Compartmentation and compartment-specific regulation of PDE5 by protein kinase G allows selective cGMP-mediated regulation of platelet functions

Current concepts and novel targets for antiplatelet therapy

Platelets have a crucial role in haemostasis and atherothrombosis. Pharmacological control of platelet hyper-reactivity has become a cornerstone in the prevention of thrombo-ischaemic complications in atherosclerotic diseases. Current antiplatelet therapies substantially improve clinical outcomes in patients with coronary artery disease, but at the cost of increased risk of bleeding. Beyond their role in thrombosis, platelets are known to regulate inflammatory (thrombo-inflammatory) and microcirculatory pathways. Therefore, controlling platelet hyper-reactivity might have implications for both tissue inflammation (myocardial ischaemia) and vascular inflammation (vulnerable plaque formation) to prevent atherosclerosis. In this Review, we summarize the pathophysiological role of platelets in acute myocardial ischaemia, vascular inflammation and atherosclerotic progression. Furthermore, we highlight current clinical concepts of antiplatelet therapy that have contributed to improving patient care and have facilitated more individualized therapy. Finally, we discuss novel therapeutic targets and compounds for antiplatelet therapy that are currently in preclinical development, some of which have a more favourable safety profile than currently approved drugs with regard to bleeding risk. These novel antiplatelet targets might offer new strategies to treat cardiovascular disease.

Novel Functional Features of cGMP Substrate Proteins IRAG1 and IRAG2

The inositol triphosphate-associated proteins IRAG1 and IRAG2 are cGMP kinase substrate proteins that regulate intracellular Ca²⁺. Previously, IRAG1 was discovered as a 125 kDa membrane protein at the endoplasmic reticulum, which is associated with the intracellular Ca²⁺ channel IP₃R-I and the PKGIβ and inhibits IP₃R-I upon PKGIβ-mediated phosphorylation. IRAG2 is a 75 kDa membrane protein homolog of IRAG1 and was recently also determined as a PKGI substrate. Several (patho-)physiological functions of IRAG1 and IRAG2 were meanwhile elucidated in a variety of human and murine tissues, e.g., of IRAG1 in various smooth muscles, heart, platelets, and other blood cells, of IRAG2 in the pancreas, heart, platelets, and taste cells. Hence, lack of IRAG1 or IRAG2 leads to diverse phenotypes in these organs, e.g., smooth muscle and platelet disorders or secretory deficiency, respectively. This review aims to highlight the recent research regarding these two regulatory proteins to envision their molecular and (patho-)physiological tasks and to unravel their functional interplay as possible (patho-)physiological counterparts.

Voltage-Gated T-Type Calcium Channel Modulation by Kinases and Phosphatases: The Old Ones, the New Ones, and the Missing Ones

Calcium (Ca2+) can regulate a wide variety of cellular fates, such as proliferation, apoptosis, and autophagy. More importantly, changes in the intracellular Ca2+ level can modulate signaling pathways that control a broad range of physiological as well as pathological cellular events, including those important to cellular excitability, cell cycle, gene-transcription, contraction, cancer progression, etc. Not only intracellular Ca2+ level but the distribution of Ca2+ in the intracellular compartments is also a highly regulated process. For this Ca2+ homeostasis, numerous Ca2+ chelating, storage, and transport mechanisms are required. There are also specialized proteins that are responsible for buffering and transport of Ca2+. T-type Ca2+ channels (TTCCs) are one of those specialized proteins which play a key role in the signal transduction of many excitable and non-excitable cell types. TTCCs are low-voltage activated channels that belong to the family of voltage-gated Ca2+ channels. Over decades, multiple kinases and phosphatases have been shown to modulate the activity of TTCCs, thus playing an indirect role in maintaining cellular physiology. In this review, we provide information on the kinase and phosphatase modulation of TTCC isoforms Cav3.1, Cav3.2, and Cav3.3, which are mostly described for roles unrelated to cellular excitability. We also describe possible potential modulations that are yet to be explored. For example, both mitogen-activated protein kinase and citron kinase show affinity for different TTCC isoforms; however, the effect of such interaction on TTCC current/kinetics has not been studied yet.

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

Development of Phosphodiesterase-Protein-Kinase Complexes as Novel Targets for Discovery of Inhibitors with Enhanced Specificity

Phosphodiesterases (PDEs) hydrolyze cyclic nucleotides to modulate multiple signaling events in cells. PDEs are recognized to actively associate with cyclic nucleotide receptors (protein kinases, PKs) in larger macromolecular assemblies referred to as signalosomes. Complexation of PDEs with PKs generates an expanded active site that enhances PDE activity. This facilitates signalosome-associated PDEs to preferentially catalyze active hydrolysis of cyclic nucleotides bound to PKs and aid in signal termination. PDEs are important drug targets, and current strategies for inhibitor discovery are based entirely on targeting conserved PDE catalytic domains. This often results in inhibitors with cross-reactivity amongst closely related PDEs and attendant unwanted side effects. Here, our approach targeted PDE–PK complexes as they would occur in signalosomes, thereby offering greater specificity. Our developed fluorescence polarization assay was adapted to identify inhibitors that block cyclic nucleotide pockets in PDE–PK complexes in one mode and disrupt protein-protein interactions between PDEs and PKs in a second mode. We tested this approach with three different systems—cAMP-specific PDE8–PKAR, cGMP-specific PDE5–PKG, and dual-specificity RegA–R_D complexes—and ranked inhibitors according to their inhibition potency. Targeting PDE–PK complexes offers biochemical tools for describing the exquisite specificity of cyclic nucleotide signaling networks in cells.

Current trends and future perspectives for heart failure treatment leveraging cGMP modifiers and the practical effector PKG

Cyclic guanosine monophosphate (cGMP), an intracellular second messenger molecule synthesized by guanylated cyclases (GCs), controls various myocardial properties, including cell growth and survival, interstitial fibrosis, endothelial permeability, cardiac contractility, and cardiovascular remodeling. These processes are mediated by the main cGMP effector protein kinase G (PKG) activation of which exerts intrinsic protective responses against the adverse effects of neurohormonal stimulation and pathological cardiac stress. Therapeutic strategies that enhance cGMP levels and PKG activation have been used for heart failure, which can be executed by reducing natriuretic peptide (NP) proteolysis, enhancing cGMP synthesis, or blocking cGMP hydrolysis. Among these, reducing NP clearance with neprilysin inhibitor combined with angiotensin receptor blocker has been shown to greatly improve the prognosis of patients with heart failure with reduced ejection fraction (HFrEF) compared to the prognosis of patients on standard therapy using angiotensin-converting enzyme inhibitors. Moreover, in a recent phase III clinical trial, soluble GC-derived cGMP generation was shown to have potential efficacy in the management of HFrEF. Despite the clinical significance of cGMP/PKG signaling activated by either soluble or particulate GCs in heart failure, the differential signaling events downstream of intracellular cGMP, which are precisely controlled not only by PKG activation but also by the changes in its targeting and compartmentalization depending on the pathophysiology of heart disease, are not yet completely understood. Hitherto, the importance of the latter PKG regulatory mechanisms in developing therapeutic strategies has not been elucidated. Further investigation of redox-based PKG modulation will aid in the successful development of clinical therapies and could also lead to the establishment of improved personalized treatments for patients with heart failure.

Two Birds with One Stone: Regular Use of PDE5 Inhibitors for Treating Male Patients with Erectile Dysfunction and Cardiovascular Diseases

Patients with cardiovascular disease (CVD) frequently have erectile dysfunction (ED) because the two conditions have similar risk factors and potential mechanisms. The therapeutic effect of CVD is strongly dependent upon long-term management of the condition. Patients with CVD tend to have poor medication compliance, and the coexistence of ED often discourages patients with CVD from continuing their long-term CVD management, thus worsening CVD treatment compliance. The two major reasons for poor compliance are that (i) the adverse effects of cardiovascular medications on erectile function drive people to reduce the prescribed dosage or even stop taking the medications to obtain satisfactory sexual arousal and (ii) a worsening mental state due to ED reduces medication compliance. The regular administration of phosphodiesterase-5 inhibitors (PDE5is) guarantees that the prescribed medication dosages are easy to comply with and that they improve the mental status of patients by enhancing their erectile function, resulting in improved long-term management of CVD through medication compliance. PDE5is themselves also play a role in reducing cardiovascular events and improving the prognosis. We recommend prescribing PDE5is for ED and suggest that PDE5i administration is a promising strategy to improve the long-term management of patients with both ED and CVD.

Protein-Protein Interactions of Phosphodiesterases

BACKGROUND

Phosphodiesterases (PDEs) are enzymes that play a key role in terminating cyclic nucleotides signalling by catalysing the hydrolysis of 3', 5'- cyclic adenosine monophosphate (cAMP) and/or 3', 5' cyclic guanosine monophosphate (cGMP), the second messengers within the cell that transport the signals produced by extracellular signalling molecules which are unable to get into the cells. However, PDEs are proteins which do not operate alone but in complexes that made up of a many proteins.

OBJECTIVE

This review highlights some of the general characteristics of PDEs and focuses mainly on the Protein-Protein Interactions (PPIs) of selected PDE enzymes. The objective is to review the role of PPIs in the specific mechanism for activation and thereby regulation of certain biological functions of PDEs.

METHODS

The article discusses some of the PPIs of selected PDEs as reported in recent scientific literature. These interactions are critical for understanding the biological role of the target PDE.

RESULTS

The PPIs have shown that each PDE has a specific mechanism for activation and thereby regulation a certain biological function.

CONCLUSION

Targeting of PDEs to specific regions of the cell is based on the interaction with other proteins where each PDE enzyme binds with specific protein(s) via PPIs.

Tadalafil for the treatment of benign prostatic hyperplasia

INTRODUCTION

In men, lower urinary tract symptoms (LUTS) are primarily attributed to benign prostatic hyperplasia (BPH). Therapeutic options are targeted to relax prostate smooth muscle and/or reduce prostate enlargement. Areas covered: This article reviews the major preclinical and clinical data on PDE5 inhibitors with a specific focus on tadalafil. It includes details of the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) - PDE5 pathway in the LUT organs (bladder and prostate) in addition to the available data on tadalafil in patients with LUTS secondary to BPH with or without erectile dysfunction (ED). Expert opinion: Preclinical and clinical data have clearly demonstrated that PDE5 inhibitors induce bladder and prostate relaxation, which contributes to the improvement seen in storage symptoms in both animal models of bladder and prostate hypercontractility. Tadalafil is effective both as a monotherapy and add-on therapy in patients with LUTS secondary to BPH. Furthermore, as LUTS-BPH and ED are urological disorders that commonly coexist in aging men, tadalafil is more advantageous than α1-adrenoceptors and should be used as the first option. Tadalafil is a safe and tolerable therapy and unlike α1- adrenoceptors and 5-alpha reductase inhibitors, which can cause sexual dysfunctions, tadalafil improves sexual function.

Phosphodiesterase type 5 inhibitors improve microvascular dysfunction markers in pulmonary arterial hypertension associated with congenital heart disease

BACKGROUND

Ideally, vasodilator therapies for pulmonary arterial hypertension (PAH) should have a favorable impact on markers of vascular dysfunction, in addition to their known effects on hemodynamics, cardiac function, and patient's physical capacity.

METHODS

We analyzed circulating (plasma) markers of endothelial and platelet activation/dysfunction (enzyme-linked immunoassays) in the specific setting of advanced PAH associated with congenital heart disease, during the course of sildenafil and tadalafil therapies. Thirty-one patients were enrolled (age 10-54 years), most of them with chronic hypoxemia and elevated hematocrit. Drugs were administered orally for 6 months (sildenafil [n = 16], 20 mg t.i.d.; tadalafil [n = 15], single daily dose of 40 mg). Measurements were performed at baseline, and 90 and 180 days.

RESULTS

Compared to controls, patients had elevated baseline β-thromboglobulin (β-TG, P = .002), P-selectin (P = .027), tissue-type plasminogen activator (t-PA, P = .009), and von Willebrand factor antigen (VWF:Ag, P = .010). Thrombomodulin was importantly reduced (TM, P < .001), while soluble CD40 Ligand was not changed (P = .320). Tadalafil administration was associated with improvement of β-TG (P = .004), t-PA (P = .003) and TM (P = .046) levels, while P-selectin was improved by sildenafil treatment only (P = .034). VWF:Ag improved transiently in the sildenafil group (P = .019). Both therapies were associated with improvement of the physical capacity (functional class and distance walked during the 6-minute test, P < .05), hematocrit and hemoglobin level (P < .05), and health-related quality of life (physical and mental components, P < .05).

CONCLUSION

In PAH associated with congenital heart disease, phosphodiesterase 5 inhibitors seem to have beneficial actions at microcirculatory level, beyond the proposed effects as vasodilators.

The oligomeric assembly of the phosphodiesterase-5 is a mixture of dimers and tetramers: A putative role in the regulation of function

BACKGROUND

Phosphodiesterases (PDEs) are a superfamily of evolutionary conserved cyclic nucleotides (cAMP/cGMP) hydrolysing enzymes, components of transduction pathways regulating crucial aspects of cell life. PDE5, one of these families, is the molecular target of several drugs used to treat erectile dysfunction and pulmonary hypertension. Despite its medical relevance, PDE5 macromolecular structure has only been solved for the isolated regulatory and catalytic domains. The definition of the quaternary structure of the full length PDE5 (MmPDE5A1), produced in large amounts in the yeast Kluyveromyces lactis, could greatly enhance the knowledge on its assembly/allosteric regulation and the development of new inhibitors for clinical-therapeutic applications.

METHODS

Small-angle X-ray scattering (SAXS), analytical ultracentrifugation (AUC), size exclusion chromatography (SEC), native polyacrylamide gel electrophoresis (PAGE) and western blot (WB) were used to assess the assembly of PDE5A1.

RESULTS

The full length MmPDE5A1 isoform is a mixture of dimers and tetramers in solution. We also report data showing that dimers and tetramers also coexist in vivo in platelets, blood components naturally containing high levels of PDE5.

CONCLUSIONS

This is the first time that structural studies on the full length protein evidenced the assembly of PDE5 in tetramers in addition to the expected dimers.

GENERAL SIGNIFICANCE

The assembly of PDE5 in tetramers in platelets, beside the dimers, opens the possibility to alternative assembly/allosteric regulation of this enzyme, as component of large signaling complexes, in all cellular districts in which PDE5 is present.

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.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.

Mathematical Modelling of Nitric Oxide/Cyclic GMP/Cyclic AMP Signalling in Platelets

Platelet activation contributes to normal haemostasis but also to pathologic conditions like stroke and cardiac infarction. Signalling by cGMP and cAMP inhibit platelet activation and are therefore attractive targets for thrombosis prevention. However, extensive cross-talk between the cGMP and cAMP signalling pathways in multiple tissues complicates the selective targeting of their activities. We have used mathematical modelling based on experimental data from the literature to quantify the steady state behaviour of nitric oxide (NO)/cGMP/cAMP signalling in platelets. The analysis provides an assessment of NO-induced cGMP synthesis and PKG activation as well as cGMP-mediated cAMP and PKA activation though modulation of phosphodiesterase (PDE2 and 3) activities. Both one- and two-compartment models of platelet cyclic nucleotide signalling are presented. The models provide new insight for understanding how NO signalling to cGMP and indirectly cAMP, can inhibit platelet shape-change, the initial step of platelet activation. Only the two-compartment models could account for the experimental observation that NO-mediated PKA activation can occur when the bulk platelet cAMP level is unchanged. The models revealed also a potential for hierarchical interplay between the different platelet phosphodiesterases. Specifically, the models predict, unexpectedly, a strong effect of pharmacological inhibitors of cGMP-specific PDE5 on the cGMP/cAMP cross-talk. This may explain the successful use of weak PDE5-inhibitors, such as dipyridamole, in anti-platelet therapy. In conclusion, increased NO signalling or PDE5 inhibition are attractive ways of increasing cGMP-cAMP cross-talk selectively in platelets.

Testosterone replacement in transgenic sickle cell mice controls priapic activity and upregulates PDE5 expression and eNOS activity in the penis

Sickle cell disease (SCD)-associated priapism is characterized by decreased nitric oxide (NO) signaling and downregulated phosphodiesterase (PDE)5 protein expression and activity in the penis. Priapism is also associated with testosterone deficiency, but molecular mechanisms underlying testosterone effects in the penis in SCD are not known. Given the critical role of androgens in erection physiology and NO synthase (NOS)/PDE5 expression, we hypothesized that testosterone replacement to eugonadal testosterone levels reduces priapism by reversing impaired endothelial (e)NOS activity and molecular abnormalities involving PDE5. Adult male transgenic Berkeley sickle cell (Sickle) and wild-type (WT) mice were implanted with testosterone pellets, which release 1.2 μg testosterone/day for 21 days, or vehicle. After 21 days, animals underwent erectile function assessment followed by collection of blood for serum testosterone measurements, penes for molecular analysis, and seminal vesicles as testosterone-responsive tissue. Serum testosterone levels were measured by radioimmunoassay; protein expressions of PDE5, α-smooth muscle actin, eNOS and nNOS, and phosphorylation of PDE5 at Ser-92, eNOS at Ser-1177, neuronal (n) NOS at Ser-1412, and Akt at Ser-473 were measured by Western blot in penile tissue. Testosterone treatment reversed downregulated serum testosterone levels and increased (p < 0.05) the weight of seminal vesicles in Sickle mice to levels comparable to that of WT mice, indicating restored testosterone levels in Sickle mice. Testosterone treatment reduced (p < 0.05) prolonged detumescence in Sickle mice and normalized downregulated P-PDE5 (Ser-92), PDE5, P-eNOS (Ser-1177), and P-Akt (Ser-473) protein expressions in the Sickle mouse penis. Testosterone treatment did not affect P-nNOS (Ser-1412), eNOS, nNOS, or α-smooth muscle actin protein expressions in the Sickle mouse penis. In conclusion, in the mouse model of human SCD, increasing testosterone to eugonadal levels reduced priapic activity and reversed impaired Akt/eNOS activity and PDE5 protein expression in the penis.

Cyclic guanosine monophosphate modulates accumulation of phosphodiesterase 5 inhibitors in human platelets

Sildenafil and tadalafil are widely-used phosphodiesterase 5 (PDE5) inhibitors for which no clear dose-response relationship could be established. Using isolated and/or recombinant PDE5, it has been demonstrated that cGMP can increase the affinity of this enzyme for sildenafil and tadalafil. We thus hypothesized that in cells expressing the nitric oxide - soluble guanylyl cyclase - cyclic guanosine monophosphate - PDE5 (NO-sGC-cGMP-PDE5) pathway such as platelets, the presence of NO increases the intracellular cGMP content and thus promotes the intracellular accumulation of sildenafil or tadalafil. As a cell model, isolated and washed human platelets were used. Platelet suspensions were incubated with sildenafil or tadalafil at different concentrations and for various time intervals with or without an NO donor to increase intraplatelet cGMP concentrations. Intracellular sildenafil or tadalafil was quantified by ultra-performance liquid chromatography tandem mass spectrometry and intracellular cGMP by an enzyme-linked immunosorbent assay. Sildenafil accumulated in platelets with an up to 4-fold higher accumulation when platelets were pretreated with an NO donor (p < .0001). Accumulation of tadalafil in platelets was even higher, whereas the increase was 2-fold when an NO donor was present (p < .001). This accumulation was time-dependent and happened concomitantly with a rise in intracellular cGMP. Our data demonstrate that intracellular cGMP increases intracellular PDE5 inhibitor concentrations most likely by raising the affinity of these compounds for PDE5. These findings suggest that PDE5 inhibitor action in humans is critically influenced by modulators of the activity of the NO pathway.

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.

Distinct phosphodiesterase 5A-containing compartments allow selective regulation of cGMP-dependent signalling in human arterial smooth muscle cells

Cyclic GMP (cGMP) translates and integrates much of the information encoded by nitric oxide (NO^·) and several natriuretic peptides, including the atrial natriuretic peptide (ANP). Previously, we reported that integration of a cGMP-specific cyclic nucleotide phosphodiesterase, namely phosphodiesterase 5A (PDE5A), into a protein kinase G (PKG)- and inositol-1,4,5-trisphosphate receptor (IP₃R)-containing endoplasmic reticulum (ER) signalosome allows localized control of PDE5A activity and of PKG-dependent inhibition of IP₃-mediated release of ER Ca²⁺ in human platelets. Herein, we report that PDE5A integrates into an analogous signalosome in human arterial smooth muscle cells (HASMC), wherein it regulates muscarinic agonist-dependent Ca²⁺ release and is activated selectively by PKG-dependent phosphorylation. In addition, we report that PDE5A also regulates HASMC functions via events independent of PKG, but rather through actions coordinated by competitive cGMP-mediated inhibition of cAMP hydrolysis by the so-called cGMP-inhibited cAMP PDE, namely phosphodiesterase 3A (PDE3A). Indeed, we show that ANP increases both cGMP and cAMP levels in HASMC and promotes phosphorylation of vasodilator-stimulated phospho-protein (VASP) at each the PKG and PKA phospho-acceptor sites. Since selective inhibition of PDE5 decreased DNA synthesis and chemotaxis of HASMC, and that PDE3A knockdown obviated these effects, our findings are consistent with a role for a PDE5A-PDE3A-PKA axis in their regulation. Our findings provide insight into the existence of distinct "pools" of PDE5A in HASMC and support the idea that these discrete compartments regulate distinct cGMP-dependent events. As a corollary, we suggest that it may be possible to target these distinct PDE5A-regulated pools and in so-doing differentially impact selected cGMP-regulated functions in these cells.

The role of cGMP and its signaling pathways in kidney disease

Cyclic nucleotide signal transduction pathways are an emerging research field in kidney disease. Activated cell surface receptors transduce their signals via intracellular second messengers such as cAMP and cGMP. There is increasing evidence that regulation of the cGMP-cGMP-dependent protein kinase 1-phosphodiesterase (cGMP-cGK1-PDE) signaling pathway may be renoprotective. Selective PDE5 inhibitors have shown potential in treating kidney fibrosis in patients with chronic kidney disease (CKD), via their downstream signaling, and these inhibitors also have known activity as antithrombotic and anticancer agents. This review gives an outline of the cGMP-cGK1-PDE signaling pathways and details the downstream signaling and regulatory functions that are modulated by cGK1 and PDE inhibitors with regard to antifibrotic, antithrombotic, and antitumor activity. Current evidence that supports the renoprotective effects of regulating cGMP-cGK1-PDE signaling is also summarized. Finally, the effects of icariin, a natural plant extract with PDE5 inhibitory function, are discussed. We conclude that regulation of cGMP-cGK1-PDE signaling might provide novel, therapeutic strategies for the worsening global public health problem of CKD.

Apelin: an antithrombotic factor that inhibits platelet function

Apelin peptide and its receptor APJ are directly implicated in various physiological processes ranging from cardiovascular homeostasis to immune signaling. Here, we show that apelin is a key player in hemostasis with an ability to inhibit thrombin- and collagen-mediated platelet activation. Mice lacking apelin displayed a shorter bleeding time and a prothrombotic profile. Their platelets exhibited increased adhesion and a reduced occlusion time in venules, and displayed a higher aggregation rate after their activation by thrombin compared with wild-type platelets. Consequently, human and mouse platelets express apelin and its receptor APJ. Apelin directly interferes with thrombin-mediated signaling pathways and platelet activation, secretion, and aggregation, but not with ADP and thromboxane A2-mediated pathways. IV apelin administration induced excessive bleeding and prevented thrombosis in mice. Taken together, these findings suggest that apelin and/or APJ agonists could potentially be useful adducts in antiplatelet therapies and may provide a promising perspective for patients who continue to display adverse thrombotic events with current antiplatelet therapies.

The Spatiotemporal Regulation of cAMP Signaling in Blood Platelets-Old Friends and New Players

Atherothrombosis, the pathology underlying numerous cardiovascular diseases, is a major cause of death globally. Hyperactive blood platelets play a key role in the atherothrombotic process through the release of inflammatory mediators and formation of thrombi. In healthy blood vessels, excessive platelet activation is restricted by endothelial-derived prostacyclin (PGI₂) through cyclic adenosine-5′-monophosphate (cAMP) and protein kinase A (PKA)-dependent mechanisms. Elevation in intracellular cAMP is associated with the control of a number of distinct platelet functions including actin polymerisation, granule secretion, calcium mobilization and integrin activation. Unfortunately, in atherosclerotic disease the protective effects of cAMP are compromised, which may contribute to pathological thrombosis. The cAMP signaling network in platelets is highly complex with the presence of multiple isoforms of adenylyl cyclase (AC), PKA, and phosphodiesterases (PDEs). However, a precise understanding of the relationship between specific AC, PKA, and PDE isoforms, and how individual signaling substrates are targeted to control distinct platelet functions is still lacking. In other cells types, compartmentalisation of cAMP signaling has emerged as a key mechanism to allow precise control of specific cell functions. A-kinase anchoring proteins (AKAPs) play an important role in this spatiotemporal regulation of cAMP signaling networks. Evidence of AKAP-mediated compartmentalisation of cAMP signaling in blood platelets has begun to emerge and is providing new insights into the regulation of platelet function. Dissecting the mechanisms that allow cAMP to control excessive platelet activity without preventing effective haemostasis may unleash the possibility of therapeutic targeting of the pathway to control unwanted platelet activity.

A Phase I proof-of-concept and safety trial of sildenafil to treat cerebral vasospasm following subarachnoid hemorrhage

OBJECTIVE

Studies show that phosphodiesterase-V (PDE-V) inhibition reduces cerebral vasospasm (CVS) and improves outcomes after experimental subarachnoid hemorrhage (SAH). This study was performed to investigate the safety and effect of sildenafil (an FDA-approved PDE-V inhibitor) on angiographic CVS in SAH patients.

METHODS

A2-phase, prospective, nonrandomized, human trial was implemented. Subarachnoid hemorrhage patients underwent angiography on Day 7 to assess for CVS. Those with CVS were given 10 mg of intravenous sildenafil in the first phase of the study and 30 mg in the second phase. In both, angiography was repeated 30 minutes after infusion. Safety was assessed by monitoring neurological examination findings and vital signs and for the development of adverse reactions. For angiographic assessment, in a blinded fashion, pre- and post-sildenafil images were graded as "improvement" or "no improvement" in CVS. Unblinded measurements were made between pre- and post-sildenafil angiograms.

RESULTS

Twelve patients received sildenafil; 5 patients received 10 mg and 7 received 30 mg. There were no adverse reactions. There was no adverse effect on heart rate or intracranial pressure. Sildenafil resulted in a transient decline in mean arterial pressure, an average of 17% with a return to baseline in an average of 18 minutes. Eight patients (67%) were found to have a positive angiographic response to sildenafil, 3 (60%) in the low-dose group and 5 (71%) in the high-dose group. The largest degree of vessel dilation was an average of 0.8 mm (range 0-2.1 mm). This corresponded to an average percentage increase in vessel diameter of 62% (range 0%-200%).

CONCLUSIONS

The results from this Phase I safety and proof-of-concept trial assessing the use of intravenous sildenafil in patients with CVS show that sildenafil is safe and well tolerated in the setting of SAH. Furthermore, the angiographic data suggest that sildenafil has a positive impact on human CVS.

Cyclic nucleotide phosphodiesterases (PDEs): coincidence detectors acting to spatially and temporally integrate cyclic nucleotide and non-cyclic nucleotide signals

The cyclic nucleotide second messengers cAMP and cGMP each affect virtually all cellular processes. Although these hydrophilic small molecules readily diffuse throughout cells, it is remarkable that their ability to activate their multiple intracellular effectors is spatially and temporally selective. Studies have identified a critical role for compartmentation of the enzymes which hydrolyse and metabolically inactivate these second messengers, the PDEs (cyclic nucleotide phosphodiesterases), in this specificity. In the present article, we describe several examples from our work in which compartmentation of selected cAMP- or cGMP-hydrolysing PDEs co-ordinate selective activation of cyclic nucleotide effectors, and, as a result, selectively affect cellular functions. It is our belief that therapeutic strategies aimed at targeting PDEs within these compartments will allow greater selectivity than those directed at inhibiting these enzymes throughout the cells.

Cyclic nucleotide phosphodiesterases: important signaling modulators and therapeutic targets

By catalyzing hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), cyclic nucleotide phosphodiesterases are critical regulators of their intracellular concentrations and their biological effects. As these intracellular second messengers control many cellular homeostatic processes, dysregulation of their signals and signaling pathways initiate or modulate pathophysiological pathways related to various disease states, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication, chronic obstructive pulmonary disease, and psoriasis. Alterations in expression of PDEs and PDE-gene mutations (especially mutations in PDE6, PDE8B, PDE11A, and PDE4) have been implicated in various diseases and cancer pathologies. PDEs also play important role in formation and function of multimolecular signaling/regulatory complexes, called signalosomes. At specific intracellular locations, individual PDEs, together with pathway-specific signaling molecules, regulators, and effectors, are incorporated into specific signalosomes, where they facilitate and regulate compartmentalization of cyclic nucleotide signaling pathways and specific cellular functions. Currently, only a limited number of PDE inhibitors (PDE3, PDE4, PDE5 inhibitors) are used in clinical practice. Future paths to novel drug discovery include the crystal structure-based design approach, which has resulted in generation of more effective family-selective inhibitors, as well as burgeoning development of strategies to alter compartmentalized cyclic nucleotide signaling pathways by selectively targeting individual PDEs and their signalosome partners.

Advances in targeting cyclic nucleotide phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) catalyse the hydrolysis of cyclic AMP and cyclic GMP, thereby regulating the intracellular concentrations of these cyclic nucleotides, their signalling pathways and, consequently, myriad biological responses in health and disease. Currently, a small number of PDE inhibitors are used clinically for treating the pathophysiological dysregulation of cyclic nucleotide signalling in several disorders, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication and chronic obstructive pulmonary disease. However, pharmaceutical interest in PDEs has been reignited by the increasing understanding of the roles of individual PDEs in regulating the subcellular compartmentalization of specific cyclic nucleotide signalling pathways, by the structure-based design of novel specific inhibitors and by the development of more sophisticated strategies to target individual PDE variants.

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

Natriuretic peptides induce weak VASP phosphorylation at Serine 239 in platelets

Cyclic guanosine-3',5'-monophoshate (cGMP) is the common second messenger for the cardiovascular effects of nitric oxide (NO) and natriuretic peptides (NP; e.g. atrial NP [ANP]), which activate soluble and particulate guanylyl cyclases, respectively. The role of NO in regulating cGMP and platelet function is well documented, whereas there is little evidence supporting a role for NPs in regulating platelet reactivity. By studying platelet aggregation and secretion in response to a PAR-1 peptide, collagen and ADP, and phosphorylation of the cGMP-dependent protein kinase (PKG) substrate vasodilator-stimulated phosphoprotein (VASP) at serine 239, we evaluated the effects of NPs in the absence or presence of the non-selective cGMP and cAMP phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Our results show that NPs, possibly through the clearance receptor (natriuretic peptide receptor-C) expressed on platelet membranes, increase VASP phosphorylation but only following PDE inhibition, indicating a small, localised cGMP synthesis. As platelet aggregation and secretion measured under the same conditions were not affected, we conclude that the magnitude of PKG activation achieved by NPs in platelets per se is not sufficient to exert functional inhibition of platelet involvement in haemostasis.

Methods for identification of cGKI substrates

The cGMP-dependent protein kinases (cGK), which belong to the family of serine/threonine kinases, exhibit their diverse functions in cells through interaction with a variety of substrate proteins. Several substrates were identified and the interactions studied using different methods inter alia co-immunoprecipitation (Co-IP) and cGMP-agarose affinity purification. In the following chapter, we will describe the preparation of cell or tissue lysates, the procedures of cGMP-agarose affinity purification and co-immunoprecipitation, and finally the separation and analysis of the protein complexes by SDS-PAGE or mass spectrometry.

Brief hyperoxia increases mitochondrial oxidation and increases phosphodiesterase 5 activity in fetal pulmonary artery smooth muscle cells

AIMS

Oxygen is a pulmonary vasodilator, but data suggest high O(2) concentrations impede that response. We previously reported 24 h of 100% O(2) increased phosphodiesterase 5 (PDE5) activity in fetal pulmonary artery smooth muscle cells (FPASMC) and in ventilated neonatal lambs. PDE5 degrades cyclic GMP (cGMP) and inhibits nitric oxide (NO)-mediated cGMP-dependent vasorelaxation. We sought to determine the mechanism by which hyperoxia initiates reactive oxygen species (ROS) production and regulates PDE5.

RESULTS

Thirty minutes of hyperoxia increased mitochondrial ROS versus normoxia (30.3±1.7% vs. 21.1±2.8%), but had no effect on cytosolic ROS, measured by roGFP, a ratiometric protein thiol redox sensor. Hyperoxia increased PDE5 activity (220±39%) and decreased cGMP responsiveness to NO (37±17%). Mitochondrial catalase overexpression attenuated hyperoxia-induced mitochondrial roGFP oxidation, compared to FPASMC infected with empty adenoviral vector (50±3% of control) or mitochondrial superoxide dismutase. MitoTEMPO, mitochondrial catalase, and DT-3, a cGMP-dependent protein kinase I alpha inhibitor, decreased PDE5 activity (32±13%, 26±21%, and 63±10% of control, respectively), and restored cGMP responsiveness to NO (147±16%,172±29%, and 189±43% of control, respectively). C57Bl6 mice exposed to 90%-100% O(2) for 45 min±mechanical ventilation had increased PA PDE5 activity (206±39% and 235±75%, respectively).

INNOVATION

This is the first description that hyperoxia induces ROS in the mitochondrial matrix prior to the cytosol. Our results indicate that short hyperoxia exposures can produce significant changes in critical cellular signaling pathways.

CONCLUSIONS

These results indicate that mitochondrial matrix oxidant signals generated during hyperoxia, specifically H(2)O(2), activate PDE5 in a cGMP-dependent protein kinase-dependent manner in pulmonary vascular smooth muscle cells.

Pathological cardiac hypertrophy alters intracellular targeting of phosphodiesterase type 5 from nitric oxide synthase-3 to natriuretic peptide signaling

BACKGROUND

In the normal heart, phosphodiesterase type 5 (PDE5) hydrolyzes cGMP coupled to nitric oxide- (specifically from nitric oxide synthase 3) but not natriuretic peptide (NP)-stimulated guanylyl cyclase. PDE5 is upregulated in hypertrophied and failing hearts and is thought to contribute to their pathophysiology. Because nitric oxide signaling declines whereas NP-derived cGMP rises in such diseases, we hypothesized that PDE5 substrate selectivity is retargeted to blunt NP-derived signaling.

METHODS AND RESULTS

Mice with cardiac myocyte inducible PDE5 overexpression (P5(+)) were crossed to those lacking nitric oxide synthase 3 (N3(-)), and each model, the double cross, and controls were subjected to transaortic constriction. P5(+) mice developed worse dysfunction and hypertrophy and enhanced NP stimulation, whereas N3(-) mice were protected. However, P5(+)/N3(-) mice behaved similarly to P5(+) mice despite the lack of nitric oxide synthase 3-coupled cGMP generation, with protein kinase G activity suppressed in both models. PDE5 inhibition did not alter atrial natriuretic peptide-stimulated cGMP in the resting heart but augmented it in the transaortic constriction heart. This functional retargeting was associated with PDE5 translocation from sarcomeres to a dispersed distribution. P5(+) hearts exhibited higher oxidative stress, whereas P5(+)/N3(-) hearts had low levels (likely owing to the absence of nitric oxide synthase 3 uncoupling). This highlights the importance of myocyte protein kinase G activity as a protection for pathological remodeling.

CONCLUSIONS

These data provide the first evidence for functional retargeting of PDE5 from one compartment to another, revealing a role for natriuretic peptide-derived cGMP hydrolysis by this esterase in diseased heart myocardium. Retargeting likely affects the pathophysiological consequence and the therapeutic impact of PDE5 modulation in heart disease.

Phosphodiesterases Regulate BAY 41-2272-Induced VASP Phosphorylation in Vascular Smooth Muscle Cells

BAY 41-2272 (BAY), a stimulator of soluble guanylyl cyclase, increases cyclic nucleotides and inhibits proliferation of vascular smooth muscle cells (VSMCs). In this study, we elucidated mechanisms of action of BAY in its regulation of vasodilator-stimulated phosphoprotein (VASP) with an emphasis on VSMC phosphodiesterases (PDEs). BAY alone increased phosphorylation of VASP(Ser239) and VASP(Ser157), respective indicators of PKG and PKA signaling. IBMX, a non-selective inhibitor of PDEs, had no effect on BAY-induced phosphorylation at VASP(Ser239) but inhibited phosphorylation at VASP(Ser157). Selective inhibitors of PDE3 or PDE4 attenuated BAY-mediated increases at VASP(Ser239) and VASP(Ser157), whereas PDE5 inhibition potentiated BAY-mediated increases only at VASP(Ser157). In comparison, 8Br-cGMP increased phosphorylation at VASP(Ser239) and VASP(Ser157) which were not affected by selective PDE inhibitors. In the presence of 8Br-cAMP, inhibition of either PDE4 or PDE5 decreased VASP(Ser239) phosphorylation and inhibition of PDE3 increased phosphorylation at VASP(Ser239), while inhibition of PDE3 or PDE4 increased and PDE5 inhibition had no effect on VASP(Ser157) phosphorylation. These findings demonstrate that BAY operates via cAMP and cGMP along with regulation by PDEs to phosphorylate VASP in VSMCs and that the mechanism of action of BAY in VSMCs is different from that of direct cyclic nucleotide analogs with respect to VASP phosphorylation and the involvement of PDEs. Given a role for VASP as a critical cytoskeletal protein, these findings provide evidence for BAY as a regulator of VSMC growth and a potential therapeutic agent against vasculoproliferative disorders.

Novel roles of cAMP/cGMP-dependent signaling in platelets

Endothelial prostacyclin and nitric oxide potently inhibit platelet functions. Prostacyclin and nitric oxide actions are mediated by platelet adenylyl and guanylyl cyclases, which synthesize cyclic AMP (cAMP) and cyclic GMP (cGMP), respectively. Cyclic nucleotides stimulate cAMP-dependent protein kinase (protein kinase A [PKA]I and PKAII) and cGMP-dependent protein kinase (protein kinase G [PKG]I) to phosphorylate a broad panel of substrate proteins. Substrate phosphorylation results in the inactivation of small G-proteins of the Ras and Rho families, inhibition of the release of Ca(2+) from intracellular stores, and modulation of actin cytoskeleton dynamics. Thus, PKA/PKG substrates translate prostacyclin and nitric oxide signals into a block of platelet adhesion, granule release, and aggregation. cAMP and cGMP are degraded by phosphodiesterases, which might restrict signaling to specific subcellular compartments. An emerging principle of cyclic nucleotide signaling in platelets is the high degree of interconnection between activating and cAMP/cGMP-dependent inhibitory signaling pathways at all levels, including cAMP/cGMP synthesis and breakdown, and PKA/PKG-mediated substrate phosphorylation. Furthermore, defects in cAMP/cGMP pathways might contribute to platelet hyperreactivity in cardiovascular disease. This article focuses on recent insights into the regulation of the cAMP/cGMP signaling network and on new targets of PKA and PKG in platelets.

Time-resolved in silico modeling of fine-tuned cAMP signaling in platelets: feedback loops, titrated phosphorylations and pharmacological modulation

Background

Hemostasis is a critical and active function of the blood mediated by platelets. Therefore, the prevention of pathological platelet aggregation is of great importance as well as of pharmaceutical and medical interest. Endogenous platelet inhibition is predominantly based on cyclic nucleotides (cAMP, cGMP) elevation and subsequent cyclic nucleotide-dependent protein kinase (PKA, PKG) activation. In turn, platelet phosphodiesterases (PDEs) and protein phosphatases counterbalance their activity. This main inhibitory pathway in human platelets is crucial for countervailing unwanted platelet activation. Consequently, the regulators of cyclic nucleotide signaling are of particular interest to pharmacology and therapeutics of atherothrombosis. Modeling of pharmacodynamics allows understanding this intricate signaling and supports the precise description of these pivotal targets for pharmacological modulation.

Results

We modeled dynamically concentration-dependent responses of pathway effectors (inhibitors, activators, drug combinations) to cyclic nucleotide signaling as well as to downstream signaling events and verified resulting model predictions by experimental data. Experiments with various cAMP affecting compounds including anti-platelet drugs and their combinations revealed a high fidelity, fine-tuned cAMP signaling in platelets without cross-talk to the cGMP pathway. The model and the data provide evidence for two independent feedback loops: PKA, which is activated by elevated cAMP levels in the platelet, subsequently inhibits adenylyl cyclase (AC) but as well activates PDE3. By multi-experiment fitting, we established a comprehensive dynamic model with one predictive, optimized and validated set of parameters. Different pharmacological conditions (inhibition, activation, drug combinations, permanent and transient perturbations) are successfully tested and simulated, including statistical validation and sensitivity analysis. Downstream cyclic nucleotide signaling events target different phosphorylation sites for cAMP- and cGMP-dependent protein kinases (PKA, PKG) in the vasodilator-stimulated phosphoprotein (VASP). VASP phosphorylation as well as cAMP levels resulting from different drug strengths and combined stimulants were quantitatively modeled. These predictions were again experimentally validated. High sensitivity of the signaling pathway at low concentrations is involved in a fine-tuned balance as well as stable activation of this inhibitory cyclic nucleotide pathway.

Conclusions

On the basis of experimental data, literature mining and database screening we established a dynamic in silico model of cyclic nucleotide signaling and probed its signaling sensitivity. Thoroughly validated, it successfully predicts drug combination effects on platelet function, including synergism, antagonism and regulatory loops.

Defining the phosphodiesterase superfamily members in rat brain microvessels

Eleven phosphodiesterase (PDE) families are known, each having several different isoforms and splice variants. Recent evidence indicates that expression of individual PDE family members is tissue-specific. Little is known concerning detailed PDE component expression in brain microvessels where the blood-brain-barrier and the local cerebral blood flow are thought to be regulated by PDEs. The present study attempted to identify PDE family members that are expressed in brain microvessels. Adult male F344 rats were sacrificed and blocks of the cerebral cortex and infratentorial areas were dissected. Microvessels were isolated using a filtration method, and total RNA was extracted. RNA quality and quantity were determined using an Agilent bioanalyzer. The isolated cortical and infratentorial microvessel total RNA amounts were 2720 ± 750 ng (n = 2) and 250 ± 40 ng (n = 2), respectively. Microarrays with 22 000 transcripts demonstrated that there were 16 PDE transcripts in the PDE superfamily, exhibiting quantifiable density in the microvessels. An additional immunofluorescent study verified that PDE4D (cAMP-specific) and PDE5A (cGMP-specific) were colocalized with RECA-1 (an endothelial marker) in the cerebral cortex using both F344 rats and Sprague-Dawley rats (n = 3-6/strain). In addition, PDE4D and PDE5A were found to be colocalized with alpha-smooth muscle actin which delineates cerebral arteries and arterioles as well as pericytes. In conclusion, a filtration method followed by microarray analyses allows PDE components to be identified in brain microvessels, and confirmed that PDE4D and PDE5A are the primary forms expressed in rat brain microvessels.

IRAG and novel PKG targeting in the cardiovascular system

Signaling by nitric oxide (NO) determines several cardiovascular functions including blood pressure regulation, cardiac and smooth muscle hypertrophy, and platelet function. NO stimulates the synthesis of cGMP by soluble guanylyl cyclases and thereby activates cGMP-dependent protein kinases (PKGs), mediating most of the cGMP functions. Hence, an elucidation of the PKG signaling cascade is essential for the understanding of the (patho)physiological aspects of NO. Several PKG signaling pathways were identified, meanwhile regulating the intracellular calcium concentration, mediating calcium desensitization or cytoskeletal rearrangement. During the last decade it emerged that the inositol trisphosphate receptor-associated cGMP-kinase substrate (IRAG), an endoplasmic reticulum-anchored 125-kDa membrane protein, is a main signal transducer of PKG activity in the cardiovascular system. IRAG interacts specifically in a trimeric complex with the PKG1β isoform and the inositol 1,4,5-trisphosphate receptor I and, upon phosphorylation, reduces the intracellular calcium release from the intracellular stores. IRAG motifs for phosphorylation and for targeting to PKG1β and 1,4,5-trisphosphate receptor I were identified by several approaches. The (patho)physiological functions for the regulation of smooth muscle contractility and the inhibition of platelet activation were perceived. In this review, the IRAG recognition, targeting, and function are summarized compared with PKG and several PKG substrates in the cardiovascular system.

Mammalian Cyclic Nucleotide Phosphodiesterases: Molecular Mechanisms and Physiological Functions

The superfamily of cyclic nucleotide (cN) phosphodiesterases (PDEs) is comprised of 11 families of enzymes. PDEs break down cAMP and/or cGMP and are major determinants of cellular cN levels and, consequently, the actions of cN-signaling pathways. PDEs exhibit a range of catalytic efficiencies for breakdown of cAMP and/or cGMP and are regulated by myriad processes including phosphorylation, cN binding to allosteric GAF domains, changes in expression levels, interaction with regulatory or anchoring proteins, and reversible translocation among subcellular compartments. Selective PDE inhibitors are currently in clinical use for treatment of erectile dysfunction, pulmonary hypertension, intermittent claudication, and chronic pulmonary obstructive disease; many new inhibitors are being developed for treatment of these and other maladies. Recently reported x-ray crystallographic structures have defined features that provide for specificity for cAMP or cGMP in PDE catalytic sites or their GAF domains, as well as mechanisms involved in catalysis, oligomerization, autoinhibition, and interactions with inhibitors. In addition, major advances have been made in understanding the physiological impact and the biochemical basis for selective localization and/or recruitment of specific PDE isoenzymes to particular subcellular compartments. The many recent advances in understanding PDE structures, functions, and physiological actions are discussed in this review.

Phosphodiesterase inhibitors: factors that influence potency, selectivity, and action

Cyclic nucleotide phosphodiesterases (PDEs) are promising targets for pharmacological intervention. The presence of multiple PDE genes, diversity of the isoforms produced from each gene, selective tissue and cellular expression of the isoforms, compartmentation within cells, and an array of conformations of PDE proteins are some of the properties that challenge the development of drugs that target these enzymes. Nevertheless, many of the characteristics of PDEs are also viewed as unique opportunities to increase specificity and selectivity when designing novel compounds for certain therapeutic indications. This chapter provides a summary of the major concepts related to the design and use of PDE inhibitors. The overall structure and properties of the catalytic domain and conformations of PDEs are summarized in light of the most recent X-ray crystal structures. The distinctive properties of catalytic domains of different families as well as the technical challenges associated with probing PDE properties and their interactions with small molecules are discussed. The effect of posttranslational modifications and protein-protein interactions are additional factors to be considered when designing PDE inhibitors. PDE inhibitor interaction with other proteins needs to be taken into account and is also discussed.

Measurement of cellular cGMP in plant cells and tissues using the endogenous fluorescent reporter FlincG

The cyclic nucleotide cGMP has been shown to play important roles in plant development and responses to abiotic and biotic stress. To date, the techniques that are available to measure cGMP in plants are limited by low spatial and temporal resolution. In addition, tissue destruction is necessary. To circumvent these drawbacks we have used the δ-FlincG fluorescent protein to create an endogenous cGMP sensor that can report cellular cGMP levels with high resolution in time and space in living plant cells. δ-FlincG in transient and stably expressing cells shows a dissociation constant for cGMP of around 200 nm giving it a dynamic range of around 20-2000 nm. Stimuli that were previously shown to alter cGMP in plant cells (nitric oxide and gibberrellic acid) evoked pronounced fluorescence signals in single cells and in root tissues, providing evidence that δ-FlincG reports changes in cellular cGMP in a physiologically relevant context.

Phosphodiesterases as targets for intermittent claudication

Intermittent claudication (IC) is one of the most frequent forms of lower extremity peripheral arterial disease (PAD) and is most commonly caused by arterial atherosclerosis. Its clinical manifestation includes fatigue, discomfort, or pain occurring in limb muscles due to exercise-induced ischemia, thus limiting the ability of IC patients to walk and exercise. In addition to lifestyle changes (diet, exercise, and smoking cessation), pharmacological treatments are needed. Pathologically, atherosclerotic lesions cause a mismatch in oxygen supply and metabolic demand in the leg muscles during walking/exercise. This subjects the muscles to repeated ischemia and reperfusion injury that can alter structure and oxidative metabolism, resulting in insufficient utilization of oxygen supply. Despite extensive research efforts, cilostazol and pentoxifylline are the only drugs indicated for relieving the symptoms of IC, with cilostazol demonstrating significant improvement in walking distance and quality of life in these patients. Originally developed as a PDE3 inhibitor, cilostazol was later found to have several other pharmacological actions, and its success has been attributed to its multifactorial actions on platelets, endothelium, smooth muscle, and lipid profiles. Using cilostazol as an example, we discuss the rationales and pitfalls of targeting PDEs in IC, and potential strategies for the development of new and more effective pharmacological treatments.

Distinct allostery induced in the cyclic GMP-binding, cyclic GMP-specific phosphodiesterase (PDE5) by cyclic GMP, sildenafil, and metal ions

The activity of many proteins orchestrating different biological processes is regulated by allostery, where ligand binding at one site alters the function of another site. Allosteric changes can be brought about by either a change in the dynamics of a protein, or alteration in its mean structure. We have investigated the mechanisms of allostery induced by chemically distinct ligands in the cGMP-binding, cGMP-specific phosphodiesterase, PDE5. PDE5 is the target for catalytic site inhibitors, such as sildenafil, that are used for the treatment of erectile dysfunction and pulmonary hypertension. PDE5 is a multidomain protein and contains two N-terminal cGMP-specific phosphodiesterase, bacterial adenylyl cyclase, FhLA transcriptional regulator (GAF) domains, and a C-terminal catalytic domain. Cyclic GMP binding to the GAFa domain and sildenafil binding to the catalytic domain result in conformational changes, which to date have been studied either with individual domains or with purified enzyme. Employing intramolecular bioluminescence resonance energy transfer, which can monitor conformational changes both in vitro and in intact cells, we show that binding of cGMP and sildenafil to PDE5 results in distinct conformations of the protein. Metal ions bound to the catalytic site also allosterically modulated cGMP- and sildenafil-induced conformational changes. The sildenafil-induced conformational change was temperature-sensitive, whereas cGMP-induced conformational change was independent of temperature. This indicates that different allosteric ligands can regulate the conformation of a multidomain protein by distinct mechanisms. Importantly, this novel PDE5 sensor has general physiological and clinical relevance because it allows the identification of regulators that can modulate PDE5 conformation in vivo.

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

Nitric oxide at a glance

In healthy blood vessels excessive platelet activation is counterbalanced by negative signalling cascades that modulate activation. This is achieved primarily through endothelial-derived nitric oxide (NO) and prostacyclin (PGI2). The biological effects of NO are mediated through stimulation soluble guanylyl cyclase (sGC) activation of cyclic AMP and GMP-mediated signalling pathways. In the present review examine our current understanding of NO-mediated regulation of platelets and highlight key issues that remain unresolved.

Dipyridamole synergizes with nitric oxide to prolong inhibition of thrombin-induced platelet shape change

We and others have previously demonstrated that nitric oxide (NO)-induced inhibition of platelet shape change is important in regulating platelet adhesion and aggregation, and therapeutic intervention of this pathway is clinically relevant for secondary prevention of stroke with dipyridamole. In the present study, we investigated whether dipyridamole affected the shape change of aspirinated platelets. Platelet shape change was inhibited using both authentic NO and sodium nitroprusside, as monitored by light scattering and mean platelet volume measurements. Dipyridamole synergized with NO, even at supra-therapeutic levels, to inhibit thrombin-induced shape change and further potentiated cAMP dependent protein kinase (PKA) mediated phosphorylation of vasodilator stimulated phosphoprotein (VASP) Ser157, even without altered levels of platelet cAMP. The effect of dipyridamole on NO-inhibited shape change depended on cGMP synthesis as evaluated by inhibition of soluble guanylyl cyclase. Measured increases in cGMP levels by dipyridamole and NO was assessed by mathematical modeling and found to be consistent with inhibition of phosphodiesterase 5 (PDE5). The model could explain the unexpected efficiency of dipyridamole in inhibiting PDE5 at the measured cGMP levels, by the majority of cGMP being bound to cGMP-dependent protein kinase (PKG). Still, selective activators of PKG failed to extend NO-mediated inhibition of the thrombin-induced platelet shape change, suggesting that PKG was not responsible for the inhibitory effect of NO and dipyridamole on shape change. The effects of dipyridamole were independent of the prostanoid and ADP pathways. Thus, the effect of dipyridamole on NO-mediated inhibition of platelet shape change may be an important and additional beneficial therapeutic effect of dipyridamole, which we suggest, is acting though localized amplification of the NO/cGMP/Phosphodiesterase3/cAMP/PKA-pathway. Probably, the efficiency of dipyridamole could be amplified clinically with NO donors.

Functional osteoclast attachment requires inositol-1,4,5-trisphosphate receptor-associated cGMP-dependent kinase substrate

Osteoclast activity is central to balanced bone turnover to maintain normal bone mass. A specialized osteoclast attachment to bone localizes acid secretion to remove bone mineral; in some cases, attachment is functionally impaired despite normal attachment proteins. The inositol-1,4,5-trisphosphate receptor-1 (IP3R1) is an intracellular calcium channel required for regulation of reversible osteoclast attachment by nitric oxide (NO), an important regulator of both normal and pathological bone degradation. In studies using human osteoclasts produced in vitro, we found that IP3R1 binds an endosomal isoform of the IP3R-associated cGMP-dependent kinase substrate (IRAG). IRAG is a substrate of cGMP-dependent kinase-1 (PKG1) and binds the PKG1 isoform PKG1β, which was the predominant form of PKG1 in human osteoclasts. Western blots of IRAG were consistent with NO-dependent serine phosphorylation of IRAG. An additional effect of PKG1β activity in osteoclasts was disassociation of IP3R1-IRAG complexes, as shown by analysis of IP3R1 complexes and by localization of the proteins within cells. IP3R1-IRAG complexes were stabilized by PKG or Src antagonists, Src activity being a requirement for IP3R1 calcium release downstream of PKG. IP3R1-mediated calcium release regulates cellular detachment in part through the calcium-dependent proteinase μ-calpain. In osteoclasts with IRAG suppressed by siRNA, activity of μ-calpain was increased relative to cells with normal IRAG, and regulation of μ-calpain by NO was lost. Furthermore, cells deficient in IRAG detached easily from substrate and had smaller attached diameters and randomly distributed podosomes, although IRAG knockdown did not affect cell viability. Our results indicate that IRAG is required for PKG1β-regulated cyclic calcium release during motility, and that disruption of the IP3R1-IRAG calcium regulation system is a novel cause of dysfunctional osteoclasts unrelated to defects in attachment proteins or acid secretion.

Decreased renal corin expression contributes to sodium retention in proteinuric kidney diseases

Patients with proteinuric kidney diseases often have symptoms of salt and water retention. It has been hypothesized that dysregulated sodium absorption is due to increased proteolytic cleavage of epithelial sodium channels (ENaCs) and increased Na,K-ATPase expression. Microarray analysis identified a reduction in kidney corin mRNA expression in rat models of puromycin aminonucleoside-induced nephrotic syndrome and acute anti-Thy1 glomerulonephritis (GN). As atrial natriuretic peptide (ANP) resistance is a mechanism accounting for volume retention, we analyzed the renal expression and function of corin; a type II transmembrane serine protease that converts pro-ANP to active ANP. Immunohistochemical analysis found that corin colocalized with ANP. The nephrotic and glomerulonephritic models exhibited concomitant increased pro-ANP and decreased ANP protein levels in the kidney consistent with low amounts of corin. Importantly, kidneys from corin knockout mice had increased amounts of renal β-ENaC and its activators, phosphodiesterase (PDE) 5 and protein kinase G II, when compared to wild-type mice. A similar expression profile was also found in cell culture suggesting the increase in PDE5 and kinase G II could account for the increase in β-ENaC seen in nephrotic syndrome and GN. Thus, we suggest that corin might be involved in the salt retention seen in glomerular diseases.

Phospholemman Ser69 phosphorylation contributes to sildenafil-induced cardioprotection against reperfusion injury

The phosphodiesterase type-5 inhibitor sildenafil has powerful cardioprotective effects against ischemia-reperfusion injury. PKG-mediated signaling has been implicated in this protection, although the mechanism and the downstream targets of this kinase remain to be fully elucidated. In this study we assessed the role of phospholemman (PLM) phosphorylation, which activates the Na⁺/K⁺-ATPase, in cardioprotection afforded by sildenafil administered during reperfusion. Isolated perfused mouse hearts were optimally protected against infarction (indexed by tetrazolium staining) by 0.1 μM sildenafil treatment during the first 10 min of reperfusion. Extended sildenafil treatment (30, 60, or 120 min at reperfusion) did not alter the degree of protection provided. This protection was PKG dependent, since it was blocked by KT-5823. Western blot analysis using phosphospecific antibodies to PLM showed that sildenafil at reperfusion did not modulate PLM Ser63 or Ser68 phosphorylation but significantly increased Ser69 phosphorylation. The treatment of isolated rat ventricular myocytes with sildenafil or 8-bromo-cGMP (PKG agonist) enhanced PLM Ser69 phosphorylation, which was bisindolylmaleimide (PKC inhibitor) sensitive. Patch-clamp studies showed that sildenafil treatment also activated the Na⁺/K⁺-ATPase, which is anticipated in light of PLM Ser69 phosphorylation. Na⁺/K⁺-ATPase activation during reperfusion would attenuate Na⁺ overload at this time, providing a molecular explanation of how sildenafil guards against injury at this time. Indeed, using flame photometry and rubidium uptake into isolated mouse hearts, we found that sildenafil enhanced Na⁺/K⁺-ATPase activity during reperfusion. In this study we provide a molecular explanation of how sildenafil guards against myocardial injury during postischemic reperfusion.

Multiple affinity states of cGMP-specific phosphodiesterase for sildenafil inhibition defined by cGMP-dependent and cGMP-independent mechanisms

cGMP-specific phosphodiesterase (PDE5) has become a target for drug development for the treatment of a number of physiological dysfunctions, affected by changes in the cGMP/cGMP-dependent protein kinase (PKG) signaling pathway. PDE5 has two highly homologous regulatory domains, GAF-A and GAF-B. We showed previously that PDE5 could be converted from a low-activity (nonactivated) state to a high-activity state upon cGMP binding to the GAF-A domain with higher sensitivities toward sildenafil (EMBO J 22:469-478, 2003). Here we investigated whether sildenafil sensitivity of PDE5 could be modified by cGMP-independent mechanisms. Individually expressed recombinant GAF-A and GAF-B proteins were tested for their ability to modulate full-length recombinant PDE5 affinity to sildenafil. The GAF-A domain protein had the most dramatic effect on the affinity of the nonactivated recombinant PDE5 for sildenafil, revealing much higher sensitivity to sildenafil inhibition. The apparent affinity for sildenafil increased from the nanomolar range to the picomolar range, providing evidence for the presence of a "super-high" sensitivity state of PDE5 for sildenafil inhibition. In human platelet, higher sensitivity of PDE5 for sildenafil inhibition has been detected after blocking cGMP-binding sites of the GAF-A domain. Thus, our data demonstrate that high sensitivity of PDE5 for sildenafil can be obtained not only through cGMP-induced activation of PDE, but also through cGMP-independent modulation of PDE5 in the nonactivated state, possibly through protein-protein interaction. Furthermore, data suggest that nonactivated PDE5 with "super-high" affinities for sildenafil inhibition may be responsible for therapeutic effects of long-term treatments with low doses of PDE5 inhibitors.