Molecular cloning and subcellular distribution of the novel PDE4B4 cAMP-specific phosphodiesterase isoform

Phosphodiesterase-4 Inhibition in Parkinson's Disease: Molecular Insights and Therapeutic Potential

Clinicians and researchers are exploring safer and novel treatment strategies for treating the ever-prevalent Parkinson's disease (PD) across the globe. Several therapeutic strategies are used clinically for PD, including dopamine replacement therapy, DA agonists, MAO-B blockers, COMT blockers, and anticholinergics. Surgical interventions such as pallidotomy, particularly deep brain stimulation (DBS), are also employed. However, they only provide temporal and symptomatic relief. Cyclic adenosine monophosphate (cAMP) is one of the secondary messengers involved in dopaminergic neurotransmission. Phosphodiesterase (PDE) regulates cAMP and cGMP intracellular levels. PDE enzymes are subdivided into families and subtypes which are expressed throughout the human body. PDE4 isoenzyme- PDE4B subtype is overexpressed in the substantia nigra of the brain. Various studies have implicated multiple cAMP-mediated signaling cascades in PD, and PDE4 is a common link that can emerge as a neuroprotective and/or disease-modifying target. Furthermore, a mechanistic understanding of the PDE4 subtypes has provided perceptivity into the molecular mechanisms underlying the adverse effects of phosphodiesterase-4 inhibitors (PDE4Is). The repositioning and development of efficacious PDE4Is for PD have gained much attention. This review critically assesses the existing literature on PDE4 and its expression. Specifically, this review provides insights into the interrelated neurological cAMP-mediated signaling cascades involving PDE4s and the potential role of PDE4Is in PD. In addition, we discuss existing challenges and possible strategies for overcoming them.

Recent advances of Phosphodiesterase 4B in cancer

INTRODUCTION

Phosphodiesterase 4B (PDE4B) is a crucial enzyme in the phosphodiesterases (PDEs), acting as a regulator of cyclic adenosine monophosphate (cAMP). It is involved in cancer process through PDE4B/cAMP signaling pathway. Cancer occurs and develops with the regulation of PDE4B in the body, suggesting that PDE4B is a promising therapeutic target.

AREAS COVERED

This review covereed the function and mechanism of PDE4B in cancer. We summarized the possible clinical applications of PDE4B, and highlighted the possible ways to develop clinical applications of PDE4B inhibitors. We also discussed some common PDEs inhibitors, and expected the development of combined targeting PDE4B and other PDEs drugs in the future.

EXPERT OPINION

The existing research and clinical data can strongly prove the role of PDE4B in cancer. PDE4B inhibition can effectively increase cell apoptosis, inhibit cell proliferation, transformation, migration, etc., indicating that PDE4B inhibition can effectively inhibit the development of cancer. Other PDEs may antagonize or coordinate this effect. As for the further study on the relationship between PDE4B and other PDEs in cancer, it is still a challenge to develop multi-targeted PDEs inhibitors.

Phosphodiesterase 4B inhibition: a potential novel strategy for treating pulmonary fibrosis

Patients with interstitial lung disease can develop a progressive fibrosing phenotype characterised by an irreversible, progressive decline in lung function despite treatment. Current therapies slow, but do not reverse or stop, disease progression and are associated with side-effects that can cause treatment delay or discontinuation. Most crucially, mortality remains high. There is an unmet need for more efficacious and better-tolerated and -targeted treatments for pulmonary fibrosis. Pan-phosphodiesterase 4 (PDE4) inhibitors have been investigated in respiratory conditions. However, the use of oral inhibitors can be complicated due to class-related systemic adverse events, including diarrhoea and headaches. The PDE4B subtype, which has an important role in inflammation and fibrosis, has been identified in the lungs. Preferentially targeting PDE4B has the potential to drive anti-inflammatory and antifibrotic effects via a subsequent increase in cAMP, but with improved tolerability. Phase I and II trials of a novel PDE4B inhibitor in patients with idiopathic pulmonary fibrosis have shown promising results, stabilising pulmonary function measured by change in forced vital capacity from baseline, while maintaining an acceptable safety profile. Further research into the efficacy and safety of PDE4B inhibitors in larger patient populations and for a longer treatment period is needed.

Clinical Implication of Phosphodiesterase-4-Inhibition

The pleiotropic function of 3′,5′-cyclic adenosine monophosphate (cAMP)-dependent pathways in health and disease led to the development of pharmacological phosphodiesterase inhibitors (PDE-I) to attenuate cAMP degradation. While there are many isotypes of PDE, a predominant role of PDE4 is to regulate fundamental functions, including endothelial and epithelial barrier stability, modulation of inflammatory responses and cognitive and/or mood functions. This makes the use of PDE4-I an interesting tool for various therapeutic approaches. However, due to the presence of PDE4 in many tissues, there is a significant danger for serious side effects. Based on this, the aim of this review is to provide a comprehensive overview of the approaches and effects of PDE4-I for different therapeutic applications. In summary, despite many obstacles to use of PDE4-I for different therapeutic approaches, the current data warrant future research to utilize the therapeutic potential of phosphodiesterase 4 inhibition.

The Molecular Biology of Phosphodiesterase 4 Enzymes as Pharmacological Targets: An Interplay of Isoforms, Conformational States, and Inhibitors

The phosphodiesterase 4 (PDE4) enzyme family plays a pivotal role in regulating levels of the second messenger cAMP. Consequently, PDE4 inhibitors have been investigated as a therapeutic strategy to enhance cAMP signaling in a broad range of diseases, including several types of cancers, as well as in various neurologic, dermatological, and inflammatory diseases. Despite their widespread therapeutic potential, the progression of PDE4 inhibitors into the clinic has been hampered because of their related relatively small therapeutic window, which increases the chance of producing adverse side effects. Interestingly, the PDE4 enzyme family consists of several subtypes and isoforms that can be modified post-translationally or can engage in specific protein-protein interactions to yield a variety of conformational states. Inhibition of specific PDE4 subtypes, isoforms, or conformational states may lead to more precise effects and hence improve the safety profile of PDE4 inhibition. In this review, we provide an overview of the variety of PDE4 isoforms and how their activity and inhibition is influenced by post-translational modifications and interactions with partner proteins. Furthermore, we describe the importance of screening potential PDE4 inhibitors in view of different PDE4 subtypes, isoforms, and conformational states rather than testing compounds directed toward a specific PDE4 catalytic domain. Lastly, potential mechanisms underlying PDE4-mediated adverse effects are outlined. In this review, we illustrate that PDE4 inhibitors retain their therapeutic potential in myriad diseases, but target identification should be more precise to establish selective inhibition of disease-affected PDE4 isoforms while avoiding isoforms involved in adverse effects. SIGNIFICANCE STATEMENT: Although the PDE4 enzyme family is a therapeutic target in an extensive range of disorders, clinical use of PDE4 inhibitors has been hindered because of the adverse side effects. This review elaborately shows that safer and more effective PDE4 targeting is possible by characterizing 1) which PDE4 subtypes and isoforms exist, 2) how PDE4 isoforms can adopt specific conformations upon post-translational modifications and protein-protein interactions, and 3) which PDE4 inhibitors can selectively bind specific PDE4 subtypes, isoforms, and/or conformations.

Dominant-Negative Attenuation of cAMP-Selective Phosphodiesterase PDE4D Action Affects Learning and Behavior

PDE4 cyclic nucleotide phosphodiesterases reduce 3′, 5′ cAMP levels in the CNS and thereby regulate PKA activity and the phosphorylation of CREB, fundamental to depression, cognition, and learning and memory. The PDE4 isoform PDE4D5 interacts with the signaling proteins β-arrestin2 and RACK1, regulators of β₂-adrenergic and other signal transduction pathways. Mutations in PDE4D in humans predispose to acrodysostosis, associated with cognitive and behavioral deficits. To target PDE4D5, we developed mice that express a PDE4D5-D556A dominant-negative transgene in the brain. Male transgenic mice demonstrated significant deficits in hippocampus-dependent spatial learning, as assayed in the Morris water maze. In contrast, associative learning, as assayed in a fear conditioning assay, appeared to be unaffected. Male transgenic mice showed augmented activity in prolonged (2 h) open field testing, while female transgenic mice showed reduced activity in the same assay. Transgenic mice showed no demonstrable abnormalities in prepulse inhibition. There was also no detectable difference in anxiety-like behavior, as measured in the elevated plus-maze. These data support the use of a dominant-negative approach to the study of PDE4D5 function in the CNS and specifically in learning and memory.

Phosphodiesterase 4B: Master Regulator of Brain Signaling

Phosphodiesterases (PDEs) are the only superfamily of enzymes that have the ability to break down cyclic nucleotides and, as such, they have a pivotal role in neurological disease and brain development. PDEs have a modular structure that allows targeting of individual isoforms to discrete brain locations and it is often the location of a PDE that shapes its cellular function. Many of the eleven different families of PDEs have been associated with specific diseases. However, we evaluate the evidence, which suggests the activity from a sub-family of the PDE4 family, namely PDE4B, underpins a range of important functions in the brain that positions the PDE4B enzymes as a therapeutic target for a diverse collection of indications, such as, schizophrenia, neuroinflammation, and cognitive function.

Phosphodiesterases as therapeutic targets for respiratory diseases

Chronic respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and asthma, affect millions of people all over the world. Cyclic adenosine monophosphate (cAMP) which is one of the most important second messengers, plays a vital role in relaxing airway smooth muscles and suppressing inflammation. Given its vast role in regulating intracellular responses, cAMP provides an attractive pharmaceutical target in the treatment of chronic respiratory diseases. Phosphodiesterases (PDEs) are enzymes that hydrolyze cyclic nucleotides and help control cyclic nucleotide signals in a compartmentalized manner. Currently, the selective PDE4 inhibitor, roflumilast, is used as an add-on treatment for patients with severe COPD associated with bronchitis and a history of frequent exacerbations. In addition, other novel PDE inhibitors are in different phases of clinical trials. The current review provides an overview of the regulation of various PDEs and the potential application of selective PDE inhibitors in the treatment of COPD and asthma. The possibility to combine various PDE inhibitors as a way to increase their therapeutic effectiveness is also emphasized.

The Past, Present, and Future of Phosphodiesterase-4 Modulation for Age-Induced Memory Loss

The purpose of this chapter is to highlight the state of progress for phosphodiesterase-4 (PDE4) modulation as a potential therapeutic for psychiatric illness, and to draw attention to particular hurdles and obstacles that must be overcome in future studies to develop PDE4-mediated therapeutics. Pathological and non-pathological related memory loss will be the focus of the chapter; however, we will at times also touch upon other psychiatric illnesses like anxiety and depression. First, we will provide a brief background of PDE4, and the rationale for its extensive study in cognition. Second, we will explore fundamental differences in individual PDE4 subtypes, and then begin to address differences between pathological and non-pathological aging. Alterations of cAMP/PDE4 signaling that occur within normal vs. pathological aging, and the potential for PDE4 modulation to combat these alterations within each context will be described. Finally, we will finish the chapter with obstacles that have hindered the field, and future studies and alternative viewpoints that need to be addressed. Overall, we hope this chapter will demonstrate the incredible complexity of PDE4 signaling in the brain, and will be useful in forming a strategy to develop future PDE4-mediated therapeutics for psychiatric illnesses.

Altered phosphorylation, electrophysiology, and behavior on attenuation of PDE4B action in hippocampus

Background

PDE4 cyclic nucleotide phosphodiesterases regulate 3′, 5′ cAMP abundance in the CNS and thereby regulate PKA activity and phosphorylation of CREB, which has been implicated in learning and memory, depression and other functions. The PDE4 isoform PDE4B1 also interacts with the DISC1 protein, implicated in neural development and behavioral disorders. The cellular functions of PDE4B1 have been investigated extensively, but its function(s) in the intact organism remained unexplored.

Results

To specifically disrupt PDE4B1, we developed mice that express a PDE4B1-D564A transgene in the hippocampus and forebrain. The transgenic mice showed enhanced phosphorylation of CREB and ERK1/2 in hippocampus. Hippocampal neurogenesis was increased in the transgenic mice. Hippocampal electrophysiological studies showed increased baseline synaptic transmission and enhanced LTP in male transgenic mice. Behaviorally, male transgenic mice showed increased activity in prolonged open field testing, but neither male nor female transgenic mice showed detectable anxiety-like behavior or antidepressant effects in the elevated plus-maze, tail-suspension or forced-swim tests. Neither sex showed any significant differences in associative fear conditioning or showed any demonstrable abnormalities in pre-pulse inhibition.

Conclusions

These data support the use of an isoform-selective approach to the study of PDE4B1 function in the CNS and suggest a probable role of PDE4B1 in synaptic plasticity and behavior. They also provide additional rationale and a refined approach to the development of small-molecule PDE4B1-selective inhibitors, which have potential functions in disorders of cognition, memory, mood and affect.

The PDE4 cAMP-Specific Phosphodiesterases: Targets for Drugs with Antidepressant and Memory-Enhancing Action

The PDE4 cyclic nucleotide phosphodiesterases are essential regulators of cAMP abundance in the CNS through their ability to regulate PKA activity, the phosphorylation of CREB, and other important elements of signal transduction. In pre-clinical models and in early-stage clinical trials, PDE4 inhibitors have been shown to have antidepressant and memory-enhancing activity. However, the development of clinically-useful PDE4 inhibitors for CNS disorders has been limited by variable efficacy and significant side effects. Recent structural studies have greatly enhanced our understanding of the molecular configuration of PDE4 enzymes, especially the "long" PDE4 isoforms that are abundant in the CNS. The new structural data provide a rationale for the development of a new generation of PDE4 inhibitors that specifically act on long PDE4 isoforms. These next generation PDE4 inhibitors may also be capable of targeting the interactions of select long forms with their "partner" proteins, such as RACK1, β-arrestin, and DISC1. They would therefore have the ability to affect cAMP levels in specific cellular compartments and target localized cellular functions, such as synaptic plasticity. These new agents might also be able to target PDE4 populations in select regions of the CNS that are implicated in learning and memory, affect, and cognition. Potential therapeutic uses of these agents could include affective disorders, memory enhancement, and neurogenesis.

PDE4B as a microglia target to reduce neuroinflammation

The importance of microglia in immune homeostasis within the brain is undisputed. Their role in a diversity of neurological and psychiatric diseases as well as CNS injury is the subject of much investigation. Cyclic adenosine monophosphate (AMP) is a critical regulator of microglia homeostasis; as the predominant negative modulator of cyclic AMP signaling within microglia, phosphodiesterase 4 (PDE4) represents a promising target for modulating immune function. PDE4 expression is regulated by inflammation, and in turn, PDE4 inhibition can alter microglia reactivity. As the prototypic PDE4 inhibitor, rolipram, was tested clinically in the 1980s, drug discovery and clinical development of PDE4 inhibitors have been severely hampered by tolerability issues involving nausea and emesis. The two PDE4 inhibitors approved for peripheral inflammatory disorders (roflumilast and apremilast) lack brain penetration and are dose-limited by side effects making them unsuitable for modulating microglial function. Subtype selective inhibitors targeting PDE4B are of high interest given the critical role PDE4B plays in immune function versus the association of PDE4D with nausea and emesis. The challenges and requirements for successful development of a novel brain-penetrant PDE4B inhibitor are discussed in the context of early clinical development strategies. Furthermore, the challenges of monitoring the state of microglia in vivo are highlighted, including a description of the currently available tools and their limitations. Continued drug discovery efforts to identify safe and well-tolerated, brain-penetrant PDE4 inhibitors are a reflection of the confidence in the rationale for modulation of this target to produce meaningful therapeutic benefit in a wide range of neurological conditions and injury. GLIA 2016;64:1698-1709.

Specific Inhibition of Phosphodiesterase-4B Results in Anxiolysis and Facilitates Memory Acquisition

Cognitive dysfunction is a core feature of dementia and a prominent feature in psychiatric disease. As non-redundant regulators of intracellular cAMP gradients, phosphodiesterases (PDE) mediate fundamental aspects of brain function relevant to learning, memory, and higher cognitive functions. Phosphodiesterase-4B (PDE4B) is an important phosphodiesterase in the hippocampal formation, is a major Disrupted in Schizophrenia 1 (DISC1) binding partner and is itself a risk gene for psychiatric illness. To define the effects of specific inhibition of the PDE4B subtype, we generated mice with a catalytic domain mutant form of PDE4B (Y358C) that has decreased ability to hydrolyze cAMP. Structural modeling predictions of decreased function and impaired binding with DISC1 were confirmed in cell assays. Phenotypic characterization of the PDE4B(Y358C) mice revealed facilitated phosphorylation of CREB, decreased binding to DISC1, and upregulation of DISC1 and β-Arrestin in hippocampus and amygdala. In behavioral assays, PDE4B(Y358C) mice displayed decreased anxiety and increased exploration, as well as cognitive enhancement across several tests of learning and memory, consistent with synaptic changes including enhanced long-term potentiation and impaired depotentiation ex vivo. PDE4B(Y358C) mice also demonstrated enhanced neurogenesis. Contextual fear memory, though intact at 24 h, was decreased at 7 days in PDE4B(Y358C) mice, an effect replicated pharmacologically with a non-selective PDE4 inhibitor, implicating cAMP signaling by PDE4B in a very late phase of consolidation. No effect of the PDE4B(Y358C) mutation was observed in the prepulse inhibition and forced swim tests. Our data establish specific inhibition of PDE4B as a promising therapeutic approach for disorders of cognition and anxiety, and a putative target for pathological fear memory.

Novel, primate-specific PDE10A isoform highlights gene expression complexity in human striatum with implications on the molecular pathology of bipolar disorder

Bipolar disorder is a highly heritable neuropsychiatric disorder affecting nearly 2.5% of the population. Prior genetic studies identified a panel of common and rare single-nucleotide polymorphisms associated with the disease that map to the first intron of the PDE10A gene. RNA sequencing of striatal brain tissue from bipolar and healthy control subjects identified a novel transcript of PDE10A, named PDE10A19, that codes for a PDE10A isoform with a unique N terminus. Genomic sequences that can encode the novel N terminus were conserved in other primates but not rodents. The RNA transcript was expressed at equal or greater levels in the human striatum compared with the two annotated transcripts, PDE10A1 and PDE10A2. The PDE10A19 transcript was detected in polysomal fractions; western blotting experiments confirmed that the RNA transcript is translated into protein. Immunocytochemistry studies using transfected mouse striatal and cortical neurons demonstrated that the PDE10A19 protein distributes to the cytosol, like PDE10A1, and unlike PDE10A2, which is associated with plasma membranes. Immunoprecipitation and immunocytochemical experiments revealed that the PDE10A19 isoform interacts physically with PDE10A2 and, when expressed at elevated levels, interferes with the plasma membrane localization of PDE10A2. These studies illustrate the complexity of PDE10A gene expression in the human brain and highlight the need to unravel the gene's complex and complete coding capabilities along with its transcriptional and translational regulation to guide the development of therapeutic agents that target the protein for the treatment of neuropsychiatric illness.

Cross-talk between PKA-Cβ and p65 mediates synergistic induction of PDE4B by roflumilast and NTHi

Phosphodiesterase 4B (PDE4B) plays a key role in regulating inflammation. Roflumilast, a phosphodiesterase (PDE)4-selective inhibitor, has recently been approved for treating severe chronic obstructive pulmonary disease (COPD) patients with exacerbation. However, there is also clinical evidence suggesting the development of tachyphylaxis or tolerance on repeated dosing of roflumilast and the possible contribution of PDE4B up-regulation, which could be counterproductive for suppressing inflammation. Thus, understanding how PDE4B is up-regulated in the context of the complex pathogenesis and medications of COPD may help improve the efficacy and possibly ameliorate the tolerance of roflumilast. Here we show that roflumilast synergizes with nontypeable Haemophilus influenzae (NTHi), a major bacterial cause of COPD exacerbation, to up-regulate PDE4B2 expression in human airway epithelial cells in vitro and in vivo. Up-regulated PDE4B2 contributes to the induction of certain important chemokines in both enzymatic activity-dependent and activity-independent manners. We also found that protein kinase A catalytic subunit β (PKA-Cβ) and nuclear factor-κB (NF-κB) p65 subunit were required for the synergistic induction of PDE4B2. PKA-Cβ phosphorylates p65 in a cAMP-dependent manner. Moreover, Ser276 of p65 is critical for mediating the PKA-Cβ-induced p65 phosphorylation and the synergistic induction of PDE4B2. Collectively, our data unveil a previously unidentified mechanism underlying synergistic up-regulation of PDE4B2 via a cross-talk between PKA-Cβ and p65 and may help develop new therapeutic strategies to improve the efficacy of PDE4 inhibitor.

Mice deficient in phosphodiesterase-4A display anxiogenic-like behavior

RATIONALE

Phosphodiesterases (PDEs) are a super family of enzymes responsible for the halting of intracellular cyclic nucleotide signaling and may represent novel therapeutic targets for treatment of cognitive disorders. PDE4 is of considerable interest to cognitive research because it is highly expressed in the brain, particularly in the cognition-related brain regions. Recently, the functional role of PDE4B and PDE4D, two of the four PDE4 subtypes (PDE4A, B, C, and D), in behavior has begun to be identified; however, the role of PDE4A in the regulation of behavior is still unknown.

OBJECTIVES

The purpose of this study was to characterize the functional role of PDE4A in behavior.

METHODS

The role of PDE4A in behavior was evaluated through a battery of behavioral tests using PDE4A knockout (KO) mice; urine corticosterone levels were also measured.

RESULTS

PDE4A KO mice exhibited improved memory in the step-through-passive-avoidance test. They also displayed anxiogenic-like behavior in elevated-plus maze, holeboard, light-dark transition, and novelty suppressed feeding tests. Consistent with the anxiety profile, PDE4A KO mice had elevated corticosterone levels compared with wild-type controls post-stress. Interestingly, PDE4A KO mice displayed no change in object recognition, Morris water maze, forced swim, tail suspension, and duration of anesthesia induced by co-administration of xylazine and ketamine (suggesting that PDE4A KO may not be emetic).

CONCLUSIONS

These results suggest that PDE4A may be important in the regulation of emotional memory and anxiety-like behavior, but not emesis. PDE4A could possibly represent a novel therapeutic target in the future for anxiety or disorders affecting memory.

Functional Proteomic Profiling of Phosphodiesterases Using SeraFILE Separations Platform

Functional proteomic profiling can help identify targets for disease diagnosis and therapy. Available methods are limited by the inability to profile many functional properties measured by enzymes kinetics. The functional proteomic profiling approach proposed here seeks to overcome such limitations. It begins with surface-based proteome separations of tissue/cell-line extracts, using SeraFILE, a proprietary protein separations platform. Enzyme kinetic properties of resulting subproteomes are then characterized, and the data integrated into proteomic profiles. As a model, SeraFILE-derived subproteomes of cyclic nucleotide-hydrolyzing phosphodiesterases (PDEs) from bovine brain homogenate (BBH) and rat brain homogenate (RBH) were characterized for cAMP hydrolysis activity in the presence (challenge condition) and absence of cGMP. Functional profiles of RBH and BBH were compiled from the enzyme activity response to the challenge condition in each of the respective subproteomes. Intersample analysis showed that comparable profiles differed in only a few data points, and that distinctive subproteomes can be generated from comparable tissue samples from different animals. These results demonstrate that the proposed methods provide a means to simplify intersample differences, and to localize proteins attributable to sample-specific responses. It can be potentially applied for disease and nondisease sample comparison in biomarker discovery and drug discovery profiling.

β-Adrenergic receptor stimulation increases surface NKCC2 expression in rat thick ascending limbs in a process inhibited by phosphodiesterase 4

The thick ascending limb of the loop of Henle (THAL) reabsorbs ∼30% of the filtered NaCl in a process mediated by the apical Na-K-2Cl cotransporter NKCC2. Stimulation of β-adrenergic receptors in the THAL enhances NaCl reabsorption and increases intracellular cAMP. We found that intracellular cAMP stimulates NKCC2 trafficking to the apical membrane via protein kinase A (PKA). Several cAMP-specific phosphodiesterases (PDE) have been identified in rat THALs, and PDE4 decreases cAMP generated by β-adrenergic stimulation in other cells. However, it is not known whether β-adrenergic receptors activation stimulates NKCC2 trafficking. Thus we hypothesized that β-adrenergic receptor stimulation enhances THAL apical membrane NKCC2 expression via the PKA pathway and PDE4 blunts this effect. THAL suspensions were obtained from Sprague-Dawley rats, and surface NKCC2 expression was measured by surface biotinylation and Western blot. Incubation of THALs with the β-adrenergic receptor agonist isoproterenol at 0.5 and 1.0 μM increased surface NKCC2 by 17 ± 1 and 29 ± 5% respectively (P < 0.05). Preventing cAMP degradation with 3-isobutyl-methylxanthine (IBMX; a nonselective phosphodiesterase inhibitor) enhanced isoproterenol-stimulated surface NKCC2 expression to 51 ± 7% (P < 0.05 vs. isoproterenol). The β-adrenergic receptor antagonist propranolol or the PKA inhibitor H-89 completely blocked isoproterenol + IBMX-induced increase on surface NKCC2, while propranolol or H-89 alone had no effect. Selective inhibition of PDE4 with rolipram (20 μM) potentiated the effect of isoproterenol on surface NKCC2 and increased cAMP levels. We concluded that β-adrenergic receptor stimulation enhances surface NKCC2 expression in the THALs via PKA and PDE4 blunts this effect.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

Effects of progesterone treatment on expression of genes involved in uterine quiescence

An important action of progesterone during pregnancy is to maintain the uterus in a quiescent state and thereby prevent preterm labor. The causes of preterm labor are not well understood, so progesterone action on the myometrium can provide clues about the processes that keep the uterus from contracting prematurely. Accordingly, we have carried out Affymetrix GeneChip analysis of progesterone effects on gene expression in immortalized human myometrial cells cultured from a patient near the end of pregnancy. Progesterone appears to inhibit uterine excitability by a number of mechanisms, including increased expression of calcium and voltage-operated K(+) channels, which dampens the electrical activity of the myometrial cell, downregulation of agents, and receptors involved in myometrial contraction, reduction in cell signal components that lead to increased intracellular Ca(2+) concentrations in response to contractile stimuli, and downregulation of proteins involved in the cross-linking of actin and myosin filaments to produce uterine contractions.

ERK and PDE4 cooperate to induce RAF isoform switching in melanoma

Melanocytes use BRAF to activate the MAP kinase (MAPK) pathway because CRAF is inhibited by the cyclic AMP (cAMP) pathway in these cells. By contrast, melanomas harboring Ras mutations use CRAF to activate the MAPK pathway. We describe the molecular mechanism of Raf isoform switching and cAMP pathway disruption, which take place during melanocyte transformation. We show that overactivation of the MAPK pathway, induced by the oncogenic Ras in melanoma, induces constitutive phosphorylation of BRAF on Ser151 by ERK, which inhibits NRAS-BRAF interaction . We also demonstrate that melanoma cells have elevated cAMP phosphodiesterase activity owing to overexpression of the cAMP-specific phosphodiesterase-4 enzymes; this activity inhibits cAMP signaling and allows CRAF reactivation in these cells. Reactivating the cAMP pathway inhibits proliferation and induces apoptosis of Ras-mutated melanoma cells, suggesting a new therapeutic approach for treating melanomas harboring Ras mutations.

S-adenosylmethionine decreases lipopolysaccharide-induced phosphodiesterase 4B2 and attenuates tumor necrosis factor expression via cAMP/protein kinase A pathway

S-Adenosylmethionine (SAM) treatment has anti-inflammatory, cytoprotective effects against endotoxin-induced organ injury. An important component of the anti-inflammatory action of SAM involves down-regulation of the lipopolysaccharide (LPS)-induced transcriptional induction of tumor necrosis factor-α (TNF) expression by monocytes/macrophages. We examined the effect of SAM on expression and activity of LPS-induced up-regulation of phosphodiesterase 4 (PDE4), which regulates cellular cAMP levels and TNF expression. LPS treatment of RAW 264.7, a mouse macrophage cell line, led to the induction of Pde4b2 mRNA expression with no effect on Pde4a or Pde4d. SAM pretreatment led to a significant decrease in LPS-induced up-regulation of Pde4b2 expression in both RAW 264.7 cells and primary human CD14(+) monocytes. Of note, the decreased Pde4b2 mRNA expression correlated with the SAM-dependent increase in the transcriptionally repressive histone H3 lysine 9 trimethylation on the Pde4b2 intronic promoter region. The SAM-mediated decrease in LPS-inducible Pde4b2 up-regulation resulted in an increase in cellular cAMP levels and activation of cAMP-dependent protein kinase A (PKA), which plays an inhibitory role in LPS-induced TNF production. In addition, SAM did not affect LPS-inducible inhibitor of nuclear factor-κB degradation or nuclear factor-κB (NF-κB)-p65 translocation into the nucleus but rather inhibited NF-κB transcriptional activity. These results demonstrate for the first time that inhibition of LPS-induced PDE4B2 up-regulation and increased cAMP-dependent PKA activation are significant mechanisms contributing to the anti-TNF effect of SAM. Moreover, these data also suggest that SAM may be used as an effective PDE4B inhibitor in the treatment of chronic inflammatory disorders in which TNF expression plays a significant pathogenic role.

Phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes

Cyclic AMP is part of an endogenous mechanism that downregulates inflammatory response, and its intracellular concentration is regulated chiefly by cyclic nucleotide phosphodiesterases type 4. The goal of the present study was to determine whether phosphodiesterases 4 are involved in the inflammatory response of astrocytes mediated by Toll-like receptors. Astrocyte cultures established from newborn rat brain were challenged with lipoteichoic acid, a ligand of Toll-like receptor 2, polyinosinic-polycytidylic acid, a ligand of Toll-like receptor 3, or lipopolysaccharide, a ligand of Toll-like receptor 4. After 24 h the expression of genes encoding phosphodiesterase 4A, phosphodiesterase 4B and phosphodiesterase 4D was determined by real time reverse transcription polymerase chain reaction. The challenge of astrocytes with the ligands profoundly up-regulated expression of the phosphodiesterase 4B mRNA, while the phosphodiesterase 4A and 4D mRNA was either unaffected or downregulated. Moreover, Toll-like receptor ligation specifically up-regulated expression of the phosphodiesterase 4B2 transcriptional variant. Thus, polyinosinic-polycytidylic acid, lipopolysaccharide and lipoteichoic acid induced approximately 7-, 5- and 4-fold up-regulation of the message, respectively. Toll-like receptor ligation also led to an over 2-fold increase in the protein level of phosphodiesterase 4B2 as revealed by immunoblot analysis. The inactivation of Rho proteins by pretreatment with toxin B form C. difficile enhanced ligation-induced up-regulation of the phosphodiesterase 4B2 message by 4-9-fold. However, in spite of this increase in the message abundance, there was no increase in the protein level compared to cells challenged with the ligands alone. These results demonstrate that the phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes, and that its up-regulation at the protein level is controlled by complex mechanisms.

Ndel1 alters its conformation by sequestering cAMP-specific phosphodiesterase-4D3 (PDE4D3) in a manner that is dynamically regulated through Protein Kinase A (PKA)

The involvement of the Nuclear distribution element-like (Ndel1; Nudel) protein in the recruitment of the dynein complex is critical for neurodevelopment and potentially important for neuronal disease states. The PDE4 family of phosphodiesterases specifically degrades cAMP, an important second messenger implicated in learning and memory functions. Here we show for the first time that Ndel1 can interact directly with PDE4 family members and that the interaction of Ndel1 with the PDE4D3 isoform is uniquely disrupted by elevation of intracellular cAMP levels. While all long PDE4 isoforms are subject to stimulatory PKA phosphorylation within their conserved regulatory UCR1 domain, specificity for release of PDE4D3 is conferred due to the PKA-dependent phosphorylation of Ser13 within the isoform-specific, unique amino-terminal domain of PDE4D3. Scanning peptide array analyses identify a common region on Ndel1 for PDE4 binding and an additional region that is unique to PDE4D3. The common site lies within the stutter region that links the second coiled-coil region to the unstable third coiled-coil regions of Ndel1. The additional binding region unique to PDE4D3 penetrates into the start of the third coiled-coil region that can undergo tail-to-tail interactions between Ndel1 dimers to form a 4 helix bundle. We demonstrate Ndel1 self-interaction in living cells using a BRET approach with luciferase- and GFP-tagged forms of Ndel1. BRET assessed Ndel1-Ndel1 self-interaction is amplified through the binding of PDE4 isoforms. For PDE4D3 this effect is ablated upon elevation of intracellular cAMP due to PKA-mediated phosphorylation at Ser13, while the potentiating effects of PDE4B1 and PDE4D5 are resistant to cAMP elevation. PDE4D long isoforms and Ndel1 show a similar sub-cellular distribution in hippocampus and cortex and locate to post-synaptic densities. We show that Ndel1 sequesters EPAC, but not PKA, in order to form a cAMP signalling complex. We propose that a key function of the Ndel1 signalling scaffold is to signal through cAMP by sequestering EPAC, whose activity may thus be specifically regulated by sequestered PDE4 that also stabilizes Ndel1-Ndel1 self-interaction. In the case of PDE4D3, its association with Ndel1 is dynamically regulated by PKA input through its ability to phosphorylate Ser13 in the unique N-terminal region of this isoform, triggering the specific release of PDE4D3 from Ndel1 when cAMP levels are elevated. We propose that Ser13 may act as a redistribution trigger in PDE4D3, allowing it to dynamically re-shape cAMP gradients in distinct intracellular locales upon its phosphorylation by PKA.

Identification and molecular characterization of a novel PDE4D11 cAMP-specific phosphodiesterase isoform

Here we report the cloning and characterization of a novel PDE4D isoform (PDE4D11) identified in mouse brain cDNA. This novel isoform has a unique isoform-specific 5'-UTR and N-terminal sequence, whereas, downstream regulatory N-terminal and catalytic C-terminal regions are homologous to other long PDE4D isoforms (Ex2-15). In silico analysis of PDE4D11 cDNA transcript identified the predicted translational start site and the use of a different transcriptional start site compared to other PDE4D isoforms. This isoform is ubiquitously expressed in different mouse tissues, particularly in the brain, liver and spleen. In the brain, PDE4D11 expression levels increased in the cerebellum, but decreased in the hippocampus with progressive age, highlighting a potential role for this isoform in the development of the brain. When transfected in vitro into murine neuroblastoma cells PDE4D11_EGFP expression is cytosolic, consistent with other long PDE4D isoforms. The appearance of cytosolic protein aggregates in discrete microdomains with this isoform, however, may represent a method of compartmentalizing PDE4D11 activity. The novel 5'-sequence of PDE4D11 is conserved among higher vertebrates including human, monkey, dog, horse and rat. Identification of this new isoform highlights the mutliplicity of unique PDE4D isoforms and their potential importance in regulating cAMP levels through compartmentalization and cell-specific expression and underscores the importance of understanding the functional role of each isoform in the development of specific drugs for the treatment of memory disorders.

Minimizing a QTL region for intramuscular fat content by characterizing the porcine Phosphodiesterase 4B (PDE4B) gene

Three isoforms of pig PDE4B were cloned and classified as two forms: PDE4B1 and PDE4B3, which contain UCR1 and UCR2; and PDE4B2, which contains only UCR2. The amino acid sequences of each isoform showed good conservation in human and rat. PDE4B2 is expressed in a wide range of tissues, but PDE4B1 and PDE4B3 are not. Using an informative SNP for the Iberian x Landrace intercross detected from intron 12, a linkage map was constructed. The location of PDE4B was estimated at 123.6 cM outside of the QTL-CI (124-128 cM) for IMF. However, the QTL-CI for IMF was reconfirmed with high significance, and its position was narrowed down to an interval of 4 cM (the region defined by markers PDE4B and SW1881). Using radiation hybrid mapping, LEPR, LEPROT, DNAJC6, AK3L1 and AK3L2 were selected as positional and/or functional candidates related to the QTL.

Production and characterization of pharmacologically active recombinant human phosphodiesterase 4B in Dictyostelium discoideum

Phosphodiesterase 4B (PDE4B) is an important therapeutic target for asthma and chronic obstructive pulmonary disease. To identify PDE4 subtype-specific compounds using high-throughput assays, full-length recombinant PDE4 proteins are needed in bulk quantity. In the present study, full-length human PDE4B2 was expressed in the cellular slime mould Dictyostelium discoideum (Dd). A cell density of 2 x 10(7) cells/mL was obtained and up to 1 mg/L recombinant PDE4B2 was purified through Ni-NTA affinity chromatography. The expressed protein was soluble and its activity was comparable to PDE4B2 protein expressed in mammalian cells (K(m)=1.7 microM). The functional significance of the Dd expression system is supported by the demonstration that, in concert with proteins expressed in mammalian systems, there are no major changes in the affinity for PDE4B2 inhibitors and substrates. These findings thus provide the first evidence that Dd can be utilized for the expression and purification of functionally active full-length human PDE4B2 in large amounts required for high-throughput screening of pharmacologically active compounds against this therapeutic target.

Anxiogenic-like behavioral phenotype of mice deficient in phosphodiesterase 4B (PDE4B)

Phosphodiesterase-4 (PDE4), an enzyme that catalyzes the hydrolysis of cyclic AMP and plays a critical role in controlling its intracellular concentration, has been implicated in depression- and anxiety-like behaviors. However, the functions of the four PDE4 subfamilies (PDE4A, PDE4B, PDE4C, and PDE4D) remain largely unknown. In animal tests sensitive to anxiolytics, antidepressants, memory enhancers, or analgesics, we examined the behavioral phenotype of mice deficient in PDE4B (PDE4B-/-). Immunoblot analysis revealed loss of PDE4B expression in the cerebral cortex and amygdala of PDE4B-/- mice. The reduction of PDE4B expression was accompanied by decreases in PDE4 activity in the brain regions of PDE4B-/- mice. Compared to PDE4B+/+ littermates, PDE4B-/- mice displayed anxiogenic-like behavior, as evidenced by decreased head-dips and time spent in head-dipping in the holeboard test, reduced transitions and time on the light side in the light-dark transition test, and decreased initial exploration and rears in the open-field test. Consistent with anxiogenic-like behavior, PDE4B-/- mice displayed increased levels of plasma corticosterone. In addition, these mice also showed a modest increase in the proliferation of neuronal cells in the hippocampal dentate gyrus. In the forced-swim test, PDE4B-/- mice exhibited decreased immobility; however, this was not supported by the results from the tail-suspension test. PDE4B-/- mice did not display changes in memory, locomotor activity, or nociceptive responses. Taken together, these results suggest that the PDE4B subfamily is involved in signaling pathways that contribute to anxiogenic-like effects on behavior.

Human PDE4A8, a novel brain-expressed PDE4 cAMP-specific phosphodiesterase that has undergone rapid evolutionary change

We have isolated cDNAs encoding PDE4A8 (phosphodiesterase 4 isoform A8), a new human cAMP-specific PDE4 isoform encoded by the PDE4A gene. PDE4A8 has a novel N-terminal region of 85 amino acids that differs from those of the related 'long' PDE4A4, PDE4A10 and PDE4A11 isoforms. The human PDE4A8 N-terminal region has diverged substantially from the corresponding isoforms in the rat and other mammals, consistent with rapid evolutionary change in this region of the protein. When expressed in COS-7 cells, PDE4A8 localized predominantly in the cytosol, but approx. 20% of the enzyme was associated with membrane fractions. Cytosolic PDE4A8 was exquisitely sensitive to inhibition by the prototypical PDE4 inhibitor rolipram (IC(50) of 11+/-1 nM compared with 1600 nM for PDE4A4), but was less sensitive to inhibition by cilomilast (IC(50) of 101+/-7 nM compared with 61 nM for PDE4A4). PDE4A8 mRNA was found to be expressed predominantly in skeletal muscle and brain, a pattern that differs from the tissue expression of other human PDE4 isoforms and also from that of rat PDE4A8. Immunohistochemical analysis showed that PDE4A8 could be detected in discrete regions of human brain, including the cerebellum, spinal cord and cerebral cortex. The unique tissue distribution of PDE4A8, combined with the evolutionary divergence of its N-terminus, suggest that this isoform may have a specific function in regulating cAMP levels in human skeletal muscle and brain.

PDE4B polymorphisms and decreased PDE4B expression are associated with schizophrenia

Schizophrenia has a complex genetic underpinning and variations in a number of candidate genes have been identified that confer risk of developing the disorder. We report in the present studies that several single nucleotide polymorphisms (SNPs) and a two-SNP haplotype in PDE4B are associated with an increased incidence of schizophrenia in two large populations of Caucasian and African American patients. The SNPs in PDE4B associated with schizophrenia occur in intronic sequences in the vicinity of a critical splice junction that gives rise to the expression of PDE4B isoforms with distinct regulation and function. We also observed specific decreases in phosphodiesterase 4B (PDE4B) isoforms in brain tissue obtained postmortem from patients diagnosed with schizophrenia and bipolar disorder. PDE4B metabolically inactivates the second messenger cAMP to regulate intracellular signaling in neurons throughout the brain. Thus, the present observations suggest that dysregulation of intracellular signaling mediated by PDE4B is a significant factor in the cause and expression, respectively, of schizophrenia and bipolar disorder and that targeting PDE4B-regulated signaling pathways may yield new therapies to treat the totality of these disorders.

Renal phosphodiesterase 4B is activated in the Dahl salt-sensitive rat

Reduced beta-adrenoreceptor signaling is associated with increased sympathoadrenal activity in hypertensive patients and animal models of hypertension. However, the mechanism that accounts for this characteristic decline in beta-adrenergic signaling is unclear. In the present study, we investigated renal phosphodiesterase 4B, which metabolizes cAMP. Immunoblot analysis detected only the phosphodiesterase 4B4 isoform present in kidney tissue from spontaneously hypertensive rats, hypertensive Dahl salt-sensitive (SS) rats, and Dahl salt-resistant rats. The phosphorylated (activated) form of the protein was present at 2-fold greater levels in Dahl SS rats than in spontaneously hypertensive rats and Dahl salt-resistant rats, whereas the unphosphorylated form of the protein was reduced by approximately one half in SS animals. In accord with immunoblot data, rolipram-inhibitable cAMP hydrolyzing activity, a measure of PDE4 activity, was approximately 3-fold greater in kidney cytosolic extracts from SS rats than in extracts from spontaneously hypertensive rats and salt-resistant rats. Phosphodiesterase 4B expression was detected by immunohistochemistry in the renal vasculature, proximal tubules, and distal tubules. These results raise the possibility that increased PDE4 activity, specifically phosphodiesterase 4B4 activity, reduces beta-adrenergic signaling in the kidney and contributes to salt-sensitive hypertension in the Dahl SS rat.

Differential distribution of PDE4B splice variant mRNAs in rat brain and the effects of systemic administration of LPS in their expression

Phosphodiesterases (PDE) control intracellular cyclic adenosine monophosphate (cAMP) levels, which appear to play an important role in the regulation of inflammation. PDE4B is especially important in this process. Using in situ hybridization histochemistry we first mapped the expression sites of the four PDE4B splicing forms in rat brain. Using the systemic administration of the bacterial endotoxin lipopolysaccharide (LPS) as an inflammation model in rats, we found an increase in PDEB2 mRNA expression in choroid plexus. The differential expression of PDE4B spliced forms and the differential regulation of PDE4B2 in an inflammatory model further supports an involvement of this splicing variant in the inflammatory response.

PDE4B5, a Novel, Super-Short, Brain-Specific cAMP Phosphodiesterase-4 Variant Whose Isoform-Specifying N-Terminal Region Is Identical to That of cAMP Phosphodiesterase-4D6 (PDE4D6)

The cAMP-specific phosphodiesterase-4 (PDE4) gene family is the target of several potential selective therapeutic inhibitors. The four PDE4 genes generate several distinct protein-coding isoforms through the use of alternative promoters and 5'-coding exons. Using mouse transcripts, we identified a novel, super-short isoform of human PDE4B encoding a novel 5' terminus, which we label PDE4B5. The protein-coding region of the novel 5' exon is conserved across vertebrates, chicken, zebrafish, and fugu. Reverse-transcription-polymerase chain reaction (PCR) and quantitative (PCR) measurements show that this isoform is brain-specific. The novel protein is 58 +/- 2 kDa; it has cAMP hydrolyzing enzymatic activity and is inhibited by PDE4-selective inhibitors rolipram and cilomilast (Ariflo). Confocal and subcellular fractionation analyses show that it is distributed predominantly and unevenly within the cytosol. The 16 novel N-terminal residues of PDE4B5 are identical to the 16 N-terminal residues of the super-short isoform of PDE4D (PDE4D6), which is also brain-specific. PDE4B5 is able to bind the scaffold protein DISC1, whose gene has been linked to schizophrenia. Microarray expression profiling of the PDE4 gene family shows that specific PDE4 genes are enriched in muscle and blood fractions; however, only by monitoring the individual isoforms is the brain specificity of the super-short PDE4D and PDE4B isoforms revealed. Understanding the distinct tissue specificity of PDE4 isoforms will be important for understanding phosphodiesterase biology and opportunities for therapeutic intervention.

Daily rhythm in pineal phosphodiesterase (PDE) activity reflects adrenergic/3',5'-cyclic adenosine 5'-monophosphate induction of the PDE4B2 variant

The pineal gland is a photoneuroendocrine transducer that influences circadian and circannual dynamics of many physiological functions via the daily rhythm in melatonin production and release. Melatonin synthesis is stimulated at night by a photoneural system through which pineal adenylate cyclase is adrenergically activated, resulting in an elevation of cAMP. cAMP enhances melatonin synthesis through actions on several elements of the biosynthetic pathway. cAMP degradation also appears to increase at night due to an increase in phosphodiesterase (PDE) activity, which peaks in the middle of the night. Here, it was found that this nocturnal increase in PDE activity results from an increase in the abundance of PDE4B2 mRNA (approximately 5-fold; doubling time, approximately 2 h). The resulting level is notably higher (>6-fold) than in all other tissues examined, none of which exhibit a robust daily rhythm. The increase in PDE4B2 mRNA is followed by increases in PDE4B2 protein and PDE4 enzyme activity. Results from in vivo and in vitro studies indicate that these changes are due to activation of adrenergic receptors and a cAMP-dependent protein kinase A mechanism. Inhibition of PDE4 activity during the late phase of adrenergic stimulation enhances cAMP and melatonin levels. The evidence that PDE4B2 plays a negative feedback role in adrenergic/cAMP signaling in the pineal gland provides the first proof that cAMP control of PDE4B2 is a physiologically relevant control mechanism in cAMP signaling.

Overview of PDEs and their regulation

Contraction and relaxation of vascular smooth muscle and cardiac myocytes are key physiological events in the cardiovascular system. These events are regulated by second messengers, cAMP and cGMP, in response to extracellular stimulants. The strength of signal transduction is controlled by intracellular cyclic nucleotide concentrations, which are determined by a balance in production and degradation of cAMP and cGMP. Degradation of cyclic nucleotides is catalyzed by 3',5'-cyclic nucleotide phosphodiesterases (PDEs), and therefore regulation of PDEs hydrolytic activity is important for modulation of cellular functions. Mammalian PDEs are composed of 21 genes and are categorized into 11 families based on sequence homology, enzymatic properties, and sensitivity to inhibitors. PDE families contain many splice variants that mostly are unique in tissue-expression patterns, gene regulation, enzymatic regulation by phosphorylation and regulatory proteins, subcellular localization, and interaction with association proteins. Each unique variant is closely related to the regulation of a specific cellular signaling. Thus, multiple PDEs function as a particular modulator of each cardiovascular function and regulate physiological homeostasis.

Chronic nicotine doses down-regulate PDE4 isoforms that are targets of antidepressants in adolescent female rats

BACKGROUND

Previous data in humans and animal models has suggested connections between anxiety, depression, smoking behavior, and nicotine dependence. The importance of these connections has been confirmed by clinical studies that led to the recent FDA approval of an anti-depressant (Zyban) for use in human smoking cessation programs. Other anti-depressants (such as rolipram) specifically inhibit PDE4 phosphodiesterases.

METHODS

We used DNA microarrays to discover gene expression changes in adolescent female rats following chronic nicotine treatments, and real-time PCR assays to confirm and extend those results.

RESULTS

We found a consistent decrease in the mRNA levels encoded by the Pde4b gene in nucleus accumbens, prefrontal cortex, and hippocampus of adolescent female rats treated with .24 mg/day nicotine, and in prefrontal cortex of adolescent female rats treated with .12 mg/day nicotine. We further show that each of these brain areas produced a different profile of Pde4b isoforms.

CONCLUSIONS

Chronic nicotine treatments produce a dose-dependent down-regulation of Pde4b, which may have an antidepressant effect. This is the first report of a link between nicotine dependence and phosphodiesterase gene expression. Our results also add to the complex interrelationships between smoking and schizophrenia, because mutations in the PDE4B gene are associated with schizophrenia.

Regulation of phosphodiesterase-4 (PDE4) expression in mouse brain by repeated antidepressant treatment: Comparison with rolipram

Cyclic nucleotide phosphodiesterase-4 (PDE4) is a component of signaling pathways involved in the mediation of antidepressant activity. Of the four PDE4 subtypes, PDE4D appears to be of particular importance, given the finding that PDE4D-deficient mice exhibit an antidepressant-like behavioral phenotype. In mouse hippocampus and cerebral cortex, the effects of repeated treatment with the antidepressants desipramine and fluoxetine or the PDE4 inhibitor rolipram on the expression of PDE4D was compared to that of PDE4A and PDE4B, the other two subtypes expressed in the brain. Expression of PDE4D was increased by all drugs tested, with the exception of desipramine in hippocampus. By contrast, these treatments affected PDE4A and PDE4B expression differentially. In hippocampus, antidepressants increased PDE4A and decreased PDE4B, whereas ROL decreased PDE4A and did not change PDE4B. In cerebral cortex, antidepressants increased PDE4A and did not change PDE4B, whereas ROL did not change PDE4A and increased PDE4B. 3H-Rolipram binding was increased in cytosolic, but not in membrane, fractions of cerebral cortex by all drugs tested; there were no changes observed in hippocampus. Overall, the present results suggest some species-dependence of the regulation of PDE4 subtypes, based on data obtained previously using rats. They also suggest that the PDE4D subtype may be of particular importance as an antidepressant target in that it is regulated by repeated treatment with both norepinephrine and serotonin reuptake inhibitors as well as by the PDE4 inhibitor rolipram, drugs that produce antidepressant effects via different neuropharmacological mechanisms.

Energy state of the liver during short-term and exhaustive exercise in C57BL/6J mice

A portal venous 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion that results in hepatic 5-aminoimidazole-4-carboxamide-1-beta-D-ribosyl-5-monophosphate (ZMP) concentrations of approximately 4 micromol/g liver increases hepatic glycogenolysis and glucose output. ZMP is an AMP analog that mimics the regulatory actions of this nucleotide. The aim of this study was to measure hepatic AMP concentrations in response to increasing energy requirements to test the hypothesis that AMP achieves concentrations during exercise, consistent with a role in stimulation of hepatic glucose metabolism. Male C57BL/6J mice (27.4+/- 0.4 g) were subjected to 35 min of rest [sedentary (SED), n=8], underwent short-term (ST, 35 min) moderate (20 m/min, 5% grade) exercise (n=8), or underwent treadmill exercise under similar conditions but until exhaustion (EXH, n=8). Hepatic AMP concentrations were 0.82+/- 0.05, 1.17+/- 0.11, and 2.52+/- 0.16 micromol/g liver in SED, ST, and EXH mice, respectively (P< 0.05). Hepatic energy charge was 0.66+/- 0.01, 0.58+/- 0.02, and 0.33+/- 0.22 in SED, ST, and EXH mice, respectively (P< 0.05). Hepatic glycogen was 11.6+/- 1.0, 8.8+/- 2.2, and 0.0+/- 0.1 mg/g liver in SED, ST, and EXH mice, respectively (P< 0.05). Hepatic AMPK (Thr(172)) phosphorylation was 1.00+/- 0.14, 1.96+/- 0.16, and 7.44+/- 0.63 arbitrary units in SED, ST, and EXH mice, respectively (P< 0.05). Thus exercise increases hepatic AMP concentrations. These data suggest that the liver is highly sensitive to metabolic demands, as evidenced by dramatic changes in cellular energy indicators (AMP) and sensors thereof (AMP-activated protein kinase). In conclusion, AMP is sensitively regulated, consistent with it having an important role in hepatic metabolism.

An Aplysia type 4 phosphodiesterase homolog localizes at the presynaptic terminals of Aplysia neuron and regulates synaptic facilitation

The cAMP-dependent signaling pathway is critically involved in memory-related synaptic plasticity. cAMP-specific type 4 phosphodiesterases (PDE4) play a role in this process by regulating the cAMP concentration. However, it is unclear how PDE4 is involved in regulating synaptic plasticity. To address this issue in Aplysia sensory-to-motor synapses, we identified a long isoform of the PDE4 homolog in Aplysia kurodai (apPDE), with genetic and biochemical properties similar to those of mammalian PDE4s. Furthermore, apPDE is localized to the membrane and presynaptic region. Both apPDE overexpression and knock-down impaired short- and long-term facilitation, indicating that an appropriate expression level of apPDE in synaptic regions is required for normal synaptic facilitation. By using fluorescence resonance energy transfer-based measurement of in vivo protein kinase A (PKA) activation, we found that the PKA activation by 5-hydroxytryptamine (5-HT) was impaired in both apPDE-overexpressed and knock-down synapses. Analogous to the inhibition of apPDE by RNA interference, chronic rolipram treatment before 5-HT stimulation also impaired the PKA activation by 5-HT, suggesting that regulation of the synaptic cAMP level by PDE4 is critical for normal synaptic facilitation. Together, we suggest that PDE4s localized in the synapses play a critical role in regulating the optimum cAMP level required for normal synaptic plasticity.

Phosphodiesterase-4 as a potential drug target

Phosphodiesterase-4 (PDE4) is the predominant enzyme in some specific cell types that is responsible for the degradation of the second messenger, cAMP. Consequently, PDE4 plays a crucial role in cell signalling and, as such, it has been the target of clinical drug development of various indications, ranging from anti-inflammation to memory enhancement. In this review, the fundamental biological role of PDE4 in intracellular signalling, its tissue distribution and regulation are described. The historical development of various chemical classes of PDE4 inhibitors and the challenges that face these inhibitors as therapeutics are also discussed. Finally, recent advances in the structural biology of PDE4 and their complexes with various inhibitors, as well as its potential impact on the rational design of potent and selective PDE4 inhibitors, are presented.

Nitric oxide induces phosphodiesterase 4B expression in rat pulmonary artery smooth muscle cells

Phosphodiesterases (PDE) metabolize cyclic nucleotides limiting the effects of vasodilators such as prostacyclin and nitric oxide (NO). In this study, DNA microarray techniques were used to assess the impact of NO on expression of PDE genes in rat pulmonary arterial smooth muscle cells (rPASMC). Incubation of rPASMC with S-nitroso-l-glutathione (GSNO) increased expression of a PDE isoform that specifically metabolizes cAMP (PDE4B) in a dose- and time-dependent manner. GSNO increased PDE4B protein levels, and rolipram-inhibitable PDE activity was 2.3 +/- 1.0-fold greater in GSNO-treated rPASMC than in untreated cells. The soluble guanylate cyclase (sGC) inhibitor, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one, and the cAMP-dependent protein kinase inhibitor, H89, prevented induction of PDE4B gene expression by GSNO, but the protein kinase G (PKG) inhibitors, Rp-8-pCPT-cGMPs and KT-5823, did not. Incubation of rPASMC with IL-1beta and tumor necrosis factor-alpha induced PDE4B gene expression, an effect that was inhibited by l-N(6)-(1-iminoethyl)lysine, an antagonist of NO synthase 2 (NOS2). The GSNO-induced increase in PDE4B mRNA levels was blocked by actinomycin D but augmented by cycloheximide. Infection of rPASMC with an adenovirus specifying a dominant negative cAMP response element binding protein (CREB) mutant inhibited the GSNO-induced increase of PDE4B gene expression. These results suggest that exposure of rPASMC to NO induces expression of PDE4B via a mechanism that requires cGMP synthesis by sGC but not PKG. The GSNO-induced increase of PDE4B gene expression is CREB dependent. These findings demonstrate that NO increases expression of a cAMP-specific PDE and provide evidence for a novel "cross talk" mechanism between cGMP and cAMP signaling pathways.

Phosphodiesterase 4B (PDE4B) and cAMP-level regulation within different tissue fractions of rat hippocampal slices during long-term potentiation in vitro

Molecular events associated with mnemonic processes and neuronal plasticity are postulated to result in functional changes in synaptic structure. One possible site is the post-synaptic density, where activity-dependent changes modulate signal transduction cascades. In this report, we detail spatial-temporal changes for phosphodiesterase 4B (PDE4B) proteins and their substrate cAMP within three neuronal fractions during early and late long-term potentiation (LTP). The cAMP-dependent protein kinase A cascade--which can be regulated by distinct PDE4B activity--is required for mnemonic processes as well as mechanisms of neuronal plasticity, such as those during the maintenance or late-LTP. Fluorescence in situ hybridization studies (FISH) identified no translocation of PDE4B3 from the soma after late-LTP induction indicating a subtle, local control of PDE4B activity. Protein changes were detected within the PSD-enriched fraction. From these results, we conclude that either the changes in PDE4B are due to modulation of pre-existing mRNA, or that the protein is specifically translocated to activated synaptic structures. Furthermore, we report late changes in cAMP levels in the somato-dendritic fraction and discuss this result with the increased PDE4B1/3 doublet in the PSD-enriched fraction.

Phosphodiesterase 4 inhibition reduces skeletal muscle atrophy

Several GTP-binding protein (G-protein)-coupled receptors that signal through Galphas (GTP-binding protein alpha stimulatory) and the cyclic adenosine monophosphate (cAMP) pathway increase skeletal muscle mass. In order to further evaluate the role of the cAMP pathway in the regulation of skeletal muscle mass, we utilized inhibitors of phosphodiesterase 4 (PDE 4), the major cAMP-modifying PDE found in skeletal muscle, to modulate skeletal muscle cAMP levels. We found that PDE 4 inhibitors reduced the loss of muscle mass and force resulting from denervation and casting in rats and mice. These studies indicate that PDE 4 inhibitors may have a role in the treatment of skeletal muscle-wasting diseases.

Fluorescence Resonance Energy Transfer–Based Analysis of cAMP Dynamics in Live Neonatal Rat Cardiac Myocytes Reveals Distinct Functions of Compartmentalized Phosphodiesterases

Cardiac myocytes have provided a key paradigm for the concept of the compartmentalized cAMP generation sensed by AKAP-anchored PKA. Phosphodiesterases (PDEs) provide the sole route for degrading cAMP in cells and are thus poised to regulate intracellular cAMP gradients. PDE3 and PDE4 represent the major cAMP degrading activities in rat ventriculocytes. By performing real-time imaging of cAMP in situ, we establish the hierarchy of these PDEs in controlling cAMP levels in basal conditions and on stimulation with a β-adrenergic receptor agonist. PDE4, rather than PDE3, appears to be responsible for modulating the amplitude and duration of the cAMP response to beta-agonists. PDE3 and PDE4 localize to distinct compartments and this may underpin their different functional roles. Our findings indicate the importance of distinctly localized PDE isoenzymes in determining compartmentalized cAMP signaling.

Regulation of the phosphodiesterase PDE4B3-isotype during long-term potentiation in the area dentata in vivo

Hippocampal long-term potentiation (LTP) is the most prominent cellular model underlying learning and memory formation. However, which cellular processes are involved in maintaining LTP remains largely unknown. We have previously detailed temporal modulations of cyclic adenosine monophosphate (cAMP) and a cAMP-specific phosphodiesterase, PDE4B3, after LTP-induction and its maintenance in hippocampal area CA1 in vitro. To test whether other hippocampal sub-structures are characterised by similar mechanisms, tissue from the area dentata of freely moving rats was analysed at different LTP-time points. The tissue was fractionated into three components, where PDE4B-levels and cAMP-concentrations were measured. In contrast with data obtained in area CA1, we now detail an LTP-specific translational, but not transcriptional regulation of PDE4B3 within the first 8 h after tetanization and present spatio-temporal changes of PDE4B proteins and cAMP that is LTP-specific.

Attenuation of the activity of the cAMP-specific phosphodiesterase PDE4A5 by interaction with the immunophilin XAP2

The cyclic AMP-specific phosphodiesterase (PDE4) isoform PDE4A5 interacted with the immunophilin XAP2 in a yeast two-hybrid assay. The interaction was confirmed in biochemical pull-down analyses. The interaction was specific, in that PDE4A5 did not interact with the closely related immunophilins AIPL1, FKBP51, or FKBP52. XAP2 also did not interact with other PDE4A isoforms or typical isoforms from the three other PDE4 subfamilies. Functionally, XAP2 reversibly inhibited the enzymatic activity of PDE4A5, increased the sensitivity of PDE4A5 to inhibition by the prototypical PDE4 inhibitor 4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone (rolipram) and attenuated the ability of cAMP-dependent protein kinase to phosphorylate PDE4A5 in intact cells. XAP2 maximally inhibited PDE4A5 by approximately 60%, with an IC50 of 120 nm, and reduced the IC50 for rolipram from 390 nm to 70-90 nm. Co-expression of XAP2 and PDE4A5 in COS7 cells showed that they could be co-immunoprecipitated and also reduced both the enzymatic activity of PDE4A5 and its IC50 for rolipram. Native XAP2 and PDE4A5 could be co-immunoprecipitated from the brain. The isolated COOH-terminal half of XAP2 (amino acids 170-330), containing its tetratricopeptide repeat domain, but not the isolated NH2-terminal half (amino acids 1-169), containing the immunophilin homology region, similarly reduced PDE4A5 activity and its IC50 for rolipram. Mutation of Arg271 to alanine, in the XAP2 tetratricopeptide repeat region, attenuated its ability to both interact with PDE4A5 in two-hybrid assays and to inhibit PDE4A5 activity. Either the deletion of a specific portion of the unique amino-terminal region or specific mutations in the regulatory UCR2 domain of PDE4A5 attenuated its ability be inhibited by XAP2. We suggest that XAP2 functionally interacts with PDE4A5 in cells.