Phosphorylation of phosphodiesterase-5 by cyclic nucleotide-dependent protein kinase alters its catalytic and allosteric cGMP-binding activities

Nitric Oxide Resistance in Priapism Associated with Sickle Cell Disease: Mechanisms, Therapeutic Challenges, and Future Directions

Patients with sickle cell disease (SCD) display priapism, a prolonged penile erection in the absence of sexual arousal. The current pharmacological treatments for SCD-associated priapism are limited and focused on acute interventions rather than prevention. Thus, there is an urgent need for new drug targets and preventive pharmacological therapies for this condition. This review focuses on the molecular mechanisms linked to the dysfunction of the NO-cyclic guanosine monophosphate (cGMP)-phosphodiesterase type 5 (PDE5) pathway implicated in SCD-associated priapism. In murine models of SCD, reduced nitric oxide (NO)-cGMP bioavailability in the corpus cavernosum is associated with elevated plasma hemoglobin levels, increased reactive oxygen species levels that inactive NO, and testosterone deficiency that leads to endothelial nitric oxide synthase downregulation. We discuss the consequences of the reduced cGMP-dependent PDE5 activity in response to these molecular changes, highlighting it as the primary pathophysiological mechanism leading to excessive corpus cavernosum relaxation, culminating in priapism. We also further discuss the impact of intravascular hemolysis on therapeutic approaches, present current pharmacological strategies targeting the NO-cGMP-PDE5 pathway in the penis, and identify potential pharmacological targets for future priapism therapies. In men with SCD and priapism, PDE5 inhibitor therapy and testosterone replacement have shown promising results. Recent preclinical research reported the beneficial effect of treatment with haptoglobin and NO donors. SIGNIFICANCE STATEMENT: This review discusses the molecular changes that reduce NO-cGMP bioavailability in the penis in SCD and highlights pharmacological targets and therapeutic strategies for the treatment of priapism, including PDE5 inhibitors, hormonal modulators, NO donors, hydroxyurea, soluble guanylate cyclase stimulators, haptoglobin, hemopexin, and antioxidants.

Targeting heme in sickle cell disease: new perspectives on priapism treatment

Men with sickle cell disease (SCD) frequently experience priapism, defined as prolonged, painful erections occurring without sexual arousal or desire. This urological emergency can lead to penile fibrosis and permanent erectile dysfunction if not treated adequately. Due to its complex pathophysiology, there is currently no effective preventative treatment for this condition. Recent studies have highlighted the dysfunction of the nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) pathway in erectile tissues as a critical mechanism in developing priapism in SCD. Additionally, further research indicates that intravascular hemolysis promotes increased smooth muscle relaxation in the corpus cavernosum and that excess heme may significantly contribute to priapism in SCD. Pharmacological treatments should ideally target the pathophysiological basis of the disease. Agents that reduce excess free heme in the plasma have emerged as potential therapeutic candidates. This review explores the molecular mechanisms underlying the excess of heme in SCD and its contribution to developing priapism. We discuss pharmacological approaches targeting the excess free heme in the plasma, highlighting it as a potential therapeutic target for future interventions in managing priapism.

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.

Soluble guanylate cyclase stimulators and activators: new horizons in the treatment of priapism associated with sickle cell disease

Priapism, defined as a prolonged and often painful penile erection occurring without sexual stimulation or desire, is a common complication in sickle cell disease (SCD), affecting up to 48% of male patients. This condition presents significant clinical challenges and can lead to erectile dysfunction if not properly managed. Current pharmacological treatments for SCD-related priapism are primarily reactive rather than preventative, highlighting a gap in effective medical intervention strategies. A critical factor in developing priapism is the reduced basal bioavailability of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) in erectile tissues. New prevention strategies should ideally target the underlying pathophysiology of the disease. Compounds that stimulate and activate soluble guanylate cyclase (sGC) emerge as potential therapeutic candidates since these compounds have the property of inducing cGMP production by sGC. This review explores the potential of sGC stimulators and activators in treating priapism associated with SCD. We discuss the advantages of these agents in the face of the challenging pathophysiology of SCD. Additionally, the review underscores the impact of intravascular hemolysis and oxidative stress on priapism pathophysiology in SCD, areas in which sGC stimulators and activators may also have beneficial therapeutic effects.

Subcellular second messenger networks drive distinct repellent-induced axon behaviors

Second messengers, including cAMP, cGMP and Ca2+ are often placed in an integrating position to combine the extracellular cues that orient growing axons in the developing brain. This view suggests that axon repellents share the same set of cellular messenger signals and that axon attractants evoke opposite cAMP, cGMP and Ca2+ changes. Investigating the confinement of these second messengers in cellular nanodomains, we instead demonstrate that two repellent cues, ephrin-A5 and Slit1, induce spatially segregated signals. These guidance molecules activate subcellular-specific second messenger crosstalk, each signaling network controlling distinct axonal morphology changes in vitro and pathfinding decisions in vivo.

Once upon a Testis: The Tale of Cyclic Nucleotide Phosphodiesterase in Testicular Cancers

Phosphodiesterases are key regulators that fine tune the intracellular levels of cyclic nucleotides, given their ability to hydrolyze cAMP and cGMP. They are critical regulators of cAMP/cGMP-mediated signaling pathways, modulating their downstream biological effects such as gene expression, cell proliferation, cell-cycle regulation but also inflammation and metabolic function. Recently, mutations in PDE genes have been identified and linked to human genetic diseases and PDEs have been demonstrated to play a potential role in predisposition to several tumors, especially in cAMP-sensitive tissues. This review summarizes the current knowledge and most relevant findings regarding the expression and regulation of PDE families in the testis focusing on PDEs role in testicular cancer development.

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.

Protein tyrosine phosphatase PTPN22 negatively modulates platelet function and thrombus formation

Protein tyrosine phosphatase nonreceptor type 22 (PTPN22) is a protein tyrosine phosphatase that negatively regulates T-cell signaling. However, whether it is expressed and functions in platelets remains unknown. Here we investigated the expression and role of PTPN22 in platelet function. We reported PTPN22 expression in both human and mouse platelets. Using PTPN22-/- mice, we showed that PTPN22 deficiency significantly shortened tail-bleeding time and accelerated arterial thrombus formation without affecting venous thrombosis and the coagulation factors VIII and IX. Consistently, PTPN22-deficient platelets exhibited enhanced platelet aggregation, granule secretion, calcium mobilization, lamellipodia formation, spreading, and clot retraction. Quantitative phosphoproteomic analysis revealed the significant difference of phosphodiesterase 5A (PDE5A) phosphorylation in PTPN22-deficient platelets compared with wild-type platelets after collagen-related peptide stimulation, which was confirmed by increased PDE5A phosphorylation (Ser92) in collagen-related peptide-treated PTPN22-deficient platelets, concomitant with reduced level and vasodilator-stimulated phosphoprotein phosphorylation (Ser157/239). In addition, PTPN22 interacted with phosphorylated PDE5A (Ser92) and dephosphorylated it in activated platelets. Moreover, purified PTPN22 but not the mutant form (C227S) possesses intrinsic serine phosphatase activity. Furthermore, inhibition of PTPN22 enhanced human platelet aggregation, spreading, clot retraction, and increased PDE5A phosphorylation (Ser92). In conclusion, our study shows a novel role of PTPN22 in platelet function and arterial thrombosis, identifying new potential targets for future prevention of thrombotic or cardiovascular diseases.

Heart Failure with Preserved Ejection Fraction and Pulmonary Hypertension: Focus on Phosphodiesterase Inhibitors

Pulmonary hypertension (PH) is common in patients with heart failure with preserved ejection fraction (HFpEF). A chronic increase in mean left atrial pressure leads to passive remodeling in pulmonary veins and capillaries and modest PH (isolated postcapillary PH, Ipc-PH) and is not associated with significant right ventricular dysfunction. In approximately 20% of patients with HFpEF, "precapillary" alterations of pulmonary vasculature occur with the development of the combined pre- and post-capillary PH (Cpc-PH), pertaining to a poor prognosis. Current data indicate that pulmonary vasculopathy may be at least partially reversible and thus serves as a therapeutic target in HFpEF. Pulmonary vascular targeted therapies, including phosphodiesterase (PDE) inhibitors, may have a valuable role in the management of patients with PH-HFpEF. In studies of Cpc-PH and HFpEF, PDE type 5 inhibitors were effective in long-term follow-up, decreasing pulmonary artery pressure and improving RV contractility, whereas studies of Ipc-PH did not show any benefit. Randomized trials are essential to elucidate the actual value of PDE inhibition in selected patients with PH-HFpEF, especially in those with invasively confirmed Cpc-PH who are most likely to benefit from such treatment.

Protein S-Nitrosation: Biochemistry, Identification, Molecular Mechanisms, and Therapeutic Applications

Protein S-nitrosation (SNO), a posttranslational modification (PTM) of cysteine (Cys) residues elicited by nitric oxide (NO), regulates a wide range of protein functions. As a crucial form of redox-based signaling by NO, SNO contributes significantly to the modulation of physiological functions, and SNO imbalance is closely linked to pathophysiological processes. Site-specific identification of the SNO protein is critical for understanding the underlying molecular mechanisms of protein function regulation. Although careful verification is needed, SNO modification data containing numerous functional proteins are a potential research direction for druggable target identification and drug discovery. Undoubtedly, SNO-related research is meaningful not only for the development of NO donor drugs but also for classic target-based drug design. Herein, we provide a comprehensive summary of SNO, including its origin and transport, identification, function, and potential contribution to drug discovery. Importantly, we propose new views to develop novel therapies based on potential protein SNO-sourced targets.

Testosterone Deficiency in Sickle Cell Disease: Recognition and Remediation

Hypogonadism is common in men with sickle cell disease (SCD) with prevalence rates as high as 25%. Testicular failure (primary hypogonadism) is established as the principal cause for this hormonal abnormality, although secondary hypogonadism and compensated hypogonadism have also been observed. The underlying mechanism for primary hypogonadism was elucidated in a mouse model of SCD, and involves increased NADPH oxidase-derived oxidative stress in the testis, which reduces protein expression of a steroidogenic acute regulatory protein and cholesterol transport to the mitochondria in Leydig cells. In all men including those with SCD, hypogonadism affects physical growth and development, cognition and mental health, sexual function, as well as fertility. However, it is not understood whether declines in physical, psychological, and social domains of health in SCD patients are related to low testosterone, or are consequences of other abnormalities of SCD. Priapism is one of only a few complications of SCD that has been studied in the context of hypogonadism. In this pathologic condition of prolonged penile erection in the absence of sexual excitement or stimulation, hypogonadism exacerbates already impaired endothelial nitric oxide synthase/cGMP/phosphodiesterase-5 molecular signaling in the penis. While exogenous testosterone alleviates priapism, it disadvantageously decreases intratesticular testosterone production. In contrast to treatment with exogenous testosterone, a novel approach is to target the mechanisms of testosterone deficiency in the SCD testis to drive endogenous testosterone production, which potentially decreases further oxidative stress and damage in the testis, and preserves sperm quality. Stimulation of translocator protein within the transduceosome of the testis of SCD mice reverses both hypogonadism and priapism, without affecting intratesticular testosterone production and consequently fertility. Ongoing research is needed to define and develop therapies that restore endogenous testosterone production in a physiologic, mechanism-specific fashion without affecting fertility in SCD men.

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

The Potential Role of Sildenafil in Cancer Management through EPR Augmentation

Enhanced permeation retention (EPR) was a significant milestone discovery by Maeda et al. paving the path for the emerging field of nanomedicine to become a powerful tool in the fight against cancer. Sildenafil is a potent inhibitor of phosphodiesterase 5 (PDE-5) used for the treatment of erectile dysfunction (ED) through the relaxation of smooth muscles and the modulation of vascular endothelial permeability. Overexpression of PDE-5 has been reported in lung, colon, metastatic breast cancers, and bladder squamous carcinoma. Moreover, sildenafil has been reported to increase the sensitivity of tumor cells of different origins to the cytotoxic effect of chemotherapeutic agents with augmented apoptosis mediated through inducing the downregulation of Bcl-xL and FAP-1 expression, enhancing reactive oxygen species (ROS) generation, phosphorylating BAD and Bcl-2, upregulating caspase-3,8,9 activities, and blocking cells at G0/G1 cell cycle phase. Sildenafil has also demonstrated inhibitory effects on the efflux activity of ATP-binding cassette (ABC) transporters such as ABCC4, ABCC5, ABCB1, and ABCG2, ultimately reversing multidrug resistance. Accordingly, there has been a growing interest in using sildenafil as monotherapy or chemoadjuvant in EPR augmentation and management of different types of cancer. In this review, we critically examine the basic molecular mechanism of sildenafil related to cancer biology and discuss the overall potential of sildenafil in enhancing EPR-based anticancer drug delivery, pointing to the outcomes of the most important related preclinical and clinical studies.

Loss of Pde6a Induces Rod Outer Segment Shrinkage and Visual Alterations in pde6aQ70X Mutant Zebrafish, a Relevant Model of Retinal Dystrophy

Retinitis pigmentosa (RP) is one of the most common forms of inherited retinal degeneration with 1/4,000 people being affected. The vision alteration primarily begins with rod photoreceptor degeneration, then the degenerative process continues with cone photoreceptor death. Variants in 71 genes have been linked to RP. One of these genes, PDE6a is responsible for RP43. To date no treatment is available and patients suffer from pronounced visual impairment in early childhood. We used the novel zebrafish pde6a^Q70X mutant, generated by N-ethyl-N-nitrosourea at the European Zebrafish Resource Centre, to better understand how PDE6a loss of function leads to photoreceptor alteration. Interestingly, zebrafish pde6a^Q70X mutants exhibited impaired visual function at 5 dpf as evidenced by the decrease in their visual motor response (VMR) compared to pde6a^WT larvae. This impaired visual function progressed with time and was more severe at 21 dpf. These modifications were associated with an alteration of rod outer segment length at 5 and 21 dpf. In summary, these findings suggest that rod outer segment shrinkage due to Pde6a deficiency begins very early in zebrafish, progresses with time. The zebrafish pde6a^Q70X mutant represents an ideal model of RP to screen relevant active small molecules that will block the progression of the disease.

Pharmacological preconditioning with phosphodiestrase inhibitor: an answer to stem cell survival against ischemic injury through JAK/STAT signaling

Stem cell transplantation in regenerative medicine has been widely used in various disorders including cardiovascular diseases (CVD) and emerging next-generation therapy. However, transplanted stem cell encountered ischemia/reperfusion (IR) injury which is a major challenge for stem cell survival. During the acute phase after myocardial infarction (MI) cytokine-rich hostile microenvironment, extensive immune cell infiltration and lack of oxygen have been a bottleneck in cell-based therapy. During prolonged ischemia, intracellular pH and ATP level decrease results in anaerobic metabolism and lactate accumulation. Consequentially, ATPase-dependent ion transport becomes dysfunctional, contributing to calcium overload and cell death by apoptosis and necrosis. Although O₂ level revitalizes upon reperfusion, a surge in the generation of reactive oxygen species (ROS) occurs with neutrophil infiltration in ischemic tissues further aggravating the injury. Ischemic preconditioning (IPC) of stem cells with a repeated short cycle of IR results in the release of chemical signals such as NO, ROS, and adenosine which triggers a cascade of signaling events that activates protein kinase C (PKC), Src protein tyrosine kinases, and nuclear factor κB (NF-κB) and subsequently increased synthesis of inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2), Heme oxygenase-1 [HO-1], aldose reductase, Mn superoxide dismutase, and anti-apoptotic genes (Mcl-1, BCl-x_L, c-FLIP_L, c-FLIP_S). Pharmacological preconditioning uses a phosphodiestrase inhibitor, another mode of protecting stem cell or heart per se from impending ischemic injury in two phases. During the early phase of cardioprotection (2 h), PC leads to increased expression of survival factors like BCl₂/Bax ratio while late phase (24 h) showed activation of the JAK/STAT survival pathway. Phosphorylation of STAT3 at two crucial residues, Tyr-705 and Ser-727, allows its entry inside the nucleus and upregulates the expression of protein kinase G-1 (PKG1) which evokes cardioprotective signaling. To confirm, heart-specific conditional STAT3 knockout mice undergone IR surgery, abolishing late-phase cardioprotective effects.

Medicinal plants as a potential source of Phosphodiesterase-5 inhibitors: A review

ETHNOPHARMACOLOGICAL RELEVANCE

The prevalence and distress caused by erectile dysfunction (ED) to both male and female partners are increasing at a steady rate. ED has now become the most treated sexual disorder for men among young and old age groups due to varying physical and psychological factors. The treatment with synthetic Phosphodiesterase-5 (PDE5) inhibitors are cost-effective but due to adverse effects such as priapism, loss of vision, heart attack and syncope, the daily life patterns of these patients are distressed and hence the need for alternative medicaments or sources are of utmost important. Therefore, the exploration of medicinal plants as PDE5 inhibitors will be worthwhile in tackling the problems as many plant extracts and fractions have been long used as aphrodisiacs and sexual stimulants which may be found to be active against PDE5 enzyme.

AIM OF THE STUDY

To provide a review on the different medicinal herbs traditionally used as natural aphrodisiacs, libido or sexual enhancers which are proven for their PDE5 inhibitory effect.

MATERIALS AND METHODS

Ethnobotanical and scientific information was procured, reviewed and compiled from the literature search of electronic databases and search engines.

RESULTS

A total of 97 medicinal plants exhibiting PDE5 inhibitory effect are reviewed in this paper which is supported by preclinical experimental evidence. Among them, 77 plants have been selected according to their traditional and ethnobotanical uses as aphrodisiacs and the rest are screened according to their effectiveness against predisposing factors responsible for ED and sexual dysfunction such as diabetes and hypertension or due to the presence of phytochemicals having structural similarity towards the identified natural PDE5 inhibitors. In addition, sixteen alkaloids, sixty-one phenolics and eight polycyclic aromatic hydrocarbons have been isolated or identified from active extracts or fractions that are exhibiting PDE5 inhibitory activity. Among them, isoflavones and biflavones are the major active constituents responsible for action, where the presence of prenyl group for isoflavones; and the methoxy group at C-5 position of flavones are considered essential for the inhibitory effect. However, the prenylated flavonol glycoside, Icariin and Icariside II isolated from Epimedium brevicornum Maxim (hory goat weed) are the most effective inhibitor, till date from natural sources. Traditional medicines or formulations containing extracts of Ginkgo biloba L., Kaempferia parviflora Wall. ex Baker, Clerodendrum colebrookianum Walp., Eurycoma longifolia Jack and Vitis vinifera L. are also found to be inhibitors of PDE5 enzyme.

CONCLUSION

The review suggests and supports the rational use of traditional medicines that can be further studied for the development of potential PDE5 inhibitors. Many traditional medicines are still used in various regions of Africa, Asia and South America that are poorly characterized and experimented. Despite the availability of a vast majority of traditional formulations as aphrodisiacs or sexual stimulants, there exists a need for systemic evaluation on the efficacy as well as the mechanism of action of the herbal constituents for the identification of novel chemical moieties that can be further developed for maximum efficacy.

Therapeutic Potential of Phosphodiesterase Inhibitors for Endothelial Dysfunction- Related Diseases

Cardiovascular diseases (CVD) are considered a major health problem worldwide, being the main cause of mortality in developing and developed countries. Endothelial dysfunction, characterized by a decline in nitric oxide production and/or bioavailability, increased oxidative stress, decreased prostacyclin levels, and a reduction of endothelium-derived hyperpolarizing factor is considered an important prognostic indicator of various CVD. Changes in cyclic nucleotides production and/ or signalling, such as guanosine 3', 5'-monophosphate (cGMP) and adenosine 3', 5'-monophosphate (cAMP), also accompany many vascular disorders that course with altered endothelial function. Phosphodiesterases (PDE) are metallophosphohydrolases that catalyse cAMP and cGMP hydrolysis, thereby terminating the cyclic nucleotide-dependent signalling. The development of drugs that selectively block the activity of specific PDE families remains of great interest to the research, clinical and pharmaceutical industries. In the present review, we will discuss the effects of PDE inhibitors on CVD related to altered endothelial function, such as atherosclerosis, diabetes mellitus, arterial hypertension, stroke, aging and cirrhosis. Multiple evidences suggest that PDEs inhibition represents an attractive medical approach for the treatment of endothelial dysfunction-related diseases. Selective PDE inhibitors, especially PDE3 and PDE5 inhibitors are proposed to increase vascular NO levels by increasing antioxidant status or endothelial nitric oxide synthase expression and activation and to improve the morphological architecture of the endothelial surface. Thereby, selective PDE inhibitors can improve the endothelial function in various CVD, increasing the evidence that these drugs are potential treatment strategies for vascular dysfunction and reinforcing their potential role as an adjuvant in the pharmacotherapy of CVD.

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.

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.

TSPO ligand FGIN-1-27 controls priapism in sickle cell mice via endogenous testosterone production

Priapism, a prolonged penile erection in the absence of sexual arousal, is common among patients with sickle cell disease (SCD). Hypogonadism is also common in patients with SCD. While the administration of exogenous testosterone reverses hypogonadism, it is contraceptive. We hypothesized that the stimulation of endogenous testosterone production decreases priapism by normalizing molecular signaling involved in penile erection without decreasing intratesticular testosterone production, which would affect fertility. Treatment of SCD mice with FGIN-1-27, a ligand for translocator protein (TSPO) that mobilizes cholesterol to the inner mitochondrial membrane, resulted in eugonadal levels of serum testosterone without decreasing intratesticular testosterone production. Normalized testosterone levels, in turn, decreased priapism. At the molecular level, TSPO restored phosphodiesterase 5 activity and decreased NADPH oxidase-mediated oxidative stress in the penis, which are major molecular signaling molecules involved in penile erection and are dysregulated in SCD. These results indicate that pharmacologic activation of TSPO could be a novel, targetable pathway for treating hypogonadal men, particularly patients with SCD, without adverse effects on fertility.

Mechanisms underlying priapism in sickle cell disease: targeting and key innovations on the preclinical landscape

Introduction: Priapism is prolonged penile erection in the absence of sexual arousal or desire and is a devastating condition affecting millions of patients with sickle cell disease (SCD) globally. Available drug treatments for SCD-related priapism remain limited and have been primarily reactive rather than preventive. Hence, there is an unmet need for new drug targets and pharmacologic therapies.Areas covered: We examine the molecular mechanisms underlying SCD-associated priapism evaluated mostly in animal models. In mouse models of SCD, molecular defects of priapism operating at the cavernous tissue level include reduced tonic NO/cGMP signaling, elevated oxidative/nitrosative stress, vascular adhesion molecule derangements, excessive adenosine and opiorphin signaling, dysregulated vasoconstrictive RhoA/ROCK signaling, and testosterone deficiency. We discuss the consequences of downregulated cGMP-dependent phosphodiesterase type 5 (PDE5) activity in response to these molecular signaling derangements, as the main effector mechanism causing unrestrained cavernous tissue relaxation that results in priapism.Expert opinion: Basic science studies are crucial for understanding the underlying pathophysiology of SCD-associated priapism. Understanding the molecular mechanisms could unearth new therapeutic targets for this condition based on these mechanisms. Treatment options should aim to improve deranged erection physiology regulatory signaling to prevent priapism and potentially restore or preserve erectile function.

Dysregulated NO/PDE5 signaling in the sickle cell mouse lower urinary tract: Reversal by oral nitrate therapy

AIMS

Nitric oxide (NO) has a critical, but not well understood, influence in the physiology of the lower urinary tract. We evaluated the effect of NO/phosphodiesterase (PDE)5 signaling in voiding dysfunction in the sickle cell disease (SCD) mouse, characterized by low NO bioavailability.

MAIN METHODS

Adult SCD (Sickle) and wild-type (WT) male mice were treated daily with sodium nitrate (10 mM) or vehicle. After 18 days, blood was obtained for nitrite measurement, urethra was collected for organ bath study, and bladder and urethra were collected for Western blot analysis of PDE5 phosphorylation (Ser-92) (activated form). Non-anesthetized mice underwent evaluation of urine volume by void spot assay. eNOS phosphorylation (Ser-1177) and nNOS phosphorylation (Ser-1412) (positive regulatory sites) were evaluated in the bladder and urethra of untreated mice.

KEY FINDINGS

Sickle mice exhibited decreased eNOS, nNOS, and PDE5 phosphorylation in the bladder and urethra, decreased plasma nitrite levels, increased relaxation of phenylephrine-contracted urethral tissue to an NO donor sodium nitroprusside, and increased total urine volume, compared with WT mice. Nitrate treatment normalized plasma nitrite levels, relaxation of urethra to sodium nitroprusside, PDE5 phosphorylation in the urethra and bladder, and urine volume in Sickle mice.

SIGNIFICANCE

Derangement in PDE5 activity associated with basally low NO bioavailability in the bladder and urethra contributes to the molecular basis for voiding abnormalities in Sickle mice. Inorganic nitrate supplementation normalized voiding in Sickle mice through mechanisms likely involving upregulation of PDE5 activity. These findings suggest that interventions targeting dysregulatory NO/PDE5 signaling may ameliorate overactive bladder in SCD.

Therapeutic targeting of 3',5'-cyclic nucleotide phosphodiesterases: inhibition and beyond

Phosphodiesterases (PDEs), enzymes that degrade 3',5'-cyclic nucleotides, are being pursued as therapeutic targets for several diseases, including those affecting the nervous system, the cardiovascular system, fertility, immunity, cancer and metabolism. Clinical development programmes have focused exclusively on catalytic inhibition, which continues to be a strong focus of ongoing drug discovery efforts. However, emerging evidence supports novel strategies to therapeutically target PDE function, including enhancing catalytic activity, normalizing altered compartmentalization and modulating post-translational modifications, as well as the potential use of PDEs as disease biomarkers. Importantly, a more refined appreciation of the intramolecular mechanisms regulating PDE function and trafficking is emerging, making these pioneering drug discovery efforts tractable.

Phosphodiesterase inhibitors say NO to Alzheimer's disease

Phosphodiesterases (PDEs) consisted of 11 subtypes (PDE1 to PDE11) and over 40 isoforms that regulate levels of cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP), the second messengers in cell functions. PDE inhibitors (PDEIs) have been attractive therapeutic targets due to their involvement in diverse medical conditions, e.g. cardiovascular diseases, autoimmune diseases, Alzheimer's disease (AD), etc. Among them; AD with a complex pathology is a progressive neurodegenerative disorder which affect mostly senile people in the world and only symptomatic treatment particularly using cholinesterase inhibitors in clinic is available at the moment for AD. Consequently, novel treatment strategies towards AD are still searched extensively. Since PDEs are broadly expressed in the brain, PDEIs are considered to modulate neurodegenerative conditions through regulating cAMP and cGMP in the brain. In this sense, several synthetic or natural molecules inhibiting various PDE subtypes such as rolipram and roflumilast (PDE4 inhibitors), vinpocetine (PDE1 inhibitor), cilostazol and milrinone (PDE3 inhibitors), sildenafil and tadalafil (PDE5 inhibitors), etc have been reported showing encouraging results for the treatment of AD. In this review, PDE superfamily will be scrutinized from the view point of structural features, isoforms, functions and pharmacology particularly attributed to PDEs as target for AD therapy.

The Phosphodiesterase-5 Inhibitors and Prostate Cancer – What We Rely Know About It?

Phosphodiesterase-5 inhibitors (PDE5Is) represent a group of drugs that are registered for the treatment of erectile dysfunctions predominantly, but recently also for treatment of pulmonary hypertension and benign prostatic hypertrophy. However, more and more research deals with possible antitumor potential of PDE5Is in different types of cancers, including prostate cancer. Prostate cancer represents the one of the most common carcinoma in the male population, whose incidence is continuously increasing. Early detection combined with radical prostatectomy increases the survival rate, but also it is necessary to keep in mind the quality of life of patients undergoing prostatectomy in light of bladder control and erectile function. Authors of various clinical studies presented the results that often lead to totally opposing conclusions. For example, Chavez and colleagues have shown that use of PDE5Is in men with erectile dysfunction decreases the risk of developing prostate cancer, while, on the other hand, Michl and colleagues pointed out the adversely effect of PDE5Is on biochemical recurrence after bilateral nerve sparing radical prostatectomy. In that sense, the aim of this review was to present as many as possible of existing results dealing with of action of PDE5Is in the field of prostatic carcinoma. Taking into account all presented data, it can be concluded that eff ect of PDE5Is on formation, development and outcome of treatment in patients with prostate carcinoma is very intriguing question, whose response requires additional both experimental and clinical research.

Synthesis and Pharmacological Screening of Pyridopyrimidines as Effective Anti-Diarrheal Agents through the Suppression of Cyclic Nucleotide Accumulation

The increased levels of cyclic nucleotides (cGMP and cAMP) in enterocytes trigger intracellular mechanisms of ion and fluid secretion into the lumen, causing secretory diarrhea. Twelve novel pyridopyrimidines derived from 5‐(3,5‐bistrifluoromethylphenyl)‐1,3‐dimethyl‐5,11‐dihydro‐1H‐indeno[2,1 : 5,6]pyrido[2,3‐d]pyrimidine‐2,4,6‐trione (FPIPP) were synthesized and evaluated on intracellular cyclic nucleotide accumulation. All compounds had no effect on either cyclic nucleotide basal levels or on pre‐contracted aortic rings. The metabolic activity and viability in T84 cells, assessed by MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide) and the LDH (lactate dehydrogenase) assays, respectively, were not affected by incubation with the compounds (50 μM). Compound VI almost abolished cGMP accumulation (94 % inhibition) induced by STa toxin in T834 cells and significantly reduced (69 %) forskolin‐induced cAMP accumulation in Jurkat cells. Compound VI was active in an in vivo model for diarrhea in rabbits. These results prompted us to perform a microscopic histopathological analysis of intestinal tissues, showing that only compound VI preserves the intestine without significant pathological changes and with a decreased inflammatory pattern in comparison to FPIPP. In vitro stability test revealed that compound VI is resistant to oxidation promoted by atmospheric oxygen.

RhoBTB1 protects against hypertension and arterial stiffness by restraining phosphodiesterase 5 activity

Mice selectively expressing PPARγ dominant negative mutation in vascular smooth muscle exhibit RhoBTB1-deficiency and hypertension. Our rationale was to employ genetic complementation to uncover the mechanism of action of RhoBTB1 in vascular smooth muscle. Inducible smooth muscle-specific restoration of RhoBTB1 fully corrected the hypertension and arterial stiffness by improving vasodilator function. Notably, the cardiovascular protection occurred despite preservation of increased agonist-mediated contraction and RhoA/Rho kinase activity, suggesting RhoBTB1 selectively controls vasodilation. RhoBTB1 augmented the cGMP response to nitric oxide by restraining the activity of phosphodiesterase 5 (PDE5) by acting as a substrate adaptor delivering PDE5 to the Cullin-3 E3 Ring ubiquitin ligase complex for ubiquitination inhibiting PDE5. Angiotensin-II infusion also caused RhoBTB1-deficiency and hypertension which was prevented by smooth muscle specific RhoBTB1 restoration. We conclude that RhoBTB1 protected from hypertension, vascular smooth muscle dysfunction, and arterial stiffness in at least two models of hypertension.

TGR5 Activation Promotes Stimulus-Secretion Coupling of Pancreatic β-Cells via a PKA-Dependent Pathway

The Takeda-G-protein-receptor-5 (TGR5) mediates physiological actions of bile acids. Since it was shown that TGR5 is expressed in pancreatic tissue, a direct TGR5 activation in β-cells is currently postulated and discussed. The current study reveals that oleanolic acid (OLA) affects murine β-cell function by TGR5 activation. Both a G_αs inhibitor and an inhibitor of adenylyl cyclase (AC) prevented stimulating effects of OLA. Accordingly, OLA augmented the intracellular cAMP concentration. OLA and two well-established TGR5 agonists, RG239 and tauroursodeoxycholic acid (TUDCA), acutely promoted stimulus-secretion coupling (SSC). OLA reduced K_ATP current and elevated current through Ca²⁺ channels. Accordingly, in mouse and human β-cells, TGR5 ligands increased the cytosolic Ca²⁺ concentration by stimulating Ca²⁺ influx. Higher OLA concentrations evoked a dual reaction, probably due to activation of a counterregulating pathway. Protein kinase A (PKA) was identified as a downstream target of TGR5 activation. In contrast, inhibition of phospholipase C and phosphoinositide 3-kinase did not prevent stimulating effects of OLA. Involvement of exchange protein directly activated by cAMP 2 (Epac2) or farnesoid X receptor (FXR2) was ruled out by experiments with knockout mice. The proposed pathway was not influenced by local glucagon-like peptide 1 (GLP-1) secretion from α-cells, shown by experiments with MIN6 cells, and a GLP-1 receptor antagonist. In summary, these data clearly demonstrate that activation of TGR5 in β-cells stimulates insulin secretion via an AC/cAMP/PKA-dependent pathway, which is supposed to interfere with SSC by affecting K_ATP and Ca²⁺ currents and thus membrane potential.

Inhibition of phosphodiesterases as a strategy to achieve neuroprotection in Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative condition, due to a mutation in the IT15 gene encoding for huntingtin. Currently, disease-modifying therapy is not available for HD, and only symptomatic drugs are administered for the management of symptoms. In the last few years, preclinical and clinical studies have indicated that pharmacological strategies aimed at inhibiting cyclic nucleotide phosphodiesterase (PDEs) may develop into a novel therapeutic approach in neurodegenerative disorders. PDEs are a family of enzymes that hydrolyze cyclic nucleotides into monophosphate isoforms. Cyclic nucleotides are second messengers that transduce the signal of hormones and neurotransmitters in many physiological processes, such as protein kinase cascades and synaptic transmission. An alteration in their balance results in the dysregulation of different biological mechanisms (transcriptional dysregulation, immune cell activation, inflammatory mechanisms, and regeneration) that are involved in neurological diseases. In this review, we discuss the action of phosphodiesterase inhibitors and their role as therapeutic agents in HD.

cAMP, cGMP and Amyloid β: Three Ideal Partners for Memory Formation

cAMP and cGMP are well established second messengers required for long-term potentiation (LTP) and memory formation/consolidation. By contrast, amyloid β (Aβ), mostly known as one of the main culprits for Alzheimer's disease (AD), has received relatively little attention in the context of plasticity and memory. Of note, however, low physiological concentrations of Aβ seem necessary for LTP induction and for memory formation. This should come as no surprise, since hormesis emerged as a central dogma in biology. Additionally, recent evidence indicates that Aβ is one of the downstream effectors for cAMP and cGMP to trigger synaptic plasticity and memory. We argue that these emerging findings depict a new scenario that should change the general view on the amyloidogenic pathway, and that could have significant implications for the understanding of AD and its pharmacological treatment in the future.

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.

The Role of Phosphodiesterase-2 in Psychiatric and Neurodegenerative Disorders

Cyclic nucleotide PDEs are a super-family of enzymes responsible for regulating intracellular levels of the second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Through their catalysis, PDEs are able to exert tight regulation over these important intracellular signaling cascades. Previously, PDEs have been implicated in learning and memory, as well as in mood disorders, such as anxiety and depression. PDE2 is of special interest due to its high level of expression in the forebrain, specifically in the isocortex, entorhinal cortex, striatum, hippocampus, amygdala, and medial habenula. Many of these brain regions are considered participants of the limbic system, which is known as the emotional regulatory center of the brain, and is important for modulating emotion and long-term memory. Therefore, PDE2s coincidental expression in these areas suggests an important role for PDE2 in these behaviors, and researchers are continuing to uncover the complex connections. It was shown that PDE2 inhibitors have pro-cognitive effects in tests of memory, including the object recognition test. PDE2 inhibitors are also protective against cognitive deficits in various models of cognitive impairment. Additionally, PDE2 inhibitors are protective against many different forms of stress-induced anxiety-like and depression-like behaviors. Currently, there is a great need for novel therapeutics for the treatment of mood and cognitive disorders, especially anxiety and depression, and other neurodegenerative diseases, such as Alzheimer's disease, and PDE2 is emerging as a viable target for future drug development for many of these diseases.

Luteinizing hormone signaling phosphorylates and activates the cyclic GMP phosphodiesterase PDE5 in mouse ovarian follicles, contributing an additional component to the hormonally induced decrease in cyclic GMP that reinitiates meiosis

Prior to birth, oocytes within mammalian ovarian follicles initiate meiosis, but then arrest in prophase until puberty, when with each reproductive cycle, one or more follicles are stimulated by luteinizing hormone (LH) to resume meiosis in preparation for fertilization. Within preovulatory follicles, granulosa cells produce high levels of cGMP, which diffuses into the oocyte to maintain meiotic arrest. LH signaling restarts meiosis by rapidly lowering the levels of cGMP in the follicle and oocyte. Part of this decrease is mediated by the dephosphorylation and inactivation the NPR2 guanylyl cyclase in response to LH, but the mechanism for the remainder of the cGMP decrease is unknown. At least one cGMP phosphodiesterase, PDE5, is activated by LH signaling, which would contribute to lowering cGMP. PDE5 exhibits increased cGMP-hydrolytic activity when phosphorylated on serine 92, and we recently demonstrated that LH signaling phosphorylates PDE5 on this serine and increases its activity in rat follicles. To test the extent to which this mechanism contributes to the cGMP decrease that restarts meiosis, we generated a mouse line in which serine 92 was mutated to alanine (Pde5-S92A), such that it cannot be phosphorylated. Here we show that PDE5 phosphorylation is required for the LH-induced increase in cGMP-hydrolytic activity, but that this increase has only a modest effect on the LH-induced cGMP decrease in mouse follicles, and does not affect the timing of meiotic resumption. Though we show that the activation of PDE5 is among the mechanisms contributing to the cGMP decrease, these results suggest that another cGMP phosphodiesterase is also activated by LH signaling.

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.

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.

3',5'-cIMP as Potential Second Messenger in the Vascular Wall

Traditionally, only the 3',5'-cyclic monophosphates of adenosine and guanosine (produced by adenylyl cyclase and guanylyl cyclase, respectively) are regarded as true "second messengers" in the vascular wall, despite the presence of other cyclic nucleotides in different tissues. Among these noncanonical cyclic nucleotides, inosine 3',5'-cyclic monophosphate (cIMP) is synthesized by soluble guanylyl cyclase in porcine coronary arteries in response to hypoxia, when the enzyme is activated by endothelium-derived nitric oxide. Its production is associated with augmentation of vascular contraction mediated by stimulation of Rho kinase. Based on these findings, cIMP appears to meet most, if not all, of the criteria required for it to be accepted as a "second messenger," at least in the vascular wall.

Phosphodiesterase type 5 and cancers: progress and challenges

Cancers are an extraordinarily heterogeneous collection of diseases with distinct genetic profiles and biological features that directly influence response patterns to various treatment strategies as well as clinical outcomes. Nevertheless, our growing understanding of cancer cell biology and tumor progression is gradually leading towards rational, tailored medical treatments designed to destroy cancer cells by exploiting the unique cellular pathways that distinguish them from normal healthy counterparts. Recently, inhibition of the activity of phosphodiesterase type 5 (PDE5) is emerging as a promising approach to restore normal intracellular cyclic guanosine monophosphate (cGMP) signalling, and thereby resulting into the activation of various downstream molecules to inhibit proliferation, motility and invasion of certain cancer cells. In this review, we present an overview of the experimental and clinical evidences highlighting the role of PDE5 in the pathogenesis and prevention of various malignancies. Current data are still not sufficient to draw conclusive statements for cancer patient management, but could provide further rational for testing PDE5-targeting drugs as anticancer agents in clinical settings.

PDE2 at the crossway between cAMP and cGMP signalling in the heart

The cyclic nucleotides cAMP and cGMP are central second messengers in cardiac cells and critical regulators of cardiac physiology as well as pathophysiology. Consequently, subcellular compartmentalization allows for spatiotemporal control of cAMP/cGMP metabolism and subsequent regulation of their respective effector kinases PKA or PKG is most important for cardiac function in health and disease. While acute cAMP-mediated signalling is a mandatory prerequisite for the physiological fight-or-flight response, sustained activation of this pathway may lead to the progression of heart failure. In contrast, acute as well as sustained cGMP-mediated signalling can foster beneficial features, e.g. anti-hypertrophic and vasodilatory effects. These two signalling pathways seem to be intuitively counteracting and there is increasing evidence for a functionally relevant crosstalk between cAMP and cGMP signalling pathways on the level of cyclic nucleotide hydrolysing phosphodiesterases (PDEs). Among this diverse group of enzymes, PDE2 may fulfill a unique integrator role. Equipped with dual substrate specificity for cAMP as well as for cGMP, it is the only cAMP hydrolysing PDE, which is allosterically activated by cGMP. Recent studies have revealed strongly remodelled cAMP/cGMP microdomains and subcellular concentration profiles in different cardiac pathologies, leading to a putatively enhanced involvement of PDE2 in cAMP/cGMP breakdown and crosstalk compared to the other cardiac PDEs. This review sums up the current knowledge about molecular properties and regulation of PDE2 and explains the complex signalling network encompassing PDE2 in order to better understand the functional role of PDE2 in distinct cell types in cardiac health and disease. Moreover, this review gives an outlook in which way PDE2 may serve as a therapeutic target to treat cardiac disease.

Regulation of Mammalian Oocyte Meiosis by Intercellular Communication Within the Ovarian Follicle

Meiotic progression in mammalian preovulatory follicles is controlled by the granulosa cells around the oocyte. Cyclic GMP (cGMP) generated in the granulosa cells diffuses through gap junctions into the oocyte, maintaining meiotic prophase arrest. Luteinizing hormone then acts on receptors in outer granulosa cells to rapidly decrease cGMP. This occurs by two complementary pathways: cGMP production is decreased by dephosphorylation and inactivation of the NPR2 guanylyl cyclase, and cGMP hydrolysis is increased by activation of the PDE5 phosphodiesterase. The cGMP decrease in the granulosa cells results in rapid cGMP diffusion out of the oocyte, initiating meiotic resumption. Additional, more slowly developing mechanisms involving paracrine signaling by extracellular peptides (C-type natriuretic peptide and EGF receptor ligands) maintain the low level of cGMP in the oocyte. These coordinated signaling pathways ensure a fail-safe system to prepare the oocyte for fertilization and reproductive success.

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.

Clinically relevant timing of antenatal sildenafil treatment reduces pulmonary vascular remodeling in congenital diaphragmatic hernia

Patients with congenital diaphragmatic hernia (CDH) suffer from severe pulmonary hypertension attributable to altered development of the pulmonary vasculature, which is often resistant to vasodilator therapy. Present treatment starts postnatally even though significant differences in the pulmonary vasculature are already present early during pregnancy. We examined the effects of prenatal treatment with the phosphodiesterase-5 inhibitor sildenafil on pulmonary vascular development in experimental CDH starting at a clinically relevant time. The well-established, nitrofen-induced CDH rodent model was treated daily with 100 mg/kg sildenafil from day 17.5 until day 20.5 of gestation (E17.5–20.5). Importantly, this timing perfectly corresponds to the developmental stage of the lung at 20 wk of human gestation, when CDH is detectable by 2D-ultrasonography and/or MRI. At E21.5 pups were delivered by caesarean section and euthanized by lethal injection of pentobarbital. The lungs were isolated and subsequently analyzed using immunostaining, real-time PCR, and volume measurements. Prenatal treatment with sildenafil improved lung morphology and attenuated vascular remodeling with reduced muscularization of the smaller vessels. Pulmonary vascular volume was not affected by sildenafil treatment. We show that prenatal treatment with sildenafil within a clinically relevant period improves pulmonary vascular development in an experimental CDH model. This may have important implications for the management of this disease and related pulmonary vascular diseases in human.

Conservation of structural fluctuations in homologous protein kinases and its implications on functional sites

Our aim is to explore the similarities in structural fluctuations of homologous kinases. Gaussian Network Model based Normal Mode Analysis was performed on 73 active conformation structures in Ser/Thr/Tyr kinase superfamily. Categories of kinases with progressive evolutionary divergence, viz. (i) Same kinase with many crystal structures, (ii) Within-Subfamily, (iii) Within-Family, (iv) Within-Group, and (v) Across-Group, were analyzed. We identified a flexibility signature conserved in all kinases involving residues in and around the catalytic loop with consistent low-magnitude fluctuations. However, the overall structural fluctuation profiles are conserved better in closely related kinases (Within-Subfamily and Within-family) than in distant ones (Within-Group and Across-Group). A substantial 65.4% of variation in flexibility was not accounted by variation in sequences or structures. Interestingly, we identified substructural residue-wise fluctuation patterns characteristic of kinases of different categories. Specifically, we recognized statistically significant fluctuations unique to families of protein kinase A, cyclin-dependent kinases, and nonreceptor tyrosine kinases. These fluctuation signatures localized to sites known to participate in protein-protein interactions typical of these kinase families. We report for the first time that residues characterized by fluctuations unique to the group/family are involved in interactions specific to the group/family. As highlighted for Src family, local regions with differential fluctuations are proposed as attractive targets for drug design. Overall, our study underscores the importance of consideration of fluctuations, over and above sequence and structural features, in understanding the roles of sites characteristic of kinases. Proteins 2016; 84:957-978. © 2016 Wiley Periodicals, Inc.

Angiotensin II-mediated hypertension impairs nitric oxide-induced NKCC2 inhibition in thick ascending limbs

In thick ascending limbs (THALs), nitric oxide (NO) decreases NaCl reabsorption via cGMP-mediated inhibition of Na-K-2Cl cotransporter (NKCC2). In angiotensin (ANG II)-induced hypertension, endothelin-1 (ET-1)-induced NO production by THALs is impaired. However, whether this alters NO's natriuretic effects and the mechanisms involved are unknown. In other cell types, ANG II augments phosphodiesterase 5 (PDE5)-mediated cGMP degradation. We hypothesized that NO-mediated inhibition of NKCC2 activity and stimulation of cGMP synthesis are blunted via PDE5 in ANG II-induced hypertension. Sprague-Dawley rats were infused with vehicle or ANG II (200 ng·kg-1·min-1) for 5 days. ET-1 reduced NKCC2 activity by 38 ± 13% (P < 0.05) in THALs from vehicle-treated rats but not from ANG II-hypertensive rats (Δ: -9 ± 13%). A NO donor yielded similar results as ET-1. In contrast, dibutyryl-cGMP significantly decreased NKCC2 activity in both vehicle-treated and ANG II-hypertensive rats (control: Δ-44 ± 15% vs.

ANG II

Δ-41 ± 10%). NO increased cGMP by 2.08 ± 0.36 fmol/μg protein in THALs from vehicle-treated rats but only 1.06 ± 0.25 fmol/μg protein in ANG II-hypertensive rats (P < 0.04). Vardenafil (25 nM), a PDE5 inhibitor, restored NO's ability to inhibit NKCC2 activity in THALs from ANG II-hypertensive rats (Δ: -60 ± 9%, P < 0.003). Similarly, NO's stimulation of cGMP was also restored by vardenafil (vehicle-treated: 1.89 ± 0.71 vs. ANG II-hypertensive: 2.02 ± 0.32 fmol/μg protein). PDE5 expression did not differ between vehicle-treated and ANG II-hypertensive rats. We conclude that NO-induced inhibition of NKCC2 and increases in cGMP are blunted in ANG II-hypertensive rats due to PDE5 activation. Defects in the response of THALs to NO may enhance NaCl retention in ANG II-induced hypertension.

Luteinizing Hormone Causes Phosphorylation and Activation of the cGMP Phosphodiesterase PDE5 in Rat Ovarian Follicles, Contributing, Together with PDE1 Activity, to the Resumption of Meiosis

The meiotic cell cycle of mammalian oocytes in preovulatory follicles is held in prophase arrest by diffusion of cGMP from the surrounding granulosa cells into the oocyte. Luteinizing hormone (LH) then releases meiotic arrest by lowering cGMP in the granulosa cells. The LH-induced reduction of cGMP is caused in part by a decrease in guanylyl cyclase activity, but the observation that the cGMP phosphodiesterase PDE5 is phosphorylated during LH signaling suggests that an increase in PDE5 activity could also contribute. To investigate this idea, we measured cGMP-hydrolytic activity in rat ovarian follicles. Basal activity was due primarily to PDE1A and PDE5, and LH increased PDE5 activity. The increase in PDE5 activity was accompanied by phosphorylation of PDE5 at serine 92, a protein kinase A/G consensus site. Both the phosphorylation and the increase in activity were promoted by elevating cAMP and opposed by inhibiting protein kinase A, supporting the hypothesis that LH activates PDE5 by stimulating its phosphorylation by protein kinase A. Inhibition of PDE5 activity partially suppressed LH-induced meiotic resumption as indicated by nuclear envelope breakdown, but inhibition of both PDE5 and PDE1 activities was needed to completely inhibit this response. These results show that activities of both PDE5 and PDE1 contribute to the LH-induced resumption of meiosis in rat oocytes, and that phosphorylation and activation of PDE5 is a regulatory mechanism.

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.

Computational Analysis of the Mechanism and Thermodynamics of Inhibition of Phosphodiesterase 5A by Synthetic Ligands

Phosphodiesterases are a large class of enzymes mediating a number of physiological processes ranging from immune response to platelet aggregation to cardiac and smooth muscle relaxation. In particular, phosphodiesterase 5 (PDE5) plays an important role in mediating sexual arousal, and it is the central molecular target in treatments of erectile dysfunction. In this study, we look at the mechanism and thermodynamics of the binding of selective inhibitors sildenafil (Viagra) and vardenafil (Levitra) to PDE5 using molecular dynamics simulations. Our simulations of PDE5 with and without sildenafil suggest a binding mechanism in which two loops surrounding the binding pocket of the enzyme (H loop, residues 660-683, and M loop, 787-812) execute sizable conformational changes (∼1 nm), clamping the ligand in the pocket. Also, we note significant changes in the coordination pattern of the divalent ions in the active site of the enzyme, as well as marked changes in the collective motions of the enzyme when the ligand is bound. Using the thermodynamic integration approach we calculate the relative free energies of binding of sildenafil, vardenafil, and demethyl-vardenafil, providing a test of the quality of the force field and the ligand parametrization used. Finally, using the single-step perturbation (SSP) technique, we calculate the relative binding free energies of these three ligands as well. In particular, we focus on critical evaluation of the SSP technique and examine the effects of computational parallelization on the efficiency of the technique. As a technical improvement, we demonstrate that an ensemble of relatively short SSP trajectories (10 × 0.5 ns) markedly outperforms a single trajectory of the same total length (1 × 5 ns) when it comes to sampling efficiency, resulting in significant real-time savings.

S-nitrosylation of PDE5 increases its ubiquitin-proteasomal degradation

Phosphodiesterase type 5 (PDE5) expression is upregulated in human failing heart, and overexpression of PDE5 in transgenic mice exacerbates stress-induced left-ventricular dysfunction, suggesting that increased PDE5 expression might contribute to the development of congestive heart failure. However, the underlying mechanisms for increased PDE5 expression are not totally understood. In the present study, we found that PDE5 activity and expression were regulated by S-nitrosylation, a covalent modification of cysteine residues by nitric oxide (NO). S-nitrosylation of PDE5 occurs at Cys220, which is located in the GAFA domain. Upon S-nitrosylation, PDE5 exhibits reduced activity and degradation via the ubiquitin-proteasome system. The decrease in PDE5 expression induced by NO could be blunted by mutation of Cys220 or the phosphorylation site of PDE5 (S102), as well as by pretreatment with H2O2. Conversely, decreased NO bioavailability by nitric oxide synthase (NOS) inhibitors or knockout of NOS3 increased PDE5 expression in cardiomyocytes. Collectively, to the best of our knowledge, our data demonstrate for the first time that S-nitrosylation is one of the mechanisms for PDE5 degradation. This novel regulatory mechanism probably accounts for the increase in PDE5 in the failing heart and other diseases in which NO bioavailability is decreased by oxidative stress.