Purification of calmodulin-stimulated cyclic nucleotide phosphodiesterase by monoclonal antibody affinity chromatography

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.

Inhibition of cyclic nucleotide phosphodiesterases by methylxanthines and related compounds

Naturally occurring methylxanthines were the first inhibitors of cyclic nucleotide (cN) phosphodiesterases (PDEs) to be discovered. To improve potency and specificity for inhibition of various PDEs in research and for treatment of diseases, thousands of compounds with related structures have now been synthesized. All known PDE inhibitors contain one or more rings that mimic the purine in the cN substrate and directly compete with cN for access to the catalytic site; this review focuses on inhibitors that contain a nucleus that is closely related to the xanthine ring of theophylline and caffeine and the purine ring of cNs. The specificity and potency of these compounds for blocking PDE action have been improved by appending groups at positions on the rings as well as by modification of the number and distribution of nitrogens and carbons in those rings. Several of these inhibitors are highly selective for particular PDEs; potent and largely selective PDE5 inhibitors are used clinically for treatment of erectile dysfunction [sildenafil (Viagra™), tadalafil (Cialis™) and vardenafil (Levitra™)] and pulmonary hypertension [sildenafil (Revatio™) and tadalafil (Adenocirca)]. Related compounds target other PDEs and show therapeutic promise for a number of maladies.

Cyclic GMP signaling in cardiovascular pathophysiology and therapeutics

Cyclic guanosine 3',5'-monophosphate (cGMP) mediates a wide spectrum of physiologic processes in multiple cell types within the cardiovascular system. Dysfunctional signaling at any step of the cascade - cGMP synthesis, effector activation, or catabolism - have been implicated in numerous cardiovascular diseases, ranging from hypertension to atherosclerosis to cardiac hypertrophy and heart failure. In this review, we outline each step of the cGMP signaling cascade and discuss its regulation and physiologic effects within the cardiovascular system. In addition, we illustrate how cGMP signaling becomes dysregulated in specific cardiovascular disease states. The ubiquitous role cGMP plays in cardiac physiology and pathophysiology presents great opportunities for pharmacologic modulation of the cGMP signal in the treatment of cardiovascular diseases. We detail the various therapeutic interventional strategies that have been developed or are in development, summarizing relevant preclinical and clinical studies.

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.

Identification of a New Variant of PDE1A Calmodulin-Stimulated Cyclic Nucleotide Phosphodiesterase Expressed in Mouse Sperm1

In mature sperm, cAMP plays an important role as a second messenger regulating functions that include capacitation, the acrosome reaction, motility, and, in some cases, chemosensing. We have cloned from mouse testis a novel calmodulin-stimulated cyclic nucleotide phosphodiesterase 1A isoform, Pde1a_v7 (mmPDE1A7), which arises from an alternative transcription start in the cyclic nucleotide phosphodiesterase 1A gene. The open reading frame is predicted to encode a polypeptide with a molecular mass of 52 kDa. Two further variants of this form, which contain two additional new exons, arise from alternative splicing. Analysis of testis cDNA by real-time polymerase chain reaction (PCR) indicates that the Pde1A_v7 transcript variant is the most abundant. The PDE1A_v7 protein uniquely lacks the first amino-terminal calmodulin-binding domain, but does possess an inhibitory domain and a second calmodulin-binding site shared with other variants. In vitro translation of the corresponding Pde1a_v7 cDNA produced a 52-kDa polypeptide having cyclic nucleotide hydrolytic activity, which was stimulated threefold by calcium-bound calmodulin. Immunoprecipitation of cyclic nucleotide phosphodiesterase 1 activity from detergent extracts of mouse sperm revealed a major protein of the size expected for PDE1A_v7, and the immunocytochemical staining for cyclic nucleotide phosphodiesterase 1A in mouse sperm showed intense immunoreactivity in the tail only. These observations, along with the PCR data, strongly suggest that this new variant PDE1A_v7 is the major form of cyclic nucleotide phosphodiesterase 1A expressed in mature sperm and is therefore likely to play an important role in cyclic nucleotide regulation of mature sperm function.

Identification, quantitation, and cellular localization of PDE1 calmodulin-stimulated cyclic nucleotide phosphodiesterases

The calmodulin-stimulated cyclic nucleotide phosphodiesterases (PDE1s) constitute a large gene family and are found in a wide variety of tissues and cells. Because of the functional diversity of PDE1 genes and the observation that these isozymes often make up a major component of the total cyclic nucleotide hydrolytic activity in certain cell types, PDE1s are of growing interest as targets for therapeutic intervention. Here we describe a series of methodologies to identify, quantitate, and determine the cellular expression of PDE1 isozymes. We describe first the resolution of different PDEs using high-performance anion-exchange chromatography and then a Western blotting methodology for identifying or authenticating PDE1 activities. Next we present an immunoprecipitation method that can be used for quantitating specific PDE1 isoforms and describe the use of RNase protection analysis for further identification of PDE1 subtypes. Finally, we provide a simple, immunocytochemical method for determining the cellular expression of PDE1 isozymes. Combined, the above methodologies should allow an investigator to identify, quantitate, and determine the cellular localization of PDE1 isozymes in any tissue with little ambiguity.

Calmodulin-stimulated cyclic nucleotide phosphodiesterase (PDE1C) is induced in human arterial smooth muscle cells of the synthetic, proliferative phenotype

The diversity among cyclic nucleotide phosphodiesterases provides multiple mechanisms for regulation of cAMP and cGMP in the cardiovascular system. Here we report that a calmodulin-stimulated phosphodiesterase (PDE1C) is highly expressed in proliferating human arterial smooth muscle cells (SMCs) in primary culture, but not in the quiescent SMCs of intact human aorta. High levels of PDE1C were found in primary cultures of SMCs derived from explants of human newborn and adult aortas, and in SMCs cultured from severe atherosclerotic lesions. PDE1C was the major cAMP hydrolytic activity in these SMCs. PDE expression patterns in primary SMC cultures from monkey and rat aortas were different from those from human cells. In monkey, high expression of PDE1B was found, whereas PDE1C was not detected. In rat SMCs, PDE1A was the only detectable calmodulin-stimulated PDE. These findings suggest that many of the commonly used animal species may not provide good models for studying the roles of PDEs in proliferation of human SMCs. More importantly, the observation that PDE1C is induced only in proliferating SMCs suggests that it may be both an indicator of proliferation and a possible target for treatment of atherosclerosis or restenosis after angioplasty, conditions in which proliferation of arterial SMCs is negatively modulated by cyclic nucleotides.

Protein kinase C in rod outer segments: effects of phosphorylation of the phosphodiesterase inhibitory subunit

The inhibitory subunit (PDE gamma) of the cGMP phosphodiesterase (PDE alpha beta gamma 2) in rod outer segments (ROS) realizes its regulatory role in phototransduction by inhibition of PDE alpha beta catalytic activity. The photoreceptor G-protein, transducin, serves as a transducer from the receptor (rhodopsin) to the effector (PDE) and eliminates the inhibitory effect of PDE gamma by direct interaction with PDE gamma. Our previous study [Udovichenko, Cunnick, Gonzalez and Takemoto (1994) J: Biol. Chem. 269, 9850-9856] has shown that PDE gamma is a substrate for protein kinase C (PKC) from ROS and that phosphorylation by PKC increases the ability of PDE gamma to inhibit PDE alpha beta catalytic activity. Here we report that transducin is less effective in activation of PDE alpha beta (gamma p)2 (a complex of PDE alpha beta with phosphorylated PDE gamma, PDE gamma p) than PDE alpha beta gamma 2. PDE gamma p also increases the rate constant of GTP hydrolysis of transducin (from 0.16 S-1 for non-phosphorylated PDE gamma to 0.21 s-1 for PDE gamma p). These data suggest that phosphorylation of the inhibitory subunit of PDE by PKC may regulate the visual transduction cascade by decreasing the photoresponse.

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.

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.

Phosphorylation of bovine rod photoreceptor cyclic GMP phosphodiesterase

The cyclic GMP phosphodiesterase (PDE) of retinal rods plays a key role in phototransduction and consists of two catalytic subunits (PDE alpha and PDE beta) and two identical inhibitory subunits (PDE gamma). Here we report that PDE alpha and PDE gamma are phosphorylated by protein kinase(s) C (PKC) from brain and rod outer segments (ROS). These same two types of PKC also phosphorylate PDE alpha in trypsin-activated PDE (without PDE gamma). In contrast, cyclic-AMP-dependent protein kinase catalytic subunit phosphorylates both PDE alpha and PDE beta, but not PDE gamma. This kinase does not phosphorylate trypsin-activated PDE. The synthetic peptides AKVISNLLGPREAAV (PDE alpha 30-44) and KQRQTRQFKSKPPKK (PDE gamma 31-45) inhibited phosphorylation of PDE by PKC from ROS. These data suggest that sites (at least one for each subunit) for phosphorylation of PDE by PKC are localized in these corresponding regions of PDE alpha and PDE gamma. Isoenzyme-specific PKC antibodies against peptides unique to the alpha, beta, gamma, delta, epsilon and zeta isoforms of protein kinase C were used to show that a major form of PKC in ROS is PKC alpha. However, other minor forms were also present.

Evidence for domain organization within the 61-kDa calmodulin-dependent cyclic nucleotide phosphodiesterase from bovine brain

The complete amino acid sequence of the 61-kDa calmodulin-dependent, cyclic nucleotide phosphodiesterase (CaM-PDE) from bovine brain has been determined. The native protein is a homodimer of N alpha-acetylated, 529-residue polypeptide chains, each of which has a calculated molecular weight of 60,755. The structural organization of this CaM-PDE has been investigated with use of limited proteolysis and synthetic peptide analogues. A site capable of interacting with CaM has been identified, and the position of the catalytic domain has been mapped. A fully active, CaM-independent fragment (Mr = 36,000), produced by limited tryptic cleavage in the absence of CaM, represents a functional catalytic domain. N-Terminal sequence and size indicate that this 36-kDa fragment is comprised of residues 136 to approximately 450 of the CaM-PDE. This catalytic domain encompasses a approximately 250 residue sequence that is conserved among PDE isozymes of diverse size, phylogeny, and function. CaM-PDE and its PDE homologues comprise a unique family of proteins, each having a catalytic domain that evolved from a common progenitor. A search of the sequence for potential CaM-binding sites revealed only one 15-residue segment with both a net positive charge and the ability to form an amphiphilic alpha-helix. Peptide analogues that include this amphiphilic segment were synthesized. Each was found to inhibit the CaM-dependent activation of the enzyme and to bind directly to CaM with high affinity in a calcium-dependent manner. This site is among the sequences cleaved from a 45-kDa chymotryptic fragment that has the complete catalytic domain but no longer binds CaM. These results indicate that residues located between position 23 and 41 of the native enzyme contribute significantly to the binding of CaM although the involvement of residues from additional sites is not excluded.

Expression of the functional cone phototransduction cascade in retinoblastoma

Retinoblastoma is a malignant intraocular tumor that primarily affects small children. These tumors are primitive neuroectodermal malignancies, however some of them show morphologic evidence of differentiation into photoreceptors. Phototransduction cascades are a series of biochemical reactions that convert a photon of light into a neural impulse in rods and cones. The components of these cascades are uniquely expressed in photoreceptors and, although functionally similar, distinct components of these cascades are expressed in rods and cones. Using HPLC anion exchange chromatography, Western blot analysis, and specific monoclonal and polyclonal antibodies, we found that the cone but not the rod cGMP phosphodiesterase is functionally expressed in all six primary retinoblastomas examined and in three continuous retinoblastoma cell lines. Morphologic evidence of differentiation did not correlate with the expression of the enzyme. Furthermore, GTP analogues could activate the phosphodiesterase activity suggesting that an intact phototransduction cascade is present in the tumors. The presence of the cone phototransduction cascade in retinoblastoma confirms that this tumor has biochemically differentiated along the cone cell lineage.

Molecular properties of cyclic nucleotide phosphodiesterase isozymes

Mammalian cells contain multiple molecular forms of cyclic nucleotide phosphodiesterase that differ in substrate specificity and kinetic and regulatory properties. Calcium/calmodulin and cyclic GMP are important regulators of the hydrolysis of cyclic AMP by either stimulating or inhibiting the activity of distinct forms of phosphodiesterase. Several isozymes of cyclic nucleotide phosphodiesterase have been purified to apparent homogeneity. Although some sequence homology is observed the isozymes appear genetically distinct by immunological criteria. Cyclic AMP- and calmodulin-dependent protein kinases can phosphorylate these enzymes and alter their kinetic and regulatory properties. Both tissue specificity and pharmacological selectivity of isozymes have been demonstrated for several drugs. In certain cases, e.g. cardiac muscle, the selective inhibition of a high affinity cAMP phosphodiesterase activity in a specific subcellular fraction correlates with pharmacologic responses. The results from molecular and pharmacologic studies of cyclic nucleotide phosphodiesterases have indeed expanded the role this system of isoenzymes exerts in the regulation of cellular function.