Differential distribution of mRNA encoding cAMP-specific phosphodiesterase isoforms in the rat brain

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.

Comparison of cAMP-specific phosphodiesterase mRNAs distribution in mouse and rat brain

There are eleven families of phosphodiesterases that regulate cellular levels of cyclic nucleotides by degradation of cAMP or cGMP. Knowledge of the expression sites of different PDE genes in brain is of special importance for studies on development of specific inhibitors considering that, for example, PDE4 inhibitor treatments exhibit profound anti-inflammatory effects. To address possible species differences we examined the expression of mRNAs coding for the cAMP specific PDE4 and PDE7 families since inhibitors have been used in clinic for schizophrenia, mood disorders, cognition and inflammatory diseases treatment. We have compared the expression of these PDEs in mouse brain by in situ hybridization histochemistry in comparison with rat brain and found that their neuroanatomical distribution differs in a few areas.

Antipsychotic induced alteration of growth and proteome of rat neural stem cells

Neural stem cells (NSCs) play a crucial role in the development and maturation of the central nervous system and therefore have the potential to target by therapeutic agents for a wide variety of diseases including neurodegenerative and neuropsychiatric illnesses. It has been suggested that antipsychotic drugs have significant effects on NSC activities. However, the molecular mechanisms underlying antipsychotic-induced changes of NSC activities, particularly growth and protein expression, are largely unknown. NSCs were treated with either haloperidol (HD; 3 μM), risperidone (RS; 3 μM) or vehicle (DMSO) for 96 h. Protein expression profiles were studied through a proteomics approach. RS promoted and HD inhibited the growth of NSCs. Proteomics analysis revealed that 15 protein spots identified as 12 unique proteins in HD-, and 20 protein spots identified as 14 proteins in RS-treated groups, were differentially expressed relative to control. When these identified proteins were compared between the two drug-treated groups, 2 proteins overlapped leaving 10 HD-specific and 12 RS-specific proteins. Further comparison of the overlapped altered proteins of 96 h treatment with the neuroleptics-induced overlapped proteins at 24 h time interval (Kashem et al. [40] in Neurochem Int 55:558-565, 2009) suggested that overlapping altered proteins expression at 24 h was decreased (17 proteins i.e. 53 % of total expressed proteins) with the increase of time (96 h) (2 proteins; 8 % of total expressed proteins). This result indicated that at early stage both drugs showed common mode of action but the action was opposite to each other while administration was prolonged. The opposite morphological pattern of cellular growth at 96 h has been associated with dominant expression of oxidative stress and apoptosis cascades in HD, and activation of growth regulating metabolic pathways in RS treated cells. These results may explain RS induced repairing of neural damage caused by a wide variety of neural diseases including schizophrenia.

Increased particulate phosphodiesterase 4 in the prefrontal cortex supports 5-HT4 receptor-induced improvement of object recognition memory in the rat

RATIONALE

Serotonin receptors (5-HT4Rs) are critical to both short-term and long-term memory processes. These receptors mainly trigger the cyclic adenosine monophosphate (cAMP)/protein kinase A signaling pathway, which is regulated by cAMP phosphodiesterases (PDEs).

OBJECTIVES

We investigated the mechanisms underlying the effect of the selective activation of 5-HT4R on information acquisition in an object recognition memory task and the putative regulation of PDE.

MATERIALS AND METHODS

The effect of RS 67333 (1 mg/kg, intraperitoneally [i.p.], injected 30 min before the sample phase) was examined at different delay intervals in an object recognition task in Sprague-Dawley rats. After the testing trial, PDE activity of brain regions implicated in this task was assayed.

RESULTS

RS 67333-treated rats spent more time exploring the novel object after a 15-min (P < 0.001) or 4-h delay (P < 0.01) but not after a 24-h delay, whereas control animals showed no preference for the novel object for delays greater than 15 min. We characterized the specific patterns and kinetic properties of PDE in the prefrontal and perirhinal cortices as well as in the hippocampus. We demonstrated that particulate PDE activities increase in both the prefrontal cortex and hippocampus following 5-HT4R stimulation. In the prefrontal cortex, PDE4 activities support the RS 67333-induced modification of PDE activities, whereas in the hippocampus, all cAMP-PDE activities varied. In contrast, particulate PDE variation in the hippocampus was not found to support improvement of recognition memory after a 4-h delay.

CONCLUSIONS

We provide evidence that the increase in particulate PDE4 activity in the prefrontal cortex supports the 5-HT4R-induced increase in information acquisition.

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.

The induction of cyclic nucleotide phosphodiesterase 4 gene (PDE4D) impairs memory in a water maze task

In this study, the effects on memory of intraperitoneal post-training administration of cyclic nucleotide phosphodiesterase (PDE) inhibitors, DC-TA 46 and rolipram, were tested using a visible/hidden-platform water maze task. The effects of these compounds on cyclic nucleotide levels in the hippocampal formation (HF) and striatum (CP) were also assessed, by enzymatic immunoassay (EIA). The results obtained from rats trained in the visible-platform task were not significantly different from controls. On the contrary, the animals trained in the hidden-platform water maze task showed a memory impairment, when injected with DC-TA 46 at maximal dose of 20mg/kg and with rolipram at 3 and 30 mg/kg doses. The effects of these drugs on cyclic nucleotide levels in HF and CP were observed at 30 min and at 24h after drug administration. Thirty minutes after drug injection, we observed an increase of cAMP level, both in HF and in CP. Twenty-four hours after the retention test, we observed that in CP the cAMP intracellular level remained high, while in the HF at effective doses both inhibitors induced cAMP PDE activity, determining a decrease of cyclic nucleotide. Semi-quantitative RT-PCR analysis, together with Western blot immunodetection, showed a mRNA and protein induction of PDE4D PDE isoforms, that may account for the increase of PDE activity observed. Our data suggest that, despite cyclic nucleotide increase at 30 min, the fundamental event causing memory impairment, came from the subsequent long time decrease of cAMP levels, due to the post-translational PDE4D induction.

mRNA expression patterns of the cGMP-hydrolyzing phosphodiesterases types 2, 5, and 9 during development of the rat brain

Recent evidence indicates that cGMP plays an important role in neural development and neurotransmission. Since cGMP levels depend critically on the activities of phosphodiesterase (PDE) enzymes, mRNA expression patterns were examined for several key cGMP-hydrolyzing PDEs (type 2 [PDE2], 5 [PDE5], and 9 [PDE9]) in rat brain at defined developmental stages. Riboprobes were used for nonradioactive in situ hybridization on sections derived from embryonic animals at 15 days gestation (E15) and several postnatal stages (P0, P5, P10, P21) until adulthood (3 months). At all stages PDE9 mRNA was present throughout the whole central nervous system, with highest levels observed in cerebellar Purkinje cells, whereas PDE2 and PDE5 mRNA expression was more restricted. Like PDE9, PDE5 mRNA was abundant in cerebellar Purkinje cells, although it was observed only on and after postnatal day 10 in these cells. In other brain regions, PDE5 mRNA expression was minimal, detected in olfactory bulb, cortical layers, and in hippocampus. PDE2 mRNA was distributed more widely, with highest levels in medial habenula, and abundant expression in olfactory bulb, olfactory tubercle, cortex, amygdala, striatum, and hippocampus. Double immunostaining of PDE2, PDE5, or PDE9 mRNAs with the neuronal marker NeuN and the glial cell marker glial fibrillary acidic protein revealed that these mRNAs were predominantly expressed in neuronal cell bodies. Our data indicate that three cGMP-hydrolyzing PDE families have distinct expression patterns, although specific cell types coexpress mRNAs for all three enzymes. Thus, it appears that differential expression of PDE isoforms may provide a mechanism to match cGMP hydrolysis to the functional demands of individual brain regions.

Differential distribution of PDE4D splice variant mRNAs in rat brain suggests association with specific pathways and presynaptical localization

cAMP plays an important role as a second-messenger molecule controlling multiple cellular processes. Its hydrolysis provides an important mechanism by which cAMP levels are regulated. This is performed by a large multigene family of cyclic nucleotide phosphodiesterases (PDEs). Members of the PDE4 enzyme family are selectively inhibited by rolipram. Five different mRNA splice forms for PDE4D have been isolated. Here, we analyzed the regional distribution of the mRNAs coding for the splice variants PDE4D1, PDE4D2, PDE4D3, PDE4D4, and PDE4D5 in the rat brain by in situ hybridization histochemistry using specific radiolabeled oligonucleotides. We found that all five splice variants showed a distinct distribution pattern and, in some cases, in association with specific brain pathways. The most relevant differences were in hippocampal formation, medial habenula, basal ganglia, and area postrema, at both the regional and cellular level. The dorsal and median raphe nuclei exclusively contained PDE4D2 mRNA transcripts, probably located on serotonergic cells. PDE4D1 mRNA was expressed in some white matter cells. PDE4D1 and PDE4D2 mRNA splice forms presented a similar distribution in the area postrema, whereas for PDE4D4 and PDE4D5 the cellular distribution presented a complementary pattern. The differential expression of PDE4D mRNA splice variants in the area postrema is consistent with their possible involvement in emesis control and suggests new molecular targets for a more selective drug design.

Characterization of r-[11C]rolipram for PET imaging of phosphodieterase-4: in vivo binding, metabolism, and dosimetry studies in rats

The high-affinity phosphodiesterase-4 (PDE4) inhibitor R-rolipram and the less potent S-enantiomer, both labeled with (11)C, were evaluated in vivo in rats. Regional brain uptake of R-[(11)C]rolipram was higher than R/S-[(11)C]rolipram, whereas S-[(11)C]rolipram retention subsided rapidly to levels below blood. Binding of R-[(11)C]rolipram was selective for PDE4 over PDE1, since treatment with PDE4 competitors Ro 20-1724, or R-, S- or R/S-rolipram, but not with the PDE1 inhibitor vinpocetine, significantly reduced radioactivity uptake to non-specific levels. R-Rolipram (ED(50) congruent with 0.04 mg/Kg) was approximately 13 fold more potent than S-rolipram at inhibiting R-[(11)C]rolipram binding in all brain regions. Nevertheless, S-[(11)C]rolipram appears to be unsuitable for measuring the non-specific binding of R-[(11)C]rolipram. Only unchanged R-[(11)C]rolipram was detected in rat brain homogenates. Additionally, the estimated absorbed radiation dose extrapolated to humans was low. These results support further investigation of R-[(11)C]rolipram in studying PDE4 in vivo by positron emission tomography imaging.

The effects of phosphodiesterase inhibition on cyclic GMP and cyclic AMP accumulation in the hippocampus of the rat

The effects of selective and non-selective 3',5'-cyclic nucleotide phosphodiesterase (PDE) inhibitors on cGMP and cAMP accumulation were studied in rat hippocampal slices incubated in vitro. The following PDE inhibitors were used: vinpocetine and calmidazolium (PDE1 selective), erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, PDE2 selective), SK&F 95654 (PDE3 selective), rolipram (PDE4 selective), SK&F 96231 (PDE5 selective), the mixed type inhibitors zaprinast and dipyridamole, and the non-selective inhibitors 3-isobutyl-1-metylxanthine (IBMX) and caffeine. cGMP levels were increased in the presence of different concentrations of IBMX, EHNA, dipyridamole, vinpocetine and rolipram. cGMP immunocytochemistry showed that incubation with different inhibitors in the presence and/or absence of sodium nitroprusside resulted in pronounced differences in the extent and regional localization of the cGMP response and indicate that PDE activity in the hippocampus is high and diverse in nature. The results suggest an interaction between cGMP and cAMP signalling pathways in astrocytes of the rat hippocampus.

Phosphodiesterase type 4 isozymes expression in human brain examined by in situ hybridization histochemistry and[3H]rolipram binding autoradiography. Comparison with monkey and rat brain

We have examined the distribution of four different cyclic AMP-specific phosphodiesterase isozyme (PDE4A, PDE4B, PDE4C and PDE4D) mRNAs in the brain of different species by in situ hybridization histochemistry and by autoradiography with [3H]rolipram. We have compared the localization of each isozyme in human brain with that in rat and monkey brain. We have found that the four PDE4 isoforms display a differential expression pattern at both regional and cellular level in the three species. PDE4A, PDE4B and PDE4D are widely distributed in human brain, with the two latter appearing more abundant. In contrast, PDE4C in human brain, presents a more restricted distribution, limited to cortex, some thalamic nuclei and cerebellum. This is at variance with the distribution of PDE4C in rat brain, where it is found exclusively in olfactory bulb. In monkey brain, the highest expression for this isoform is found in the claustrum, and at lower levels in cortical areas and cerebellum. PDE4B presented a broad distribution, being expressed in both neuronal and non neuronal cell populations. In general, the distribution of binding sites visualized with [3H]rolipram correlated well with the expression of each PDE4 isozyme.

Cyclic AMP phosphodiesterases in the zebra finch: distribution, cloning and characterization of a PDE4B homolog

Songbirds are important animal models for studying neural mechanisms underlying learning and memory. While evidence has emerged that cAMP plays a significant role in invertebrate and mammalian learning, little is known about the role of cAMP pathways in regulating neuronal function in birds. With the goal of identifying important components of this pathway, we report the first cloning of a cAMP-specific, Type IV phosphodiesterase (PDE4) in a non-mammalian vertebrate. A combination of PCR analysis and cDNA library screening was used to show that homologs of the four known mammalian PDE4 genes also exist in zebra finch. A full-length cDNA representing the zebra finch homolog of PDE4B1 was isolated from a telencephalic library. Expression of this cDNA in human embryonic kidney 293 (HEK) cells yielded an enzyme that hydrolyzed cAMP with a low K(m) and was inhibited by micromolar concentrations of rolipram; these properties are typical of all known mammalian PDE4s. In brain, northern blots revealed transcripts of 3.6 and 4.4 kb in adults, but only the 3.6 kb transcript in juveniles, suggesting that PDE4 expression is developmentally regulated. In situ hybridization of tissue sections demonstrated that PDE4 message was distributed widely throughout the adult zebra finch brain, including regions controlling the learning of songs and the acquisition of spatial memories. These data suggest that PDE4 enzymes may influence a variety of brain functions in these birds and play a role in learning.

New phosphodiesterase inhibitors as therapeutics for the treatment of chronic lung disease

Phosphodiesterase 4 (PDE4) is a member of the growing family cyclic AMP and cyclic GMP. Earliest described inhibitors of PDE4, such as rolipram, demonstrate marked anti-inflammatory and bronchodilatory effects in vitro and in vivo. The clinical utility of these earlier compounds was limited by their propensity to elicit gastrointestinal side effects. This has led to an extensive effort to identify novel PDE4 inhibitors that maintain the anti-inflammatory activity and bronchodilatory activity of rolipram but with a reduced potential to produce side effects. This article summarizes the evidence supporting the utility of selective PDE4 inhibitors in the treatment of asthma and chronic obstructive pulmonary disease, discusses the recent results obtained in clinical trials with second-generation inhibitors, and presents two approaches designed to identify additional novel selective PDE4 inhibitors.

Phosphodiesterase 4 (PDE4) inhibitors in asthma and chronic obstructive pulmonary disease (COPD)

Phosphodiesterases (PDE) are a family of enzymes responsible for the metabolism of the intracellular second messengers cyclic AMP and cyclic GMP. PDE4 is a cyclic AMP specific PDE that is the major if not sole cyclic AMP metabolizing enzymes found in inflammatory and immune cells, and contributes significantly to cyclic AMP metabolism in smooth muscles. Based on its cellular and tissue distribution and the demonstration that selective inhibitors of this isozyme reduce bronchoconstriction in animals and suppress the activation of inflammatory cells, PDE4 has become an important molecular target for the development of novel therapies for asthma and COPD. This chapter will review the evidence demonstrating the ability of PDE4 inhibitors to modify airway obstruction, airway inflammation and airway remodelling and hyperreactivity, will present some preliminary findings obtained with theses compounds in clinical trials and and will discuss experimental approaches designed to identify novel compounds that maintain the beneficial activity of the initial selective PDE4 inhibitors but with a reduced tendency of elicit the gastrointestinal side effects observed with this class of compounds.

Characterization of the rolipram-sensitive, cyclic AMP-specific phosphodiesterases: identification and differential expression of immunologically distinct forms in the rat brain

To determine the properties of the cAMP-specific, rolipram-sensitive phosphodiesterases (cAMP-PDEs) that are expressed in different organs, monoclonal and polyclonal antibodies were raised against different epitopes present in the cAMP-PDE sequences. Of the several antibodies generated against peptides and fusion proteins, one monoclonal and four polyclonal antibodies recognized both the native cAMP-PDEs as well as the denatured proteins on Western immunoblot analysis. An immunoprecipitation assay demonstrated that these antibodies recognized the recombinant rat PDE4A, PDE4B, and PDE4D proteins with different avidity. The polyclonal antibody K118 and the monoclonal M3S1 were most specific for rat PDE4B and PDE4D forms, respectively, whereas the AC55 antiserum displayed the highest affinity for PDE4A forms. This selectivity was confirmed by Western blot analysis using recombinant rat PDE4A, PDE4B, and PDE4D proteins expressed in a heterologous system. These antibodies were used to characterize the cAMP-PDEs expressed in the rat brain. An immunoblot of extract of cortex and cerebellum demonstrated that at least seven different polypeptides specifically cross-reacted with the different antibodies, indicating that multiple cAMP-PDEs are expressed in this tissue. On the basis of cross-reactivity with PDE4D but not PDE4A or PDE4B antibodies, 93- and 105-kDa PDE4D species were detected in the cortex and cerebellum extract. These forms are different from the 68-kDa PDE4D form expressed in endocrine cells after hormonal stimulation. Although the 93-kDa form was recovered in both the soluble and particulate fractions, the 105-kDa polypeptide was mostly particulate in the cortex and cerebellum extracts. PDE4B forms of 90-87 kDa were recovered in both soluble and particulate compartments of the brain extract. These forms were different from the previously identified PDE4A variants of 110 and 75 kDa. These data demonstrate that the presence of multiple cAMP-PDE genes is translated into cAMP-PDE proteins of different sizes and distinct immunological properties and that multiple variants derived from these cAMP-PDE genes are expressed in different regions of the brain and different subcellular compartments. These immunological tools will be useful to identify different cAMP-PDE forms expressed in organs targeted for pharmacological intervention with PDE4 inhibitors.

The role of cyclic AMP as a precursor of extracellular adenosine in the rat hippocampus

Extracellular adenosine 3':5'-cyclic monophosphate (cAMP) is a potential source of the inhibitory neuromodulator adenosine in the brain. Previous work has demonstrated that cAMP, which is formed intracellularly, can be transported into the extracellular space and subsequently catabolized to adenosine. However, the physiological conditions under which cAMP release might lead to adenosine formation and activation of adenosine receptors are not well understood. In this study we demonstrate that superfusion of hippocampal slices with cAMP or forskolin led to the formation of extracellular adenosine which activated adenosine receptors in a manner comparable to that seen with adenosine superfusion. In contrast, application of brief pulses of cAMP onto the cell bodies of CA1 pyramidal neurons failed to produce an adenosine receptor-mediated response, while application of brief pulses of adenosine or AMP elicited significant responses. These data suggest that large, prolonged increases in extracellular cAMP levels can result in the formation of extracellular adenosine and the activation of adenosine receptors, but brief increases in cAMP levels in the vicinity of individual neurons cannot. These findings imply that increases in cAMP levels may lead to relatively slow increases in extracellular adenosine, as opposed to the fast, spatially restricted increases that would occur following the release of other adenine nucleotides.