (PDE) isozymes, contributes to the pathogenesis of HF, and interventions targeted to enhancing cGMP have proven effective in preclinical models and patients. Numerous PDE isozymes coordinate the regulation of cardiac cGMP in the context of HF; PDE2 expression and activity are up-regulated in experimental and human HF, but a well-defined role for this isoform in pathogenesis has yet to be established, certainly in terms of cGMP signaling. Herein, using a selective pharmacological inhibitor of PDE2, BAY 60-7550, and transgenic mice lacking either NO-sensitive GC-1α (GC-1α^-/-) or natriuretic peptide-responsive GC-A (GC-A^-/-), we demonstrate that the blockade of PDE2 promotes cGMP signaling to offset the pathogenesis of experimental HF (induced by pressure overload or sympathetic hyperactivation), reversing the development of left ventricular hypertrophy, compromised contractility, and cardiac fibrosis. Moreover, we show that this beneficial pharmacodynamic profile is maintained in GC-A^-/- mice but is absent in animals null for GC-1α or treated with a NO synthase inhibitor, revealing that PDE2 inhibition preferentially enhances NO/GC/cGMP signaling in the setting of HF to exert wide-ranging protection to preserve cardiac structure and function. These data substantiate the targeting of PDE2 in HF as a tangible approach to maximize myocardial cGMP signaling and enhancing therapy.

Roles of PDE1 in Pathological Cardiac Remodeling and Dysfunction

Pathological cardiac hypertrophy and dysfunction is a response to various stress stimuli and can result in reduced cardiac output and heart failure. Cyclic nucleotide signaling regulates several cardiac functions including contractility, remodeling, and fibrosis. Cyclic nucleotide phosphodiesterases (PDEs), by catalyzing the hydrolysis of cyclic nucleotides, are critical in the homeostasis of intracellular cyclic nucleotide signaling and hold great therapeutic potential as drug targets. Recent studies have revealed that the inhibition of the PDE family member PDE1 plays a protective role in pathological cardiac remodeling and dysfunction by the modulation of distinct cyclic nucleotide signaling pathways. This review summarizes recent key findings regarding the roles of PDE1 in the cardiac system that can lead to a better understanding of its therapeutic potential.

Measurement of cGMP-generating and -degrading activities and cGMP levels in cells and tissues: Focus on FRET-based cGMP indicators

The intracellular messenger molecule cGMP has an established function in the regulation of numerous physiological events. Yet for the identification of further biological cGMP-mediated functions, precise information whether a cGMP response exists in a certain cell type or tissue is mandatory. In this review, the techniques to measure cGMP i.e. cGMP-formation, -degradation or levels are outlined and discussed. As a superior method to measure cGMP, the article focusses on FRET-based cGMP indicators, describes the different cGMP indicators and discusses their advantages and drawbacks. Finally, the successful applications of these cGMP indicators to measure cGMP responses in cells and tissues are outlined and summarized. Hopefully, with the availability of the FRET-based cGMP indicators, the knowledge about the cGMP responses in special cells or tissues is going to increase thereby allowing to assess further cGMP-mediated functional responses and possibly to address their pathophysiology with the available guanylyl cyclase activators, stimulators and PDE inhibitors.

Screening of Human Proteins for Fluoride and Aluminum Binding

Previous studies showed that prolonged exposure to fluoride (F-) and aluminum (Al3+) ions is associated with numerous diseases including neurological disorders. They don't have any known biological function. But they can bind with proteins that interact with ions similar to them. Such unwanted interactions affect the normal biological function of the target proteins, as well as their downstream protein-protein interactions. Several studies show the detrimental effects posed by them including Alzheimer's disease. However, their target proteins have never been reported. Here, we have screened for the human protein targets subjected to F- and Al3+ interactions by using data-driven prediction tools. We have identified 20 different proteins that directly bind with them (10 interact with fluoride and 10 with aluminum). In addition, protein-protein interaction has been explored to find the proteins that indirectly interact with F- and Al3+. We have found 86 indirect targets for F- and 90 for Al3+. Furthermore, 19 common protein targets have been identified, including proteins (9 out of 19) associated with neurodegenerative disorders. However, wet lab experiments are beyond our scopes to validate the binding networks. Additional studies must be warranted.

Cardiac Phosphodiesterases and Their Modulation for Treating Heart Disease

An important hallmark of cardiac failure is abnormal second messenger signaling due to impaired synthesis and catabolism of cyclic adenosine 3',5'- monophosphate (cAMP) and cyclic guanosine 3',5'- monophosphate (cGMP). Their dysregulation, altered intracellular targeting, and blunted responsiveness to stimulating pathways all contribute to pathological remodeling, muscle dysfunction, reduced cell survival and metabolism, and other abnormalities. Therapeutic enhancement of either cyclic nucleotides can be achieved by stimulating their synthesis and/or by suppressing members of the family of cyclic nucleotide phosphodiesterases (PDEs). The heart expresses seven of the eleven major PDE subtypes - PDE1, 2, 3, 4, 5, 8, and 9. Their differential control over cAMP and cGMP signaling in various cell types, including cardiomyocytes, provides intriguing therapeutic opportunities to counter heart disease. This review examines the roles of these PDEs in the failing and hypertrophied heart and summarizes experimental and clinical data that have explored the utility of targeted PDE inhibition.

Identification of murine phosphodiesterase 5A isoforms and their functional characterization in HL-1 cardiac cell line

Phosphodiesterase 5A (PDE5A) specifically degrades the ubiquitous second messenger cGMP and experimental and clinical data highlight its important role in cardiac diseases. To address PDE5A role in cardiac physiology, three splice variants of the PDE5A were cloned for the first time from mouse cDNA library (mPde5a1, mPde5a2, and mPde5a3). The predicted amino acidic sequences of the three murine isoforms are different in the N-terminal regulatory domain. mPDE5A isoforms were transfected in HEK293T cells and they showed high affinity for cGMP and similar sensitivity to sildenafil inhibition. RT-PCR analysis showed that mPde5a1, mPde5a2, and mPde5a3 had differential tissue distribution. In the adult heart, mPde5a1 and mPde5a2 were expressed at different levels whereas mPde5a3 was undetectable. Overexpression of mPDE5As induced an increase of HL-1 number cells which progress into cell cycle. mPDE5A1 and mPDE5A3 overexpression increased the number of polyploid and binucleated cells, mPDE5A3 widened HL-1 areas, and modulated hypertrophic markers more efficiently respect to the other mPDE5A isoforms. Moreover, mPDE5A isoforms had differential subcellular localization: mPDE5A1 was mainly localized in the cytoplasm, mPDE5A2 and mPDE5A3 were also nuclear localized. These results demonstrate for the first time the existence of three PDE5A isoforms in mouse and highlight their potential role in the induction of hypertrophy.

Mechanisms of the cyclic nucleotide cross-talk signaling network in cardiac L-type calcium channel regulation

Regulation of L-type Calcium (Ca2+) Channel (LCC) gating is critical to shaping the cardiac action potential (AP) and triggering the initiation of excitation-contraction (EC) coupling in cardiac myocytes. The cyclic nucleotide (cN) cross-talk signaling network, which encompasses the β-adrenergic and the Nitric Oxide (NO)/cGMP/Protein Kinase G (PKG) pathways and their interaction (cross-talk) through distinctively-regulated phosphodiesterase isoenzymes (PDEs), regulates LCC current via Protein Kinase A- (PKA) and PKG-mediated phosphorylation. Due to the tightly-coupled and intertwined biochemical reactions involved, it remains to be clarified how LCC gating is regulated by the signaling network from receptor to end target. In addition, the large number of EC coupling-related phosphorylation targets of PKA and PKG makes it difficult to quantify and isolate changes in L-type Ca2+ current (ICaL) responses regulated by the signaling network. We have developed a multi-scale, biophysically-detailed computational model of LCC regulation by the cN signaling network that is supported by experimental data. LCCs are modeled with functionally distinct PKA- and PKG-phosphorylation dependent gating modes. The model exhibits experimentally observed single channel characteristics, as well as whole-cell LCC currents upon activation of the cross-talk signaling network. Simulations show 1) redistribution of LCC gating modes explains changes in whole-cell current under various stimulation scenarios of the cN cross-talk network; 2) NO regulation occurs via potentiation of a gating mode characterized by prolonged closed times; and 3) due to compensatory actions of cross-talk and antagonizing functions of PKA- and PKG-mediated phosphorylation of LCCs, the effects of individual inhibitions of PDEs 2, 3, and 4 on ICaL are most pronounced at low levels of β-adrenergic stimulation. Simulations also delineate the contribution of the following two mechanisms to overall LCC regulation, which have otherwise been challenging to distinguish: 1) regulation of PKA and PKG activation via cN cross-talk (Mechanism 1); and 2) LCC interaction with activated PKA and PKG (Mechanism 2). These results provide insights into how cN signals transduced via the cN cross-talk signaling network are integrated via LCC regulation in the heart.

Nanodomain Regulation of Cardiac Cyclic Nucleotide Signaling by Phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) form an 11-member superfamily comprising 100 different isoforms that regulate the second messengers cyclic adenosine or guanosine 3',5'-monophosphate (cAMP or cGMP). These PDE isoforms differ with respect to substrate selectivity and their localized control of cAMP and cGMP within nanodomains that target specific cellular pools and synthesis pathways for the cyclic nucleotides. Seven PDE family members are physiologically relevant to regulating cardiac function, disease remodeling of the heart, or both: PDE1 and PDE2, both dual-substrate (cAMP and cGMP) esterases; PDE3, PDE4, and PDE8, which principally hydrolyze cAMP; and PDE5A and PDE9A, which target cGMP. New insights regarding the different roles of PDEs in health and disease and their local signaling control are broadening the potential therapeutic utility for PDE-selective inhibitors. In this review, we discuss these PDEs, focusing on the different mechanisms by which they control cardiac function in health and disease by regulating intracellular nanodomains.

Old dog, new tricks: novel cardiac targets and stress regulation by protein kinase G

The second messenger cyclic guanosine 3'5' monophosphate (cGMP) and its downstream effector protein kinase G (PKG) have been discovered more than 40 years ago. In vessels, PKG1 induces smooth muscle relaxation in response to nitric oxide signalling and thus lowers systemic and pulmonary blood pressure. In platelets, PKG1 stimulation by cGMP inhibits activation and aggregation, and in experimental models of heart failure (HF), PKG1 activation by inhibiting cGMP degradation is protective. The net effect of the above-mentioned signalling is cardiovascular protection. Yet, while modulation of cGMP-PKG has entered clinical practice for treating pulmonary hypertension or erectile dysfunction, translation of promising studies in experimental HF to clinical success has failed thus far. With the advent of new technologies, novel mechanisms of PKG regulation, including mechanosensing, redox regulation, protein quality control, and cGMP degradation, have been discovered. These novel, non-canonical roles of PKG1 may help understand why clinical translation has disappointed thus far. Addressing them appears to be a requisite for future, successful translation of experimental studies to the clinical arena.

Roles of phosphodiesterases in the regulation of the cardiac cyclic nucleotide cross-talk signaling network

The balanced signaling between the two cyclic nucleotides (cNs) cAMP and cGMP plays a critical role in regulating cardiac contractility. Their degradation is controlled by distinctly regulated phosphodiesterase isoenzymes (PDEs), which in turn are also regulated by these cNs. As a result, PDEs facilitate communication between the β-adrenergic and Nitric Oxide (NO)/cGMP/Protein Kinase G (PKG) signaling pathways, which regulate the synthesis of cAMP and cGMP respectively. The phenomena in which the cAMP and cGMP pathways influence the dynamics of each other are collectively referred to as cN cross-talk. However, the cross-talk response and the individual roles of each PDE isoenzyme in shaping this response remain to be fully characterized. We have developed a computational model of the cN cross-talk network that mechanistically integrates the β-adrenergic and NO/cGMP/PKG pathways via regulation of PDEs by both cNs. The individual model components and the integrated network model replicate experimentally observed activation-response relationships and temporal dynamics. The model predicts that, due to compensatory interactions between PDEs, NO stimulation in the presence of sub-maximal β-adrenergic stimulation results in an increase in cytosolic cAMP accumulation and corresponding increases in PKA-I and PKA-II activation; however, the potentiation is small in magnitude compared to that of NO activation of the NO/cGMP/PKG pathway. In a reciprocal manner, β-adrenergic stimulation in the presence of sub-maximal NO stimulation results in modest cGMP elevation and corresponding increase in PKG activation. In addition, we demonstrate that PDE2 hydrolyzes increasing amounts of cAMP with increasing levels of β-adrenergic stimulation, and hydrolyzes increasing amounts of cGMP with decreasing levels of NO stimulation. Finally, we show that PDE2 compensates for inhibition of PDE5 both in terms of cGMP and cAMP dynamics, leading to cGMP elevation and increased PKG activation, while maintaining whole-cell β-adrenergic responses similar to that prior to PDE5 inhibition. By defining and quantifying reactions comprising cN cross-talk, the model characterizes the cross-talk response and reveals the underlying mechanisms of PDEs in this non-linear, tightly-coupled reaction system.

Everything you ever wanted to know about phosphodiesterase 5 inhibitors and the heart (but never dared ask): How do they work?

INTRODUCTION

Phosphodiesterase 5 inhibitors (PDE5i) were developed while investigating novel treatments for coronary artery disease, but their andrological side effects shifted their indication toward the management of erectile dysfunction. Although PDE5i are now also indicated for pulmonary arterial hypertension and there are mounting preclinical and clinical evidences about their potentially beneficial cardiac effects, their use remains controversial and the involved mechanisms remain unclear.

MATERIALS AND METHODS

This review aimed to analyze the effects of PDE5i administration in various animal and humans models of cardiovascular diseases.

RESULTS

Animal studies have shown that PDE5i have protective effects in several models of cardiac disease. In humans, some studies showed that PDE5i improves microvascular and endothelial dysfunction and exerts positive effects in different samples of cardiovascular (CV) impairment. In contrast, other studies found no benefit (and no harm) in heart failure with preserved ejection fraction. The discrepancies in these findings are likely related to the fact that the mechanisms targeted by PDE5i in human disease are still poorly understood and the target population not yet identified. The mechanisms of actions herein reviewed suggest that hypertrophy, microvascular impairment, and inflammation, should be variably present for PDE5i to work. All these conditions frequently coexist in diabetes. A gender responsiveness has also been recently proposed.

CONCLUSIONS

Continuous PDE5 inhibition may exert cardioprotective effects, improving endothelial function and counteracting cardiac remodeling in some but not all conditions. A better patient selection could help to clarify the controversies on PDE5i use for CV disorders.

Cardiac Adrenergic Nervous System and Left Ventricular Remodeling

Heightened cardiac adrenergic nervous system (ANS) activity and progression of left ventricular (LV) remodeling are temporally related in patients with systolic heart failure. Whether cardiac ANS activation directly contributes to or merely accompanies LV remodeling remains an unresolved issue. Human and experimental data that directly link cardiac ANS activation to LV remodeling and worsening heart failure are first reviewed, including cardiac norepinephrine spillover. Alterations of beta adrenergic receptor signaling pathways are then addressed with emphasis on the mechanisms that may mediate the beneficial effect of beta adrenergic receptor blockade on LV remodeling. Lastly, alternative approaches to beta adrenergic receptor blockade for lessening cardiac ANS activation and reversing cardiac ANS-induced LV remodeling are discussed. A large body of work now links LV remodeling to cardiac ANS activation. However, the precise mechanisms that link cardiac ANS activation to LV remodeling are still to be fully understood. Fully understanding of these mechanisms may uncover new therapeutic approaches.