Alternative splicing of cAMP-specific phosphodiesterase mRNA transcripts. Characterization of a novel tissue-specific isoform, RNPDE4A8

Computational investigation of the dynamic control of cAMP signaling by PDE4 isoform types

Cyclic adenosine monophosphate (cAMP) is a generic signaling molecule that, through precise control of its signaling dynamics, exerts distinct cellular effects. Consequently, aberrant cAMP signaling can have detrimental effects. Phosphodiesterase 4 (PDE4) enzymes profoundly control cAMP signaling and comprise different isoform types of which the enzymatic activity is modulated by differential feedback mechanisms. Because these feedback dynamics are non-linear and occur coincidentally, their effects are difficult to examine experimentally, but can be well simulated computationally. Through understanding the role of PDE4 isoform types in regulating cAMP signaling, PDE4-targeted therapeutic strategies can be better specified. Here, we established a computational model to study how feedback mechanisms on different PDE4 isoform types lead to dynamic, isoform-specific control of cAMP signaling. Ordinary differential equations describing cAMP dynamics were implemented in the VirtualCell (VCell) environment. Simulations indicated that long PDE4 isoforms exert the most profound control on oscillatory cAMP signaling, as opposed to the PDE4-mediated control of single cAMP input pulses. Moreover, elevating cAMP levels or decreasing PDE4 levels revealed different effects on downstream signaling. Together these results underline that cAMP signaling is distinctly regulated by different PDE4 isoform types and that this isoform-specificity should be considered in both computational and experimental follow-up studies to better define PDE4 enzymes as therapeutic targets in diseases in which cAMP signaling is aberrant.

PAN-selective inhibition of cAMP-phosphodiesterase 4 (PDE4) induces gastroparesis in mice

Inhibitors of cAMP-phosphodiesterase 4 (PDE4) exert a number of promising therapeutic benefits, but adverse effects, in particular emesis and nausea, have curbed their clinical utility. Here, we show that PAN-selective inhibition of PDE4, but not inhibition of PDE3, causes a time- and dose-dependent accumulation of chow in the stomachs of mice fed ad libitum without changing the animals' food intake or the weight of their intestines, suggesting that PDE4 inhibition impairs gastric emptying. Indeed, PDE4 inhibition induced gastric retention in an acute model of gastric motility that traces the passage of a food bolus through the stomach over a 30 minutes time period. In humans, abnormal gastric retention of food is known as gastroparesis, a syndrome predominated by nausea (>90% of cases) and vomiting (>80% of cases). We thus explored the abnormal gastric retention induced by PDE4 inhibition in mice under the premise that it may represent a useful correlate of emesis and nausea. Delayed gastric emptying was produced by structurally distinct PAN-PDE4 inhibitors including Rolipram, Piclamilast, Roflumilast, and RS25344, suggesting that it is a class effect. PDE4 inhibitors induced gastric retention at similar or below doses commonly used to induce therapeutic benefits (e.g., 0.04 mg/kg Rolipram), thus mirroring the narrow therapeutic window of PDE4 inhibitors in humans. YM976, a PAN-PDE4 inhibitor that does not efficiently cross the blood-brain barrier, induced gastroparesis only at significantly higher doses (≥1 mg/kg). This suggests that PDE4 inhibition may act in part through effects on the autonomic nervous system regulation of gastric emptying and that PDE4 inhibitors that are not brain-penetrant may have an improved safety profile. The PDE4 family comprises four subtypes, PDE4A, B, C, and D. Selective ablation of any of these subtypes in mice did not induce gastroparesis per se, nor did it protect from PAN-PDE4 inhibitor-induced gastroparesis, indicating that gastric retention may result from the concurrent inhibition of multiple PDE4s. Thus, potentially, any of the four PDE4 subtypes may be targeted individually for therapeutic benefits without inducing nausea or emesis. Acute gastric retention induced by PDE4 inhibition is alleviated by treatment with the widely used prokinetic Metoclopramide, suggesting a potential of this drug to alleviate the side effects of PDE4 inhibitors. Finally, given that the cause of gastroparesis remains largely idiopathic, our findings open the possibility that a physiologic or pathophysiologic downregulation of PDE4 activity/expression may be causative in a subset of patients.

Phosphodiesterases as therapeutic targets for respiratory diseases

Chronic respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and asthma, affect millions of people all over the world. Cyclic adenosine monophosphate (cAMP) which is one of the most important second messengers, plays a vital role in relaxing airway smooth muscles and suppressing inflammation. Given its vast role in regulating intracellular responses, cAMP provides an attractive pharmaceutical target in the treatment of chronic respiratory diseases. Phosphodiesterases (PDEs) are enzymes that hydrolyze cyclic nucleotides and help control cyclic nucleotide signals in a compartmentalized manner. Currently, the selective PDE4 inhibitor, roflumilast, is used as an add-on treatment for patients with severe COPD associated with bronchitis and a history of frequent exacerbations. In addition, other novel PDE inhibitors are in different phases of clinical trials. The current review provides an overview of the regulation of various PDEs and the potential application of selective PDE inhibitors in the treatment of COPD and asthma. The possibility to combine various PDE inhibitors as a way to increase their therapeutic effectiveness is also emphasized.

A. Phosphodiesterases and cAMP Pathway in Pituitary Diseases

Human phosphodiesterases (PDEs) comprise a complex superfamily of enzymes derived from 24 genes separated into 11 PDE gene families (PDEs 1-11), expressed in different tissues and cells, including heart and brain. The isoforms PDE4, PDE7, and PDE8 are specific for the second messenger cAMP, which is responsible for mediating diverse physiological actions involving different hormones and neurotransmitters. The cAMP pathway plays an important role in the development and function of endocrine tissues while phosphodiesterases are responsible for ensuring the appropriate intensity of the actions of this pathway by hydrolyzing cAMP to its inactive form 5'-AMP. PDE1, PDE2, PDE4, and PDE11A are highly expressed in the pituitary, and overexpression of some PDE4 isoforms have been demonstrated in different pituitary adenoma subtypes. This observed over-expression in pituitary adenomas, although of unknown etiology, has been considered a compensatory response to tumorigenesis. PDE4A4/5 has a unique interaction with the co-chaperone aryl hydrocarbon receptor-interacting protein (AIP), a protein implicated in somatotroph tumorigenesis via germline loss-of-function mutations. Based on the association of low PDE4A4 expression with germline AIP-mutation-positive samples, the available data suggest that lack of AIP hinders the upregulation of PDE4A4 protein seen in sporadic somatotrophinomas. This unique disturbance of the cAMP-PDE pathway observed in the majority of AIP-mutation positive adenomas could contribute to their well-described poor response to somatostatin analogs and may support a role in tumorigenesis.

Reduced protein expression of the phosphodiesterases PDE4A4 and PDE4A8 in AIP mutation positive somatotroph adenomas

Type 4 phosphodiesterases (PDE4s) of the large PDE enzyme superfamily have unique specificity for cAMP and may, therefore, be relevant for somatotroph tumorigenesis. Somatotroph adenomas typically overexpress PDEs probably as part of a compensatory mechanism to reduce cAMP levels. The rat PDE4A5 isoform (human homolog PDE4A4) interacts with the AIP protein, coded by a tumour suppressor gene mutated in a subgroup of familial isolated pituitary adenomas (FIPAs). PDE4A8 is the closest related isoform of PDE4A4. We aimed to evaluate the expression of both PDE4A4 and PDE4A8 in GH cells of AIP-mutated adenomas and compare their expression with that in GH cells from sporadic AIP-mutation negative GH-secreting adenomas, where we had shown previously that both PDE4A4 and PDE4A8 isoforms had been over-expressed. Confocal immunofluorescence analysis showed that both PDE4A8 and PDE4A4 had lower expression in AIP-mutated somatotropinoma samples compared to sporadic GH-secreting tumours (P < 0.0001 for both). Based on the association of low PDE4A4 and PDE4A8 expression with germline AIP-mutations positive samples we suggest that lack of AIP hinders the upregulation of PDE4A8 and PDE4A4 protein seen in sporadic somatotrophinomas. These data point to a unique disturbance of the cAMP-PDE pathway in AIP-mutation positive adenomas, which may help to explain their well-described poor response to somatostatin analogues.

The Past, Present, and Future of Phosphodiesterase-4 Modulation for Age-Induced Memory Loss

The purpose of this chapter is to highlight the state of progress for phosphodiesterase-4 (PDE4) modulation as a potential therapeutic for psychiatric illness, and to draw attention to particular hurdles and obstacles that must be overcome in future studies to develop PDE4-mediated therapeutics. Pathological and non-pathological related memory loss will be the focus of the chapter; however, we will at times also touch upon other psychiatric illnesses like anxiety and depression. First, we will provide a brief background of PDE4, and the rationale for its extensive study in cognition. Second, we will explore fundamental differences in individual PDE4 subtypes, and then begin to address differences between pathological and non-pathological aging. Alterations of cAMP/PDE4 signaling that occur within normal vs. pathological aging, and the potential for PDE4 modulation to combat these alterations within each context will be described. Finally, we will finish the chapter with obstacles that have hindered the field, and future studies and alternative viewpoints that need to be addressed. Overall, we hope this chapter will demonstrate the incredible complexity of PDE4 signaling in the brain, and will be useful in forming a strategy to develop future PDE4-mediated therapeutics for psychiatric illnesses.

Expression of PDE11A in Normal and Malignant Human Tissues

Cyclic nucleotide phosphodiesterase 11A (PDE11A) is the newest member in the PDE family. Although the tissue distribution of PDE11A mRNA has been shown, its protein expression pattern has not been well studied. The goal of this report is to investigate the distribution of PDE11A proteins in a wide range of normal and malignant human tissues. We utilized a polyclonal antibody that recognized all four PDE11A isoforms. Its specificity was demonstrated by Western blot analysis on a recombinant human PDE11A protein and native PDE11A proteins in various human tissues. Immunohistochemistry showed that PDE11A is widely expressed. Various degrees of immunoreactivity were observed in the epithelial cells, endothelial cells, and smooth muscle cells of all tissues examined. The highest expression was in the epithelial, endothelial, and smooth muscle cells of the prostate, Leydig, and spermatogenic cells of the testis, the tubule epithelial cells in the kidney, the epithelial and endothelial cells in the adrenal, the epithelial cells and macrophages in the colon, and the epidermis in the skin. Furthermore, PDE11A expression was also detected in several human carcinomas. Our results suggest that PDE11A might be involved in multiple physiological processes in various organs via its ability to modulate intracellular cAMP and cGMP levels.

cAMP-specific PDE4 phosphodiesterases and AIP in the pathogenesis of pituitary tumors

PDE4 cyclic nucleotide phosphodiesterases regulate cAMP abundance in cells and therefore regulate numerous processes, including cell growth and differentiation. The rat PDE4A5 isoform (human homolog PDE4A4) interacts with the AIP protein (also called XAP2 or ARA-9). Germline mutations in AIP occur in approximately 20% of patients with Familial Isolated Pituitary Adenoma (FIPA) and 20% of childhood-onset simplex somatotroph adenomas. We therefore examined the protein expression of PDE4A4 and the closely related isoform PDE4A8 in normal human pituitary tissue and in pituitary adenomas. PDE4A4 had low expression in normal pituitary but was significantly overexpressed in somatotroph, lactotroph, corticotroph and clinically nonfunctioning gonadotroph adenomas (P<0.0001 for all subtypes). Likewise, PDE4A8 was expressed in normal pituitary and was also significantly overexpressed in the adenoma subtypes (P<0.0001 for all). Among the different adenoma subtypes, corticotroph and lactotroph adenomas were the highest and lowest expressed for PDE4A4, respectively, whereas the opposite was observed for PDE4A8. Naturally occurring oncogenic variants in AIP were shown by a two-hybrid assay to disrupt the ability of AIP to interact with PDE4A5. A reverse two-hybrid screen identified numerous additional variants in the tetratricopeptide repeat (TPR) region of AIP that also disrupted its ability to interact with PDE4A5. The expression of PDE4A4 and PDE4A8 in normal pituitary, their increased expression in adenomatous pituitary cells where AIP is meant to participate, and the disruption of the PDE4A4-AIP interaction by AIP mutants may play a role in pituitary tumorigenesis.

Mice deficient in phosphodiesterase-4A display anxiogenic-like behavior

RATIONALE

Phosphodiesterases (PDEs) are a super family of enzymes responsible for the halting of intracellular cyclic nucleotide signaling and may represent novel therapeutic targets for treatment of cognitive disorders. PDE4 is of considerable interest to cognitive research because it is highly expressed in the brain, particularly in the cognition-related brain regions. Recently, the functional role of PDE4B and PDE4D, two of the four PDE4 subtypes (PDE4A, B, C, and D), in behavior has begun to be identified; however, the role of PDE4A in the regulation of behavior is still unknown.

OBJECTIVES

The purpose of this study was to characterize the functional role of PDE4A in behavior.

METHODS

The role of PDE4A in behavior was evaluated through a battery of behavioral tests using PDE4A knockout (KO) mice; urine corticosterone levels were also measured.

RESULTS

PDE4A KO mice exhibited improved memory in the step-through-passive-avoidance test. They also displayed anxiogenic-like behavior in elevated-plus maze, holeboard, light-dark transition, and novelty suppressed feeding tests. Consistent with the anxiety profile, PDE4A KO mice had elevated corticosterone levels compared with wild-type controls post-stress. Interestingly, PDE4A KO mice displayed no change in object recognition, Morris water maze, forced swim, tail suspension, and duration of anesthesia induced by co-administration of xylazine and ketamine (suggesting that PDE4A KO may not be emetic).

CONCLUSIONS

These results suggest that PDE4A may be important in the regulation of emotional memory and anxiety-like behavior, but not emesis. PDE4A could possibly represent a novel therapeutic target in the future for anxiety or disorders affecting memory.

Cyclic AMP-specific phosphodiesterase, PDE8A1, is activated by protein kinase A-mediated phosphorylation

The cyclic AMP-specific phosphodiesterase PDE8 has been shown to play a pivotal role in important processes such as steroidogenesis, T cell adhesion, regulation of heart beat and chemotaxis. However, no information exists on how the activity of this enzyme is regulated. We show that under elevated cAMP conditions, PKA acts to phosphorylate PDE8A on serine 359 and this action serves to enhance the activity of the enzyme. This is the first indication that PDE8 activity can be modulated by a kinase, and we propose that this mechanism forms a feedback loop that results in the restoration of basal cAMP levels.

Phosphodiesterase-4D knock-out and RNA interference-mediated knock-down enhance memory and increase hippocampal neurogenesis via increased cAMP signaling

Phosphodiesterase-4 (PDE4) plays an important role in mediating memory via the control of intracellular cAMP signaling; inhibition of PDE4 enhances memory. However, development of PDE4 inhibitors as memory enhancers has been hampered by their major side effect of emesis. PDE4 has four subtypes (PDE4A-D) consisting of 25 splice variants. Mice deficient in PDE4D displayed memory enhancement in radial arm maze, water maze, and object recognition tests. These effects were mimicked by repeated treatment with rolipram in wild-type mice. In addition, similarly as rolipram-treated wild-type mice, PDE4D-deficient mice also displayed increased hippocampal neurogenesis and phosphorylated cAMP response element-binding protein (pCREB). Furthermore, microinfusion of lentiviral vectors that contained microRNAs (miRNAs) targeting long-form PDE4D isoforms into bilateral dentate gyri of the mouse hippocampus downregulated PDE4D4 and PDE4D5, enhanced memory, and increased hippocampal neurogenesis and pCREB. Finally, while rolipram and PDE4D deficiency shortened α2 adrenergic receptor-mediated anesthesia, a surrogate measure of emesis, miRNA-mediated PDE4D knock-down in the hippocampus did not. The present results suggest that PDE4D, in particular long-form PDE4D, plays a critical role in the mediation of memory and hippocampal neurogenesis, which are mediated by cAMP/CREB signaling; reduced expression of PDE4D, or at least PDE4D4 and PDE4D5, in the hippocampus enhances memory but appears not to cause emesis. These novel findings will aid in the development of PDE4 subtype- or variant-selective inhibitors for treatment of disorders involving impaired cognition, including Alzheimer's disease.

p62 (SQSTM1) and cyclic AMP phosphodiesterase-4A4 (PDE4A4) locate to a novel, reversible protein aggregate with links to autophagy and proteasome degradation pathways

Chronic challenge of cyclic AMP phosphodiesterase-4A4 (PDE4A4) with certain PDE4 selective inhibitors causes it to reversibly form intracellular aggregates that are not membrane-encapsulated. These aggregates are neither stress granules (SGs) nor processing bodies (PBs) as they contain neither PABP-1 nor Dcp1a, respectively. However, the PDE4 inhibitor rolipram decreases arsenite-induced SGs and increases the amount of PBs, while arsenite challenge ablates rolipram-induced PDE4A4 aggregates. PDE4A4 aggregates are neither autophagic vesicles (autophagosomes) nor aggresomes, although microtubule disruptors ablate PDE4A4 aggregate formation. PDE4A4 constitutively co-immunoprecipitates with p62 protein (sequestosome1, SQSTM1), which locates to both PDE4A4 aggregates and autophagosomes in cells constitutively challenged with rolipram. The mTor inhibitor, rapamycin, activates autophagy, prevents PDE4A4 from forming intracellular aggregates and triggers the loss of bound p62 from PDE4A4. siRNA-mediated knockdown of p62 attenuates PDE4A4 aggregate formation. The p62-binding protein, light chain 3 (LC3), is not found in PDE4A4 aggregates. Blockade of proteasome activity and activation of autophagy with MG132 both increases the level of ubiquitinated proteins found associated with PDE4A4 and inhibits PDE4A4 aggregate formation. Activation of autophagy with either thapsigargin or ionomycin inhibits PDE4A4 aggregate formation. Inhibition of autophagy with either wortmannin or LY294002 activates PDE4A4 aggregate formation. The protein kinase C inhibitors, RO 320432 and GO 6983, and the ERK inhibitors UO 126 and PD 98059 all activated PDE4A4 aggregate formation, whilst roscovitine, thalidomide and the tyrosine kinase inhibitors, genistein and AG17, all inhibited this process. We suggest that the fate of p62-containing protein aggregates need not necessarily be terminal, through delivery to autophagic vesicles and aggresomes. Instead, we propose a novel regulatory mechanism where a sub-population of p62-containing protein aggregates would form in a rapid, reversible manner so as to sequester specific cargo away from their normal, functionally important site(s) within the cell. Thus an appropriate conformational change in the target protein would confer reversible recruitment into a sub-population of p62-containing protein aggregates and so provide a regulatory function by removing these cargo proteins from their functionally important site(s) in a cell.

Apremilast, a cAMP phosphodiesterase-4 inhibitor, demonstrates anti-inflammatory activity in vitro and in a model of psoriasis

BACKGROUND AND PURPOSE

Apremilast is an orally administered phosphodiesterase-4 inhibitor, currently in phase 2 clinical studies of psoriasis and other chronic inflammatory diseases. The inhibitory effects of apremilast on pro-inflammatory responses of human primary peripheral blood mononuclear cells (PBMC), polymorphonuclear cells, natural killer (NK) cells and epidermal keratinocytes were explored in vitro, and in a preclinical model of psoriasis.

EXPERIMENTAL APPROACH

Apremilast was tested in vitro against endotoxin- and superantigen-stimulated PBMC, bacterial peptide and zymosan-stimulated polymorphonuclear cells, immunonoglobulin and cytokine-stimulated NK cells, and ultraviolet B light-activated keratinocytes. Apremilast was orally administered to beige-severe combined immunodeficient mice, xenotransplanted with normal human skin and triggered with human psoriatic NK cells. Epidermal skin thickness, proliferation index and inflammation markers were analysed.

KEY RESULTS

Apremilast inhibited PBMC production of the chemokines CXCL9 and CXCL10, cytokines interferon-gamma and tumour necrosis factor (TNF)-alpha, and interleukins (IL)-2, IL-12 and IL-23. Production of TNF-alpha by NK cells and keratinocytes was also inhibited. In vivo, apremilast significantly reduced epidermal thickness and proliferation, decreased the general histopathological appearance of psoriasiform features and reduced expression of TNF-alpha, human leukocyte antigen-DR and intercellular adhesion molecule-1 in the lesioned skin.

CONCLUSIONS AND IMPLICATIONS

Apremilast displayed a broad pattern of anti-inflammatory activity in a variety of cell types and decreased the incidence and severity of a psoriasiform response in vivo. Inhibition of TNF-alpha, IL-12 and IL-23 production, as well as NK and keratinocyte responses by this phosphodiesterase-4 inhibitor suggests a novel approach to the treatment of psoriasis.

Grueneberg ganglion olfactory subsystem employs a cGMP signaling pathway

The mammalian olfactory sense employs several olfactory subsystems situated at characteristic locations in the nasal cavity to detect and report on different classes of odors. These olfactory subsystems use different neuronal signal transduction pathways, receptor expression repertoires, and axonal projection targets. The Grueneberg ganglion (GG) is a newly appreciated olfactory subsystem with receptor neurons located just inside of the nostrils that project axons to a unique domain of interconnected glomeruli in the caudal olfactory bulb. It is not well understood how the GG relates to other olfactory subsystems in contributing to the olfactory sense. Furthermore, the range of chemoreceptors and the signal transduction cascade utilized by the GG have remained mysterious. To resolve these unknowns, we explored the molecular relationship between the GG and the GC-D neurons, another olfactory subsystem that innervates similarly interconnected glomeruli in the same bulbar region. We found that mouse GG neurons express the cGMP-associated signaling proteins phosphodiesterase 2a, cGMP-dependent kinase II, and cyclic nucleotide gated channel subunit A3 coupled to a chemoreceptor repertoire of cilia-localized particulate guanylyl cyclases (pGC-G and pGC-A). The primary cGMP signaling pathway of the GG is shared with the GC-D neurons, unifying their target glomeruli as a unique center of olfactory cGMP signal transduction. However, the distinct chemoreceptor repertoire in the GG suggests that the GG is an independent olfactory subsystem. This subsystem is well suited to detect a unique set of odors and to mediate behaviors that remained intact in previous olfactory perturbations.

Human PDE4A8, a novel brain-expressed PDE4 cAMP-specific phosphodiesterase that has undergone rapid evolutionary change

We have isolated cDNAs encoding PDE4A8 (phosphodiesterase 4 isoform A8), a new human cAMP-specific PDE4 isoform encoded by the PDE4A gene. PDE4A8 has a novel N-terminal region of 85 amino acids that differs from those of the related 'long' PDE4A4, PDE4A10 and PDE4A11 isoforms. The human PDE4A8 N-terminal region has diverged substantially from the corresponding isoforms in the rat and other mammals, consistent with rapid evolutionary change in this region of the protein. When expressed in COS-7 cells, PDE4A8 localized predominantly in the cytosol, but approx. 20% of the enzyme was associated with membrane fractions. Cytosolic PDE4A8 was exquisitely sensitive to inhibition by the prototypical PDE4 inhibitor rolipram (IC(50) of 11+/-1 nM compared with 1600 nM for PDE4A4), but was less sensitive to inhibition by cilomilast (IC(50) of 101+/-7 nM compared with 61 nM for PDE4A4). PDE4A8 mRNA was found to be expressed predominantly in skeletal muscle and brain, a pattern that differs from the tissue expression of other human PDE4 isoforms and also from that of rat PDE4A8. Immunohistochemical analysis showed that PDE4A8 could be detected in discrete regions of human brain, including the cerebellum, spinal cord and cerebral cortex. The unique tissue distribution of PDE4A8, combined with the evolutionary divergence of its N-terminus, suggest that this isoform may have a specific function in regulating cAMP levels in human skeletal muscle and brain.

PDE4B5, a Novel, Super-Short, Brain-Specific cAMP Phosphodiesterase-4 Variant Whose Isoform-Specifying N-Terminal Region Is Identical to That of cAMP Phosphodiesterase-4D6 (PDE4D6)

The cAMP-specific phosphodiesterase-4 (PDE4) gene family is the target of several potential selective therapeutic inhibitors. The four PDE4 genes generate several distinct protein-coding isoforms through the use of alternative promoters and 5'-coding exons. Using mouse transcripts, we identified a novel, super-short isoform of human PDE4B encoding a novel 5' terminus, which we label PDE4B5. The protein-coding region of the novel 5' exon is conserved across vertebrates, chicken, zebrafish, and fugu. Reverse-transcription-polymerase chain reaction (PCR) and quantitative (PCR) measurements show that this isoform is brain-specific. The novel protein is 58 +/- 2 kDa; it has cAMP hydrolyzing enzymatic activity and is inhibited by PDE4-selective inhibitors rolipram and cilomilast (Ariflo). Confocal and subcellular fractionation analyses show that it is distributed predominantly and unevenly within the cytosol. The 16 novel N-terminal residues of PDE4B5 are identical to the 16 N-terminal residues of the super-short isoform of PDE4D (PDE4D6), which is also brain-specific. PDE4B5 is able to bind the scaffold protein DISC1, whose gene has been linked to schizophrenia. Microarray expression profiling of the PDE4 gene family shows that specific PDE4 genes are enriched in muscle and blood fractions; however, only by monitoring the individual isoforms is the brain specificity of the super-short PDE4D and PDE4B isoforms revealed. Understanding the distinct tissue specificity of PDE4 isoforms will be important for understanding phosphodiesterase biology and opportunities for therapeutic intervention.

Expression of the cAMP-phosphodiesterase PDE4D isoforms and age-related changes in follicle-stimulating hormone-stimulated PDE4 activities in immature rat sertoli cells

Major changes in the cAMP-dependent signal transduction pathway triggered by FSH take place during transition of rat Sertoli cells from proliferative to the quiescent/terminally differentiated state. Using Sertoli cell cultures isolated from 10-, 20-, and 30-day-old rats, we recorded a specific increase in PDE4 activity in both the soluble and particulate subcellular fractions of 20-day-old Sertoli cells, which also displayed the highest cAMP response to FSH and the highest FSH-induced increase in PDE4 activity in both subcellular compartments. RT-PCR and immunoblotting experiments showed that almost all the PDE4D isoforms, known as the main cAMP-regulated rolipram-sensitive PDE in Sertoli cells, were expressed throughout the early postpartum period, whereas only the short PDE4D isoforms (PDE4D1 and PDE4D2) were transcriptionally regulated by FSH. Unexpectedly, the immunoblot data also revealed that the soluble PDE4 activities were mainly related to the long PDE4D isoforms and that short PDE4D1 was predominantly particulate. The subcellular distribution and expression of PDE4D proteins were unaffected by the developmental status of the Sertoli cells. Only the expression of short PDE4D1 appeared to be upregulated by FSH and only in 20-day-old Sertoli cells, which suggests phenotype-dependent differential regulation of Pde4d1 mRNA translation. Resensitization of the cAMP response to FSH in 20-day-old Sertoli cells was also associated with the highest FSH-induced transient increase in both soluble and particulate PDE4 activities, which suggests developmental changes in the PKA-mediated upregulation of the catalytic activities of long PDE4D. Such alterations may be involved in the phenotype-dependent alterations in FSH receptor coupling with its associated G proteins in rat Sertoli cells.

Differential expression and regulation of the cAMP-selective phosphodiesterase type 4A splice variants in rat brain by chronic antidepressant administration

Chronic antidepressant treatment up-regulates the cAMP cascade in limbic brain regions, suggesting that activation of this pathway contributes to the therapeutic efficacy of antidepressants. A role for cAMP signaling is supported by the finding that rolipram, a selective inhibitor of cAMP-specific phosphodiesterases type 4 (PDE4), has antidepressant efficacy in behavioral models of depression and in clinical trials. To elucidate further the role of PDE4 isozymes, we characterized the expression and regulation of PDE4A splice variants (i.e. PDE4A1, PDE4A5, PDE4A8 and PDE4A10) in rat brain by chronic antidepressant treatment. Initial in situ hybridization studies (ISH) revealed high levels of PDE4A1 mRNA in medial septum, diagonal band, olfactory system, hippocampus and cerebellum. PDE4A5 mRNA expression was restricted to the olfactory nuclei, deep cortical layers, dentate and CA1 pyramidal layers. PDE4A10 mRNA was localized in the dentate gyrus and CA1 pyramidal layers. PDE4A8 mRNA was absent in rat brain. We determined the influence of chronic fluoxetine or electroconvulsive seizure (ECS) treatments on PDE4A splice variants expression in various brain regions. ISH analysis indicated that chronic fluoxetine or ECS treatments significantly increased PDE4A1, but not PDE4A5 or PDE4A10, mRNA levels in frontal and parietal cortices. ECS increased PDE4A5 levels in the anterior cingulate and frontoparietal cortices, CA1 and dentate gyrus, whereas chronic fluoxetine or ECS treatment increased PDE4A10 levels in the hippocampus. The differential up-regulation of PDE4A splice variants suggests compensatory region-specific responses to the antidepressant-induced increase in cAMP signaling and suggests that these splice variants may be relevant as targets for antidepressant intervention.

Intracellular targeting of phosphodiesterase-4 underpins compartmentalized cAMP signaling

The phosphodiesterase-4 (PDE4) enzyme belongs to a family of cAMP-dependent phosphodiesterases that provide the major means of hydrolyzing and, thereby, inactivating the key intracellular second messenger, cAMP. As such, PDE4s are central to the regulation of many diverse signaling processes that allow cells to respond to external stimuli. Four genes (4A, 4B, 4C, and 4D) encode around 20 distinct isoform members of the PDE4 family. Each isoform is characterized by a unique N-terminal region. PDE4s are multidomain metallohydrolases with each domain serving particular roles allowing them to be targeted to varying regions and organelles of intracellular space and regulated in distinct fashions by phosphorylation and protein-protein interaction. Although identical in catalytic function, each isoform locates to distinct regions within the cell so as to create and manage spatially distinct pools of cAMP. The multiplicity of partners associating with members of the four gene PDE4 family places these enzymes in key regulatory positions, permitting them to channel complex biological signals via fundamental signaling cohorts such as G-protein-coupled receptors (GPCRs), arrestins, A-kinase-anchoring proteins (AKAPs), and tyrosyl family kinases. The cAMP cascade has long been linked to cellular growth and embryogenesis and with this comes the implication that PDE4 may play considerable roles in the regulation of progeny development in maturing cells and tissues.

Modulation of matrix metalloproteinase production from human lung fibroblasts by type 4 phosphodiesterase inhibitors

Over-expression of matrix metalloproteinases by lung fibroblasts has been blamed for much of the tissue destruction associated with airway inflammation. Because cyclic AMP is known to regulate fibroblast proliferation, as well as cytokine and extracellular matrix protein production, the current study was designed to evaluate the ability of three selective phosphodiesterase (PDE) type 4 inhibitors, rolipram, cilomilast and CI-1044, to inhibit extracellular matrix degradation. Using zymography and ELISA, we found that pro-MMP-2 release was enhanced following 24 h treatment of human lung fibroblast (MRC-5) with TGF-beta1 (10 ng/ml) or TNF-alpha (10 ng/ml), whereas PMA (0.02 microM) had no effect. One hour of pre-incubation with PDE4 inhibitors (10 microM) induced an inhibition of TNF-alpha-stimulated pro-MMP-2 release. Zymography and immunoblotting revealed that fibroblasts cultured with PMA or TNF-alpha released increased amounts of pro-MMP-1, whereas TGF-beta1 had no effect. Incubation with CI-1044 or cilomilast significantly prevented the TNF-alpha increase in pro-MMP-1. These results suggest that PDE4 inhibitors are effective in inhibiting the pro-MMP-2 and pro-MMP-1 secretion induced by TNF-alpha and might underline a potential therapeutic benefit of selective PDE4 inhibitors in lung diseases associated with abnormal tissue remodelling.

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

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

Attenuation of the activity of the cAMP-specific phosphodiesterase PDE4A5 by interaction with the immunophilin XAP2

The cyclic AMP-specific phosphodiesterase (PDE4) isoform PDE4A5 interacted with the immunophilin XAP2 in a yeast two-hybrid assay. The interaction was confirmed in biochemical pull-down analyses. The interaction was specific, in that PDE4A5 did not interact with the closely related immunophilins AIPL1, FKBP51, or FKBP52. XAP2 also did not interact with other PDE4A isoforms or typical isoforms from the three other PDE4 subfamilies. Functionally, XAP2 reversibly inhibited the enzymatic activity of PDE4A5, increased the sensitivity of PDE4A5 to inhibition by the prototypical PDE4 inhibitor 4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone (rolipram) and attenuated the ability of cAMP-dependent protein kinase to phosphorylate PDE4A5 in intact cells. XAP2 maximally inhibited PDE4A5 by approximately 60%, with an IC50 of 120 nm, and reduced the IC50 for rolipram from 390 nm to 70-90 nm. Co-expression of XAP2 and PDE4A5 in COS7 cells showed that they could be co-immunoprecipitated and also reduced both the enzymatic activity of PDE4A5 and its IC50 for rolipram. Native XAP2 and PDE4A5 could be co-immunoprecipitated from the brain. The isolated COOH-terminal half of XAP2 (amino acids 170-330), containing its tetratricopeptide repeat domain, but not the isolated NH2-terminal half (amino acids 1-169), containing the immunophilin homology region, similarly reduced PDE4A5 activity and its IC50 for rolipram. Mutation of Arg271 to alanine, in the XAP2 tetratricopeptide repeat region, attenuated its ability to both interact with PDE4A5 in two-hybrid assays and to inhibit PDE4A5 activity. Either the deletion of a specific portion of the unique amino-terminal region or specific mutations in the regulatory UCR2 domain of PDE4A5 attenuated its ability be inhibited by XAP2. We suggest that XAP2 functionally interacts with PDE4A5 in cells.

Prenatal cigarette smoke decreases lung cAMP and increases airway hyperresponsiveness

Epidemiologic studies suggest that in utero exposure to tobacco smoke, primarily through maternal smoking, increases the risk for asthma in children; however, the mechanism of this phenomenon is not clear. Cyclic adenosine monophosphate relaxes airway smooth muscles in the lung and acts as an antiasthmatic. In this study, we examined the effects of in utero cigarette smoke exposure of Balb/c mice on airway responsiveness, as determined by Penh measurements. Animals exposed prenatally but not postnatally to cigarette smoke exhibited increased airway hyperresponsiveness after a single intratracheal injection of Aspergillus fumigatus extract. The increased airway hyperresponsiveness was not associated with increased leukocyte migration or mucous production in the lung but was causally related to decreased lung cyclic adenosine monophosphate levels, increased phosphodiesterase-4 enzymatic activity, and phosphodiesterase-4D (PDE4D) isoform-specific messenger ribonucleic acid expression in the lung. Exposure of adult mice to cigarette smoke did not significantly alter airway responsiveness, cyclic adenosine monophosphate levels, or the phosphodiesterase activity. These results suggest that prenatal exposure to cigarette smoke affects lung airway reactivity by modulating the lung cyclic adenosine monophosphate levels through changes in phosphodiesterase-4D activity, and these effects are independent of significant mucous production or leukocyte recruitment into the lung.

Molecular cloning and subcellular distribution of the novel PDE4B4 cAMP-specific phosphodiesterase isoform

We have isolated cDNAs encoding PDE4B4, a new cAMP-specific phosphodiesterase (PDE4) isoform with novel properties. The amino acid sequence of PDE4B4 demonstrates that it is encoded by the PDE4B gene, but that it differs from the previously isolated PDE4B1, PDE4B2 and PDE4B3 isoforms by the presence of a novel N-terminal region of 17 amino acids. PDE4B4 contains both of the upstream conserved region 1 (UCR1) and UCR2 regulatory units that are characteristic of 'long' PDE4 isoforms. RNase protection demonstrated that PDE4B4 mRNA is expressed preferentially in liver, skeletal muscle and various regions of the brain, which differs from the pattern of tissue distribution of the other known PDE4B long forms, PDE4B1 and PDE4B3. Expression of PDE4B4 cDNA in COS7 cells produced a protein of 85 kDa under denaturing conditions. Subcellular fractionation of recombinant, COS7-cell expressed PDE4B4 showed that the protein was localized within the cytosol, which was confirmed by confocal microscopic analysis of living COS7 cells transfected with a green fluorescent protein-PDE4B4 chimaera. PDE4B4 exhibited a K(m) for cAMP of 5.4 microM and a V(max), relative to that of the long PDE4B1 isoform, of 2.1. PDE4B4 was inhibited by the prototypical PDE4 inhibitor rolipram [4-[3-(cyclopentoxyl)-4-methoxyphenyl]-2-pyrrolidinone] with an IC(50) of 83 nM. Treatment of COS7 cells with forskolin, to elevate cAMP levels, produced activation of PDE4B4, which was associated with the phosphorylation of PDE4B4 on Ser-56 within UCR1. The unique tissue distribution and intracellular targeting of PDE4B4 suggests that this isoform may have a distinct functional role in regulating cAMP levels in specific cell types.

Type IV phosphodiesterase activity specifically regulates cAMP-stimulated casein secretion in the rat mammary gland

This study investigates the regulation of cAMP-stimulated casein secretion in rat mammary explants by cAMP phosphodiesterase (cAMP-PDE) activity. cAMP-PDE activity of the lactating rat mammary gland is shown to be provided by three families, types II, III and IV. In mammary explants, general inhibition of the cAMP-PDE activity significantly increased the rate of cAMP-stimulated casein secretion. This effect could be mimicked using the type-IV specific inhibitor rolipram but not by the specific, or combined, inhibition of the type II and type III activity. Only type IV activity significantly affected intracellular accumulation of cAMP whereas all three cAMP-PDE activities were shown to influence the PKA activation ratio in cells. RtPCR analysis showed that the mammary gland apparently expresses just three type IV isozymes, RNPDE4A5, RNPDE4A8 and RNPDE4D3. A specific role for type IV cAMP-PDE activity in the regulation of casein secretion is suggested and possible mechanisms for the effects of PDEIV activity discussed.

Long PDE4 cAMP specific phosphodiesterases are activated by protein kinase A-mediated phosphorylation of a single serine residue in Upstream Conserved Region 1 (UCR1)

1. Challenge of COS1 cells with the adenylyl cyclase activator forskolin led to the activation of recombinant PDE4A8, PDE4B1, PDE4C2 and PDE4D5 cAMP-specific phosphodiesterase long isoforms. 2. Forskolin challenge did not activate mutant long PDE4 isoforms where the serine target residue (STR) within the protein kinase A (PKA) consensus phosphorylation site in Upstream Conserved Region 1 (UCR1) was mutated to alanine. 3. The PKA inhibitor, H89, ablated forskolin activation of wild-type long PDE4 isoforms. 4. Activated PKA caused the in vitro phosphorylation of recombinant wild-type long PDE4 isoforms, but not those where the STR was mutated to alanine. 5. An antiserum specific for the phosphorylated form of the STR detected a single immunoreactive band for recombinant long PDE4 isoforms expressed in COS1 cells challenged with forskolin. This was not evident in forskolin-challenged cells treated with H89. Neither was it evident in forskolin-challenged cells expressing long isoforms where the STR had been mutated to alanine. 6. In transfected COS cells challenged with forskolin, only the phosphorylated PDE4D3 long form showed a decrease in mobility in Western blotting analysis. This decreased mobility of PDE4D3 was ablated upon mutation of either of the two serine targets for PKA phosphorylation in this isoform, namely Ser54 in UCR1 and Ser13 in the isoform-specific N-terminal region. 7. Activation by forskolin challenge did not markedly alter the sensitivity of PDE4A8, PDE4B1, PDE4C2 and PDE4D5 to inhibition by rolipram. 8. Long PDE4 isoforms from all four sub-families can be phosphorylated by protein kinase A (PKA). This leads to an increase in their activity and may thus contribute to cellular desensitization processes in cells where these isoforms are selectively expressed.

In addition to the SH3 binding region, multiple regions within the N-terminal noncatalytic portion of the cAMP-specific phosphodiesterase, PDE4A5, contribute to its intracellular targeting

The long cyclic AMP (cAMP)-specific phosphodiesterase isoform, PDE4A5 (PDE4A subfamily isoform variant 5), when transiently expressed in COS-7 cells, was shown in subcellular fractionation studies to be associated with both membrane and cytosol fractions, with immunofluorescence analyses identifying PDE4A5 as associated both with ruffles at the cell margin and also at a distinct perinuclear localisation. Deletion of the first nine amino acids of PDE4A5 (1) ablated its ability to interact with the SH3 domain of the tyrosyl kinase, LYN; (2) reduced, but did not ablate, membrane association; and (3) disrupted the focus of PDE4A5 localisation within ruffles at the cell margin. This deleted region contained a Class I SH3 binding motif of similar sequence to those identified by screening a phage display library with the LYN-SH3 domain. Truncation to remove the PDE4A5 isoform-specific N-terminal region caused a further reduction in membrane association and ablated localisation at the cell margin. Progressive truncation to delete the PDE4A long isoform common region and then the long isoform-specific UCR1 did not cause any further change in membrane association or intracellular distribution. However, deletion up to the super-short form splice junction generated an entirely soluble 'core' PDE4A species. We propose that multiple sites in the N-terminal noncatalytic portion of PDE4A5 have the potential to associate with intracellular structures and thus define its intracellular localisation. At least two such sites lie within the PDE4A5 isoform-specific N-terminal region and these appear to be primarily responsible for targeting PDE4A5 to, and organising it within, the cell margin; one is an SH3 binding motif able to interact with LYN kinase and the other lies within the C-terminal portion of the PDE4A5 unique region. A third membrane association region is located within the N-terminal portion of UCR2 and appears to be primarily responsible for targeting to the perinuclear region. Progressive N-terminal truncation, to delete defined regions of PDE4A5, identified activity changes occurring upon deletion of the SH3 binding site region and then upon deletion of the membrane association site region located within UCR2. This suggests that certain of these anchor sites may not only determine intracellular targeting but may also transduce regulatory effects on PDE4A5 activity.

The novel long PDE4A10 cyclic AMP phosphodiesterase shows a pattern of expression within brain that is distinct from the long PDE4A5 and short PDE4A1 isoforms

In situ hybridisation methods were used to map the distribution of the novel long PDE4A10 isoform in the brain. PDE4A10 distribution was compared to that of the long PDE4A5 isoform and the short PDE4A1 isoform using probes specific for unique sequences within each of these isoforms. Coronal sections of the brain, taken at the level of the olfactory bulb, prefrontal cortex, striatum, thalamus, hippocampus and cerebellum, were analysed. Strongest expression of PDE4A isoforms was found in the olfactory bulb granular layer with high signals also in the piriform cortex, the dentate gyrus and the CA1 and CA2 pyramidal cells. For the two long forms, level general staining was noted throughout the striatum, thalamus and hippocampus but no signal was evident in the cerebellum. The long PDE4A10 and PDE4A5 isoforms localised to essentially the same regions throughout the brain, although PDE4A10 was uniquely expressed in the major island of Calleja. A signal for the short PDE4A1 isoform was found in regions in which the two long isoforms were both expressed, with the exception of the medial nucleus of the amygdala where weak signals for PDE4A5 and PDE4A10 were detected but PDE4A1 was absent. Uniquely, strong signals for PDE4A1 were detected in the glomerular layer of the olfactory bulb, the CA3 pyramidal cell region and the cerebellum; areas where signals for the two long forms were not evident. PDE4A transcripts for both PDE4A5 and PDE4A10 were not apparent in the brain stem and those for PDE4A1 were low. PDE4A isoforms are present in several key areas of the brain and therefore present valid targets for therapeutic interventions. Whilst the two long PDE4A isoforms show a remarkably similar distribution, in at least three regions there is clear segregation between their pattern of expression and that of the PDE4A1 short form. This identifies differential regulation of the expression of PDE4A long and short isoforms. We suggest that specific PDE4A isoforms may have distinct functional roles in the brain, indicating that PDE4A isoform-selective inhibitors may have specific therapeutic and pharmacologic properties.

Development of rolipram-sensitive, cyclic AMP phosphodiesterase (PDE4) in rat primary neuronal cultures

The development of PDE4 was examined in primary neuronal cultures of rat cerebral cortex. Three days after culturing, neurons exhibited relatively low PDE4 activity (i.e., rolipram-sensitive PDE activity). It gradually increased over time, approximately doubling by day 12. The increase in activity was accompanied by an increase in the expression of the PDE4A variants, PDE4A5 and PDE4A1, as well as of the synaptic marker protein synapsin I. There was a strong correlation between the expression of the PDE4A variants with that of synapsin I, which suggests that as neurons develop and signal transduction increases there is a regulated increase in PDE4 expression and activity. Consistent with this interpretation, it was found that treatment with the sodium channel blocker tetrodotoxin, which inhibits depolarization-induced neurotransmitter release, reduced the expression of the PDE4A variants. These data demonstrate the developmental regulation of PDE4 in neurons and offer a manner by which the association of PDE4 variants with particular signal transduction pathways may be studied in vitro.

Identification of rat cyclic nucleotide phosphodiesterase 11A (PDE11A): comparison of rat and human PDE11A splicing variants

We have isolated and characterized rat cyclic nucleotide phosphodiesterase (PDE)11A, which exhibits properties of a dual-substrate PDE, and its splice variants (RNPDE11A2, RNPDE11A3, and RNPDE11A4). The deduced amino-acid sequence of the longest form of rat PDE11A splice variant, RNPDE11A4, was 94% identical with that of the human variant (HSPDE11A4). Rat PDE11A splice variants were expressed in a tissue-specific manner. RNPDE11A4 showed unique tissue distribution distinct from HSPDE11A4, which is specifically expressed in the prostate. Rat PDE11A splice variants were expressed in COS-7 cells, and their enzymatic characteristics were compared. Although the Km values for cAMP and cGMP were similar for all of them (1.3-1.6 and 2.1-3.9 microM, respectively), the Vmax values differed significantly (RNPDE11A4 >> RNPDE11A2 > RNPDE11A3). Human PDE11A variants also displayed very similar Km values and significantly different Vmax values (HSPDE11A4 >> HSPDE11A2 > HSPDE11A3 >> HSPDE11A1). The Vmax values of HSPDE11A4 for cAMP and cGMP were at least 100 times higher than those of HSPDE11A1. These observations indicate unique characteristics of PDE11A splicing variants.

Molecular cloning, genomic positioning, promoter identification, and characterization of the novel cyclic amp-specific phosphodiesterase PDE4A10

We describe the cloning and expression of HSPDE4A10, a novel long form splice variant of the human cAMP phosphodiesterase PDE4A gene. The 825 amino acid HSPDE4A10 contains a unique N terminus of 46 amino acids encoded by a unique 5' exon. Exon-1(4A10) lies approximately 11 kilobase pairs (kb) downstream of exon-1(4A4) and approximately 13.5 kb upstream of the PDE4A common exon 2. We identify a rat PDE4A10 ortholog and reveal a murine ortholog by nucleotide sequence database searching. PDE4A10 transcripts were detected in various human cell lines and tissues. The 5' sequence flanking exon-1(4A10) exhibited promoter activity with the minimal functional promoter region being highly conserved in the corresponding mouse genomic sequence. Transient expression of the engineered human PDE4A10 open reading frame in COS7 cells allowed detection of a 121-kDa protein in both soluble and particulate fractions. PDE4A10 was localized primarily to the perinuclear region of COS7 cells. Soluble and particulate forms exhibited similar K(m) values for cAMP hydrolysis (3-4 microM) and IC(50) values for inhibition by rolipram (50 nM) but the V(max) value of the soluble form was approximately 3-fold greater than that of the particulate form. At 55 degrees C, soluble HSPDE4A10 was more thermostable (T(0.5) = 11 min) than the particulate enzyme (T(0.5) = 5 min). HSPDE4A10 and HSPDE4A4B are shown here to be similar in size and exhibit similar maximal activities but differ with respect to sensitivity to inhibition by rolipram, thermostability, interaction with the SRC homology 3 domain of LYN, an SRC family tyrosyl kinase, and subcellular localization. We suggest that the unique N-terminal regions of PDE4A isoforms confer distinct properties upon them.

Diazepam and rolipram differentially inhibit cyclic AMP-specific phosphodiesterases PDE4A1 and PDE4B3 in the mouse

Cyclic AMP is hydrolyzed by members of at least eight classes of cyclic nucleotide phosphodiesterases (PDEs). Although it has been reported that cyclic AMP PDE activity in mammalian tissues can be inhibited by benzodiazepines, it has not been conclusively demonstrated that members of the class of cyclic AMP-specific, rolipram-inhibitable PDEs (PDE4s) are targets for these drugs. Moreover, no PDE4s expressed in mice have been characterized. To address these issues, we isolated two cDNAs representing homologues of PDE4A1 and PDE4B3 from a mouse brain library. After transient transfection in human embryonic kidney (HEK) 293 cells, the mouse PDEs hydrolyzed cyclic AMP with a low K(m) and were inhibited by rolipram; both are properties typical of other mammalian PDE4 enzymes. In addition, we found that diazepam inhibited cyclic AMP hydrolysis by the mouse PDE4 subtypes. Interestingly, PDE4B was significantly more sensitive to inhibition by both rolipram and diazepam than the PDE4A subtype. This is the first demonstration that recombinantly expressed PDE4s are inhibited by diazepam, and should facilitate future studies with mouse models of depression and anxiety.

Membrane localization of cyclic nucleotide phosphodiesterase 3 (PDE3). Two N-terminal domains are required for the efficient targeting to, and association of, PDE3 with endoplasmic reticulum

Subcellular localization of cyclic nucleotide phosphodiesterases (PDEs) may be important in compartmentalization of cAMP/cGMP signaling responses. In 3T3-L1 adipocytes, mouse (M) PDE3B was associated with the endoplasmic reticulum (ER) as indicated by its immunofluorescent colocalization with the ER protein BiP and subcellular fractionation studies. In transfected NIH 3006 or COS-7 cells, recombinant wild-type PDE3A and PDE3B isoforms were both found almost exclusively in the ER. The N-terminal portion of PDE3 can be arbitrarily divided into region 1 (aa 1-300), which contains a large hydrophobic domain with six predicted transmembrane helices, followed by region 2 (aa 301-500) containing a smaller hydrophobic domain (of approximately 50 aa). To investigate the role of regions 1 and 2 in membrane association, we examined the subcellular localization of a series of catalytically active, Flag-tagged N-terminal-truncated human (H) PDE3A and MPDE3B recombinants, as well as a series of fragments from regions 1 and 2 of MPDE3B synthesized as enhanced green fluorescent (EGFP) fusion proteins in COS-7 cells. In COS-7 cells, the localization of a mutant HPDE3A, lacking the first 189 amino acids (aa) and therefore four of the six predicted transmembrane helices (H3A-Delta189), was virtually identical to that of the wild type. M3B-Delta302 (lacking region 1) and H3A-Delta397 (lacking region 1 as well as part of region 2) retained, to different degrees, the ability to associate with membranes, albeit less efficiently than H3A-Delta189. Proteins that lacked both regions 1 and 2, H3A-Delta510 and M3B-Delta604, did not associate with membranes. Consistent with these findings, region 1 EGFP-MPDE3B fusion proteins colocalized with the ER, whereas region 2 EGFP fusion proteins were diffusely distributed. Thus, some portion of the N-terminal hydrophobic domain in region 1 plus a second domain in region 2 are important for efficient membrane association/targeting of PDE3.

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.

Isolation and characterization of two novel phosphodiesterase PDE11A variants showing unique structure and tissue-specific expression

cDNAs encoding a novel phosphodiesterase, phosphodiesterase 11A (PDE11A), were isolated by a combination of reverse transcriptase-polymerase chain reaction using degenerate oligonucleotide primers and rapid amplification of cDNA ends. Their catalytic domain was identical to that of PDE11A1 (490 amino acids) reported during the course of this study. However, the cDNAs we isolated had N termini distinct from PDE11A1, indicating two novel N-terminal variants of PDE11A. PDE11A3 cDNA encoded a 684-amino acid protein including one complete and one incomplete GAF domain in the N-terminal region. PDE11A4 was composed of 934 amino acids including two complete GAF domains and shared 630 C-terminal amino acids with PDE11A3 but had a distinct N terminus containing the putative phosphorylation sites for cAMP- and cGMP-dependent protein kinases. PDE11A3 transcripts were specifically expressed in testis, whereas PDE11A4 transcripts were particularly abundant in prostate. Recombinant PDE11A4 expressed in COS-7 cells hydrolyzed cAMP and cGMP with K(m) values of 3.0 and 1.4 microm, respectively, and the V(max) value with cAMP was almost twice that with cGMP. Although PDE11A3 showed the same K(m) values as PDE11A4, the relative V(max) values of PDE11A3 were approximately one-sixth of those of PDE11A4. PDE11A4, but not PDE11A3, was phosphorylated by both cAMP- and cGMP-dependent protein kinases in vitro. Thus, the PDE11A gene undergoes tissue-specific alternative splicing that generates structurally and functionally distinct gene products.

Sub-family selective actions in the ability of Erk2 MAP kinase to phosphorylate and regulate the activity of PDE4 cyclic AMP-specific phosphodiesterases

Expressed in intact cells and in vitro, PDE4B and PDE4C isoenzymes of cyclic nucleotide phosphodiesterase (PDE), in common with PDE4D isoenzymes, are shown to provide substrates for C-terminal catalytic unit phosphorylation by the extracellular signal-regulated kinase Erk2 (p42(MAPK)). In contrast, PDE4A isoenzymes do not provide substrates for C-terminal catalytic unit phosphorylation by Erk2. Mutant PDE4 enzymes were generated to show that Erk2 phosphorylation occurs at a single, cognate serine residue located within the C-terminal portion of the PDE4 catalytic unit. PDE4 long-form isoenzymes were markedly inhibited by Erk2 phosphorylation. The short-form PDE4B2 isoenzyme was activated by Erk2 phosphorylation. These functional changes in PDE activity were mimicked by mutation of the target serine for Erk2 phosphorylation to the negatively charged amino acid, aspartic acid. Epidermal growth factor (EGF) challenge caused diametrically opposed changes in cyclic AMP levels in COS1 cells transfected to express the long PDE4B1 isoenzyme compared to cells expressing the short PDE4B2 isoenzyme. We suggest that PDE4 enzymes may provide a pivotal point for integrating cyclic AMP and Erk signal transduction in cells with 4 genes encoding enzymes that are either insensitive to Erk2 action or may either be activated or inhibited. This indicates that PDE4 isoenzymes have distinct functional roles, giving credence to the notion that distinct therapeutic benefits may accrue using either PDE4 subfamily or isoenzyme-selective inhibitors.

The cAMP-specific phosphodiesterase PDE4A5 is cleaved downstream of its SH3 interaction domain by caspase-3. Consequences for altered intracellular distribution

The unique N-terminal region of the cAMP-specific phosphodiesterase PDE4A5, which confers an ability to bind to certain protein SH3 domains, is cleaved during apoptosis in both Rat-1 fibroblasts and PC12 cells. Cleavage was abolished by the caspase-3-selective inhibitor, z-DEVD-CHO but not the caspase-1 selective inhibitor, z-YVAD-CHO. Caspase-3 treatment of PDE4A5, expressed either transiently in COS cells or generated in vitro by coupled transcription translation, generated a similar cleavage product of 100 kDa compared with the native 110-kDa PDE4A5. This product could be detected immunochemically with an antibody raised to a C-terminal PDE4A5 peptide but not an antibody raised to the N terminus of PDE4A5, indicating that caspase-3 caused N-terminal cleavage of PDE4A5. Deletion of the putative caspase-3 cleavage site, (69)DAVD(72), in PDE4A5, or generation of either the D72A or the D69A mutants, ablated the ability of caspase-3 to cause cleavage. The N-terminal truncate PDE4A5-DeltaP3 was engineered to mimic the caspase-cleaved product of PDE4A5. This showed altered catalytic activity and, unlike PDE4A5, was unable to interact with the SH3 domain of the tyrosyl kinase, LYN. Although both PDE4A5 and PDE4A5-DeltaP3 were localized at cell cortical regions (ruffles), the distinct perinuclear association noted for both PDE4A5 and LYN was not seen for PDE4A5-DeltaP3. Staurosporine-induced apoptosis caused a marked redistribution of PDE4A5 but not PDE4A8 in stably transfected Rat-1 cells. The PDE4-selective inhibitor, rolipram together with the adenylyl cyclase activator forskolin, caused a synergistic increase in the apoptosis of Rat-1 cells. Overexpression of PDE4A5 in Rat-1 cells protected against staurosporine-induced apoptosis in contrast to overexpression of PDE4A8, which potentiated apoptosis. PDE4A5 may be the sole PDE4 family member to provide a substrate for caspase-3 cleavage and this action serves to remove the SH3 binding domain that is unique to this isoform within the PDE4A family and to alter its intracellular targeting.

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.

Functions of the N-terminal region of cyclic nucleotide phosphodiesterase 3 (PDE 3) isoforms

The N-terminal portion of phosphodiesterase (PDE) 3 was arbitrarily divided into region 1 (amino acids 1-300), which contains a large hydrophobic domain with six predicted transmembrane helices, and region 2 (amino acids 301-500), with a smaller hydrophobic domain ( approximately 50 residues). To analyze these regions, full-length human (H)PDE3A and mouse (M)PDE3B and a series of N-terminal truncated mutants were synthesized in Sf9 cells. Activities of HPDE3A, H3A-Delta189, MPDE3B, and M3B-Delta196, which retained all or part of the hydrophobic domain in region 1, were recovered almost entirely in particulate fractions. H3A-Delta321 and M3B-Delta302, containing region 2, were recovered essentially equally in particulate and cytosolic fractions. H3A-Delta397 and H3A-Delta457, lacking both hydrophobic domains, were predominantly cytosolic. H3A-Delta510 and M3B-Delta604, lacking both regions 1 and 2, were virtually completely cytosolic. M3B-Delta196 eluted as a large aggregated complex during gel filtration. With removal of greater amounts of N-terminal sequence, aggregation of PDE3 decreased, and H3A-Delta607, H3A-Delta721, and M3B-Delta604 eluted as dimers. Truncated HPDE3A proteins were more sensitive than full-length HPDE3A to inhibition by lixazinone. These results suggest that the hydrophobic domains in regions 1 and 2 contain structural determinants important for association of PDE3 with intracellular membranes, as well for self-association or aggregation during gel filtration and sensitivity to a specific inhibitor.

Striatum- and testis-specific phosphodiesterase PDE10A isolation and characterization of a rat PDE10A

PDE10A, a phosphodiesterase (PDE) exhibiting properties of a cAMP PDE and a cAMP-inhibited cGMP PDE, was cloned and investigated in detail in rats. PDE10A transcripts were abundant in the brain and testis. In situ hybridization analysis using a PDE10A riboprobe demonstrated the presence of PDE10A transcripts in the neurons of the striatum and the olfactory tubercle regions of the brain. Rat PDE10A cDNAs were isolated from a brain cDNA library and nucleotide sequence analysis revealed several N-terminal variants. The deduced amino-acid sequence of one of the major variant forms contained 794 amino acids, and it was 96% identical to that of the human PDE10A2. The other major form has a distinct N-terminal sequence that is not found in humans. PDE10A was partially purified from rat striatum and testis, and characterized with respect to Km, inhibitor sensitivity and immunoreactivity to an anti-PDE10A serum. These findings indicate that PDE10A functions in these tissues.

The unique N-terminal domain of the cAMP phosphodiesterase PDE4D4 allows for interaction with specific SH3 domains

Of the five PDE4D isoenzymes, only the PDE4D4 cAMP specific phosphodiesterase was able to bind to SH3 domains. Only PDE4D4 and PDE4A5, but not any other PDE4A, B, C and D isoforms expressed in rat brain, bound to src, lyn and fyn kinase SH3 domains. Purified PDE4D4 could bind to purified lyn SH3. PDE4D4 and PDE4A5 both exhibited selectivity for binding the SH3 domains of certain proteins. PDE4D4 did not bind to WW domains. We suggest that an important function of the unique N-terminal region of PDE4D4 may be to allow for association with certain SH3 domain-containing proteins.

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.

Selective up-regulation of phosphodiesterase-4 cyclic adenosine 3',5'-monophosphate (cAMP)-specific phosphodiesterase variants by elevated cAMP content in human myometrial cells in culture

In human myometrium, the modulation of intracellular cAMP content resulting from agonist-mediated stimulation of the receptor-adenylyl cyclase complex is largely influenced by the rate of cAMP hydrolysis by phosphodiesterase (PDE) isoenzymes. We have previously shown that the PDE4 family contributes to the predominant cAMP-hydrolyzing activity in human myometrium and that elevation of the PDE4B2 messenger RNA steady state level occurs in pregnant myometrial tissue. In the present study, we used a model of human myometrial cells in culture to determine whether an elevated cAMP concentration could influence PDE expression. As in myometrial tissue, high levels of PDE4 activity were detected in these smooth muscle cells. Long term treatment with 8-bromo-cAMP or forskolin resulted in a selective induction of PDE4B and of PDE4D short form messenger RNA variants. Concurrently, an increased immunoreactive signal for the PDE4B- and PDE4D-related isoenzymes was detected. This induction was consistent with an observed significant up-regulation of PDE4 activity. Accordingly, our results demonstrate that in human cultured myometrial cells, cAMP-elevating agents manipulate PDE4 activity through selective induction of synthesis of PDE4B and PDE4D short forms. Such a mechanism might have physiological importance during pregnancy by dampening hormonal stimulation and could thereby be involved in tolerance to the tocolytic effect of beta-adrenoceptor agonists.

Genomic origin and transcriptional regulation of two variants of cGMP-binding cGMP-specific phosphodiesterases

We have reported alternative splice variants of cGMP-binding cGMP-specific phosphodiesterases (PDE5A), i.e. rat PDE5A2, human PDE5A1, canine PDE5A1 and PDE5A2, which possess distinct N-terminal sequences. In this study, the DNA sequences corresponding to the unique N-terminal portions of PDE5A1 and PDE5A2 were shown to be tandemly located upstream of exons encoding the common region of PDE5A in both human and rat PDE5A genes. The presence of human PDE5A2 and rat PDE5A1 transcripts in lung was confirmed by reverse transcriptase-PCR. These results indicated that two variant forms of PDE5A exist in humans, canines and rats. We examined the tissue distribution of the two variants of human PDE5A in adult and fetal humans. The patterns of expression of the two alternatively spliced transcripts of human PDE5A in human tissues differed. Many putative regulatory elements including cAMP response elements were observed in the 5'-untranslated region and intron of the PDE5A gene. The levels of the PDE5A transcripts, especially the PDE5A2 transcripts, were increased by a cAMP analogue in cultured rat vascular smooth muscle cells, indicating that the PDE5A2 is an inducible variant of PDE5A in rats.

The RACK1 signaling scaffold protein selectively interacts with the cAMP-specific phosphodiesterase PDE4D5 isoform

The WD-repeat protein receptor for activated C-kinase (RACK1) was identified by its interaction with the cyclic AMP-specific phosphodiesterase (PDE4) isoform PDE4D5 in a yeast two-hybrid screen. The interaction was confirmed by co-immunoprecipitation of native RACK1 and PDE4D5 from COS7, HEK293, 3T3-F442A, and SK-N-SH cell lines. The interaction was unaffected by stimulation of the cells with the phorbol ester phorbol 2-myristate 3-acetate. PDE4D5 did not interact with two other WD-repeat proteins, beta'-coatomer protein and Gsbeta, in two-hybrid tests. RACK1 did not interact with other PDE4D isoforms or with known PDE4A, PDE4B, and PDE4C isoforms. PDE4D5 and RACK1 interacted with high affinity (Ka approximately 7 nM) [corrected] when they were expressed and purified from Escherichia coli, demonstrating that the interaction does not require intermediate proteins. The binding of the E. coli-expressed proteins did not alter the kinetics of cAMP hydrolysis by PDE4D5 but caused a 3-4-fold change in its sensitivity to inhibition by the PDE4 selective inhibitor rolipram. The subcellular distributions of RACK1 and PDE4D5 were extremely similar, with the major amount of both proteins (70%) in the high speed supernatant (S2) fraction. Analysis of constructs with specific deletions or single amino acid mutations in PDE4D5 demonstrated that a small cluster of amino acids in the unique amino-terminal region of PDE4D5 was necessary for its interaction with RACK1. We suggest that RACK1 may act as a scaffold protein to recruit PDE4D5 and other proteins into a signaling complex.

Cyclic nucleotide phosphodiesterase (PDE) inhibitors and immunomodulation

Intracellular levels of cyclic nucleotide second messengers are regulated predominantly by the complex superfamily of cyclic nucleotide phosphodiesterase (PDE) enzymes. Recent advances in our understanding of the molecular pharmacology of these enzymes has led to their identification as biologic regulators of certain disease states and the development of isozyme-selective inhibitors as potential therapeutic agents. A large body of in vitro and preclinical data suggests the therapeutic utility of PDE4 inhibitors as potent anti-inflammatory agents. Early clinical trials with selective PDE inhibitors substantiate this approach while highlighting pharmacodynamic and toxicologic pitfalls inherent to the inhibition of specific PDE isozymes. This commentary will review our current understanding of PDE inhibitors as immunomodulatory agents.