Differential regulation of bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase isoenzymes by cyclic AMP-dependent protein kinase and calmodulin-dependent phosphatase

Genetic mechanisms for impaired synaptic plasticity in schizophrenia revealed by computational modelling

Schizophrenia phenotypes are suggestive of impaired cortical plasticity in the disease, but the mechanisms of these deficits are unknown. Genomic association studies have implicated a large number of genes that regulate neuromodulation and plasticity, indicating that the plasticity deficits have a genetic origin. Here, we used biochemically detailed computational modelling of post-synaptic plasticity to investigate how schizophrenia-associated genes regulate long-term potentiation (LTP) and depression (LTD). We combined our model with data from post-mortem mRNA expression studies (CommonMind gene-expression datasets) to assess the consequences of altered expression of plasticity-regulating genes for the amplitude of LTP and LTD. Our results show that the expression alterations observed post mortem, especially those in anterior cingulate cortex, lead to impaired PKA-pathway-mediated LTP in synapses containing GluR1 receptors. We validated these findings using a genotyped EEG dataset where polygenic risk scores for synaptic and ion channel-encoding genes as well as modulation of visual evoked potentials (VEP) were determined for 286 healthy controls. Our results provide a possible genetic mechanism for plasticity impairments in schizophrenia, which can lead to improved understanding and, ultimately, treatment of the disorder.

Hippocampal AMPA- and NMDA-induced cGMP signals are mainly generated by NO-GC2 and are under tight control by PDEs 1 and 2

In the central nervous system, the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signalling cascade has an established role in fine-tuning of synaptic transmission. In the present study, we asked which isoform of NO-sensitive guanylyl cyclase, NO-GC1 or NO-GC2, is responsible for generation of N-methyl-d-aspartate (NMDA)- and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-induced cGMP signals and which of the phosphodiesterases (PDEs) is responsible for degradation. To this end, we performed live cell fluorescence measurements of primary hippocampal neurons isolated from NO-GC isoform-deficient mice. Although both isoforms contributed to the NMDA- and AMPA-induced cGMP signals, NO-GC2 clearly played the predominant role. Whereas under PDE-inhibiting conditions the cGMP levels elicited by both glutamatergic ligands were comparable, NMDA-induced cGMP signals were clearly higher than the AMPA-induced ones in the absence of PDE inhibitors. Thus, AMPA-induced cGMP signals are more tightly controlled by PDE-mediated degradation than NMDA-induced signals. In addition, these findings are compatible with the existence of at least two different pools of cGMP in both of which PDE1 and PDE2-known to be highly expressed in the hippocampus-are mainly responsible for cGMP degradation. The finding that distinct pools of cGMP are equipped with different amounts of PDEs highlights the importance of PDEs for the shape of NO-induced cGMP signals in the central nervous system.

FRET-based cyclic GMP biosensors measure low cGMP concentrations in cardiomyocytes and neurons

Several FRET (fluorescence resonance energy transfer)-based biosensors for intracellular detection of cyclic nucleotides have been designed in the past decade. However, few such biosensors are available for cGMP, and even fewer that detect low nanomolar cGMP concentrations. Our aim was to develop a FRET-based cGMP biosensor with high affinity for cGMP as a tool for intracellular signaling studies. We used the carboxyl-terminal cyclic nucleotide binding domain of Plasmodium falciparum cGMP-dependent protein kinase (PKG) flanked by different FRET pairs to generate two cGMP biosensors (Yellow PfPKG and Red PfPKG). Here, we report that these cGMP biosensors display high affinity for cGMP (EC50 of 23 ± 3 nM) and detect cGMP produced through soluble guanylyl cyclase and guanylyl cyclase A in stellate ganglion neurons and guanylyl cyclase B in cardiomyocytes. These biosensors are therefore optimal tools for real-time measurements of low concentrations of cGMP in living cells.

Phosphodiesterase 1: A Unique Drug Target for Degenerative Diseases and Cognitive Dysfunction

The focus of this chapter is on the cyclic nucleotide phosphodiesterase 1 (PDE1) family. PDE1 is one member of the 11 PDE families (PDE 1-11). It is the only phosphodiesterase family that is calcium/calmodulin activated. As a result, whereas other families of PDEs 2-11 play a dominant role controlling basal levels of cyclic nucleotides, PDE1 is involved when intra-cellular calcium levels are elevated and, thus, has an "on demand" or activity-dependent involvement in the control of cyclic nucleotides in excitatory cells including neurons, cardiomyocytes and smooth muscle. As a Class 1 phosphodiesterase, PDE1 hydrolyzes the 3' bond of 3'-5'-cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Here, we review evidence for this family of enzymes as drug targets for development of therapies aimed to address disorders of the central nervous system (CNS) and of degenerative diseases. The chapter includes sections on the potential for cognitive enhancement in mental disorders, as well as a review of PDE1 enzyme structure, enzymology, tissue distribution, genomics, inhibitors, pharmacology, clinical trials, and therapeutic indications. Information is taken from public databases. A number of excellent reviews of the phosphodiesterase family have been written as well as reviews of the PDE1 family. References cited here are not comprehensive, rather pointing to major reviews and key publications.

Cyclic nucleotide phosphodiesterase 1 and vascular aging

VSMCs (vascular smooth muscle cells) play critical roles in arterial remodelling with aging, hypertension and atherosclerosis. VSMCs exist in diverse phenotypes and exhibit phenotypic plasticity, e.g. changing from a quiescent/contractile phenotype to an active myofibroblast-like, often called ‘synthetic’, phenotype. Synthetic VSMCs are able to proliferate, migrate and secrete ECM (extracellular matrix) proteinases and ECM proteins. In addition, they produce pro-inflammatory molecules, providing an inflammatory microenvironment for leucocyte penetration, accumulation and activation. The aging VSMCs have also shown changes in cellular phenotype, responsiveness to contracting and relaxing mediators, replicating potential, matrix synthesis, inflammatory mediators and intracellular signalling. VSMC dysfunction plays a key role in age-associated vascular remodelling. Cyclic nucleotide PDEs (phosphodiesterases), by catalysing cyclic nucleotide hydrolysis, play a critical role in regulating the amplitude, duration and compartmentalization of cyclic nucleotide signalling. Abnormal alterations of PDEs and subsequent changes in cyclic nucleotide homoeostasis have been implicated in a number of different diseases. In the study published in the latest issue of Clinical Science, Bautista Niño and colleagues have shown that, in cultured senescent human VSMCs, PDE1A and PDE1C mRNA levels are significantly up-regulated and inhibition of PDE1 activity with vinpocetine reduced cellular senescent makers in senescent VSMCs. Moreover, in the premature aging mice with genomic instability (Ercc1^d/−), impaired aortic ring relaxation in response to SNP (sodium nitroprusside), an NO (nitric oxide) donor, was also largely improved by vinpocetine. More interestingly, using data from human GWAS (genome-wide association studies), it has been found that PDE1A single nucleotide polymorphisms is significantly associated with diastolic blood pressure and carotid intima–media thickening, two hallmarks of human vascular dysfunction in aging. These findings establish a strong relationship between PDE1 expression regulation and vascular abnormalities in aging.

Phosphodiesterase 10A Is Tethered to a Synaptic Signaling Complex in Striatum

Phosphodiesterase 10A (PDE10A) is a dual substrate PDE that can hydrolyze both cGMP and cAMP. In brain, PDE10A is almost exclusively expressed in the striatum. In several studies, PDE10A has been implicated in regulation of striatal output using either specific inhibitors or PDE10A knock-out mice and has been suggested as a promising target for novel antipsychotic drugs. In striatal medium spiny neurons, PDE10A is localized at the plasma membrane and in dendritic spines close to postsynaptic densities. In the present study, we identify PDE10A as the major cAMP PDE in mouse striatum and monitor PKA-dependent PDE10A phosphorylation. With recombinantly expressed PDE10A we demonstrate that phosphorylation does not alter PDE10A activity. In striatum, PDE10A was found to be associated with the A kinase anchoring protein AKAP150 suggesting the existence of a multiprotein signaling complex localizing PDE10A to a specific functional context at synaptic membranes. Furthermore, the cAMP effector PKA, the NMDA receptor subunits NR2A and -B, as well as PSD95, were tethered to the complex. In agreement, PDE10A was almost exclusively found in multiprotein complexes as indicated by migration in high molecular weight fractions in size exclusion chromatography. Finally, affinity of PDE10A to the signaling complexes formed around AKAP150 was reduced by PDE10A phosphorylation. The data indicate that phosphorylation of PDE10 has an impact on the interaction with other signaling proteins and adds an additional line of complexity to the role of PDE10 in regulation of synaptic transmission.

Phosphodiesterases and cyclic GMP regulation in heart muscle

The cyclic nucleotide cGMP and its corresponding activated kinase cGK-1 serve as a counterbalance to acute and chronic myocardial stress. cGMP hydrolysis by several members of the phosphodiesterase (PDE) superfamily, PDE1, PDE2, and PDE5, regulate this signaling in the heart. This review details new insights regarding how these PDEs modulate cGMP and cGK-1 to influence heart function and chronic stress responses, and how their inhibition may provide potential therapeutic benefits.

Nitric oxide synthase and cyclic GMP signaling in cardiac myocytes: from contractility to remodeling

Cyclic guanosine 3'5'monophosphate (cGMP) is the common downstream second messenger of natriuretic peptides and nitric oxide. In cardiac myocytes, the physiological effects of cGMP are exerted through the activation of protein kinase G (PKG) signaling, and the activation and/or inhibition of phosphodiesterases (PDEs), providing an integration point between cAMP and cGMP signals. Specificity of cGMP signals is achieved through compartmentalization of cGMP synthesis by guanylate cyclases, and cGMP hydrolysis by PDEs. Increasing evidence suggests that cGMP-dependent signaling pathways play an important role in inhibiting cardiac remodeling, through the inhibition Ca(2+) handling upstream of pathological Ca(2+)-dependent signaling pathways. Thus, enhancing cardiac myocyte cGMP signaling represents a promising therapeutic target for treatment of cardiovascular disease. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

Mammalian Cyclic Nucleotide Phosphodiesterases: Molecular Mechanisms and Physiological Functions

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

Allosteric-site and catalytic-site ligand effects on PDE5 functions are associated with distinct changes in physical form of the enzyme

Native phosphodiesterase-5 (PDE5) homodimer contains distinct non-catalytic cGMP allosteric sites and catalytic sites for cGMP hydrolysis. Purified recombinant PDE5 was activated by pre-incubation with cGMP. Relatively low concentrations of cGMP produced a Native PAGE gel shift of PDE5 from a single band position (lower band) to a band with decreased mobility (upper band); higher concentrations of cGMP produced a band of intermediate mobility (middle band) in addition to the upper band. Two point mutations (G659A and G659P) near the catalytic site that reduced affinity for cGMP substrate retained allosteric cGMP-binding affinity like that of WT PDE5 but displayed cGMP-induced gel shift only to the middle-band position. The upper band could represent a form produced by cGMP binding to the catalytic site, while the middle band could represent a form produced by cGMP binding to the allosteric site. Millimolar cGMP was required for gel shift of PDE5 when added to the pre-incubation before Native PAGE, presumably due to removal of most of the cGMP during electrophoresis, but micromolar cGMP was sufficient for this effect if cGMP was included in the native gel buffer. cGMP-induced gel shift was associated with stimulation of PDE5 catalytic activity, and the rates of onset and reversibility of this effect suggested that it was due to cGMP binding to the allosteric site. Incubation of PDE5 with non-hydrolyzable, catalytic site-specific, substrate analogs such as the inhibitors sildenafil and tadalafil, followed by dilution, did not produce activation of catalytic activity like that obtained with cGMP, although both inhibitors produced a similar gel shift to the upper band as that obtained with cGMP. This implied that occupation of the catalytic site alone can produce a gel shift to the upper band. PDE5 activation or gel shift was reversed by lowering cGMP with dilution followed by at least 1h of incubation. Such slow reversibility could prolong effects of cGMP on PDE5 in cells after decline of this nucleotide. Reversal was also achieved by Mg(++) addition to the pre-incubation mixture to promote cGMP degradation, but Mg(++) addition did not reverse the gel shift caused by sildenafil, which is not hydrolyzed by PDE5. Upon extensive dilution, the effect of tadalafil, a potent PDE5 inhibitor, to enhance catalytic-site affinity for this inhibitor was rapidly reversed. Thus, kinetic effect of binding of a high-affinity PDE5 inhibitor to the catalytic site is more readily reversible than that obtained by cGMP binding to the allosteric site. It is concluded that cGMP or PDE5 inhibitor binding to the catalytic site, or ligand binding to both the catalytic site and allosteric site simultaneously, changes PDE5 to a similar physical form; this form is distinct from that produced by cGMP binding to the allosteric site, which activates the enzyme and reverses more slowly.

ABCD of the phosphodiesterase family: interaction and differential activity in COPD

Phosphodiesterases (PDEs) are important enzymes that hydrolyze the cyclic nucleotides adenosine 3'5'-cyclic monophosphate (cAMP) and guanosine 3'5'-cyclic monophosphate (cGMP) to their inactive 5' monophosphates. They are highly conserved across species and as well as their role in signal termination, they also have a vital role in intra-cellular localization of cyclic nucleotide signaling and integration of the cyclic nucleotide pathways with other signaling pathways. Because of their pivotal role in intracellular signaling, they are now of considerable interest as therapeutic targets in a wide variety diseases, including COPD where PDE inhibitors may have bronchodilator, anti-inflammatory and pulmonary vasodilator actions. This review examines the diversity and cellular localization of the isoforms of PDE, the known and speculative relevance of this to the treatment of COPD, and the range of PDE inhibitors in development together with a discussion of their possible role in treating COPD.

Biochemistry and physiology of cyclic nucleotide phosphodiesterases: essential components in cyclic nucleotide signaling

Although cyclic nucleotide phosphodiesterases (PDEs) were described soon after the discovery of cAMP, their complexity and functions in signaling is only recently beginning to become fully realized. We now know that at least 100 different PDE proteins degrade cAMP and cGMP in eukaryotes. A complex PDE gene organization and a large number of PDE splicing variants serve to fine-tune cyclic nucleotide signals and contribute to specificity in signaling. Here we review some of the major concepts related to our understanding of PDE function and regulation including: (a) the structure of catalytic and regulatory domains and arrangement in holoenzymes; (b) PDE integration into signaling complexes; (c) the nature and function of negative and positive feedback circuits that have been conserved in PDEs from prokaryotes to human; (d) the emerging association of mutant PDE alleles with inherited diseases; and (e) the role of PDEs in generating subcellular signaling compartments.

Cyclic Nucleotide Phosphodiesterases: Molecular Regulation to Clinical Use

Cyclic nucleotide phosphodiesterases (PDEs) are enzymes that regulate the cellular levels of the second messengers, cAMP and cGMP, by controlling their rates of degradation. There are 11 different PDE families, with each family typically having several different isoforms and splice variants. These unique PDEs differ in their three-dimensional structure, kinetic properties, modes of regulation, intracellular localization, cellular expression, and inhibitor sensitivities. Current data suggest that individual isozymes modulate distinct regulatory pathways in the cell. These properties therefore offer the opportunity for selectively targeting specific PDEs for treatment of specific disease states. The feasibility of these enzymes as drug targets is exemplified by the commercial and clinical successes of the erectile dysfunction drugs, sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra). PDE inhibitors are also currently available or in development for treatment of a variety of other pathological conditions. In this review the basic biochemical properties, cellular regulation, expression patterns, and physiological functions of the different PDE isoforms will be discussed. How these properties relate to the current and future development of PDE inhibitors as pharmacological agents is especially considered. PDEs hold great promise as drug targets and recent research advances make this an exciting time for the field of PDE research.

Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents

Cyclic nucleotide phosphodiesterases (PDEs), which are ubiquitously distributed in mammalian tissues, play a major role in cell signaling by hydrolyzing cAMP and cGMP. Due to their diversity, which allows specific distribution at cellular and subcellular levels, PDEs can selectively regulate various cellular functions. Their critical role in intracellular signaling has recently designated them as new therapeutic targets for inflammation. The PDE superfamily represents 11 gene families (PDE1 to PDE11). Each family encompasses 1 to 4 distinct genes, to give more than 20 genes in mammals encoding the more than 50 different PDE proteins probably produced in mammalian cells. Although PDE1 to PDE6 were the first well-characterized isoforms because of their predominance in various tissues and cells, their specific contribution to tissue function and their regulation in pathophysiology remain open research fields. This concerns particularly the newly discovered families, PDE7 to PDE11, for which roles are not yet established. In many pathologies, such as inflammation, neurodegeneration, and cancer, alterations in intracellular signaling related to PDE deregulation may explain the difficulties observed in the prevention and treatment of these pathologies. By inhibiting specifically the up-regulated PDE isozyme(s) with newly synthesized potent and isozyme-selective PDE inhibitors, it may be potentially possible to restore normal intracellular signaling selectively, providing therapy with reduced adverse effects.

Ca2+-calmodulin-dependent phosphodiesterase (PDE1): Current perspectives

Ca2+-calmodulin-dependent phosphodiesterases (PDE1), like Ca2+-sensitive adenylyl cyclases (AC), are key enzymes that play a pivotal role in mediating the cross-talk between cAMP and Ca2+ signalling. Our understanding of how ACs respond to Ca2+ has advanced greatly, with significant breakthroughs at both the molecular and functional level. By contrast, little is known of the mechanisms that might underlie the regulation of PDE1 by Ca2+ in the intact cell. In living cells, Ca2+ signals are complex and diverse, exhibiting different spatial and temporal properties. The potential therefore exists for dynamic changes in the subcellular distribution and activation of PDE1 in relation to intracellular Ca2+ dynamics. PDE1s are a large family of multiply-spliced gene products. Therefore, it is possible that a cell-type specific response to elevation in [Ca2+]i can occur, depending on the isoform of PDE1 expressed. In this article, we summarize current knowledge on Ca2+ regulation of PDE1 in the intact cell and discuss approaches that might be undertaken to delineate the responses of this important group of enzymes to changes in [Ca2+]i.

Phosphodiesterases in the vascular system

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a variety of extracellular stimuli in several tissues. In the vascular system, these nucleotides play important roles in the regulation of vascular tone and in the maintenance of the mature contractile phenotype in smooth muscle cells. Given that cyclic nucleotide signaling regulates a wide variety of cellular functions, it is not surprising that cyclic nucleotide phosphodiesterases (PDEs). In paticular, the accumulating data showing that there are a large number of different PDE isozymes have triggered an equally large increase in interest about these enzymes. At least 11 different gene families of PDEs are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the important aspects about the structure, cellular localization, and regulation of each family of PDEs. Particular emphasis is placed on new information obtained in the last few years about vascular disease. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locations will lead to new therapeutic uses for this class of drugs in diseases of the vascular system.

Phosphodiesterase 1B knock-out mice exhibit exaggerated locomotor hyperactivity and DARPP-32 phosphorylation in response to dopamine agonists and display impaired spatial learning

Using homologous recombination, we generated mice lacking phosphodiesterase-mediated (PDE1B) cyclic nucleotide-hydrolyzing activity. PDE1B(-/-) mice showed exaggerated hyperactivity after acute D-methamphetamine administration. Striatal slices from PDE1B(-/-) mice exhibited increased levels of phospho-Thr34 DARPP-32 and phospho-Ser845 GluR1 after dopamine D1 receptor agonist or forskolin stimulation. PDE1B(-/-) and PDE1B(+/-) mice demonstrated Morris maze spatial-learning deficits. These results indicate that enhancement of cyclic nucleotide signaling by inactivation of PDE1B-mediated cyclic nucleotide hydrolysis plays a significant role in dopaminergic function through the DARPP-32 and related transduction pathways.

Phosphodiesterase 1B knock-out mice exhibit exaggerated locomotor hyperactivity and DARPP-32 phosphorylation in response to dopamine agonists and display impaired spatial learning

Using homologous recombination, we generated mice lacking phosphodiesterase-mediated (PDE1B) cyclic nucleotide-hydrolyzing activity. PDE1B(-/-) mice showed exaggerated hyperactivity after acute D-methamphetamine administration. Striatal slices from PDE1B(-/-) mice exhibited increased levels of phospho-Thr34 DARPP-32 and phospho-Ser845 GluR1 after dopamine D1 receptor agonist or forskolin stimulation. PDE1B(-/-) and PDE1B(+/-) mice demonstrated Morris maze spatial-learning deficits. These results indicate that enhancement of cyclic nucleotide signaling by inactivation of PDE1B-mediated cyclic nucleotide hydrolysis plays a significant role in dopaminergic function through the DARPP-32 and related transduction pathways.

Functional plasticity of cyclic AMP hydrolysis in rat adenohypophysial corticotroph cells

Characterisation of cyclic nucleotide-hydrolysing phosphodiesterases (PDEs) in recombinant systems has highlighted regulatory properties indicative of distinct physiological roles for these enzymes. The present study investigated the role of PDEs in the adenosine 3'5'-monophosphate (cAMP) response to the hypothalamic neuropeptides corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) in acutely dispersed rat adenohypophysial cells. Ca(2+)-activated PDE (PDE1) and Ca(2+)-independent, rolipram-sensitive PDE (PDE4) accounted for close to 90% of cAMP-hydrolysing activity in the adenohypophysis. Messenger RNA transcripts of PDE1 (isotypes 1A and 1C) and PDE4 (isotypes B and D3) were detected by RT-PCR. The PDE blockers rolipram and IBMX enhanced cAMP accumulation induced by CRF or CRF and AVP. Vinpocetine, an inhibitor of low K(m) PDE1 isotypes, did not alter the response to CRF but enhanced the effect of the combined CRF/AVP stimulus. Thus, PDE4s terminate the cAMP response to moderate stimulation, while low-affinity PDE1 becomes important when the concentrations of CRF and AVP are characteristic of exposure to intensive stress.

Isolation and differential tissue distribution of two human cDNAs encoding PDE1 splice variants

A cDNA selection technique has been used to isolate full-length human cDNAs of the phosphodiesterase 1 (PDE1) calcium calmodulin (CaM)-regulated phosphodiesterase gene family. We isolated cDNAs representing multiple splice variants of PDE1A, 1B and 1C from a variety of tissues. Included among these were two novel splice variants for PDE1A and 1B. The first, PDE1A5, encodes a 519-residue protein, which is different from PDE1A1 by the insertion of 14 residues, a divergent carboxy terminus and also differs from PDE1A3 through a divergent amino terminus. Our second novel splice variant represents the first occurrence of a splice variant of the PDE1B gene. PDE1B2 encodes a 516-residue protein and diverges from PDE1B1 by the replacement of the first 38 residues by an alternative 18, which is predicted to be functionally significant. Using the splice variant sequence differences to perform comparative Northern analysis, we have demonstrated that each variant has a differential tissue distribution.

Calmodulin-dependent cyclic nucleotide phosphodiesterase in an experimental rat model of cardiac ischemia-reperfusion

In the present study, we investigated the activity and expression of calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE) and the effects of calpains in rat heart after ischemia and reperfusion. Immunohistochemical studies indicated that CaMPDE in normal heart is localized in myocardial cells. Rat ischemic heart showed a decrease in CaMPDE activity in the presence of Ca2+ and calmodulin; however, in ischemic-reperfusion tissue a progressive increase in Ca2+ and calmodulin-independent cyclic nucleotide phosphodiesterase (CaM-independent PDE) activity was observed. Perfusion of hearts with cell-permeable calpain inhibitor suppressed the increase of Ca2+ and CaM-independent PDE activity. Protein expression of CaMPDE was uneffected by hypoxic injury to rat myocardium. The purified heart CaMPDE was proteolyzed by calpains into a 45 kDa immunoreactive fragment in vitro. Based on these results, we propose that hypoxic injury to rat myocardium results in the generation of CaM-independent PDE by calpain mediated proteolysis, allowing the maintenance of cAMP concentrations within the physiological range.

Identification of human PDE7B, a cAMP-specific phosphodiesterase

We isolated a human cAMP-specific phosphodiesterase (PDE7B) cDNA from human caudate nucleus. The human PDE7B was composed of 450 amino acid residues with a molecular mass of 51,835 Da. The deduced amino acid sequence of human PDE7B was 64.1% identical to that of human PDE7A (67.1% identity in the catalytic region). Northern blot analysis demonstrated that PDE7B transcripts were abundantly expressed in the putamen, caudate nucleus, and heart followed by skeletal muscle, pancreas, and occipital pole. Recombinant PDE7B expressed in transfected COS-7 cells had a low cAMP K(m) value of 0. 13 microM, which is similar to the K(m) value of recombinant human PDE7A expressed in transfected COS-7 cells. Interestingly, the relative V(max) value of recombinant PDE7B was half to one-third of recombinant PDE7A. The PDE7B activity was inhibited by dipyridamole and SCH51866, with IC(50) values of 1.1 microM and 1.5 microM, respectively. Thus, the PDE7B exhibited unique tissue distribution in humans and kinetic profiles. Human PDE7B showed the lowest K(m) values compared to the other cAMP-hydrolyzing PDEs which have been reported to be expressed in the brain. Therefore, human PDE7B may be involved in the control of cAMP-mediated neural activity and cAMP metabolism in the brain.

Characterization and phosphorylation of PDE10A2, a novel alternative splice variant of human phosphodiesterase that hydrolyzes cAMP and cGMP

We have isolated a novel alternative splice variant of human cAMP- and cGMP-hydrolyzing phosphodiesterase (PDE10A2) from human fetal lung. The N-terminal sequence of human PDE10A2 differed from that of human PDE10A1 reported previously. PDE10A1 and PDE10A2 expressed in COS-7 cells have cGMP K(m) values of 14 and 13 microM, low cAMP K(m) values of 0.28 and 0.22 microM, and high cAMP K(m) values of 0.96 and 1.1 microM, respectively, at high concentrations of cGMP and cAMP. PCR analysis demonstrated that both PDE10A1 and PDE10A2 transcripts are present in various human tissues and that PDE10A2 transcripts are a major form in some human tissues. The unique N-terminus of PDE10A2 has a putative phosphorylation site by cAMP-dependent protein kinase (cAK), but PDE10A1 does not. The recombinant PDE10A2 protein is preferentially phosphorylated by cAK, although the recombinant PDE10A1 protein is not phosphorylated by cAK.

Isolation and characterization of PDE10A, a novel human 3', 5'-cyclic nucleotide phosphodiesterase

A gene encoding a novel human 3', 5'-cyclic nucleotide phosphodiesterase (PDE) was identified and characterized. PDE10A1 encodes a protein that is 779 amino acids in length. An incomplete cDNA for a second 5'-splice variant, PDE10A2, was isolated. The proteins encoded by the two variants share 766 amino acids in common. This common region includes an amino-terminal domain with partial homology to the cGMP-binding domains of PDE2, PDE5 and PDE6 as well as a carboxy-terminal region with homology to the catalytic regions of mammalian PDEs. Northern analysis revealed that PDE10A is widely expressed. The PDE10A gene was mapped to three yeast artificial chromosomes (YACs) that contain human DNA from chromosome 6q26-27. A recombinant protein corresponding to the 766 amino acid region common to PDE10A1 and PDE10A2 was expressed in yeast. It hydrolyzed both cAMP and cGMP. Inhibitors that are selective for other PDE families are poor inhibitors of PDE10A; however, PDE10A is inhibited by the non-specific PDE inhibitor, IBMX.

Calcineurin. Structure, function, and inhibition

Calcineurin is a serine-threonine specific Ca(2+)-calmodulin-activated protein phosphatase that is conserved from yeast to humans. Remarkably, this enzyme is the common target for two novel and structurally unrelated immunosuppressive antifungal drugs, cyclosporin A and FK506. Both drugs form complexes with abundant intracellular binding proteins, cyclosporin A with cyclophilin A and FK506 with FKBP 12, which bind to and inhibit calcineurin. The X-ray structure of an FKPB12-FK506-calcineurin AB ternary complex reveals that FKBP12-FK506 binds in a hydophobic groove between the calcineurin A catalytic and the regulatory B subunit, in accord with biochemical and genetic studies on inhibitor action. Calcineurin plays a key role in regulating the transcription factor NF-AT during T-cell activation, and in mediating responses of microorganisms to cation stress. These findings highlight the potential of yeast genetic studies to define novel drug targets and elucidate conserved elements of signal transduction cascades.

Activation of a cGMP-stimulated cAMP phosphodiesterase by protein kinase C in a liver Golgi-endosomal fraction

The ability of Ca2+/phospholipid-dependent protein kinase (protein kinase C, PKC) to stimulate cAMP phosphodiesterase (PDE) activity in a liver Golgi-endosomal (GE) fraction was examined in vivo and in a cell-free system. Injection into rats of 4 beta-phorbol 12-myristate 13-acetate, a known activator of PKC, caused a rapid and marked increase in PKC activity (+325% at 10 min) in the GE fraction, along with an increase in the abundance of the PKC alpha-isoform as seen on Western immunoblots. Concurrently, 4 beta-phorbol 12-myristate 13-acetate treatment caused a time-dependent increase in cAMP PDE activity in the GE fraction (96% at 30 min). Addition of the catalytic subunit of protein kinase A (PKA) to GE fractions from control and 4 beta-phorbol 12-myristate 13-acetate-treated rats led to a comparable increase (130-150%) in PDE activity, suggesting that PKA is probably not involved in the in-vivo effect of 4 beta-phorbol 12-myristate 13-acetate. In contrast, addition of purified PKC increased (twofold) PDE activity in GE fractions from control rats but affected only slightly the activity in GE fractions from 4 beta-phorbol 12-myristate 13-acetate-treated rats. About 50% of the Triton-X-100-solubilized cAMP PDE activity in the GE fraction was immunoprecipitated with an anti-PDE3 antibody. On DEAE-Sephacel chromatography, three peaks of PDE were sequentially eluted: one early peak, which was stimulated by cGMP and inhibited by erythro-9 (2-hydroxy-3-nonyl) adenine (EHNA); a selective inhibitor of type 2 PDEs; and two retarded peaks of activity, which were potently inhibited by cGMP and cilostamide, an inhibitor of type 3 PDEs. Further characterization of peak I by HPLC resolved a major peak which was activated (threefold) by 5 microM cGMP and inhibited (87%) by 25 microM EHNA, and a minor peak which was insensitive to EHNA and cilostamide. 4 beta-Phorbol 12-myristate 13-acetate treatment caused a selective increase (2.5-fold) in the activity associated with DEAE-Sephacel peak I, without changing the K(m) value. These results suggest that PKC selectively activates a PDE2, cGMP-stimulated isoform in the GE fraction.

In vitro generation of an active calmodulin-independent phosphodiesterase from brain calmodulin-dependent phosphodiesterase (PDE1A2) by m-calpain

In the present study we have shown that bovine brain 60-kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme (CaMPDE - PDE1A2) is proteolyzed by a Ca2+-dependent cysteine protease, m-calpain. The proteolysis of PDE1A2 by m-calpain results in its conversion to a totally calmodulin (CaM)-independent form accompanied by degradation of PDE1A2 into a 45-kDa catalytic fragment and a 15-kDa fragment. The activity of PDE1A2 is unaffected by the presence or absence of CaM during cleavage, suggesting that the interaction between CaM and PDE1A2 does not alter substrate recognition by calpain. Furthermore, we provide evidence, based on the studies of CaM overlay and phosphorylation, that the cleavage site is not present either in the CaM-binding domain or phosphorylation site. N-terminal sequence analysis of the 45-kDa fragment indicated that cleavage occurs between residues 126Gln and 127Ala, and eliminates the CaM-dependent activity of carboxy termini PDE1A2. The present findings suggest that limited proteolysis in the brain through calpains could be an alternate mechanism for activating CaMPDE(s) and for regulating intracellular levels of cAMP.

Phosphorylation and inhibition of olfactory adenylyl cyclase by CaM kinase II in Neurons: a mechanism for attenuation of olfactory signals

Acute desensitization of olfactory signaling is a critical property of the olfactory system that allows animals to detect and respond to odorants. Correspondingly, an important feature of odorant-stimulated cAMP increases is their transient nature, a phenomenon that may be attributable to the unique regulatory properties of the olfactory adenylyl cyclase (AC3). AC3 is stimulated by receptor activation and inhibited by Ca2+ through Ca2+/calmodulin kinase II (CaMKII) phosphorylation at Ser-1076. Since odorant-stimulated cAMP increases are accompanied by elevated intracellular Ca2+, CaMKII inhibition of AC3 may contribute to termination of olfactory signaling. To test this hypothesis, we generated a polyclonal antibody specific for AC3 phosphorylated at Ser-1076. A brief exposure of mouse olfactory cilia or primary olfactory neurons to odorants stimulated phosphorylation of AC3 at Ser-1076. This phosphorylation was blocked by inhibitors of CaMKII, which also ablated cAMP decreases associated with odorant-stimulated cAMP transients. These data define a novel mechanism for termination of olfactory signaling that may be important in olfactory responses.

Novel protein inhibitor of calmodulin-dependent cyclic nucleotide phosphodiesterase from glioblastoma multiforme

Previous investigations from our laboratory have demonstrated a significant reduction in the catalytic function of the 60 kDa and 63 kDa isozymes of calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE) when comparing human cerebral tissue that was free of tumor and glioblastoma multiforme (GBM) and gliosarcoma [Lal S., Raju R. V. S., Macaulay R. B. J., and Sharma R. K. (1996) Can. J. Neurol. Sci., 23, 245-250]. The results suggested the possibility of an endogenously produced inhibitor of CaMPDE expressed in these tumors. Further investigation has initially characterized the presence of a heat-labile, protein inhibitor of both the 60 kDa and 63 kDa isozymes of CaMPDE. Sephacryl S-200 gel filtration column chromatography indicated that the inhibitor has an apparent molecular weight of 22 kDa and experimental evidence demonstrates that this inhibitor protein may act independently of calmodulin, and is therefore a novel CaMPDE inhibitor. Previous work on non-CNS tumors has shown high levels of CaMPDE activity and absence of an inhibitor. This suggests that a different mechanism may exist for the proliferation of these subsets of tumors.

Binding of cGMP to both allosteric sites of cGMP-binding cGMP-specific phosphodiesterase (PDE5) is required for its phosphorylation

cGMP-binding phosphodiesterases contain two homologous allosteric cGMP-binding sites (sites a and b) that are arranged in tandem; they constitute a superfamily of mammalian cyclic nucleotide receptors distinct from the cyclic nucleotide-dependent protein kinases/cation channels family. The functional role of each of these two sites in the phosphodiesterases is not known. The cGMP-binding sites of one of these phosphodiesterases, the cGMP-binding cGMP-specific phosphodiesterase (cGB-PDE, PDE5), have been analysed by using site-directed mutagenesis. Mutations that affect cGMP binding to either one or both allosteric sites do not influence cGMP hydrolysis in the catalytic site under the conditions used. However, compared with wild-type enzyme, the D289A, D478A and D289A/D478A mutants, which are defective in cGMP binding to either site a or site b, or both allosteric sites, require much higher cGMP concentrations for the allosteric stimulation of phosphorylation by the catalytic subunit of cAMP-dependent protein kinase. The cGMP effect is on the cGB-PDE rather than on the catalytic subunit of the protein kinase because the latter enzyme does not require cGMP for activity. The D289N mutant, which has higher binding affinity for cGMP than does the wild-type enzyme, is phosphorylated at lower concentrations of cGMP than is the wild-type enzyme. It is concluded that cGMP binding to the allosteric sites of cGB-PDE does not directly affect catalysis, but binding to both of these sites regulates phosphorylation of this enzyme.

Age-related changes in the activation of aortic cholesteryl ester hydrolases by protein kinases in rats

Age-related changes in the activities of acid and neutral cholesteryl ester hydrolases (ACEH and NCEH) and their activation by protein kinase A (PKA) and also by protein kinase C (PKC) were examined in the aortae of 4-, 8-, 12- and 20-week-old rats in relation to their aortic lipid and lipid peroxides and lipid contents. The physiological basal activity as well as total activities of the ACEH and NCEH activated by the two kinases, which were high in the aortae of the 4- and 8-week-old rats, decreased gradually with increasing age to about 40% (ACEH) and 50% (NCEH) by 20 weeks of age. The vitamin E intake and ad libitum-diet intake of the rats each modified the age-related decline of CEH activities. The aortic PKA and PKC activities were reflected by the CEH activities to some degree. The in vitro exposure of the aortic CEH to active oxygen (AO) generators revealed the PKC-mediated activation of CEH, which was inhibited by superoxide dismutase and catalase. These results suggested that the activities of ACEH and NCEH and their regulatory enzymes may be modulated by the dual effect of endogenous AO; an activation of CEH at low doses and an inactivation at high doses, or upon a long-term exposure in aging to a low level of endogenous AO.

In vitro phosphorylation of bovine cardiac muscle high molecular weight calmodulin binding protein by cyclic AMP-dependent protein kinase and dephosphorylation by calmodulin-dependent phosphatase

High molecular weight calmodulin binding protein (HMWCaMBP) is one of the major proteins expressed in bovine cardiac muscle. In this study, we report the phosphorylation and dephosphorylation of HMWCaMBP in vitro with a view to understand the function of this protein. The HMWCaMBP was phosphorylated by cAMP-dependent protein kinase with the incorporation of 2.30 mol of phosphate/mol of protein in the presence of EGTA. When phosphorylation was carried out in the presence of Ca2+/calmodulin (CaM), the incorporation of phosphate was reduced to 1.40 mol of phosphate/mol of protein. The decrease in the stoichometry of phosphorylation by Ca2+/CaM appears to be substrate directed i.e. due to the interaction of Ca2+/CaM with HMWCaMBP. The phosphorylated HMWCaMBP was unable to compete for free CaM in a CaM-dependent cyclic nucleotide phosphodiesterase (CaMPDE) assay. These results suggest that the phosphorylation sites may reside in or in proximity to the CaM-binding domain on HMWCaMBP since phosphorylated HMWCaMBP did not inhibit CaMPDE activity. HMWCaMBP was dephosphorylated by CaM-dependent phosphatase, calcineurin.

Identification and characterisation of a human calmodulin-stimulated phosphodiesterase PDE1B1

A cDNA encoding a calmodulin-stimulated 3',5'-cyclic nucleotide phosphodiesterase (PDE) was isolated from a human brain cDNA library. The cDNA, designated HSPDE1B1, encoded a protein of 536 amino acids that shared 96% sequence identity with the bovine "63 kDa" calmodulin-stimulated PDE. The recombinant protein had cyclic nucleotide phosphodiesterase activity that was stimulated approximately 2-fold by Ca2+/calmodulin and preferred cGMP as substrate. In addition, the enzymatic activity of HSPDE1B1 was inhibited by phosphodiesterase inhibitors with potencies similar to that displayed toward the bovine PDE1 enzymes: IBMX approximately equal to 8-methoxymethyl-IBMX > vinpocetine approximately equal to zaprinast > cilostamide > rolipram. HSPDE1B1 mRNA was found predominantly in the brain. Lower mRNA levels were found in heart and skeletal muscle. In situ hybridisation of brain revealed expression of HSPDE1B1 predominately in neuronal cells of the cerebellum, hippocampus and caudate. The HSPDE1B1 gene was mapped to human chromosome 12. A partial genomic sequence of HSPDE1B1 was isolated and shown to contain two splice junctions that are conserved in the rat PDE4 and the Drosophila dunce genes.

Inhibitory cross-talk by cAMP kinase on the calmodulin-dependent protein kinase cascade

The calmodulin-dependent kinase (CaM-K) cascade, a Ca2+-triggered system involving phosphorylation and activation of CaM-KI and CaM-KIV by CaM kinase kinase (CaM-KK), regulates transcription through direct phosphorylation of transcription factors such as cAMP response element-binding protein. We have shown previously that activated CaM-KIV can activate the mitogen-activated protein kinases (Enslen, H., Tokumitsu, H., Stork, P. J. S., Davis, R. J., and Soderling, T. R. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 10803-10808), and the present paper describes a novel regulatory cross-talk between cAMP kinase (PKA) and CaM-KK. PKA gave rapid phosphorylation in vitro and in cells of recombinant CaM-KK, resulting in 50-75% inhibition of CaM-KK activity, part of which was due to suppression of CaM-binding by phosphorylation of Ser458 in the CaM-binding domain. However, the Ser458 --> Ala mutant, or a truncation mutant in which the CaM-binding and autoinhibitory domains were deleted, was still partially suppressed by PKA-mediated phosphorylation. The second inhibitory site was identified as Thr108 by site-specific mutagenesis. Treatments of COS-7, PC12, hippocampal, or Jurkat cells with the PKA activators forskolin or isoproterenol gave 30-90% inhibition of either endogenous or transfected CaM-KK and/or CaM-KIV activities. These results demonstrate that the CaM kinase cascade is negatively regulated in cells by the cAMP/PKA pathway.

Calmodulin-dependent cyclic nucleotide phosphodiesterase from bovine eye: high calmodulin affinity isozyme immunologically related to the brain 60-kDa isozyme

Calmodulin-dependent cyclic nucleotide phosphodiesterase was identified in and purified to apparent homogeneity from the total calmodulin-binding protein fraction of bovine eye in a single step by immunoaffinity chromatography. The bovine eye calmodulin-dependent cyclic nucleotide phosphodiesterase is immunologically similar to the bovine brain 60-kDa isozyme. The purified enzyme had higher affinity for calmodulin than the 60-kDa phosphodiesterase isozyme from bovine brain, but similar affinity to that of the heart isozyme. When the Ca(2+)-dependence of the eye enzyme was compared to cardiac calmodulin-dependent cyclic nucleotide phosphodiesterase at an identical concentration of calmodulin, the bovine eye calmodulin-dependent cyclic nucleotide phosphodiesterase was activated at the same Ca2+ concentration as the bovine heart calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme.

Amantadine: an antiparkinsonian agent inhibits bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme

The effect of amantadine (an antiparkinsonian agent) on calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes was investigated. Amantadine inhibited bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase but not the bovine brain 63 kDa, heart and lung calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes. The inhibition of bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase was overcome by increasing the concentration of calmodulin. This suggests that amantadine may be an antagonist of calmodulin or act specifically and reversibly on the action of calmodulin. The bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme is predominantly expressed in the brain and its inhibition may result in increased intracellular levels of cyclic AMP (cAMP). The increased intracellular levels of cAMP have a protective role for dopaminergic neurons. The present findings suggest that amantadine may be a valuable tool to investigate the physiological role of 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme in the progression of Parkinson's disease and gives a new insight into the action of this drug.

Src tyrosine kinase activity in rat thecal-interstitial cells and mouse TM3 Leydig cells is positively associated with cAMP-specific phosphodiesterase activity

Phosphodiesterases (PDEs) play a critical role in the regulation of intracellular cyclic nucleotide concentration and, consequently, regulate the state of cellular differentiation. We have reported that the Src-selective tyrosine kinase inhibitor, herbimycin A, potentiates luteinizing hormone (LH)-stimulated cAMP accumulation in culture media by ovarian thecal-interstitial cells (TIC; see Taylor, C and Terranova, P.F. (1995) Lipopolysaccharide inhibits rat ovarian thecal-interstitial cell steroid secretion in vitro. Endocrinology 136, 5527-5532). The present study was conducted to investigate the effects of herbimycin, and changes in Src tyrosine kinase activity, on PDE activity in rat TIC an in the mouse TM3 Leydig cell line. Treatment of TIC with herbimycin (1 microM) for 24 h inhibited basal and LH-stimulated PDE activity (approximately 50 and 70%, respectively) and was associated with an increase in cAMP and progesterone accumulation in culture media. Treatment of TM3 cells with herbimycin inhibited PDE activity and increased cAMP accumulation in a dose- and time-dependent manner. TM3 cell cultures challenged with herbimycin had lower Src tyrosine kinase activity than controls (approximately 50%); however, protein kinase A activity was unaffected. TM3 cells stably transfected with a dominant negative Src tyrosine kinase (TM3Srck-) had lower PDE activity than cells transfected with a G418 resistance gene alone (TM3pSV2neo) which served as control cells. Conversely, TM3 cells expressing a temperature-sensitive Src kinase had significantly greater PDE activity at the Src active temperature (35 degrees C; the temperature at which the enzyme is active) than TM3pSV2neo control cells grown at the same temperature. TM3 cell lysates hydrolyzed minimal amounts of cGMP, indicating a cAMP-specific PDE. Phosphodiesterase activity in both TM3 and rat TIC was sensitive to the PDE4-selective inhibitor RO20-1724, indicating the predominant active enzyme is probably a member of the cAMP-specific PDE4 family. From the present data, we conclude that a tyrosine kinase of the Src family may play an important role in regulating phosphodiesterase activity in thecal and Leydig cells, and thus regulate intracellular cAMP and the state of cellular differentiation.

Calmodulin-dependent cyclic nucleotide phosphodiesterase in human cerebral cortex and glioblastoma multiforme

BACKGROUND

Calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE) has been extensively studied and characterized in normal mammalian tissues; however very little is known about this enzyme in human brain tumors. It has been established that high levels of this enzyme exist in non-central nervous system tumors, PDE inhibitors or cAMP analogues have been used to treat them. This study has examined the levels of CaMPDE in glioblastoma multiforme from six patients and has compared these to the levels of CaMPDE in four patients with normal cerebral tissue. In addition, an enzyme immune assay method (EIA) was developed in this study for the detection of CaMPDE in human cerebral tissue. This method is proposed to be used as an adjunct to the spectrophotometric method presently utilized. This would be beneficial in cases where small tissue samples, for example in stereotactic biopsy, are available.

METHODS

The CaMPDE activity and corresponding levels of expression in cerebral tissue from temporal lobectomies and both surgical extraction or stereotactic biopsy in patients with primary tumors were determined by spectrophotometric and EIA, respectively. The EIA was developed from the production of a polyclonal antibody against bovine brain 60 kDa CaMPDE isozyme. Cross reactivity of the antibody with human was confirmed using transblot and immunohistochemistry.

RESULTS

Utilising the EIA, there was found to be significant reduction in both catalytic activity (p < 0.001) and in quantitative protein expression (p < 0.001) in glioblastoma multiforme from patients when compared to normal cerebral cortex. Immunoblotting experiments and immunohistochemistry demonstrated that CaMPDE in glioblastoma multiforme failed to react with a polyclonal antibody raised against bovine brain 60 kDa CaMPDE isozyme, whereas the enzyme from normal tissue reacted with antibody.

CONCLUSIONS

Contrary to other studies on non-CNS tumors, the catalytic activity and the protein expression of CaMPDE is reduced in glioblastoma multiforme. The EIA method is a more sensitive in detecting CaMPDE than in the spectrophotometric method, especially when a small amount of tissue is available. Immunohistochemistry and the EIA may be useful in the future to use as markers for other types of brain tumors and not for glioblastoma multiforme as demonstrated.

Phosphorylation and activation of a cAMP-specific phosphodiesterase by the cAMP-dependent protein kinase. Involvement of serine 54 in the enzyme activation

A cAMP-specific phosphodiesterase (PDE4D3) is activated in rat thyroid cells by TSH through a cAMP-dependent phosphorylation (Sette, C., Iona, S., and Conti, M.(1994) J. Biol. Chem. 269, 9245-9252). This short term activation may be involved in the termination of the hormonal stimulation and/or in the induction of desensitization. Here, we have further characterized the protein kinase A (PKA)-dependent phosphorylation of this PDE4D3 variant and identified the phosphorylation site involved in the PDE activation. The PKA-dependent incorporation of phosphate in the partially purified, recombinant rat PDE4D3 followed a time course similar to that of activation. Half-maximal activation of the enzyme was obtained with 0.6 microM ATP and 30 nM of the catalytic subunit of PKA. Phosphorylation altered the Vmax of the PDE without affecting the Km for cAMP. Phosphorylation also modified the Mg2+ requirements and the pattern of inhibition by rolipram. Cyanogen bromide cleavage of the 32P-labeled rat PDE4D3 yielded two or three major phosphopeptide bands, providing a first indication that the enzyme may be phosphorylated at multiple sites in a cell-free system. Site-directed mutagenesis was performed on the serine residues present at the amino terminus of this PDE in the context of preferred motifs for PKA phosphorylation. The PKA-dependent incorporation of 32P was reduced to the largest extent in mutants with both Ser13 --> Ala and Ser54 --> Ala substitutions, confirming the presence of more than one phosphorylation site in rat PDE4D3. While substitution of serine 13 with alanine did not affect the activation by PKA, substitution of Ser54 completely suppressed the kinase activation. Similar conclusions were reached with wild type and mutated PDE4D3 proteins expressed in MA-10 cells, where the endogenous PKA was activated by dibutyryl cAMP. Again, the PDE with the Ser54 --> Ala substitution could not be activated by the endogenous PKA in the intact cell. These findings support the hypothesis that the PDE4D3 variant contains a regulatory domain target for phosphorylation at the amino terminus of the protein and that Ser54 in this domain plays a crucial role in activation.

Beta-adrenergic regulation of synaptic NMDA receptors by cAMP-dependent protein kinase

To identify the protein kinases regulating synaptic NMDA receptors, as well as the conditions favoring enhancement of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) by phosphorylation, we studied the effects of kinase activation and inhibition in hippocampal neurons. Inhibition of cAMP-dependent protein kinase (PKA) prevented recovery of NMDA receptors from calcineurin-mediated dephosphorylation induced by synaptic activity, suggesting that tonically active PKA phosphorylates receptors during quiescent periods. Conversely, elevation of PKA activity by forskolin, cAMP analogs, or the beta-adrenergic receptor agonists norepinephrine and isoproterenol overcame the ability of calcineurin to depress the amplitude of NMDA EPSCs. Thus, stimulation of beta-adrenergic receptors during excitatory synaptic transmission can increase charge transfer and Ca2+ influx through NMDA receptors.

Identification of inhibitory and calmodulin-binding domains of the PDE1A1 and PDE1A2 calmodulin-stimulated cyclic nucleotide phosphodiesterases

Using a bovine 61-kDa (PDE1A2) calmodulin-stimulated phosphodiesterase (CaM-PDE) cDNA and a bovine lung 59-kDa (PDE1A1) CaM-PDE cDNA reported here, we have identified two new regions within the primary structure of these two related isozymes that are important for regulation by Ca2+/CaM. PDE1A1 is identical to the PDE1A2 isozyme except for the amino-terminal 18 residues. In agreement with earlier studies, the CaM concentration required for half-maximal activation (KCaM) of recombinant PDE1A1 (0.3 nM) was approximately 10-fold less than the KCaM for recombinant PDE1A2 (4 nM). A series of deletion mutations of the PDE1A2 cDNA removing nucleotide sequence encoding the first 46-106 aminoterminal residues were constructed and expressed using the baculovirus system. Deletion of the amino acids encompassing a previously identified, putative CaM-binding domain (residues 4-46) produced a polypeptide that was still activated 3-fold by CaM (KCaM approximately 3 nM). However, complete CaM-independent activation occurred when residues 4-98 were deleted. To determine the location of the additional CaM-binding domain(s), the inhibitory potency of seven overlapping, synthetic peptides spanning amino acids 76-149 of PDE1A2 was tested using the CaM-activated enzyme. One peptide spanning amino acids 114-137 of PDE1A2 appeared to be the most potent inhibitor of CaM-stimulated activity. These results reveal the existence of a CaM-binding domain located approximately 90 residues carboxyl-terminal to the putative CaM-binding domains previously identified within the PDE1A1 and PDE1A2 isozymes. Moreover, a discrete segment important for holding these CaM-PDEs in a less active state at low Ca2+ concentrations is located between the two CaM-binding domains.

Signal transduction: regulation of cAMP concentration in cardiac muscle by calmodulin-dependent cyclic nucleotide phosphodiesterase

The bovine heart calmodulin-dependent phosphodiesterase can be phosphorylated by cAMP-dependent protein kinase, resulting in a decrease in the enzyme's affinity for calmodulin. The phosphorylation of calmodulin-dependent phosphodiesterase is blocked by Ca2+ and calmodulin and reversed by the calmodulin-dependent phosphatase. The dephosphorylation is accompanied by an increase in the affinity of the phosphodiesterase for calmodulin. The CaM-dependent phosphodiesterase isozymes of heart and brain are regulated by calmodulin, but the affinity for calmodulin are different. Furthermore, the bovine heart CaM-dependent phosphodiesterase isozyme in stimulated at much lower Ca2+ concentration than the bovine brain isozymes. Results from this study suggest that the activity of this phosphodiesterase is precisely regulated by cross-talk between Ca2+ and cAMP signalling pathways.

Interferon-gamma and lipopolysaccharide reduce cAMP responses in cultured glial cells: reversal by a type IV phosphodiesterase inhibitor

The aim of the present study was to determine whether two classical macrophage activators, bacterial lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) could affect the accumulation of the second messenger cAMP in cultured rat microglia and astrocytes. Purified microglia and astrocyte secondary cultures obtained from the neonatal rat were grown for 3 days in basal medium Eagle (BME) + 10% fetal calf serum (FCS). Exposure of microglia to LPS resulted into a dose- and time-dependent decrease in the accumulation of cAMP induced by receptor-mediated (isoproterenol or prostaglandin E2) or direct (forskolin) activation of adenylate cyclase. The inhibitory effect of LPS was rapid (a 10 min preincubation was sufficient to approach a maximal effect), occurred at low doses (IC50 = 1.2 ng/ml), and was not abrogated by pertussis toxin. A selective inhibitor of type IV phosphodiesterase (rolipram, 100 nM) prevented the effect of LPS on cAMP accumulation, while inhibitors of other forms of phosphodiesterase were unable to do so. IFN-gamma (100 u/ml) also caused a depression of the evoked cAMP accumulation in microglia after a 10 min preincubation, and its effect was prevented by rolipram, as in the case of LPS. Astrocytes differed from microglia in that LPS (1-100 ng/ml) did not inhibit the accumulation of cAMP induced by either isoproterenol or forskolin; on the other hand, IFN-gamma did have an inhibitory effect (though less pronounced than in microglia) that could be prevented by rolipram.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin: effects of cell stimuli and drugs on cellular activation

The activity, localization and cellular content of CaM can be regulated by drugs, hormones and neurotransmitters. Regulation of physiological responses of CaM can depend upon local Ca(2+)-entry domains in the cells and phosphorylation of CaM target proteins, which would either decrease responsiveness of CaM target enzymes or increase CaM availability for binding to other target proteins. Despite the abundance of CaM in many cells, persistent cellular activation by a variety of substances can lead to an increase in CaM, reflected both in the nucleus and other cellular compartments. Increases in CaM-binding proteins can accompany stimuli-induced increases in CaM. A role for CaM in vesicular or protein transport, cell morphology, secretion and other cytoskeletal processes is emerging through its binding to cytoskeletal proteins and myosins in addition to the more often investigated activation of target enzymes. More complete knowledge of the physiological regulation of CaM can lead to a greater understanding of its role in physiological processes and ways to alter its actions through pharmacology.

Expression and regulation of human and rat phosphodiesterase type IV isogenes

Type IV phosphodiesterases (PDE IV) specifically hydrolyze cAMP and are inhibited by rolipram. RT-PCR was applied to analyze the expression patterns of mRNAs for four cloned human and rat phosphodiesterase type IV isogenes (PDE IV-A, -B, -C and -D). Although these patterns were mostly coincident for the human and rat PDE IV genes, some differences were found between the two species. PDE IV-A expression was detectable in human blood but not in rat blood, suggesting a species-specific difference in the expression of this PDE IV isogene. PDE IV-C was neither detected in human or rat blood nor in different cell populations of the human immune system. It is further demonstrated that the PDE IV isogene expression is differentially regulated by cAMP in different cell types.