Activation of phosphodiesterase IV during desensitization of the A2A adenosine receptor-mediated cyclic AMP response in rat pheochromocytoma (PC12) cells

The D2 Dopamine Receptor Interferes With the Protective Effect of the A2A Adenosine Receptor on TDP-43 Mislocalization in Experimental Models of Motor Neuron Degeneration

The A2A adenosine receptor (A2AR) and D2 dopamine receptor (D2R) are two G-protein-coupled receptors that can form dimers and negatively regulate their partners. TAR DNA-binding protein (TDP-43) is a nuclear protein that has been implicated in amyotrophic lateral sclerosis (ALS). Mislocalization of TDP-43 from the nucleus to the cytoplasm is an early step of TDP-43 proteinopathy. Our previous studies indicated that A2AR is a potential drug target for ALS because treatment with an A2AR agonist (JMF1907; a T1-11 analog) prevents reactive oxygen species (ROS)-induced TDP-43 mislocalization in a motor neuron cell line (NSC34) and delays motor impairment in a TDP-43 transgenic ALS mouse model. Here, we set out to assess whether activation of D2R interferes with the beneficial effects of an A2AR agonist on motor neurons. We first demonstrated that A2AR and D2R are both located in motor neurons of mouse and human spinal cords and human iPSC-derived motor neurons. Expression of A2AR and D2R in NSC34 cells led to dimer formation without affecting the binding affinity of A2AR toward T1-11. Importantly, activation of D2R reduced T1-11-mediated activation of cAMP/PKA signaling and subsequent inhibition of TDP-43 mislocalization in NSC34 cells. Treatment with quinpirole (a D2 agonist) blunted the rescuing effect of T1-11 on TDP-43 mislocalization and impaired grip strength in a mouse model of ALS. Our findings suggest that D2R activation may limit the beneficial responses of an A2AR agonist in motor neurons and may have an important role in ALS pathogenesis.

GSK3β negatively regulates TRAX, a scaffold protein implicated in mental disorders, for NHEJ-mediated DNA repair in neurons

Translin-associated protein X (TRAX) is a scaffold protein with various functions and has been associated with mental illnesses, including schizophrenia. We have previously demonstrated that TRAX interacts with a Gsα protein-coupled receptor, the A_2A adenosine receptor (A_2AR), and mediates the function of this receptor in neuritogenesis. In addition, stimulation of the A_2AR markedly ameliorates DNA damage evoked by elevated oxidative stress in neurons derived from induced pluripotent stem cells (iPSCs). Here, we report that glycogen synthase kinase 3 beta (GSK3β) and disrupted-in-schizophrenia 1 (DISC1) are two novel interacting proteins of TRAX. We present evidence to suggest that the stimulation of A_2AR markedly facilitated DNA repair through the TRAX/DISC1/GSK3β complex in a rat neuronal cell line (PC12), primary mouse neurons, and human medium spiny neurons derived from iPSCs. A_2AR stimulation led to the inhibition of GSK3β, thus dissociating the TRAX/DISC1/GSK3β complex and facilitating the non-homologous end-joining pathway (NHEJ) by enhancing the activation of a DNA-dependent protein kinase via phosphorylation at Thr²⁶⁰⁹. Similarly, pharmacological inhibition of GSK3β by SB216763 also facilitated the TRAX-mediated repair of oxidative DNA damage. Collectively, GSK3β binds with TRAX and negatively affects its ability to facilitate NHEJ repair. The suppression of GSK3β by A_2AR activation or a GSK3β inhibitor releases TRAX for the repair of oxidative DNA damage. Our findings shed new light on the molecular mechanisms underlying diseases associated with DNA damage and provides a novel target (i.e., the TRAX/DISC1/GSK3β complex) for future therapeutic development for mental disorders.

Phosphodiesterase 2 negatively regulates adenosine-induced transcription of the tyrosine hydroxylase gene in PC12 rat pheochromocytoma cells

Adenosine induces expression of the tyrosine hydroxylase (TH) gene in PC12 cells. However, it is suggested that atrial natriuretic peptide (ANP) inhibits expression of this gene. Using real-time PCR and luciferase reporter assays we found that ANP significantly decreases the adenosine-induced transcription of the TH gene. Results of measurements of cyclic nucleotide concentrations indicated that ANP-induced accumulation of cGMP inhibits the adenosine-induced increase in cAMP level. Using selective phosphodiesterase 2 (PDE2) inhibitors and a synthetic cGMP analog activating PDE2, we found that PDE2 is involved in coupling the ANP-triggered signal to the cAMP metabolism. We have established that ANP-induced elevated levels of cGMP as well as cGMP analog stimulate hydrolytic activity of PDE2, leading to inhibition of adenosine-induced transcription of the TH gene. We conclude that ANP mediates negative regulation of TH gene expression via stimulation of PDE2-dependent cAMP breakdown in PC12 cells.

A novel Gαs-binding protein, Gas-2 like 2, facilitates the signaling of the A2A adenosine receptor

The A2A adenosine receptor (A2AR) is a G-protein-coupled receptor that contains a long cytoplasmic carboxyl terminus (A2AR-C). We report here that Gas-2 like 2 (G2L2) is a new interacting partner of A2AR-C. The interaction between A2AR and G2L2 was verified by GST pull-down, co-immunoprecipitation, immunocytochemical staining, and fluorescence resonance energy transfer. Expression of G2L2 increased the intracellular cAMP content evoked by A2AR in an A2AR-C-dependent manner. Immunoprecipitation and pull-down assays demonstrated that G2L2 selectively bound to A2AR-C and the inactive form of Gαs to facilitate the recruitment of the trimeric G protein complex to the proximal position of A2AR for efficient activation. Collectively, G2L2 is a new effector that controls the action of A2AR by modulating its ability to regulate the Gαs-mediated cAMP contents.

The A2A adenosine receptor rescues neuritogenesis impaired by p53 blockage via KIF2A, a kinesin family member

The A2A adenosine receptor (A2AR) is a G-protein-coupled receptor. We previously reported that the C terminus of the A2AR binds to translin-associated protein X (TRAX) and modulates nerve growth factor (NGF)-evoked neurite outgrowth in PC12 cells. Herein, we show that neuritogenesis of primary hippocampal neurons requires p53 because blockage of p53 suppressed neurite outgrowth. The impaired neuritogenesis caused by p53 blockage was rescued by activation of the A2AR (designated the A2A rescue effect) in a TRAX-dependent manner. Importantly, suppression of a TRAX-interacting protein (kinesin heavy chain member 2A, KIF2A) inhibited the A2A rescue effect, whereas overexpression of KIF2A caused a rescue effect. Expression of a KIF2A fragment (KIF2A514), which disturbed the interaction between KIF2A and TRAX, blocked the rescue effect. Transient colocalization of TRAX and KIF2A was detected in the nucleus of PC12 cells upon NGF treatment. These data suggest that functional interaction between KIF2A and TRAX is critical for the A2A rescue effect. Moreover, p53 blockage during NGF treatment prevented the redistribution of KIF2A from the nucleus to the cytoplasmic region. Expression of a nuclear-retained KIF2A variant (NLS-KIF2A) did not rescue the impaired neurite outgrowth as did the wild-type KIF2A. Therefore, redistribution of KIF2A to the cytoplasmic fraction is a prerequisite for neurite outgrowth. Collectively, we demonstrate that KIF2A functions downstream of p53 to mediate neuritogenesis of primary hippocampal neurons and PC12 cells. Stimulation of the A2AR rescued neuritogenesis impaired by p53 blockage via an interaction between TRAX and KIF2A.

Adenosine receptor desensitization and trafficking

As with the majority of G-protein-coupled receptors, all four of the adenosine receptor subtypes are known to undergo agonist-induced regulation in the form of desensitization and trafficking. These processes can limit the ability of adenosine receptors to couple to intracellular signalling pathways and thus reduce the ability of adenosine receptor agonists as well as endogenous adenosine to produce cellular responses. In addition, since adenosine receptors couple to multiple signalling pathways, these pathways may desensitize differentially, while the desensitization of one pathway could even trigger signalling via another. Thus, the overall picture of adenosine receptor regulation can be complex. For all adenosine receptor subtypes, there is evidence to implicate arrestins in agonist-induced desensitization and trafficking, but there is also evidence for other possible forms of regulation, including second messenger-dependent kinase regulation, heterologous effects involving G proteins, and the involvement of non-clathrin trafficking pathways such as caveolae. In this review, the evidence implicating these mechanisms is summarized for each adenosine receptor subtype, and we also discuss those issues of adenosine receptor regulation that remain to be resolved as well as likely directions for future research in this field.

Adenosine receptors and inflammation

Extracellular adenosine is produced in a coordinated manner from cells following cellular challenge or tissue injury. Once produced, it serves as an autocrine- and paracrine-signaling molecule through its interactions with seven-membrane-spanning G-protein-coupled adenosine receptors. These signaling pathways have widespread physiological and pathophysiological functions. Immune cells express adenosine receptors and respond to adenosine or adenosine agonists in diverse manners. Extensive in vitro and in vivo studies have identified potent anti-inflammatory functions for all of the adenosine receptors on many different inflammatory cells and in various inflammatory disease processes. In addition, specific proinflammatory functions have also been ascribed to adenosine receptor activation. The potent effects of adenosine signaling on the regulation of inflammation suggest that targeting specific adenosine receptor activation or inactivation using selective agonists and antagonists could have important therapeutic implications in numerous diseases. This review is designed to summarize the current status of adenosine receptor signaling in various inflammatory cells and in models of inflammation, with an emphasis on the advancement of adenosine-based therapeutics to treat inflammatory disorders.

Subcellular localization of cyclic nucleotide phosphodiesterase type 10A variants, and alteration of the localization by cAMP-dependent protein kinase-dependent phosphorylation

Our previous studies have suggested that two phosphodiesterase type 10A (PDE10A) variants, PDE10A1 and PDE10A2 transcripts, are mainly expressed in humans and that PDE10A2 and PDE10A3 transcripts are major variants in rats. In the present study, immunoblot analysis demonstrated that PDE10A proteins, especially PDE10A2, are more abundant in membrane fractions than in cytosolic fractions of rat striatum. Recombinant PDE10A1 and PDE10A3 were produced only in cytosolic fractions of transfected PC12h cells. By contrast, recombinant PDE10A2 was present mainly in membrane fractions. This finding agreed well with the result of subcellular fractionation of PDE10A in rat striatum. Immunocytochemical analysis showed that PDE10A2 was localized in the Golgi apparatus of transfected PC12h cells. PDE10A2 was phosphorylated by cAMP-dependent protein kinase (PKA) at Thr16. Interestingly, recombinant protein of wild-type PDE10A2, but not PDE10A2 mutant with an Ala replacement at Thr16, was distributed to cytosolic fractions by co-transfection with a plasmid encoding the catalytic subunit of PKA. A PDE10A2 mutant with Glu substitution at Thr16, which can be a mimic of phosphorylation, was localized in the cytosolic fractions of transfected PC12h cells. These observations implied that phosphorylation of PDE10A2 at Thr16 by PKA caused alteration of subcellular localization of PDE10A2 from the Golgi apparatus to cytosol. It is hypothesized that cAMP signaling in the Golgi area and the cytosol in neurons is controlled through alteration of subcellular localization of PDE10A brought by activation of PKA in response to intracellular elevations of cAMP.

PDE4 cAMP phosphodiesterases: modular enzymes that orchestrate signalling cross-talk, desensitization and compartmentalization

cAMP is a second messenger that controls many key cellular functions. The only way to inactivate cAMP is to degrade it through the action of cAMP phosphodiesterases (PDEs). PDEs are thus poised to play a key regulatory role. PDE4 cAMP-specific phosphodiesterases appear to have specific functions with selective inhibitors serving as potent anti-inflammatory agents. The recent elucidation of the structure of the PDE4 catalytic unit allows for molecular insight into the mode of catalysis as well as substrate and inhibitor selectivity. The four PDE4 genes encode over 16 isoforms, each of which is characterized by a unique N-terminal region. PDE4 isoforms play a pivotal role in controlling functionally and spatially distinct pools of cAMP by virtue of their unique intracellular targeting. Targeting occurs by association with proteins, such as arrestins, SRC family tyrosyl kinases, A-kinase anchoring proteins ('AKAPs') and receptor for activated C kinase 1 ('RACK1'), and, in the case of isoform PDE4A1, by a specific interaction (TAPAS-1) with phosphatidic acid. PDE4 isoforms are 'designed' to be regulated by extracellular-signal-related protein kinase (ERK), which binds to anchor sites on the PDE4 catalytic domain that it phosphorylates. The upstream conserved region 1 (UCR1) and 2 (UCR2) modules that abut the PDE4 catalytic unit confer regulatory functions by orchestrating the functional outcome of phosphorylation by cAMP-dependent protein kinase ('PKA') and ERK. PDE4 enzymes stand at a crossroads that allows them to integrate various signalling pathways with that of cAMP in spatially distinct compartments.

Essential role of cAMP-response element-binding protein activation by A2A adenosine receptors in rescuing the nerve growth factor-induced neurite outgrowth impaired by blockage of the MAPK cascade

We found in the present study that stimulation of the A(2A) adenosine receptor (A(2A)-R) using an A(2A)-selective agonist (CGS21680) rescued the blockage of nerve growth factor (NGF)-induced neurite outgrowth when the NGF-evoked MAPK cascade was suppressed by an MEK inhibitor (PD98059) or by a dominant-negative MAPK mutant (dnMAPK). This action of A(2A)-R (designated as the A(2A)-rescue effect) can be blocked by two inhibitors of protein kinase A (PKA) and was absent in a PKA-deficient PC12 variant. Activation of the cAMP/PKA pathway by forskolin exerted the same effect as that by A(2A)-R stimulation. PKA, thus, appears to mediate the A(2A)-rescue effect. Results from cAMP-response element-binding protein (CREB) phosphorylation at serine 133, trans-reporting assays, and overexpression of two dominant-negative CREB mutants revealed that A(2A)-R stimulation led to activation of CREB in a PKA-dependent manner and subsequently reversed the damage of NGF-evoked neurite outgrowth by PD98059 or dnMAPK. Expression of an active mutant of CREB readily rescued the NGF-induced neurite outgrowth impaired by dnMAPK, further strengthening the importance of CREB in the NGF-mediated neurite outgrowth process. Moreover, simultaneous activation of the A(2A)-R/PKA/CREB-mediated and the phosphatidylinositol 3-kinase pathways caused neurite outgrowth that was not suppressed by a selective inhibitor of TrkA, indicating that transactivation of TrkA was not involved. Collectively, CREB functions in conjunction with the phosphatidylinositol 3-kinase pathway to mediate the neurite outgrowth process in PC12 cells.

Age differences in phosphodiesterase type-IV and its functional response to dopamine D1 receptor modulation in the living brain: a PET study in conscious monkeys

The present study demonstrated the age-related changes in the striatal dopamine D1 receptor binding and its related cAMP second-messenger system in the living brains of conscious young (6.4 +/- 1.8 years old) and aged (19.5 +/- 3.3 years old) monkeys (Macaca mulatta) using positron emission tomography (PET). For quantitative analysis of D1 receptors, [11C]SCH23390 was used and phosphodiesterase type-IV (PDE-IV) activity, as an index of cAMP system, was estimated by two scans with R- and S-[11C]rolipram. Significant age-related decreases in D1 receptor binding were observed in the striatum and frontal cortex. Analysis of uptake of R- and S-[11C]rolipram indicated age-related decreases in PDE-IV activity showing 22.0 and 25.2% decreases in the striatum and frontal cortex, respectively, while no significant changes were observed in the cerebellum. With systemic preadministration of the dopamine D1 receptor antagonist SCH23390 (0.2, 0.6, and 2 mg/kg), the PDE-IV activities in the striatum and frontal cortex were dose-dependently suppressed in both age groups. However, the degree of suppression by SCH23390 was more marked in young than in aged monkeys. These results demonstrate that the striatal cAMP second-messenger system activity as well as its functional response to dopamine D1 antagonist showed age-related impairment in the brain.

The antidepressant and antiinflammatory effects of rolipram in the central nervous system

Rolipram is a selective inhibitor of phosphodiesterases (PDE) IV, especially of the subtype PDE IVB. These phosphodiesterases are responsible for hydrolysis of the cyclic nucleotides cAMP and cGMP, particularly in nerve and immune cells. Consequences of rolipram-induced elevation of intracellular cAMP are increased synthesis and release of norepinephrine, which enhance central noradrenergic transmission, and suppress expression of proinflammatory cytokines and other mediators of inflammation. In humans and animals rolipram produces thereby a variety of biological effects. These effects include attenuation of endogenous depression and inflammation in the central nervous system (CNS), both effects are of potential clinical relevance. There are some discrepancies between in vitro and in vivo effects of rolipram, as well as between results obtained in animal models and clinical studies. The clinical use of rolipram is limited because of its behavioral and other side effects. Newly developed selective PDE IV inhibitors with presumably higher potency and lower toxicity are currently under investigation.

Nerve growth factor inhibits PC12 cell PDE 2 phosphodiesterase activity and increases PDE 2 binding to phosphoproteins

Nerve growth factor (NGF) has been shown to increase cyclic AMP in PC12 cells and to potentiate the actions of other agents that raise cyclic AMP. In our studies, NGF causes over 50% loss of PDE 2 activity (cyclic GMP-stimulated cyclic nucleotide phosphodiesterase) in PC12 cells within 24 h. After 72 h of NGF treatment, cyclic AMP hydrolysis in PC12 extracts is no longer cyclic GMP-stimulated. NGF deprivation increases the phosphodiesterase activity of treated cells. NGF does not decrease either PDE 2 mRNA or immunoreactivity of PDE 2A2 protein. Incubation of whole cells with micromolar Na(3)VO(4) mimics NGF treatment, reducing PDE 2 activity in PC12 cells by over 50% after 24 h, suggesting a phosphoprotein-mediated regulation of PDE 2 activity. Protein kinase inhibitor effects were difficult to assess due to their direct interaction with the PDE in cell lysates. To study phosphorylation in PDE 2 regulation, PDE 2A2 was epitope-tagged, and stable clonal PC12 cell transfectants were isolated (PC12B cells). When combined with metabolically labeled (32)P-phosphoproteins in vivo or in vitro, phosphoproteins of 108, 90, 64, 43, 33 and 19 kDa coprecipitated with epitope-tagged PDE 2A2 in an NGF sensitive manner. A 23-kDa phosphoprotein containing immunoreactive phosphoserine associated with the complex in an NGF independent manner. Phosphothreonine plus phosphotyrosine immunoreactivity at 23, 24, and 64 kDa as well as the phosphotyrosine immunoreactivity at 108, 90, 64, 43, 33, and 19 kDa required NGF or orthovanadate treatment. These proteins are hypothesized to be part of an NGF-regulated complex controlling PDE 2A2 activity.

Nerve growth factor induces sphingomyelin accumulation in pheochromocytoma cells

The pheochromocytoma cells are a well-known model for studying the nerve growth factor (NGF)-induced molecular changes during the differentiation process. The involvement of sphingomyelin (SM) was studied using the fluorescent analogue of ceramide, i.e. N-lissamine rhodaminyl-(12-aminododecanoyl) D-erythro-sphingosine (C12-LRh-Cer). This fluorescent analogue is metabolically active and can be used to follow the biosynthesis of SM in intact cells. NGF induces a 4-fold increase of fluorescent SM content in PC12 cells, when loaded with C12-LRh-Cer. Treatment of PC12 cells with actinomycin D or cycloheximide completely abolishes the NGF-induced elevation of SM. Inhibition of p140(trkA) receptor by AG-879 prevents extracellular signal-regulated kinase 1/2 phosphorylation and suppresses the increase of SM. Inhibition of protein kinase C (PKC), protein kinase A (PKA) and phosphatidylinositol 3-kinase does not have any effect on NGF-induced C12-LRh-SM accumulation. On the other hand, activation of PKA or PKC with simultaneous treatment with NGF has a synergistic effect on increase of SM content. The NGF-induced SM increase in PC12 cells is an effect promoted by other differentiating agents like dibutyryl cyclic AMP or fibroblast growth factor-2 but not by a mitogenic agent like epidermal growth factor.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

Effects of noradrenergic lesions on the development of rolipram-sensitive, low-K(m), cyclic AMP specific phosphodiesterase in rat brain

Rolipram-sensitive, low-K(m)80% loss of norepinephrine in cerebral cortex) without affecting dopaminergic systems. The lesions resulted in temporary reduction of PDE4 activity in cerebral cortex, cerebellum and brainstem. Lesions in the adult rats, on the other hand, did not alter PDE4 activity. Decreased PDE4 activity by neonatal noradrenergic lesions was due to a decrease in the V(max) of cAMP hydrolysis by PDE4, and not a change in the K(m) values. Immunoblot analysis showed that decreased PDE4 activity in cerebellum was associated with reduced expression of PDE4A5, PDE4A1, and several PDE4B variants. No change in the expression of any PDE4 subtype in cerebral cortex was detected with the antibodies used in this study. Neither the permanent loss of noradrenergic innervation in cerebral cortex, nor the permanent noradrenergic hyperinnervation in brainstem was accompanied by any permanent change in PDE4 activity. Decreasing PDE4 activity early after neonatal noradrenergic lesions might be important in maintaining constant concentrations of cAMP, which is critical for the cellular proliferation and differentiation that is active during this period.

The N terminus domain of type VI adenylyl cyclase mediates its inhibition by protein kinase C

Previous results from our laboratory have shown that phosphorylation of type VI adenylyl cyclase (ACVI) by protein kinase C (PKC) caused suppression of adenylyl cyclase activity. In the present study, we investigated the role of the N terminus cytosolic domain of ACVI in this PKC-mediated inhibition of ACVI. Removal of amino acids 1 to 86 of ACVI or mutation of Ser(10) (a potential PKC phosphorylation site) into alanine significantly relieved the PKC-mediated inhibition and markedly reduced the PKC-evoked protein phosphorylation. PKC also effectively phosphorylated a recombinant N terminus cytosolic domain (amino acids 1-160) protein of ACVI and a synthetic peptide representing Ser(10). In addition, the amino acids 1 to 86 truncated mutant exhibited kinetic properties similar to those of the wild type. Taken together, these data demonstrate that the highly variable N terminus cytoplasmic domain of ACVI is a regulatory domain with a critical role in PKC-mediated suppression, which is a hallmark of this adenylyl cyclase isozyme. In addition, Ser(10) was found to serve as an acceptor for the PKC-mediated phosphorylating transfer of ACVI.

Adenosine A2A receptor interactions with receptors for other neurotransmitters and neuromodulators

Adenosine, by activating adenosine A2A receptors, seems to have a crucial function in regulating the activation of multiple receptors that affect neurotransmitter release and/or synaptic transmission, in particular receptors for neuropeptides (calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP)), and NMDA receptors, metabotropic glutamate receptors, nicotinic autofacilitatory receptors, dopamine receptors and adenosine A1 receptors. The manner in which these A2A receptors are involved in interactions with the receptors for other neurotransmitters and or neuromodulators opens novel avenues for the action of this 'omnipresent' nucleoside. Either by direct receptor-receptor modulation or by post-receptor mechanisms, adenosine, in its 'obsession' to protect cells from insults, uses as many receptor systems as possible to synchronize synaptic transmission, in order to exert what seems to be the 'destiny' of this nucleoside--protection of the nervous system.

Involvement of P1 receptors in the effect of forskolin on cyclic AMP accumulation and export in PC12 cells

In PC12 cells, forskolin as well as the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased intracellular adenosine-3',5'-cyclic monophosphate (cyclic AMP) levels, which peaked at 45-60 minutes and declined thereafter. Maximum levels were 3000 and 1700 pmol/10(6) cells during treatment with 10 microM forskolin or 0.1 microM NECA, respectively. Extracellular cyclic AMP rose with time, at mean rates of 24.7 (forskolin) and 11.3 (NECA) pmol/min/10(6) cells. With either drug, a linear correlation was obtained between the calculated time integral of intracellular cyclic AMP and the measured extracellular cyclic AMP levels, indicating that the outflow of cyclic AMP was sustained by a nonsaturated transport system. The ability of forskolin to increase intracellular and extracellular cyclic AMP levels was hindered in a concentration-dependent manner by 8-(p-sulfophenyl)theophylline (8-SPT). A similar inhibition was exerted by other two adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine and 3,7-dimethyl-1-propargylxanthine. The concentration-response curve to adenosine was shifted to the right by 25 microM 8-SPT, whereas that of forskolin was shifted downwards. Adenosine deaminase (ADA, EC 3.5.44, 1 U/mL) reduced the intracellular cyclic AMP response to forskolin by 68%, whereas the adenosine transport inhibitor, dipyridamole (10 microM), significantly increased 1 and 10 microM forskolin-dependent cyclic AMP accumulation. Erythro-9-(2-hydroxy-3-nonyl)adenine (10 microM), an inhibitor of ADA, and alpha,beta-methyleneadenosine 5'-diphosphate (100 microM), an inhibitor of ecto-5'-nucleotidase, did not alter forskolin activity. These results demonstrate that a cyclic AMP extrusion system operates in PC12 cells during adenylyl cyclase stimulation by forskolin and that this stimulation involves a synergistic interaction with endogenous adenosine. However, extruded cyclic AMP does not appear to significantly contribute to the formation of the endogenous adenosine pool.

Imaging of cAMP-specific phosphodiesterase-IV: comparison of [11C]rolipram and [11C]Ro 20-1724 in rats

The phosphodiesterase type IV (PDEIV) family of enzymes contributes to the metabolism of cAMP formed by the stimulation of beta-adrenergic, A2-adenosine, and H2-histamine receptors in the brain. Disturbances in cAMP-mediated signaling have been implicated in several neuropsychiatric disorders, and there is evidence that increasing cAMP levels through PDEIV inhibition improves the symptoms of these disorders. In the present study, the selective PDEIV inhibitors rolipram and Ro 20-1724, labeled with C-11, were evaluated in vivo in rats, as potential radioligands for imaging PDEIV enzymes with positron emission tomography (PET). Biodistribution experiments revealed a greater than threefold increased regional brain retention of [11C]rolipram as compared to [11C]Ro 20-1724. [nC]Rolipram uptake was higher in rat brain areas (e.g., cortical regions and olfactory system) showing higher expression of PDEIV enzymes, as determined previously using [3H]rolipram autoradiography or molecular genetic techniques. Binding of [n1C]rolipram and [11C]Ro 20-1724 was specific, since coadministration of high doses of unlabeled rolipram (10 mg/Kg, i.v.) or Ro 20-1724 (30 mg/Kg with [11C]rolipram and 10 mg/Kg with [11C]Ro 20-1724, i.v.) reduced radioactivity uptake in brain regions. Pretreatment with high doses of the PDEI selective inhibitor vinpocetine (10 mg/Kg, i.p., 15 min prior), or the noradrenaline reuptake inhibitor desipramine (10 mg/Kg, i.p., 30 min prior), or coinjection with the adenylyl cyclase activator forskolin (6.5 or 15 mg/Kg, i.v.), did not inhibit [11C]rolipram uptake in brain areas, suggesting binding selectivity for PDEIV enzymes. We conclude that [11C]rolipram, but not [11C]Ro 20-1724, is a promising radioligand for imaging the PDEIV enzymes with PET.

Expression of type VI adenylyl cyclase in the central nervous system: implication for a potential regulator of multiple signals in different neurotransmitter systems

The aim of this study was to investigate the distribution of a calcium-inhibitable adenylyl cyclase type VI (type VI AC) in the central nervous system using an antiserum directed against the N-terminus of type VI AC. Our results indicate that type VI AC immunoreactivity is generally expressed in many brain regions with different levels of intensity. Most interestingly, the majority of the detected type VI AC immunoreactivity is present in cells of neuronal phenotype. Double immunostaining of type VI AC and markers of various neurotransmitter systems suggest that type VI AC might participate in regulation of the classical neurotransmitter systems and therefore appeared to play a very important role in the central nervous system.