Sequence comparison of the 63-, 61-, and 59-kDa calmodulin-dependent cyclic nucleotide phosphodiesterases

Role of phosphodiesterase 1 in the pathophysiology of diseases and potential therapeutic opportunities

Cyclic nucleotide phosphodiesterases (PDEs) are superfamily of enzymes that regulate the spatial and temporal relationship of second messenger signaling in the cellular system. Among the 11 different families of PDEs, phosphodiesterase 1 (PDE1) sub-family of enzymes hydrolyze both 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) in a mutually competitive manner. The catalytic activity of PDE1 is stimulated by their binding to Ca²⁺/calmodulin (CaM), resulting in the integration of Ca²⁺ and cyclic nucleotide-mediated signaling in various diseases. The PDE1 family includes three subtypes, PDE1A, PDE1B and PDE1C, which differ for their relative affinities for cAMP and cGMP. These isoforms are differentially expressed throughout the body, including the cardiovascular, central nervous system and other organs. Thus, PDE1 enzymes play a critical role in the pathophysiology of diseases through the fundamental regulation of cAMP and cGMP signaling. This comprehensive review provides the current research on PDE1 and its potential utility as a therapeutic target in diseases including the cardiovascular, pulmonary, metabolic, neurocognitive, renal, cancers and possibly others.

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.

Calmodulin-dependent cyclic nucleotide phosphodiesterase (PDE1) splice variants from bovine cardiac muscle

Calmodulin-dependent cyclic nucleotide phopshodiesterase (PDE1) has been extensively characterized and is a key enzyme involved in the complex interaction between cyclic nucleotide and Ca(2+) second-messenger systems. It is well established that PDE1 exists in different isozymes. For example, bovine brain tissue has two PDE1 isozymes (PDE1A2 and PDE1B1) whereas only one form (PDE1A1) is reported in bovine cardiac tissue. In this study, we report the cloning of two cDNA splice variants of PDE1: PDE1-small and PDE1-large, from bovine cardiac tissue. Their amino acid sequence similarity to PDE1 sequences from other mammalian species showed that all are very conserved, suggesting their importance in cellular functions. Interestingly, compared to other mammalian species, bovine PDE1A, PDE-small and PDE-large show a deletion at the C-terminal end of the catalytic domain of the gene. Although the significance of this deletion at this crucial location of the gene is not known, we have successfully over-expressed both PDE1-small and PDE1-large splice variants in E. coli and these splice variants are characterized in terms of Western blot, biotinylated calmodulin overlay and peptide mass fingerprinting. Results from these studies suggested that these two splice variants belong to the PDE1 superfamily. To our knowledge, this is the first report on cloning and characterization of these cDNA variants from bovine cardiac tissue. Since there are at least two isoforms of PDE1 in bovine heart tissue, this merits further in-depth study.

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.

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.

Cloning of dog heart PDE1A - a first detailed characterization at the molecular level in this species

The dog, as a model for cardiovascular function, has been widely used in the pharmacological analysis of PDE inhibitors, particularly those thought to target the heart. However biochemical analyses of dog heart PDE have been largely performed on mixed enzyme populations, sequence information is lacking and no PDE from dog heart has been cloned. We have characterized a completely purified PDE1 enzyme from dog heart using dye-affinity, Mono-Q and calmodulin-affinity chromatography. The enzyme was stimulated 3-4-fold by calmodulin ([S]=0.5 microM) and, in the absence of calmodulin, exhibited biphasic kinetics with a low K(m) of 1.2 microM and 0.53 microM for cAMP and cGMP, with respective V(max) values of 283 and 146 nmoles min(-1) mg(-1). Internal peptides from this enzyme were used to design degenerate PCR primers. Subsequent 3'-RACE, 5'-RACE and high fidelity PCR were then used to produce a full length gene identified as PDE1A1 by sequence identity to human and bovine sequences. Northern analysis using the dog heart cDNA as a probe suggested the presence of an additional form of PDE1, in heart only, separate from the PDE1A group which was present in both heart and skeletal muscle. Multiple forms of human PDE1A are known to exist and PDE1B is present in human heart muscle. The findings here extend the PDE1 data to the dog and contribute to our understanding of the molecular biology of PDE1A in this species.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

The molecular biology of cyclic nucleotide phosphodiesterases

Recent progress in the field of cyclic nucleotides has shown that a large array of closely related proteins is involved in each step of the signal transduction cascade. Nine families of adenylyl cyclases catalyze the synthesis of the second messenger cAMP, and protein kinases A, the intracellular effectors of cAMP, are composed of four regulatory and three catalytic subunits. A comparable heterogeneity has been discovered for the enzymes involved in the inactivation of cyclic nucleotide signaling. In mammals, 19 different genes encode the cyclic nucleotide phosphodiesterases (PDEs), the enzymes that hydrolyze and inactivate cAMP and cGMP. This is only an initial level of complexity, because each PDE gene contains several distinct transcriptional units that give rise to proteins with subtle structural differences, bringing the number of the PDE proteins close to 50. The molecular biology of PDEs in Drosophila and Dictyostelium has shed some light on the role of PDE diversity in signaling and development. However, much needs to be done to understand the exact function of these enzymes, particularly during mammalian development and cell differentiation. With the identification and mapping of regulatory and targeting domains of the PDEs, modularity of the PDE structure is becoming an established tenet in the PDE field. The use of different transcriptional units and exon splicing of a single PDE gene generates proteins with different regulatory domains joined to a common catalytic domain, therefore expanding the array of isoforms with subtle differences in properties and sensitivities to different signals. The physiological context in which these different isoforms function is still largely unknown and undoubtedly will be a major area of expansion in the years to come.

Isolation, expression and analysis of splice variants of a human Ca2+/calmodulin-stimulated phosphodiesterase (PDE1A)

The PDE1A gene encodes a Ca2+/calmodulin-stimulated 3',5'-cyclic nucleotide phosphodiesterase (PDE). We have performed 5' and 3' RACE and identified two additional 5'-splice variants and one additional 3'-splice variant of the human PDE1A gene. The three known 5'-splice variants and the two known 3'-splice variants combine to generate six different PDE1A mRNAs. However, one of the 5'-splice variants exhibits alternate splicing in the 5' untranslated region. Thus the six mRNAs encode four different PDE1A proteins. Recombinant forms of the different human PDE1A isoforms were expressed in Sf9 cells. The kinetic properties and inhibitor sensitivities of the four PDE1A isoforms are very similar to one another.

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.

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.

Inactivation of Recombinant Monocyte cAMP-Specific Phosphodiesterase by cAMP Analog, 8-[(4-Bromo-2,3-Dioxobutyl)thio]Adenosine 3′,5′-Cyclic Monophosphate

Two cAMP analogs, 8- and 2- [(4-bromo-2,3-dioxobutyl) thio]adenosine 3′,5′-cyclic monophosphate (8- and 2-BDB-TcAMP) have been used in probing the catalytic site of recombinant monocyte cAMP-specific phosphodiesterase (PDE4a). 2-BDB-TcAMP is a reversible and competitive inhibitor (Ki = 5.5 μmol/L) of cAMP hydrolysis by PDE4a. 8-BDB-TcAMP irreversibly inactivates the enzyme in a time- and concentration-dependent manner with a second order rate constant of 0.022 mmol/L−1min−1. The rate of inactivation of PDE4a is reduced by the presence of the substrate cAMP and specific inhibitors, rolipram and denbufylline, but not by cGMP or AMP. Reduction of the enzyme-inhibitor complex with sodium [3H]borohydride shows that 1.2 mol of the affinity label/mol of enzyme was incorporated. The radiolabeled peptide is composed of 10 amino acid residues (697 to 706) located near the carboxyl end of the proposed catalytic domain. The peptide (GPGHPPLPDK) has seven nonpolar and aliphatic residues, of which four are proline, giving the peptide a highly structured conformation. This peptide is the first to be identified in the putative catalytic domain involved in substrate recognition.

Receptor-mediated stimulation of lipid signalling pathways in CHO cells elicits the rapid transient induction of the PDE1B isoform of Ca2+/calmodulin-stimulated cAMP phosphodiesterase

Chinese hamster ovary cells (CHO cells) do not exhibit any Ca2+/calmodulin-stimulated cAMP phosphodiesterase (PDE1) activity. Challenge of CHO cells with agonists for endogenous P2-purinoceptors, lysophosphatidic acid receptors and thrombin receptors caused a similar rapid transient induction of PDE1 activity in each instance. This was also evident on noradrenaline challenge of a cloned CHO cell line transfected so as to overexpress alpha 1B-adrenoceptors. This novel PDE1 activity appeared within about 15 min of exposure to ligands, rose to a maximum value within 30 min to 1 h and then rapidly decreased. In each case, the expression of novel PDE1 activity was blocked by the transcriptional inhibitor actinomycin D. Challenge with insulin of either native CHO cells or a CHO cell line transfected so as to overexpress the human insulin receptor failed to induce PDE1 activity. Reverse transcriptase-PCR analyses, using degenerate primers able to detect the PDE1C isoform, did not amplify any fragment from RNA preparations of CHO cells expressing PDE1 activity, although they did so from the human thyroid carcinoma FTC133 cell line. Reverse transcriptase-PCR analyses, using degenerate primers able to detect the PDE1A and PDE1B isoforms, successfully amplified a fragment of the predicted size from RNA preparations of both CHO cells expressing PDE1 activity and human Jurkat T-cells. Sequencing of the PCR products, generated using the PDE1A/B primers, yielded a novel sequence which, by analogy with sequences reported for bovine and murine PDE1B forms, suggests that the PDE1 species induced in CHO cells through protein kinase C activation and that expressed in Jurkat T-cells are PDE1B forms.

The calmodulin-dependent phosphodiesterase gene PDE1C encodes several functionally different splice variants in a tissue-specific manner

We report here the identification of cDNAs for three new mouse PDE1C splice variants and the characterization of their kinetics, regulation by Ca2+, sensitivities to inhibitors, and tissue/cellular expression patterns. Sequence analysis indicated that these three cDNAs (PDE1C1, PDE1C4, and PDE1C5), together with our previously reported PDE1C2 and PDE1C3, are alternative splice products of the PDE1C gene. The results from RNase protection analysis and in situ hybridization indicated that the expression of the different PDE1C splice variants is differentially regulated in a tissue/cell-specific manner. Particularly, high levels of PDE1C mRNAs were found in the olfactory epithelium, testis, and several regions of mouse brain such as cerebellar granule cells. All of these splice variants have similar kinetic properties, showing high affinities and approximately the same relative Vmax values for both cAMP and cGMP. However, they responded to Ca2+ stimulation differently. In addition, they show different sensitivities to the calmodulin-dependent phosphodiesterase inhibitors, KS505a and SCH51866. Substrate competition experiments suggested the presence of only one catalytic site on these PDE1C isozymes for both cAMP and cGMP. In summary, these findings suggest that the PDE1C gene undergoes tissue-specific alternative splicing that generates structurally and functionally diverse gene products.

KS-505a, an isoform-selective inhibitor of calmodulin-dependent cyclic nucleotide phosphodiesterase

The effects of KS-505a, a novel microbial metabolite, on the activity of calmodulin-dependent cyclic nucleotide phosphodiesterase (CaM-PDE) were investigated. (1) KS-505a potently inhibited the purified 61 kDa isoenzyme of CaM-PDE from bovine brain and required much higher doses to inhibit the purified 59 kDa isoenzyme of CaM-PDE from bovine heart. The inhibition of both isoenzymes was observed only in the presence of calcium-activated calmodulin (Ca2+/CaM). The IC50 values for the 61 and 59 kDa isoenzymes were 0.17 and 13 microM respectively with 20 microM cAMP as a substrate. (2) Kinetic analysis indicated that the inhibitory mode of KS-505a for the 61 kDa isoenzyme was competitive with respect to Ca2+/CaM; the K1 for KS-505a was 0.089 microM. The inhibition was not competitive with respect to the substrates cAMP or cGMP. (3) KS-505a did not interfere with the interaction between Ca2+/CaM and n-phenyll-naphthylamine, a hydrophobic fluorescent probe, nor was it adsorbed to CaM-conjugated gels in the presence of Ca2+, thereby indicating that KS-505a does not bind to Ca2+/CaM. (4) Trypsin-activated 61 kDa isoenzyme, which lacked the Ca2+/CaM-binding domain, was not inhibited by KS-505a at less than micromolar concentrations. Taken together, these results suggest that KS-505a apparently bound to a site in the Ca2+/CaM-binding domain of the 61 kDa isoenzyme and selectively inhibited Ca2+/CaM-activated 61 kDa isoenzyme activity. (5) In rat hippocampal slices, KS-505a at 10 micronM increased the intracellular cAMP concentration to approximately three times the basal level, whereas in rat striatal slices it had no effect on the cAMP concentration at concentrations of 1.0-10 microM, suggesting that each CaM-PDE isoenzyme functions differentially in these regions. These results demonstrate that KS-505a is a highly potent selective inhibitor both in vitro and in vivo and distinguishes between subfamily members within the CaM-PDE family.

Isolation and characterization of cDNAs corresponding to two human calcium, calmodulin-regulated, 3',5'-cyclic nucleotide phosphodiesterases

cDNAs corresponding to two human calcium, calmodulin (CaM)-regulated 3',5'-cyclic nucleotide phosphodiesterases (PDEs) were isolated. One, Hcam1 (PDE1A3), corresponds to the bovine 61-kDa CaM PDE (PDE1A2). The second, Hcam3 (PDE1C), represents a novel phosphodiesterase gene. Hcam1 encodes a 535-amino acid protein that differs most notably from the bovine 61-kDa CaM PDE by the presence of a 14-amino acid insertion and a divergent carboxyl terminus. RNase protection studies indicated that Hcam1 is represented in human RNA from several tissues, including brain, kidney, testes, and heart. Two carboxyl-terminal splice variants for Hcam3 were isolated. One, Hcam3b (PDE1C1), encodes a protein 634 amino acids (72 kDa) in length. The other, Hcam3a (PDE1C3), diverges from Hcam3b 4 amino acids from the carboxyl terminus of Hcam3b, and extends an additional 79 amino acids. All the cDNAs isolated for Hcam3a are incomplete; they do not include the 5'-end of the open reading frame. Northern analysis revealed that both splice variants were expressed in several tissues, including brain and heart, and that there may be additional splice variants. Amino-truncated recombinant proteins were expressed in yeast and characterized biochemically. Hcam3a has a high affinity for both cAMP and cGMP and thus has distinctly different kinetic parameters from Hcam1, which has a higher affinity for cGMP than for cAMP. Both PDE1C enzymes were inhibited by isobutylmethylxanthine, 8-methoxymethyl isobutylmethylxanthine, zaprinast, and vinpocetine.

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.

Molecular cloning and characterization of a calmodulin-dependent phosphodiesterase enriched in olfactory sensory neurons

The sensing of an odorant by an animal must be a rapid but transient process, requiring an instant response and also a speedy termination of the signal. Previous biochemical and electrophysiological studies suggest that one or more phosphodiesterases (PDEs) may play an essential role in the rapid termination of the odorant-induced cAMP signal. Here we report the molecular cloning, expression, and characterization of a cDNA from rat olfactory epithelium that encodes a member of the calmodulin-dependent PDE family designated as PDE1C. This enzyme shows high affinity for cAMP and cGMP, having a Km for cAMP much lower than that of any other neuronal Ca2+/calmodulin-dependent PDE. The mRNA encoding this enzyme is highly enriched in olfactory epithelium and is not detected in six other tissues tested. However, RNase protection analyses indicate that other alternative splice variants related to this enzyme are expressed in several other tissues. Within the olfactory epithelium, this enzyme appears to be expressed exclusively in the sensory neurons. The high affinity for cAMP of this Ca2+/calmodulin-dependent PDE and the fact that its mRNA is highly concentrated in olfactory sensory neurons suggest an important role for it in a Ca(2+)-regulated olfactory signal termination.

Molecular cloning of a novel splice variant of human type IVA (PDE-IVA) cyclic AMP phosphodiesterase and localization of the gene to the p13.2-q12 region of human chromosome 19 [corrected]

We have isolated from a human T-cell Jurkat cDNA library a novel human cDNA (2EL) that is closely related to the human type-IV PDE splice variant family 'A' (PDE-IVA) cDNA characterized previously by us [Sullivan, Egerton, Shakur, Marquardsen and Houslay (1994) Cell. Signalling 6, 793-812]; (h6.1, PDE-IVA/h6.1; HSPDE4A7). (PDE stands for cyclic nucleotide phosphodiesterase). The novel cDNA 2EL (PDE-IVA/2EL; HSPDE4A8) contains two regions of unique sequence not found in PDE-IVA/h6.1. These are a distinct 5'-end and a 34 bp insert which occurs within a domain thought to encode the type-IV PDE catalytic site and which can be expected to result in premature truncation of any expressed protein. HSPDE4A8 appeared to be catalytically inactive. Isolation and characterization of a human genomic cosmid clone revealed that 2EL and h6.1 represent alternative splice variants of the human PDE-IVA gene. Using a unique sequence found at the 5'-end of the 2EL cDNA, a probe was generated which was used to screen the DNA of human-hamster hybrids. This located the human gene for PDE-IVA to human chromosome 19. Through both the analysis of genomic DNAs from a human-hamster somatic cell hybrid panel and also using fluorescent in situ hybridization, it was shown that the human PDE-IVA gene is located on human chromosome 19, between p13.2 [corrected] and q12. This region on chromosome 19 has been shown to be related to genetic diseases such as the autosomal dominant cerebrovascular disease CADASIL, susceptibility to late-onset Alzheimer's disease and changes seen in benign pituitary and thyroid adenomas.

Identification and characterization of the developmentally regulated pattern of expression in the testis of a mouse gene exhibiting similarity to the family of phosphodiesterases

A cDNA for a rat brain phosphodiesterase (PDE) was used to screen a mouse testis library to identify the murine PDEs which are expressed in this tissue. A clone of 981 bp, p4-6, was isolated and shown to exhibit limited identity at the amino acid level to the rat brain PDE (20%). The putative protein encoded by clone p4-6 also contains multiple potential modification sites, for phosphorylation, myristylation, and glycosylation, many of which are located at positions similar to those found for rat brain PDE. The gene identified by p4-6 yields 3 transcripts, an abundant 1.9 kb transcript, and less abundant transcripts of 3.8 and 6.7 kb. Of the nine tissues examined in this study, the expression of the corresponding gene was limited to the adult mouse testis. Furthermore, the expression in the testis was most abundant in the germ cell lineage, although low levels were detected in somatic cells of the testis as well. Analysis of RNA from testes at different stages of development suggested that the p4-6 gene is most abundantly expressed in germ cells that have completed the meiotic divisions.

Cyclic GMP and regulation of cyclic nucleotide hydrolysis

Several of the different PDE isozyme families have the ability in vitro to hydrolyze cGMP. In particular they include the CaM-dependent PDEs, the cGMP-stimulated PDEs, and the cGMP binding, cGMP-specific PDEs. Existing evidence suggests or demonstrates that in different cell types, each of these can be important determinants for the control of cGMP steady-state levels. Each of these enzymes is differentially expressed and regulated; moreover, the amount of the enzyme expressed and the mode of regulation determine to a large extent the rate of rise, maximal level, rate of fall, and duration of the cGMP signal in the cell. In addition to enzymes that function to degrade cGMP at least two also are regulated by cGMP both in vitro and in the intact cell. The cGMP-stimulated PDE has the ability to decrease cAMP levels in response to cGMP and the cGMP-inhibited PDE can increase cAMP levels in response to cGMP. We are just beginning to define how many different isozymes of PDE exist in mammalian tissues, where they are located, and how they are regulated. Selective inhibitors to each are being developed and studies designed to define structural features that determine the mechanisms of action and regulation of the PDEs have been initiated. It is expected that in the next few years more PDEs will be discovered and the functions of the new an existing ones with be more clearly defined.

The role of protein phosphorylation in the regulation of cyclic nucleotide phosphodiesterases

The cyclic nucleotide phosphodiesterases constitute a complex superfamily of enzymes responsible for catalyzing the hydrolysis of cyclic nucleotides. Regulation of cyclic nucleotide phosphodiesterases is one of the two major mechanisms by which intracellular cyclic nucleotide levels are controlled. In many cases the fluctuations in cyclic nucleotide levels in response to hormones is due to the hormone responsiveness of the phosphodiesterase. Isozymes of the cGMP-inhibited, cAMP-specific, calmodulin-stimulated and cGMP-binding phosphodiesterases have been demonstrated to be substrates for protein kinases. Here we review the evidence that hormonally responsive phosphorylation acts to regulate cyclic nucleotide phosphodiesterases. In particular, the cGMP-inhibited phosphodiesterases, which can be phosphorylated by at least two different protein kinases, are activated as a result of phosphorylation. In contrast, phosphorylation of the calmodulin-stimulated phosphodiesterases, which coincides with a decreased sensitivity to activation by calmodulin, results in decreased phosphodiesterase activity.

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.

Solution structure of a calmodulin-target peptide complex by multidimensional NMR

The three-dimensional solution structure of the complex between calcium-bound calmodulin (Ca(2+)-CaM) and a 26-residue synthetic peptide comprising the CaM binding domain (residues 577 to 602) of skeletal muscle myosin light chain kinase, has been determined using multidimensional heteronuclear filtered and separated nuclear magnetic resonance spectroscopy. The two domains of CaM (residues 6 to 73 and 83 to 146) remain essentially unchanged upon complexation. The long central helix (residues 65 to 93), however, which connects the two domains in the crystal structure of Ca(2+)-CaM, is disrupted into two helices connected by a long flexible loop (residues 74 to 82), thereby enabling the two domains to clamp residues 3 to 21 of the bound peptide, which adopt a helical conformation. The overall structure of the complex is globular, approximating an ellipsoid of dimensions 47 by 32 by 30 angstroms. The helical peptide is located in a hydrophobic channel that passes through the center of the ellipsoid at an angle of approximately 45 degrees with its long axis. The complex is mainly stabilized by hydrophobic interactions which, from the CaM side, involve an unusually large number of methionines. Key residues of the peptide are Trp4 and Phe17, which serve to anchor the amino- and carboxyl-terminal halves of the peptide to the carboxyl- and amino-terminal domains of CaM, respectively. Sequence comparisons indicate that a number of peptides that bind CaM with high affinity share this common feature containing either aromatic residues or long-chain hydrophobic ones separated by a stretch of 12 residues, suggesting that they interact with CaM in a similar manner.

Regulation and function of cyclic nucleotides

Recent work has greatly expanded our knowledge of the structure, regulation and diversity of enzymes involved in the synthesis and degradation of cyclic nucleotides. This review focuses on recent work that provides insight into the structure and function of the cyclases and phosphodiesterases that regulate cyclic nucleotide metabolism. Particular emphasis is given to the roles played by multiple isoforms of each enzyme system.