Scanning peptide array analyses identify overlapping binding sites for the signalling scaffold proteins, beta-arrestin and RACK1, in cAMP-specific phosphodiesterase PDE4D5

Ndel1 alters its conformation by sequestering cAMP-specific phosphodiesterase-4D3 (PDE4D3) in a manner that is dynamically regulated through Protein Kinase A (PKA)

The involvement of the Nuclear distribution element-like (Ndel1; Nudel) protein in the recruitment of the dynein complex is critical for neurodevelopment and potentially important for neuronal disease states. The PDE4 family of phosphodiesterases specifically degrades cAMP, an important second messenger implicated in learning and memory functions. Here we show for the first time that Ndel1 can interact directly with PDE4 family members and that the interaction of Ndel1 with the PDE4D3 isoform is uniquely disrupted by elevation of intracellular cAMP levels. While all long PDE4 isoforms are subject to stimulatory PKA phosphorylation within their conserved regulatory UCR1 domain, specificity for release of PDE4D3 is conferred due to the PKA-dependent phosphorylation of Ser13 within the isoform-specific, unique amino-terminal domain of PDE4D3. Scanning peptide array analyses identify a common region on Ndel1 for PDE4 binding and an additional region that is unique to PDE4D3. The common site lies within the stutter region that links the second coiled-coil region to the unstable third coiled-coil regions of Ndel1. The additional binding region unique to PDE4D3 penetrates into the start of the third coiled-coil region that can undergo tail-to-tail interactions between Ndel1 dimers to form a 4 helix bundle. We demonstrate Ndel1 self-interaction in living cells using a BRET approach with luciferase- and GFP-tagged forms of Ndel1. BRET assessed Ndel1-Ndel1 self-interaction is amplified through the binding of PDE4 isoforms. For PDE4D3 this effect is ablated upon elevation of intracellular cAMP due to PKA-mediated phosphorylation at Ser13, while the potentiating effects of PDE4B1 and PDE4D5 are resistant to cAMP elevation. PDE4D long isoforms and Ndel1 show a similar sub-cellular distribution in hippocampus and cortex and locate to post-synaptic densities. We show that Ndel1 sequesters EPAC, but not PKA, in order to form a cAMP signalling complex. We propose that a key function of the Ndel1 signalling scaffold is to signal through cAMP by sequestering EPAC, whose activity may thus be specifically regulated by sequestered PDE4 that also stabilizes Ndel1-Ndel1 self-interaction. In the case of PDE4D3, its association with Ndel1 is dynamically regulated by PKA input through its ability to phosphorylate Ser13 in the unique N-terminal region of this isoform, triggering the specific release of PDE4D3 from Ndel1 when cAMP levels are elevated. We propose that Ser13 may act as a redistribution trigger in PDE4D3, allowing it to dynamically re-shape cAMP gradients in distinct intracellular locales upon its phosphorylation by PKA.

The RACK1 signal anchor protein from Trypanosoma brucei associates with eukaryotic elongation factor 1A: a role for translational control in cytokinesis

RACK1 is a WD-repeat protein that forms signal complexes at appropriate locations in the cell. RACK1 homologues are core components of ribosomes from yeast, plants and mammals. In contrast, a cryo-EM analysis of trypanosome ribosomes failed to detect RACK1, thus eliminating an important translational regulatory mechanism. Here we report that TbRACK1 from Trypanosoma brucei associates with eukaryotic translation elongation factor-1a (eEF1A) as determined by tandem MS of TAP-TbRACK1 affinity eluates, co-sedimentation in a sucrose gradient, and co-precipitation assays. Consistent with these observations, sucrose gradient purified 80S monosomes and translating polysomes each contained TbRACK1. When RNAi was used to deplete cells of TbRACK1, a shift in the polysome profile was observed, while the phosphorylation of a ribosomal protein increased. Under these conditions, cell growth became hypersensitive to the translational inhibitor anisomycin. The kinetoplasts and nuclei were misaligned in the postmitotic cells, resulting in partial cleavage furrow ingression during cytokinesis. Overall, these findings identify eEF1A as a novel TbRACK1 binding partner and establish TbRACK1 as a component of the trypanosome translational apparatus. The synergy between anisomycin and TbRACK1 RNAi suggests that continued translation is required for complete ingression of the cleavage furrow.

Identification and molecular characterization of a novel PDE4D11 cAMP-specific phosphodiesterase isoform

Here we report the cloning and characterization of a novel PDE4D isoform (PDE4D11) identified in mouse brain cDNA. This novel isoform has a unique isoform-specific 5'-UTR and N-terminal sequence, whereas, downstream regulatory N-terminal and catalytic C-terminal regions are homologous to other long PDE4D isoforms (Ex2-15). In silico analysis of PDE4D11 cDNA transcript identified the predicted translational start site and the use of a different transcriptional start site compared to other PDE4D isoforms. This isoform is ubiquitously expressed in different mouse tissues, particularly in the brain, liver and spleen. In the brain, PDE4D11 expression levels increased in the cerebellum, but decreased in the hippocampus with progressive age, highlighting a potential role for this isoform in the development of the brain. When transfected in vitro into murine neuroblastoma cells PDE4D11_EGFP expression is cytosolic, consistent with other long PDE4D isoforms. The appearance of cytosolic protein aggregates in discrete microdomains with this isoform, however, may represent a method of compartmentalizing PDE4D11 activity. The novel 5'-sequence of PDE4D11 is conserved among higher vertebrates including human, monkey, dog, horse and rat. Identification of this new isoform highlights the mutliplicity of unique PDE4D isoforms and their potential importance in regulating cAMP levels through compartmentalization and cell-specific expression and underscores the importance of understanding the functional role of each isoform in the development of specific drugs for the treatment of memory disorders.

EPAC and PKA allow cAMP dual control over DNA-PK nuclear translocation

We identify a compartmentalized signaling system that identifies a functional role for the GTP exchange factor, exchange protein activated by cAMP (EPAC) coupled to Rap2 in the nucleus. In this system, cAMP regulates the nuclear/cytoplasmic trafficking of DNA-dependent protein kinase (DNA-PK), a critical kinase that acts to repair double-stranded breaks (DSBs) in damaged DNA and to phosphorylate the cell survival kinase, PKB/Akt. Intersecting regulatory inputs for cAMP employ EPAC to transduce positive effects, namely the Rap2-dependent nuclear exit and activation of DNA-PK, whereas protein kinase A (PKA) provides the negative input by antagonizing these actions. We identify this as a compartmentalized regulatory system where modulation of cAMP input into the stimulatory, EPAC and inhibitory, PKA intersecting arms is provided by spatially discrete, cAMP degradation systems. The distribution of DNA-PK between nuclear and cytoplasmic compartments can thus potentially be influenced by relative inputs of cAMP signaling through the EPAC and PKA pathways. Through this signaling system EPAC activation can thereby impact on the Ser-473 phosphorylation status of PKB/Akt and the repair of etoposide-induced DSBs.

PDE4 associates with different scaffolding proteins: modulating interactions as treatment for certain diseases

cAMP is an ubiquitous second messenger that is crucial to many cellular processes. The sole means of terminating the cAMP signal is degradation by cAMP phosphodiesterases (PDEs). The PDE4 family is of particular interest because PDE4 inhibitors have therapeutic potential for the treatment of various inflammatory and auto-immune diseases and also have anti-depressant and memory-enhancing effects. The subcellular targeting of PDE4 isoforms is fundamental to the compartmentalization of cAMP signaling pathways and is largely achieved via proteinprotein interactions. Increased knowledge of these protein-protein interactions and their regulatory properties could aid in the design of novel isoform-specific inhibitors with improved efficacy and fewer prohibitive side effects.

Mutations of beta-arrestin 2 that limit self-association also interfere with interactions with the beta2-adrenoceptor and the ERK1/2 MAPKs: implications for beta2-adrenoceptor signalling via the ERK1/2 MAPKs

FRET (fluorescence resonance energy transfer) and co-immunoprecipitation studies confirmed the capacity of beta-arrestin 2 to self-associate. Amino acids potentially involved in direct protein-protein interaction were identified via combinations of spot-immobilized peptide arrays and mapping of surface exposure. Among potential key amino acids, Lys(285), Arg(286) and Lys(295) are part of a continuous surface epitope located in the polar core between the N- and C-terminal domains. Introduction of K285A/R286A mutations into beta-arrestin 2-eCFP (where eCFP is enhanced cyan fluorescent protein) and beta-arrestin 2-eYFP (where eYFP is enhanced yellow fluorescent protein) constructs substantially reduced FRET, whereas introduction of a K295A mutation had a more limited effect. Neither of these mutants was able to promote beta2-adrenoceptor-mediated phosphorylation of the ERK1/2 (extracellular-signal-regulated kinase 1/2) MAPKs (mitogen-activated protein kinases). Both beta-arrestin 2 mutants displayed limited capacity to co-immunoprecipitate ERK1/2 and further spot-immobilized peptide arrays indicated each of Lys(285), Arg(286) and particularly Lys(295) to be important for this interaction. Direct interactions between beta-arrestin 2 and the beta2-adrenoceptor were also compromised by both K285A/R286A and K295A mutations of beta-arrestin 2. These were not non-specific effects linked to improper folding of beta-arrestin 2 as limited proteolysis was unable to distinguish the K285A/R286A or K295A mutants from wild-type beta-arrestin 2, and the interaction of beta-arrestin 2 with JNK3 (c-Jun N-terminal kinase 3) was unaffected by the K285A/R286A or L295A mutations. These results suggest that amino acids important for self-association of beta-arrestin 2 also play an important role in the interaction with both the beta2-adrenoceptor and the ERK1/2 MAPKs. Regulation of beta-arrestin 2 self-association may therefore control beta-arrestin 2-mediated beta2-adrenoceptor-ERK1/2 MAPK signalling.

Tyrosine 302 in RACK1 is essential for insulin-like growth factor-I-mediated competitive binding of PP2A and beta1 integrin and for tumor cell proliferation and migration

Insulin-like growth factor (IGF)-I regulates a mutually exclusive interaction of PP2A and beta1 integrin with the WD repeat scaffolding protein RACK1. This interaction is required for the integration of IGF-I receptor (IGF-IR) and adhesion signaling. Here we investigated the nature of the binding site for PP2A and beta1 integrin in RACK1. A WD7 deletion mutant of RACK1 did not associate with PP2A but retained some interaction with beta1 integrin, whereas a WD6/WD7 mutant lost the ability to bind to both PP2A and beta1 integrin. Using immobilized peptide arrays representing the entire RACK1 protein, we identified a common cluster of amino acids (FAGY) at positions 299-302 within WD7 of RACK1 which were essential for binding of both PP2A and beta1 integrin to RACK1. PP2A showed a higher level of association with a peptide in which Tyr-302 was phosphorylated compared with an unphosphorylated peptide, whereas beta1 integrin binding was not affected by phosphorylation. RACK1 mutants in which either the FAGY cluster or Tyr-302 were mutated to AAAF, or Phe, respectively, did not interact with either PP2A or beta1 integrin. These mutants were unable to rescue the decrease in PP2A activity caused by suppression of RACK1 in MCF-7 cells with small interfering RNA. MCF-7 cells and R+ (IGF-IR-overexpressing fibroblasts) expressing these mutants exhibited decreased proliferation and migration, whereas R- cells (IGF-IR null fibroblasts) were unaffected. Taken together, the data demonstrate that Tyr-302 in RACK1 is required for interaction with PP2A and beta1 integrin, for regulation of PP2A activity, and for IGF-I-mediated cell migration and proliferation.

Endosomal colocalization of melanocortin-3 receptor and beta-arrestins in CAD cells with altered modification of AKT/PKB

The melanocortin 3-receptor is involved in regulating energy metabolism, body fluid composition and inflammatory responses. Melanocortin receptors function by activating membrane bound adenylate cyclase. However, the literature reports indicate that some G protein coupled receptors (GPCRs) can also activate mitogen activated protein kinase (MAPK) or phosphoinositide 3 kinase (PI3K) signaling pathways consequent to their endocytosis. These studies were undertaken to evaluate the role of these pathways in MC3R signaling in brain-stem neuronal cells. Recruitment of arrestins is implicated in the activation of secondary pathways by GPCRs and our data shows the colocalization of either arrestin B1 or B2 with MC3R in endosomes. An alteration in PKB phosphorylation pattern was observed in MC3R expressing cells independent of agonist stimulation. MC3R transfectants exhibited increased proliferation rates and inhibition of PKB pathway with triciribine abrogated cell proliferation in both vector control and MC3R transfectants. PKB is constitutively active in proliferating CAD cells but could be further activated by culturing the cells in differentiation medium. These studies suggest that the AKT/PKB pathway plays an important role in the proliferation of CAD cells and suggest a link between MC3R and cell growth pathways that may involve the alteration of AKT/PKB signaling pathway.

Relations between the mitogen-activated protein kinase and the cAMP-dependent protein kinase pathways: comradeship and hostility

Inter- and intracellular communications and responses to environmental changes are pivotal for the orchestrated and harmonious operation of multi-cellular organisms. These well-tuned functions in living organisms are mediated by the action of signal transduction pathways, which are responsible for receiving a signal, transmitting and amplifying it, and eliciting the appropriate cellular responses. Mammalian cells posses numerous signal transduction pathways that, rather than acting in solitude, interconnect with each other, a phenomenon referred to as cross-talk. This allows cells to regulate the distribution, duration, intensity and specificity of the response. The cAMP/cAMP-dependent protein kinase (PKA) pathway and the mitogen-activated protein kinase (MAPK) cascades modulate common processes in the cell and multiple levels of cross-talk between these signalling pathways have been described. The first- and best-characterized interconnections are the PKA-dependent inhibition of the MAPKs ERK1/2 mediated by RAF-1, and PKA-induced activation of ERK1/2 interceded through B-RAF. Recently, novel interactions between components of these pathways and new mechanisms for cross-talk have been elucidated. This review discusses both known and novel interactions between compounds of the cAMP/PKA and MAPKs signalling pathways in mammalian cells.

cAMP-specific phosphodiesterase-4D5 (PDE4D5) provides a paradigm for understanding the unique non-redundant roles that PDE4 isoforms play in shaping compartmentalized cAMP cell signalling

The PDE4 (phosphodiesterase-4) enzyme family consists of a distinct array of N-terminal splice variant isoforms arising from four subfamily genes (4A, 4B, 4C and 4D). These all hydrolyse specifically the intracellular second messenger cAMP. Although identical in catalytic function, each isoform appears to serve a non-superfluous regulatory role. For example, a beta-arrestin-sequestered subpopulation of the PDE4D5 isoform specifically regulates the phosphorylation of the beta(2)-AR (beta(2)-adrenergic receptor) by PKA (protein kinase A; also called cAMP-dependent protein kinase). This was elucidated by the use of novel technologies, including dominant-negative approaches, siRNA (small interfering RNA) knockdown and spot-immobilized peptide array analyses. Functional phenotypes uncovered using these methodologies have shown that beta-arrestin-sequestered PDE4D5 shapes the spatial cAMP gradient around the membrane-bound beta(2)-AR, regulating its phosphorylation by PKA and its ability to activate ERK (extracellular-signal-regulated kinase) through G(i) in cardiomyocytes and HEK-293 (human embryonic kidney)-B2 cells. This approach has provided the very first identification of a non-redundant and specific role for a PDE isoform. The fact that phenotypes can be uncovered by displacing PDE4 isoforms from specific anchor sites using dominant-negative constructs and cell-permeable peptides points to novel means for developing therapeutics aimed at disrupting specifically sequestered PDE isoforms and even specifically sequestered subpopulations of individual isoforms.

Direct AKAP-mediated protein-protein interactions as potential drug targets

A-kinase-anchoring proteins (AKAPs) are a diverse family of about 50 scaffolding proteins. They are defined by the presence of a structurally conserved protein kinase A (PKA)-binding domain. AKAPs tether PKA and other signalling proteins such as further protein kinases, protein phosphatases and phosphodiesterases by direct protein-protein interactions to cellular compartments. Thus, AKAPs form the basis of signalling modules that integrate cellular signalling processes and limit these to defined sites. Disruption of AKAP functions by gene targeting, knockdown approaches and, in particular, pharmacological disruption of defined AKAP-dependent protein-protein interactions has revealed key roles of AKAPs in numerous processes, including the regulation of cardiac myocyte contractility and vasopressin-mediated water reabsorption in the kidney. Dysregulation of such processes causes diseases, including cardiovascular and renal disorders. In this review, we discuss AKAP functions elucidated by gene targeting and knockdown approaches, but mainly focus on studies utilizing peptides for disruption of direct AKAP-mediated protein-protein interactions. The latter studies point to direct AKAP-mediated protein-protein interactions as targets for novel drugs.

Structures of the four subfamilies of phosphodiesterase-4 provide insight into the selectivity of their inhibitors

PDE4 (phosphodiesterase-4)-selective inhibitors have attracted much attention as potential therapeutics for the treatment of both depression and major inflammatory diseases, but their practical application has been compromised by side effects. A possible cause for the side effects is that current PDE4-selective inhibitors similarly inhibit isoforms from all four PDE4 subfamilies. The development of PDE4 subfamily-selective inhibitors has been hampered by a lack of structural information. In the present study, we rectify this by providing the crystal structures of the catalytic domains of PDE4A, PDE4B and PDE4D in complex with the PDE4 inhibitor NVP {4-[8-(3-nitrophenyl)-[1,7]naphthyridin-6-yl]benzoic acid} as well as the unliganded PDE4C structure. NVP binds in the same conformation to the deep cAMP substrate pocket and interacts with the same residues in each instance. However, detailed structural comparison reveals significant conformational differences. Although the active sites of PDE4B and PDE4D are mostly comparable, PDE4A shows significant displacements of the residues next to the invariant glutamine residue that is critical for substrate and inhibitor binding. PDE4C appears to be more distal from other PDE4 subfamilies, with certain key residues being disordered. Our analyses provide the first structural basis for the development of PDE4 subfamily-selective inhibitors.

1H NMR structural and functional characterisation of a cAMP-specific phosphodiesterase-4D5 (PDE4D5) N-terminal region peptide that disrupts PDE4D5 interaction with the signalling scaffold proteins, beta-arrestin and RACK1

The unique 88 amino acid N-terminal region of cAMP-specific phosphodiesterase-4D5 (PDE4D5) contains overlapping binding sites conferring interaction with the signaling scaffold proteins, betaarrestin and RACK1. A 38-mer peptide, whose sequence reflected residues 12 through 49 of PDE4D5, encompasses the entire N-terminal RACK1 Interaction Domain (RAID1) together with a portion of the beta-arrestin binding site. (1)H NMR and CD analyses indicate that this region has propensity to form a helical structure. The leucine-rich hydrophobic grouping essential for RACK1 interaction forms a discrete hydrophobic ridge located along a single face of an amphipathic alpha-helix with Arg34 and Asn36, which also play important roles in RACK1 binding. The Asn22/Pro23/Trp24/Asn26 grouping, essential for RACK1 interaction, was located at the N-terminal head of the amphipathic helix that contained the hydrophobic ridge. RAID1 is thus provided by a distinct amphipathic helical structure. We suggest that the binding of PDE4D5 to the WD-repeat protein, RACK1, may occur in a manner akin to the helix-helix interaction shown for G(gamma) binding to the WD-repeat protein, G(beta). A more extensive section of the PDE4D5 N-terminal sequence (Thr11-Ala85) is involved in beta-arrestin binding. Several residues within the RAID1 helix contribute to this interaction however. We show here that these residues form a focused band around the centre of the RAID1 helix, generating a hydrophobic patch (from Leu29, Val30 and Leu33) flanked by polar/charged residues (Asn26, Glu27, Asp28, Arg34). The interaction with beta-arrestin exploits a greater circumference on the RAID1 helix, and involves two residues (Glu27, Asp28) that do not contribute to RACK1 binding. In contrast, the interaction of RACK1 with RAID1 is extended over a greater length of the helix and includes Leu37/Leu38, which do not contribute to beta-arrestin binding. A membrane-permeable, stearoylated Val12-Ser49 38-mer peptide disrupted the interaction of both beta-arrestin and RACK1 with endogenous PDE4D5 in HEKB2 cells, whilst a cognate peptide with a Glu27Ala substitution selectively failed to disrupt PDE4D5/RACK1 interaction. The stearoylated Val12-Ser49 38-mer peptide enhanced the isoprenaline-stimulated PKA phosphorylation of the beta(2)-adrenergic receptors (beta(2)AR) and its activation of ERK, whilst the Glu27Ala peptide was ineffective in both these regards.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

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

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

Dynamic regulation, desensitization, and cross-talk in discrete subcellular microdomains during beta2-adrenoceptor and prostanoid receptor cAMP signaling

Dynamic and localized actions of cAMP are central to the generation of discrete cellular events in response to a range of G(s)-coupled receptor agonists. In the present study we have employed a cyclic nucleotide-gated channel sensor to report acute changes in cAMP in the restricted cellular microdomains adjacent to two different G(s)-coupled receptor pathways, beta(2)-adrenoceptors and prostanoid receptors that are expressed endogenously in HEK293 cells. We probed by either selective small interference RNA-mediated knockdown or dominant negative overexpression the contribution of key signaling components in the rapid attenuation of the local cAMP signaling and subsequent desensitization of each of these G-protein-coupled receptor signaling pathways immediately following receptor activation. Direct measurements of cAMP changes just beneath the plasma membrane of single HEK293 cells reveal novel insights into key regulatory roles provided by protein kinase A-RII, beta-arrestin2, cAMP phosphodiesterase-4D3, and cAMP phosphodiesterase-4D5. We provide new evidence for distinct modes of cAMP down-regulation in these two G(s)-linked pathways and show that these distinct G-protein-coupled receptor signaling systems are subject to unidirectional, heterologous desensitization that allows for limited cross-talk between distinct, dynamically regulated pools of cAMP.

p75 neurotrophin receptor regulates tissue fibrosis through inhibition of plasminogen activation via a PDE4/cAMP/PKA pathway

Clearance of fibrin through proteolytic degradation is a critical step of matrix remodeling that contributes to tissue repair in a variety of pathological conditions, such as stroke, atherosclerosis, and pulmonary disease. However, the molecular mechanisms that regulate fibrin deposition are not known. Here, we report that the p75 neurotrophin receptor (p75^NTR), a TNF receptor superfamily member up-regulated after tissue injury, blocks fibrinolysis by down-regulating the serine protease, tissue plasminogen activator (tPA), and up-regulating plasminogen activator inhibitor-1 (PAI-1). We have discovered a new mechanism in which phosphodiesterase PDE4A4/5 interacts with p75^NTR to enhance cAMP degradation. The p75^NTR-dependent down-regulation of cAMP results in a decrease in extracellular proteolytic activity. This mechanism is supported in vivo in p75^NTR-deficient mice, which show increased proteolysis after sciatic nerve injury and lung fibrosis. Our results reveal a novel pathogenic mechanism by which p75^NTR regulates degradation of cAMP and perpetuates scar formation after injury.

Isoform-selective susceptibility of DISC1/phosphodiesterase-4 complexes to dissociation by elevated intracellular cAMP levels

Disrupted-in-schizophrenia 1 (DISC1) is a genetic susceptibility factor for schizophrenia and related severe psychiatric conditions. DISC1 is a multifunctional scaffold protein that is able to interact with several proteins, including the independently identified schizophrenia risk factor phosphodiesterase-4B (PDE4B). Here we report that the 100 kDa full-length DISC1 isoform (fl-DISC1) can bind members of each of the four gene, cAMP-specific PDE4 family. Elevation of intracellular cAMP levels, so as to activate protein kinase A, caused the release of PDE4D3 and PDE4C2 isoforms from fl-DISC1 while not affecting binding of PDE4B1 and PDE4A5 isoforms. Using a peptide array strategy, we show that PDE4D3 binds fl-DISC1 through two regions found in common with PDE4B isoforms, the interaction of which is supplemented because of the presence of additional PDE4B-specific binding sites. We propose that the additional binding sites found in PDE4B1 underpin its resistance to release during cAMP elevation. We identify, for the first time, a functional distinction between the 100 kDa long DISC1 isoform and the short 71 kDa isoform. Thus, changes in the expression pattern of DISC1 and PDE4 isoforms offers a means to reprogram their interaction and to determine whether the PDE4 sequestered by DISC1 is released after cAMP elevation. The PDE4B-specific binding sites encompass point mutations in mouse Disc1 that confer phenotypes related to schizophrenia and depression and that affect binding to PDE4B. Thus, genetic variation in DISC1 and PDE4 that influence either isoform expression or docking site functioning may directly affect psychopathology.

Effects of RACK1 on cell migration and IGF-I signalling in cardiomyoctes are not dependent on an association with the IGF-IR

RACK1 can act as a scaffolding protein to integrate IGF-IR and integrin signalling in transformed cells but its actions in regulating IGF-IR signalling in non-transformed cells are less well understood. Here, we investigated the function of RACK1 in the non-transformed cardiomyocyte cell line H9c2. Overexpression of RACK1 in H9c2 cells was sufficient to increase cell size, increase adhesion to collagen 1, enhance protection from hydrogen peroxide-induced cell death, and increase cell migration. However, cell proliferation was decreased in these cells. Small interfering RNA (siRNA)-mediated suppression of RACK1 in H9c2 cells resulted in decreased cell adhesion and migration, but had no effect on cell proliferation or size. Increased basal and IGF-I-mediated Erk phosphorylation was observed in RACK1-overexpressing H9c2 cells. Interestingly, contrary to observations in transformed cells, RACK1 was not observed to interact with the IGF-IR in H9c2 cells. Also in contrast to observations in transformed cells, IGF-I promoted recruitment of Src to RACK1 as well as recruitment of PKCalpha, and PKCepsilon to RACK1. Overall, the data indicate that in H9c2 cells RACK1 can influence cell size, cell survival, adhesion, migration, but its responses to IGF-I are independent of an association with the IGF-IR. Thus, the composition of the RACK1 scaffolding complex and its effects on IGF-I signalling may be different in transformed and non-transformed cells.

Numerous distinct PKA-, or EPAC-based, signalling complexes allow selective phosphodiesterase 3 and phosphodiesterase 4 coordination of cell adhesion

By activating two distinct classes of effector enzymes, namely Protein Kinases A [PKA] or Exchange Proteins Activated by cAMP [EPAC], the ubiquitous second messenger cAMP selectively coordinates numerous events simultaneously in virtually all cells. Studies focused on dissecting the manner by which cAMP simultaneously regulates multiple cellular events have shown that cAMP activates its effectors non-uniformly in cells and that this localized cAMP-mediated signalling is made possible, at least in part, by anchoring of cAMP effectors to selected subcellular structures. In the work described here, we report that HEK293T cells ["293T"] contain several PKA- and EPAC1-based signalling complexes. Interestingly, our data do not identify signalling complexes in which both PKA and EPAC are each present but rather are consistent with the idea that these two effectors operate in distinct complexes in these cells. Similarly, we report that while individual PKA- or EPAC-containing complexes can contain either phosphodiesterase 3B, [PDE3B] or phosphodiesterase 4D [PDE4D], they do not contain both these phosphodiesterases. Indeed, although PDE4D enzymes were identified in both PKA- and EPAC-based complexes, PDE3B was largely identified in EPAC-based complexes. Using a combination of approaches, we identified that integration of PDE3B into EPAC-based complexes occurred through its amino terminal fragment [PDE3B(AT)]. Consistent with the idea that integration of PDE3B within EPAC-based complexes was dynamic and regulated PDE3 inhibitor-mediated effects on cellular functions, expression of PDE3B(AT) competed with endogenous PDE3B for integration into EPAC-based complexes and antagonized PDE3 inhibitor-based cell adhesion. Our data support the concept that cells can contain several non-overlapping PKA- and EPAC-based signalling complexes and that these complexes may also represent sites within cells were the effects of family-selective PDE inhibitors could be integrated to affect cell functions, including adhesion.

A major functional role for phosphodiesterase 4D5 in human airway smooth muscle cells

Relaxation of airway smooth muscle is dependent predominantly upon elevation of cell cAMP content. Although the processes involved in elevation of cell cAMP content are reasonably well established, the mechanisms governing subsequent control of cAMP turnover are less clear. Breakdown of cAMP is solely regulated by phosphodiesterase (PDE) isoenzymes. We have previously reported that PDE4 family members are likely to be important in this process, and that expression of PDE4D variants is actively regulated at the transcriptional level. Here, we demonstrate a key role for PDE4D5 in the control of beta(2)-adrenoceptor (beta(2)AR)-stimulated cAMP activity in human airway smooth muscle cells using splice variant-specific small interfering RNA knockdown. Furthermore, we show, using an Epac (exchange protein directly activated by cAMP)-based, cAMP-sensitive fluorescent probe, that these intracellular cAMP gradients are controlled both temporally and dynamically by PDE4D5. Elevation of cAMP within the cytoplasm after beta(2)AR stimulation is rapid and shows no distinct spatial compartmentalization in these cells. These data suggest that PDE4D5, despite being a minor component of the tissue PDE pool, is the key physiological regulator of beta(2)AR-induced cAMP turnover within human airway smooth muscle.

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.

Mapping binding sites for the PDE4D5 cAMP-specific phosphodiesterase to the N- and C-domains of beta-arrestin using spot-immobilized peptide arrays

Beta2-ARs (beta2-adrenoceptors) become desensitized rapidly upon recruitment of cytosolic beta-arrestin. PDE4D5 (family 4 cAMP-specific phosphodiesterase, subfamily D, isoform 5) can be recruited in complex with beta-arrestin, whereupon it regulates PKA (cAMP-dependent protein kinase) phosphorylation of the beta2-AR. In the present study, we have used novel technology, employing a library of overlapping peptides (25-mers) immobilized on cellulose membranes that scan the entire sequence of beta-arrestin 2, to define the interaction sites on beta-arrestin 2 for binding of PDE4D5 and the cognate long isoform, PDE4D3. We have identified a binding site in the beta-arrestin 2 N-domain for the common PDE4D catalytic unit and two regions in the beta-arrestin 2 C-domain that confer specificity for PDE4D5 binding. Alanine-scanning peptide array analysis of the N-domain binding region identified severely reduced interaction with PDE4D5 upon R26A substitution, and reduced interaction upon either K18A or T20A substitution. Similar analysis of the beta-arrestin 2 C-domain identified Arg286 and Asp291, together with the Leu215-His220 region, as being important for binding PDE4D5, but not PDE4D3. Transfection with wild-type beta-arrestin 2 profoundly decreased isoprenaline-stimulated PKA phosphorylation of the beta2-AR in MEFs (mouse embryo fibroblasts) lacking both beta-arrestin 1 and beta-arrestin 2. This effect was negated using either the R26A or the R286A mutant form of beta-arrestin 2 or a mutant with substitution of an alanine cassette for Leu215-His220, which showed little or no PDE4D5 binding, but was still recruited to the beta2-AR upon isoprenaline challenge. These data show that the interaction of PDE4D5 with both the N- and C-domains of beta-arrestin 2 are essential for beta2-AR regulation.

cAMP-Specific Phosphodiesterase-4 Enzymes in the Cardiovascular System

Cyclic AMP regulates a vast number of distinct events in all cells. Early studies established that its hydrolysis by cyclic nucleotide phosphodiesterases (PDEs) controlled both the magnitude and the duration of its influence. Recent evidence shows that PDEs also act as coincident detectors linking cyclic-nucleotide- and non-cyclic-nucleotide-based cellular signaling processes and are tethered with great selectively to defined intracellular structures, thereby integrating and spatially restricting their cellular effects in time and space. Although 11 distinct families of PDEs have been defined, and cells invariably express numerous individual PDE enzymes, a large measure of our increased appreciation of the roles of these enzymes in regulating cyclic nucleotide signaling has come from studies on the PDE4 family. Four PDE4 genes encode more than 20 isoforms. Alternative mRNA splicing and the use of different promoters allows cells the possibility of expressing numerous PDE4 enzymes, each with unique amino-terminal-targeting and/or regulatory sequences. Dominant negative and small interfering RNA-mediated knockdown strategies have proven that particular isoforms can uniquely control specific cellular functions. Thus the protein kinase A phosphorylation status of the beta(2) adrenoceptor and, thereby, its ability to switch its signaling to extracellular signal-regulated kinase activation, is uniquely regulated by PDE4D5 in cardiomyocytes. We describe how cardiomyocytes and vascular smooth muscle cells selectively vary both the expression and the catalytic activities of PDE4 isoforms to regulate their various functions and how altered regulation of these processes can influence the development, or resolution, of cardiovascular pathologies, such as heart failure, as well as various vasculopathies.

Alterations in proteoglycan synthesis selectively impair FSH-induced particulate cAMP-phosphodiesterase 4 (PDE4) activation in immature rat Sertoli cells

FSH-induced upregulation of cAMP-PDE4 activities was decreased in cultured Sertoli cells when alteration of cell proteoglycans (PGs) metabolism was simultaneously induced either by para-nitrophenyl beta-d-xyloside (PNPX) or by sodium chlorate. This effect was restricted to the particulate PDE4 activities and its timing was consistent with the half-life of Sertoli cell PGs. It did not result from alterations in Pde4d variants expression, the major FSH-regulated PDE4 in Sertoli cells. Moreover, lack of changes in the particulate levels of major immunoreactive 75 kDa and 90 kDa PDE4D proteins, corresponding likely to short PDE4D1 and long PDE4D3/D8/D9 isoforms respectively, suggested that the decrease in FSH-stimulated of PDE4 activities in chlorate- and PNPX-treated cells at the end of the 24-h incubation period resulted from the increased reversal of the activated particulate PDE4(D) activities back to unstimulated levels. By controlling FSH-stimulated particulate PDE4 inactivation through a still unknown mechanism (sustained activation of PKA or reduction of phosphoprotein phosphatase activities), cell PGs could be involved in the alteration of cAMP response to FSH accompanying the transition of Sertoli cells from proliferative to non-proliferative differentiated state.

Overview of PDEs and their regulation

Contraction and relaxation of vascular smooth muscle and cardiac myocytes are key physiological events in the cardiovascular system. These events are regulated by second messengers, cAMP and cGMP, in response to extracellular stimulants. The strength of signal transduction is controlled by intracellular cyclic nucleotide concentrations, which are determined by a balance in production and degradation of cAMP and cGMP. Degradation of cyclic nucleotides is catalyzed by 3',5'-cyclic nucleotide phosphodiesterases (PDEs), and therefore regulation of PDEs hydrolytic activity is important for modulation of cellular functions. Mammalian PDEs are composed of 21 genes and are categorized into 11 families based on sequence homology, enzymatic properties, and sensitivity to inhibitors. PDE families contain many splice variants that mostly are unique in tissue-expression patterns, gene regulation, enzymatic regulation by phosphorylation and regulatory proteins, subcellular localization, and interaction with association proteins. Each unique variant is closely related to the regulation of a specific cellular signaling. Thus, multiple PDEs function as a particular modulator of each cardiovascular function and regulate physiological homeostasis.

Compartmentalization of cAMP-dependent signaling by phosphodiesterase-4D is involved in the regulation of vasopressin-mediated water reabsorption in renal principal cells

The cAMP/protein kinase A (PKA)-dependent insertion of water channel aquaporin-2 (AQP2)-bearing vesicles into the plasma membrane in renal collecting duct principal cells (AQP2 shuttle) constitutes the molecular basis of arginine vasopressin (AVP)-regulated water reabsorption. cAMP/PKA signaling systems are compartmentalized by A kinase anchoring proteins (AKAP) that tether PKA to subcellular sites and by phosphodiesterases (PDE) that terminate PKA signaling through hydrolysis of localized cAMP. In primary cultured principal cells, AVP causes focal activation of PKA. PKA and cAMP-specific phosphodiesterase-4D (PDE4D) are located on AQP2-bearing vesicles. The selective PDE4 inhibitor rolipram increases AKAP-tethered PKA activity on AQP2-bearing vesicles and enhances the AQP2 shuttle and thereby the osmotic water permeability. AKAP18delta, which is located on AQP2-bearing vesicles, directly interacts with PDE4D and PKA. In response to AVP, PDE4D and AQP2 translocate to the plasma membrane. Here PDE4D is activated through PKA phosphorylation and reduces the osmotic water permeability. Taken together, a novel, compartmentalized, and physiologically relevant cAMP-dependent signal transduction module on AQP2-bearing vesicles, comprising anchored PDE4D, AKAP18delta, and PKA, has been identified.