Electron microscopic immunocytochemical evidence that the calmodulin-dependent cyclic nucleotide phosphodiesterase is localized predominantly at postsynaptic sites in the rat brain

Temporal and spatial regulation of cAMP signaling in disease: role of cyclic nucleotide phosphodiesterases

Since its discovery, cAMP has been proposed as one of the most versatile second messengers. The remarkable feature of cAMP to tightly control highly diverse physiological processes, including metabolism, homeostasis, secretion, muscle contraction, cell proliferation and migration, immune response, and gene transcription, is reflected by millions of different articles worldwide. Compartmentalization of cAMP in space and time, maintained by mainly phosphodiesterases, contributes to the maintenance of equilibrium inside the cell where one signal can trigger many different events. Novel cAMP sensors seem to carry out certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Measuring space and time events with biosensors will increase our current knowledge on the pathophysiology of diseases, such as chronic obstructive pulmonary disease, asthma, cognitive impairment, cancer, and renal and heart failure. Further insights into the cAMP dynamics will help to optimize the pharmacological treatment for these diseases.

Characterization of mRNA species that are associated with postsynaptic density fraction by gene chip microarray analysis

We previously reported the partial identification by random sequencing of mRNA species that are associated with the postsynaptic density (PSD) fraction prepared from the rat forebrain [Tian et al., 1999. Mol. Brain Res. 72, 147-157]. We report here further characterization by gene chip analysis of the PSD fraction-associated mRNAs, which were prepared in the presence of RNase inhibitor. We found that mRNAs encoding various postsynaptic proteins, such as channels, receptors for neurotransmitters and neuromodulators, proteins involved in signaling, scaffold and adaptor proteins and cytoskeletal proteins, were highly concentrated in the PSD fraction, whereas those encoding housekeeping proteins, such as enzymes in the glycolytic pathway, were not. We extracted approximately 1900 mRNA species that were highly concentrated in the PSD fraction. mRNAs related to certain neuronal diseases were also enriched in the PSD fraction. We also constructed a cDNA library using the PSD fraction-associated mRNAs as templates, and identified 1152 randomly selected clones by sequencing. Our data suggested that the PSD fraction-associated mRNAs are a very useful resource, in which a number of as yet uncharacterized mRNAs are concentrated. Identification and functional characterization of them are essential for complete understanding of synaptic function.

Synaptic concentration of type-I adenylyl cyclase in cerebellar neurons

Specific subcellular targeting and spatial arrangement of signaling molecules are important for efficient signal transduction. The neuro-specific type-I adenylyl cyclase (AC1) is stimulated by Ca2+, and plays an essential role in neurodevelopment and neuroplasticity. We generated hemagglutinin (HA)-tagged AC1 to study its subcellular localization in cultured neurons. The HA-tagged AC1 has similar enzymatic activity and regulatory properties to that of non-tagged protein. HA-AC1 targeted to both apical and basolateral domains in the epithelial Madin-Darby canine kidney (MDCK) cells, and it was found in both axons and dendrites in cultured hippocampal neurons as well as in cerebellar granule neurons. Interestingly, AC1 showed a distinct punctate form of immunostaining in MDCK cells and transfected neurons, suggesting it targets to specific subcellular domains. By immunostaining with different synaptic markers, we found that AC1 puncta were located at the excitatory synapses in cerebellar granule neurons. Our data provide a possible cellular mechanism for the physiological role of AC1 in neuroplasticity.

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.

Development and molecular organization of dendritic spines and their synapses

Nearly all excitatory input in the hippocampus impinges on dendritic spines which serve as multifunctional compartments that can, at the very least, selectively isolate and amplify incoming signals. Their importance to normal brain function is highlighted by the severe mental impairment observed in most individuals having poorly developed spines (Purpura, Science 1974;186:1126-1128). Distinct groups of membrane proteins, cytoskeletal elements, scaffolding proteins, and second messenger-related proteins are concentrated particularly in dendritic spines, but their ability to generate, maintain, and coordinately regulate spine structure or function is poorly understood. Here we review the unique molecular composition of dendritic spines along with the factors known to influence dendritic spine development in order to construct a model of dendritic spine development in relation to synaptogenesis.

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

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

Cyclic-3',5'-nucleotide phosphodiesterase isozymes in cell biology and pathophysiology of the kidney

Investigations of recent years revealed that isozymes of cyclic-3', 5'-nucleotide phosphodiesterase (PDE) are a critically important component of the cyclic-3',5'-adenosine monophosphate (cAMP) protein kinase A (PKA) signaling pathway. The superfamily of cyclic-3', 5'-phosphodiesterase (PDE) isozymes consists of at least nine gene families (types): PDE1 to PDE9. Some PDE families are very diverse and consist of several subtypes and numerous PDE isoform-splice variants. PDE isozymes differ in molecular structure, catalytic properties, intracellular regulation and location, and sensitivity to selective inhibitors, as well as differential expression in various cell types. A number of type-specific "second-generation" PDE inhibitors have been developed. Current evidence indicates that PDE isozymes play a role in several pathobiologic processes in kidney cells. In rat mesangial cells, PDE3 and PDE4 compartmentalize cAMP signaling to the PDE3-linked cAMP-PKA pathway that modulates mitogenesis and PDE4-linked cAMP-PKA pathway that modulates generation of reactive oxygen species. Administration of selective PDE isozyme inhibitors in vivo suppresses proteinuria and pathologic changes in experimental anti-Thy-1.1 mesangial proliferative glomerulonephritis in rats. Increased activity of PDE5 (and perhaps also PDE9) in glomeruli and in cells of collecting ducts in sodium-retaining states, such as nephrotic syndrome, accounts for renal resistance to atriopeptin; diminished ability to excrete sodium can be corrected by administration of the selective PDE5 inhibitor zaprinast. Anomalously high PDE4 activity in collecting ducts is a basis of unresponsiveness to vasopressin in mice with hereditary nephrogenic diabetes insipidus. Apparently, PDE isozymes apparently also play an important role in the pathogenesis of acute renal failure of different origins. Administration of PDE isozyme-selective inhibitors suppresses some components of immune responses to allograft transplant and improves preservation and survival of transplanted organ. PDE isozymes are a target for action of numerous novel selective PDE inhibitors, which are key components in the design of novel "signal transduction" pharmacotherapies of kidney diseases.

Phosphodiesterase type 4 that regulates cAMP level in cortical neurons shows high sensitivity to rolipram

To characterize the role of phosphodiesterase type 4 (a cAMP-specific and rolipram-sensitive phosphodiesterase) among phosphodiesterases in the regulation of the intracellular cAMP level in cortical neurons, we investigated the effects of phosphodiesterase inhibitors on the intracellular cAMP levels in primary cultured rat cortical neurons. Selective inhibitors of phosphodiesterase type 4 and type 2 significantly enhanced beta-adrenoceptor-mediated cAMP increase. Selective inhibitors of phosphodiesterase type 1, type 3 and type 5/6 had no effect on the cAMP level. Rolipram enhanced the beta-adrenoceptor-mediated cAMP increase in cortical neurons, astrocytes and vascular smooth muscle cells at different minimum effective concentrations (10, 100 and 1000 nM, respectively). These findings indicate that phosphodiesterase type 4, showing a high-sensitivity to rolipram, plays a major role in regulating cAMP in the cortical neurons, and that rolipram at low doses enhances the intracellular cAMP increase in the cortical neurons selectively.

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

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

Induction of cyclic AMP and cyclic GMP 3':5'-cyclic nucleotide phosphodiesterase activities in neuroblastoma lines under differentiating conditions

It is now widely accepted that cyclic nucleotide phosphodiesterases (PDEs) play fundamental roles in signal transduction pathways; they show a remarkable molecular complexity, different tissue distribution and complex regulatory mechanisms. Here we report PDE isoforms expression in two dibutyryl cyclic AMP differentiated murine cell lines: the hybrid neuroblastoma-glioma 108CC15 and the parental neuroblastoma N18TG2. They differ in the ability to establish functional synapses, a feature present only in the former. Ionic exchange chromatography elution profiles of N18TG2 and 108CC15 undifferentiated cell extracts show two main peaks of activity. The first one hydrolyzes cyclic GMP and is specifically inhibited by Zaprinast, thus representing a member of the PDE5 family. The second peak hydrolyzes cyclic AMP and is significantly inhibited by rolipram, as all the PDE4 family members. The induction of differentiation by dibutyryl cyclic AMP in both clonal lines results in an increase of PDE activities only after 3 hr of treatment, suggesting that protein neosynthesis is involved. Interestingly in both clones, besides the increase in cyclic AMP hydrolyzing specific activity (3.1-fold in 108CC15 and 2.5-fold in N18TG2), we also observed an increase in cyclic GMP hydrolyzing activity (1.7-fold in 108CC15 and 4.3-fold in N18TG2). While the induction of PDE4, previously reported also in other cellular systems, could be considered as a feedback response to the higher cyclic AMP levels, this is not true for the isoform that hydrolyzes cyclic GMP. These data suggest that the induction of PDE isoforms in neuroblastoma cells could be related to the activation of neuronal differentiative pathway.

Effect of papaverine on synaptic transmission in the guinea-pig ileum

1. The effect of papaverine, a well known smooth muscle relaxant, was investigated on neural transmission within the enteric nervous system. Segments of guinea-pig ileum were placed in a partitioned bath to enable drugs, including papaverine, to be applied to enteric nerve pathways without interfering with the recording of the smooth muscle contraction. Ascending excitatory enteric nerve pathways were activated by electrical field stimulation in the anal compartment (10 Hz for 2 s, 45 mA, 0.5 ms pulse duration) and the resulting contraction of the intestinal circular muscle in the oral compartment was recorded isotonically. 2. Tetrodotoxin (0.6 microM) and hexamethonium (100 microM) both abolished, or greatly reduced, the contractions when applied to either compartment indicating that nicotinic synapses are involved in this pathway. 3. Papaverine (0.3-30 microM) applied independently to each compartment depressed in a concentration-dependent manner, the nerve-mediated contractions. The IC50 of this inhibitory effect was 3.53 microM for the oral and 4.76 microM for the anal compartments, respectively. Two other phosphodiesterase (PDE) inhibitors, 3-isobutyl-1-methylxanthine (IBMX 10-300 microM) and theophylline (30-1000 microM) added to the anal compartment also inhibited the nerve mediated contractions. Papaverine applied to the anal bath, after IBMX 100 microM (or theophylline 300 microM) further inhibited the nerve-mediated contractions, but was less effective than when applied alone. 4. Phentolamine (1 microM), an alpha-adrenoceptor antagonist, reduced the inhibitory effect of papaverine, but not that of IBMX (100 microM) or theophylline (300 microM). A combination of phentolamine and IBMX (or theophylline) prevented the inhibitory effect of papaverine. 5. Tetrodotoxin, but not papaverine or hexamethonium, inhibited the contraction elicited by electrical stimulation just anal to the partition indicating that papaverine did not affect the generation or conduction of nerve action potentials. 6. Verapamil (1 microM) and nifedipine (1 microM), two smooth muscle relaxants which act by blocking L-type calcium channels, only inhibited the contractions when applied directly to the recording (oral) compartment. This indicates that L-type Ca2+ channels are probably not involved in synaptic transmission in these ascending pathways and thus that the PDE inhibitors do not inhibit synaptic transmission by acting on these channels. omega-Conotoxin GVIA (10 nM), a potent inhibitor of the N-type Ca2+ channels, blocked the nerve-mediated contractions applied to either compartment. Whether the PDE inhibitors exert their inhibitory actions via these channels remains to be established. 7. The results indicate that the PDE inhibitors, papaverine, IBMX and theophylline inhibit excitatory enteric neural pathways by depressing synaptic transmission. The inhibitory effect of papaverine (but not IMBX or theophylline) involves, at least in part, the release of noradrenaline from sympathetic nerves acting on alpha-adrenoceptors on enteric neurones.

A novel calmodulin-binding protein, belonging to the WD-repeat family, is localized in dendrites of a subset of CNS neurons

A rat brain synaptosomal protein of 110,000 M(r) present in a fraction highly enriched in adenylyl cyclase activity was microsequenced (Castets, F., G. Baillat, S. Mirzoeva, K. Mabrouk, J. Garin, J. d'Alayer, and A. Monneron. 1994. Biochemistry. 33:5063-5069). Peptide sequences were used to clone a cDNA encoding a novel, 780-amino acid protein named striatin. Striatin is a member of the WD-repeat family (Neer, E.J., C.J. Schmidt, R. Nambudripad, and T.F. Smith. 1994. Nature (Lond.). 371:297-300), the first one known to bind calmodulin (CaM) in the presence of Ca++. Subcellular fractionation shows that striatin is a membrane-associated, Lubrol-soluble protein. As analyzed by Northern blots, in situ hybridization, and immunocytochemistry, striatin is localized in the central nervous system, where it is confined to a subset of neurons, many of which are associated with the motor system. In particular, striatin is conspicuous in the dorsal part of the striatum, as well as in motoneurons. Furthermore, striatin is essentially found in dendrites, but not in axons, and is most abundant in dendritic spines. We propose that striatin interacts, through its WD-repeat domain and in a CaM/Ca(++)-dependent manner, with one or several members of a surrounding cluster of molecules engaged in a Ca(++)-signaling pathway specific to excitatory synapses.

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

We studied the distributions of four different cyclic AMP-specific phosphodiesterase isoform mRNAs (APDE1-4) and compared them with that of 63 kDa calmodulin-stimulated phosphodiesterase (CPDE) in the rat brain by in situ hybridization histochemistry using specific radiolabeled oligonucleotides. The distribution patterns were unique for all the APDE isoforms examined here. Although no significant signals for APDE1 could be detected anywhere in the rat brain, all other isoforms were expressed ubiquitously but unevenly and showed overlapping distribution patterns. Among all the APDE isoforms studied here, APDE3 showed the strongest and the most extensive expression. Its distribution pattern implies that it may modulate different cellular processes associated with learning and memory. Compared to APDE3, the levels of expression of APDE2 and APDE4 were weaker, the latter showing the weakest expression. Our study suggests that different isoforms of APDE are expressed together in the same class of neurons implying complex interactions among different signaling pathways, thereby mediating distinct and specific functions.

Adenylate cyclases: critical foci in neuronal signaling

Current findings show that adenylate cyclases comprise a heterogeneous multigene family, members of which are variously regulated by the alpha and beta gamma subunits of G proteins, by Ca2+ and by protein kinases. In the CNS, individual isoforms of adenylate cyclase are expressed discretely in select regions of the brain. At the subcellular level, adenylate cyclases can be concentrated into dendritic spines, thereby increasing their susceptibility to multiple regulatory influences. Altogether, such findings greatly expand knowledge of the potential role of this archetypical signaling system in the modulation of neuronal function.

Immunohistochemical localization of adenylyl cyclase in rat brain indicates a highly selective concentration at synapses

Only three isoforms of adenylyl cyclase (EC 4.6.1.1) mRNAs (AC1, -2, and -5) are expressed at high levels in rat brain. AC1 occurs predominantly in hippocampus and cerebellum, AC5 is restricted to the basal ganglia, whereas AC2 is more widely expressed, but at much lower levels. The distribution and abundance of adenylyl cyclase protein were examined by immunohistochemistry with an antiserum that recognizes a peptide sequence shared by all known mammalian adenylyl cyclase isoforms. The immunoreactivity in striatum and hippocampus could be readily interpreted within the context of previous in situ hybridization studies. However, extending the information that could be gathered by comparisons with in situ hybridization analysis, it was apparent that staining was confined to the neuropil--corresponding to immunoreactive dendrites and axon terminals. Electron microscopy indicated a remarkably selective subcellular distribution of adenylyl cyclase protein. In the CA1 area of the hippocampus, the densest immunoreactivity was seen in postsynaptic densities in dendritic spine heads. Labeled presynaptic axon terminals were also observed, indicating the participation of adenylyl cyclase in the regulation of neurotransmitter release. The selective concentration of adenylyl cyclases at synaptic sites provides morphological data for understanding the pre- and postsynaptic roles of adenylyl cyclase in discrete neuronal circuits in rat brain. The apparent clustering of adenylyl cyclases, coupled with other data that suggest higher-order associations of regulatory elements including G proteins, N-methyl-D-aspartate receptors, and cAMP-dependent protein kinases, suggests not only that the primary structural information has been encoded to render the cAMP system responsive to the Ca(2+)-signaling system but also that higher-order strictures are in place to ensure that Ca2+ signals are economically delivered and propagated.

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.

Is cyclic AMP involved in excitatory amino acid-evoked adenosine release from rat cortical slices?

Activation of both N-methyl-D-aspartate (NMDA) and non-NMDA receptors releases endogenous adenosine from superfused rat cortical slices. NMDA-evoked adenosine release is Ca(2+)-dependent and results from the extracellular degradation of a released nucleotide, whereas non-NMDA receptor activation releases adenosine per se in a Ca(2+)-independent manner. IBMX selectively inhibits NMDA- but not non-NMDA-evoked adenosine release. Forskolin, but not 1,9-dideoxy-forskolin, produced a slight but significant increase in NMDA-evoked adenosine release, suggesting that the formation of cyclic AMP may somehow be involved. The inhibition of NMDA-evoked adenosine release by IBMX is not accompanied by enhanced cyclic AMP recovery in superfusates, nor is release diminished when cyclic AMP transport is inhibited by probenecid, suggesting that the adenosine is not derived from the extracellular metabolism of released cyclic AMP. It is possible that 5'AMP, derived from the intracellular conversion of cyclic AMP by phosphodiesterase, might be released during NMDA receptor activation. However, more selective inhibitors of the specific phosphodiesterase isozymes known to be located in the cortex failed to diminish NMDA-evoked adenosine release. Therefore, the effects of both forskolin and IBMX on NMDA-evoked adenosine release could be nonspecific, coincidental and unrelated to their actions on cyclic AMP levels in the cortex. However, it is also possible that a novel IBMX-sensitive phosphodiesterase plays a primary role in converting cyclic AMP to 5'AMP intracellularly during NMDA receptor activation; the 5'AMP could then exit the cells and be converted to adenosine extracellularly.

Characterization of 3':5' cyclic nucleotide phosphodiesterase activities of mouse neuroblastoma N18TG2 cells

Characterization of 'low Km' 3':5' cyclic nucleotide phosphodiesterase activities (PDE) expressed in mouse N18TG2 neuroblastoma cells is reported. At least 3 peaks of activity were isolated by DEAE chromatography, none of which was calcium-calmodulin stimulated and cGMP stimulated or inhibited. A first peak elutes at 200 mM sodium acetate; it specifically hydrolyzes cGMP with a Km of 4.7 microM and shows sensitivity to zaprinast [M&B 22948] (1.8 microM). A second peak eluting at 410 mM sodium acetate hydrolyzes both cyclic nucleotides. A third peak, specific for cAMP hydrolysis, elutes at 580 mM sodium acetate, has a Km of 3.2 microM and is sensitive to RO 20 1724 (7.6 microM) and rolipram (2 microM). Hydrodynamic analysis showed for the first peak a Stokes radius of 5.3 nm with a sedimentation coefficient of 8.1 S, a frictional ratio (f/fo) of 1.41 and a native molecular mass of 182 kDa. The same analysis for peak 3 showed a Stokes radius of 4.1 nm with a sedimentation coefficient of 3.2 S, a frictional ratio of 1.63 and a native molecular mass of 56 kDa. The biochemical features reported for the enzyme eluting in the first peak, and its cGMP-binding activity stimulated by inhibitors of phosphodiesterase activity, demonstrate that it belongs to the PDE V subfamily; on the other hand the cAMP specific enzyme eluting in the third peak can be assigned to the 'RO 20 1724 inhibited' form. The significance of these findings is discussed in relation to the functional characteristics of the N18TG2 cell line.

Molecular cloning of DNA encoding a calmodulin-dependent phosphodiesterase enriched in striatum

A murine cDNA for the 63-kDa calmodulin-dependent phosphodiesterase (CaM-PDE), PDE1B-1, was isolated by using polymerase chain reaction with degenerate primers followed by the cloning of a full-length cDNA from a whole-brain phage library. The nucleotide sequence of 2986 base pairs contains an open reading frame encoding a protein of 535 amino acids (M(r) = 61,231) with a predicted isoelectric point of 5.54. The deduced protein sequence shows approximately 60% identity with that of the 61-kDa isoform (PDE1A2), consistent with the proposal that these proteins arise from two separate genes [Novack, J. P., Charbonneau, H., Bentley, J. K., Walsh, K. A. & Beavo, J. A. (1991) Biochemistry 30, 7940-7947]. Southern blot analysis suggests high nucleotide-sequence conservation of the PDE1B1 gene among mammalian and avian species. A single approximately 3600-nucleotide mRNA transcript was seen in all brain regions, with striatum containing 4- to 30-fold higher levels than other areas. In nonneural tissues, low amounts of PDE1B1 mRNA were detected in lung, spleen, thymus, and testis; hybridization to several larger mRNA species was also seen in thymus and testis. By using nucleic acid probes for PDE1B1, the mechanisms that control its highly selective gene expression can now be studied at the molecular level.