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

Enantioselective pharmacodynamics of propranolol in HUVEC cells: a study using chiral 2D gel electrophoresis and mass spectrometry

Propranolol (PRO), a nonselective β-adrenergic receptor (β-AR) antagonist, has two enantiomers, R(+)-PRO and S(-)-PRO, which have diverse biological effects. For example, S(-)-PRO blocks the β-receptor ~100 times more strongly than R(+)-PRO. However, the signaling pathway that causes this difference remains unclear. This pathway may affect the expression of numerous proteins, some of which play key roles during the drug action process. Therefore, we treated human umbilical vein endothelial cells (HUVECs) with R(+)-PRO and S(-)-PRO in order to identify differentially expressed proteins and to determine their functions in the drug action process. Of the 22 differentially expressed protein spots investigated, 14 demonstrated higher expression levels in the R(+)-PRO-treated cells, while 8 demonstrated lower expression levels in the same cells. Mass spectrometry identified 10 of the differentially expressed proteins: 4 signaling molecules, 2 metabolic enzymes, 3 heat shock proteins and 1 cytoskeleton protein. Our results suggest that these differentially expressed proteins, particularly guanine nucleotide-binding protein subunit β-2-like 1 (GBLP), are the key biomacromolecules underlying the mechanism by which PRO enantiomers induce stereoselective cellular responses. The results aid in clarifying the role of PRO in the treatment of arrhythmia and angina.

Non-neuronal functions of the m2 muscarinic acetylcholine receptor

Acetylcholine is an important neurotransmitter whose effects are mediated by two classes of receptors. The nicotinic acetylcholine receptors are ion channels, whereas the muscarinic receptors belong to the large family of G protein coupled seven transmembrane helix receptors. Beyond its function in neuronal systems, it has become evident that acetylcholine also plays an important role in non-neuronal cells such as epithelial and immune cells. Furthermore, many cell types in the periphery are capable of synthesizing acetylcholine and express at least some of the receptors. In this review, we summarize the non-neuronal functions of the muscarinic acetylcholine receptors, especially those of the M₂ muscarinic receptor in epithelial cells. We will review the mechanisms of signaling by the M₂ receptor but also the cellular trafficking and ARF6 mediated endocytosis of this receptor, which play an important role in the regulation of signaling events. In addition, we provide an overview of the M₂ receptor in human pathological conditions such as autoimmune diseases and cancer.

RACK1 depletion in a mouse model causes lethality, pigmentation deficits and reduction in protein synthesis efficiency

The receptor for activated C-kinase 1 (RACK1) is a conserved structural protein of 40S ribosomes. Strikingly, deletion of RACK1 in yeast homolog Asc1 is not lethal. Mammalian RACK1 also interacts with many nonribosomal proteins, hinting at several extraribosomal functions. A knockout mouse for RACK1 has not previously been described. We produced the first RACK1 mutant mouse, in which both alleles of RACK1 gene are defective in RACK1 expression (ΔF/ΔF), in a pure C57 Black/6 background. In a sample of 287 pups, we observed no ΔF/ΔF mice (72 expected). Dissection and genotyping of embryos at various stages showed that lethality occurs at gastrulation. Heterozygotes (ΔF/+) have skin pigmentation defects with a white belly spot and hypopigmented tail and paws. ΔF/+ have a transient growth deficit (shown by measuring pup size at P11). The pigmentation deficit is partly reverted by p53 deletion, whereas the lethality is not. ΔF/+ livers have mild accumulation of inactive 80S ribosomal subunits by polysomal profile analysis. In ΔF/+ fibroblasts, protein synthesis response to extracellular and pharmacological stimuli is reduced. These results highlight the role of RACK1 as a ribosomal protein converging signaling to the translational apparatus.

Biological roles of cAMP: variations on a theme in the different kingdoms of life

Cyclic AMP (cAMP) plays a key regulatory role in most types of cells; however, the pathways controlled by cAMP may present important differences between organisms and between tissues within a specific organism. Changes in cAMP levels are caused by multiple triggers, most affecting adenylyl cyclases, the enzymes that synthesize cAMP. Adenylyl cyclases form a large and diverse family including soluble forms and others with one or more transmembrane domains. Regulatory mechanisms for the soluble adenylyl cyclases involve either interaction with diverse proteins, as happens in Escherichia coli or yeasts, or with calcium or bicarbonate ions, as occurs in mammalian cells. The transmembrane cyclases can be regulated by a variety of proteins, among which the α subunit and the βγ complex from G proteins coupled to membrane receptors are prominent. cAMP levels also are controlled by the activity of phosphodiesterases, enzymes that hydrolyze cAMP. Phosphodiesterases can be regulated by cAMP, cGMP or calcium-calmodulin or by phosphorylation by different protein kinases. Regulation through cAMP depends on its binding to diverse proteins, its proximal targets, this in turn causing changes in a variety of distal targets. Specifically, binding of cAMP to regulatory subunits of cAMP-dependent protein kinases (PKAs) affects the activity of substrates of PKA, binding to exchange proteins directly activated by cAMP (Epac) regulates small GTPases, binding to transcription factors such as the cAMP receptor protein (CRP) or the virulence factor regulator (Vfr) modifies the rate of transcription of certain genes, while cAMP binding to ion channels modulates their activity directly. Further studies on cAMP signalling will have important implications, not only for advancing fundamental knowledge but also for identifying targets for the development of new therapeutic agents.

β-Arrestin-kinase scaffolds: turn them on or turn them off?

G-protein-coupled receptors (GPCRs) can signal through heterotrimeric G-proteins or through β-arrestins to elicit responses to a plethora of extracellular stimuli. While the mechanisms underlying G-protein signaling is relatively well understood, the mechanisms by which β-arrestins regulate the diverse set of proteins with which they associate remain unclear. Multi-protein complexes are a common feature of β-arrestin-dependent signaling. The first two such complexes discovered were the mitogen-activated kinases modules associated with extracellular regulated kinases (ERK1/2) and Jnk3. Subsequently a number of other kinases have been shown to undergo β-arrestin-dependent regulation, including Akt, phosphatidylinositol-3kinase (PI3K), Lim-domain-containing kinase (LIMK), calcium calmodulin kinase II (CAMKII), and calcium calmodulin kinase kinase β (CAMKKβ). Some are positively and some negatively regulated by β-arrestin association. One of the missing links to understanding these pathways is the molecular mechanisms by which the activity of these kinases is regulated. Do β-arrestins merely serve as scaffolds to bring enzyme and substrate together or do they have a direct effect on the enzymatic activities of target kinases? Recent evidence suggests that both mechanisms are involved and that the mechanisms by which β-arrestins regulate kinase activity varies with the target kinase. This review discusses recent advances in the field focusing on 5 kinases for which considerable mechanistic detail and specific sites of interaction have been elucidated.

Specific interactions between Epac1, β-arrestin2 and PDE4D5 regulate β-adrenergic receptor subtype differential effects on cardiac hypertrophic signaling

β1 and β2 adrenergic receptors (βARs) are highly homologous but fulfill distinct physiological and pathophysiological roles. Here we show that both βAR subtypes activate the cAMP-binding protein Epac1, but they differentially affect its signaling. The distinct effects of βARs on Epac1 downstream effectors, the small G proteins Rap1 and H-Ras, involve different modes of interaction of Epac1 with the scaffolding protein β-arrestin2 and the cAMP-specific phosphodiesterase (PDE) variant PDE4D5. We found that β-arrestin2 acts as a scaffold for Epac1 and is necessary for Epac1 coupling to H-Ras. Accordingly, knockdown of β-arrestin2 prevented Epac1-induced histone deacetylase 4 (HDAC4) nuclear export and cardiac myocyte hypertrophy upon β1AR activation. Moreover, Epac1 competed with PDE4D5 for interaction with β-arrestin2 following β2AR activation. Dissociation of the PDE4D5-β-arrestin2 complex allowed the recruitment of Epac1 to β2AR and induced a switch from β2AR non-hypertrophic signaling to a β1AR-like pro-hypertrophic signaling cascade. These findings have implications for understanding the molecular basis of cardiac myocyte remodeling and other cellular processes in which βAR subtypes exert opposing effects.

Novel cAMP signalling paradigms: therapeutic implications for airway disease

Since its discovery over 50 years ago, cAMP has been the archetypal second messenger introducing students to the concept of cell signalling at the simplest level. As explored in this review, however, there are many more facets to cAMP signalling than the path from Gs-coupled receptor to adenylyl cyclase (AC) to cAMP to PKA to biological effect. After a brief description of this canonical cAMP signalling pathway, a snapshot is provided of the novel paradigms of cAMP signalling. As in the airway the cAMP pathway relays the major bronchorelaxant signal and as such is the target for frontline therapy for asthma and COPD, particular emphasis is given to airway disease and therapy. Areas discussed include biased agonism, continued signalling following internalization, modulation of cAMP by AC, control of cAMP degradation, cAMP and calcium crosstalk, Epac-mediated signalling and finally the implications of altered genotypes will be considered. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

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.

Synthetic peptide arrays for investigating protein interaction domains

Synthetic peptide array technology was first developed in the early 1990s by Ronald Frank. Since then the technique has become a powerful tool for high throughput approaches in biology and biochemistry. Here, we focus on peptide arrays applied to investigate the binding specificity of protein interaction domains such as WW, SH3, and PDZ domains. We describe array-based methods used to reveal domain networks in yeast, and briefly review rules as well as ideas about the synthesis and application of peptide arrays. We also provide initial results of a study designed to investigate the nature and evolution of SH3 domain interaction networks in eukaryotes.

p75 neurotrophin receptor regulates glucose homeostasis and insulin sensitivity

Insulin resistance is a key factor in the etiology of type 2 diabetes. Insulin-stimulated glucose uptake is mediated by the glucose transporter 4 (GLUT4), which is expressed mainly in skeletal muscle and adipose tissue. Insulin-stimulated translocation of GLUT4 from its intracellular compartment to the plasma membrane is regulated by small guanosine triphosphate hydrolases (GTPases) and is essential for the maintenance of normal glucose homeostasis. Here we show that the p75 neurotrophin receptor (p75(NTR)) is a regulator of glucose uptake and insulin resistance. p75(NTR) knockout mice show increased insulin sensitivity on normal chow diet, independent of changes in body weight. Euglycemic-hyperinsulinemic clamp studies demonstrate that deletion of the p75(NTR) gene increases the insulin-stimulated glucose disposal rate and suppression of hepatic glucose production. Genetic depletion or shRNA knockdown of p75(NTR) in adipocytes or myoblasts increases insulin-stimulated glucose uptake and GLUT4 translocation. Conversely, overexpression of p75(NTR) in adipocytes decreases insulin-stimulated glucose transport. In adipocytes, p75(NTR) forms a complex with the Rab5 family GTPases Rab5 and Rab31 that regulate GLUT4 trafficking. Rab5 and Rab31 directly interact with p75(NTR) primarily via helix 4 of the p75(NTR) death domain. Adipocytes from p75(NTR) knockout mice show increased Rab5 and decreased Rab31 activities, and dominant negative Rab5 rescues the increase in glucose uptake seen in p75(NTR) knockout adipocytes. Our results identify p75(NTR) as a unique player in glucose metabolism and suggest that signaling from p75(NTR) to Rab5 family GTPases may represent a unique therapeutic target for insulin resistance and diabetes.

Oxygen-dependent cleavage of the p75 neurotrophin receptor triggers stabilization of HIF-1α

Homeostatic control of oxygen availability allows cells to survive oxygen deprivation. Although the transcription factor hypoxia-inducible factor 1α (HIF-1α) is the main regulator of the hypoxic response, the upstream mechanisms required for its stabilization remain elusive. Here, we show that p75 neurotrophin receptor (p75(NTR)) undergoes hypoxia-induced γ-secretase-dependent cleavage to provide a positive feed-forward mechanism required for oxygen-dependent HIF-1α stabilization. The intracellular domain of p75(NTR) directly interacts with the evolutionarily conserved zinc finger domains of the E3 RING ubiquitin ligase Siah2 (seven in absentia homolog 2), which regulates HIF-1α degradation. p75(NTR) stabilizes Siah2 by decreasing its auto-ubiquitination. Genetic loss of p75(NTR) dramatically decreases Siah2 abundance, HIF-1α stabilization, and induction of HIF-1α target genes in hypoxia. p75(NTR-/-) mice show reduced HIF-1α stabilization, vascular endothelial growth factor (VEGF) expression, and neoangiogenesis after retinal hypoxia. Thus, hypoxia-induced intramembrane proteolysis of p75(NTR) constitutes an apical oxygen-dependent mechanism to control the magnitude of the hypoxic response.

RACK1, A multifaceted scaffolding protein: Structure and function

The Receptor for Activated C Kinase 1 (RACK1) is a member of the tryptophan-aspartate repeat (WD-repeat) family of proteins and shares significant homology to the β subunit of G-proteins (Gβ). RACK1 adopts a seven-bladed β-propeller structure which facilitates protein binding. RACK1 has a significant role to play in shuttling proteins around the cell, anchoring proteins at particular locations and in stabilising protein activity. It interacts with the ribosomal machinery, with several cell surface receptors and with proteins in the nucleus. As a result, RACK1 is a key mediator of various pathways and contributes to numerous aspects of cellular function. Here, we discuss RACK1 gene and structure and its role in specific signaling pathways, and address how posttranslational modifications facilitate subcellular location and translocation of RACK1. This review condenses several recent studies suggesting a role for RACK1 in physiological processes such as development, cell migration, central nervous system (CN) function and circadian rhythm as well as reviewing the role of RACK1 in disease.

Tools used to study how protein complexes are assembled in signaling cascades

Most proteins do not function on their own but as part of large signaling complexes that are arranged in every living cell in response to specific environmental cues. Proteins interact with each other either constitutively or transiently and do so with different affinity. When identifying the role played by a protein inside a cell, it is essential to define its particular cohort of binding partners so that the researcher can predict what signaling pathways the protein is engaged in. Once identified and confirmed, the information might allow the interaction to be manipulated by pharmacological inhibitors to help fight disease. In this review, we discuss protein-protein interactions and how they are essential to propagate signals in signaling pathways. We examine some of the high-throughput screening methods and focus on the methods used to confirm specific protein-protein interactions including; affinity tagging, co-immunoprecipitation, peptide array technology and fluorescence microscopy.

Compartmentalization of beta-adrenergic signals in cardiomyocytes

Activation of adrenergic receptors (AR) represents the primary mechanism to increase cardiac performance under stress. Activated βAR couple to Gs protein, leading to adenylyl cyclase-dependent increases in secondary-messenger cyclic adenosine monophosphate (cAMP) to activate protein kinase A. The increased protein kinase A activities promote phosphorylation of diversified substrates, ranging from the receptor and its associated partners to proteins involved in increases in contractility and heart rate. Recent progress with live-cell imaging has drastically advanced our understanding of the βAR-induced cAMP and protein kinase A activities that are precisely regulated in a spatiotemporal fashion in highly differentiated myocytes. Several features stand out: membrane location of βAR and its associated complexes dictates the cellular compartmentalization of signaling; βAR agonist dose-dependent equilibrium between cAMP production and cAMP degradation shapes persistent increases in cAMP signals for sustained cardiac contraction response; and arrestin acts as an agonist dose-dependent master switch to promote cAMP diffusion and propagation into intracellular compartments by sequestrating phosphodiesterase isoforms associated with the βAR signaling cascades. These features and the underlying molecular mechanisms of dynamic regulation of βAR complexes with adenylyl cyclase and phosphodiesterase enzymes and the implication in heart failure are discussed.

Phosphorylation of cAMP-specific PDE4A5 (phosphodiesterase-4A5) by MK2 (MAPKAPK2) attenuates its activation through protein kinase A phosphorylation

cAMP-specific PDE (phosphodiesterase) 4 isoforms underpin compartmentalized cAMP signalling in mammalian cells through targeting to specific signalling complexes. Their importance is apparent as PDE4 selective inhibitors exert profound anti-inflammatory effects and act as cognitive enhancers. The p38 MAPK (mitogen-activated protein kinase) signalling cascade is a key signal transduction pathway involved in the control of cellular immune, inflammatory and stress responses. In the present study, we show that PDE4A5 is phosphorylated at Ser147, within the regulatory UCR1 (ultraconserved region 1) domain conserved among PDE4 long isoforms, by MK2 (MAPK-activated protein kinase 2, also called MAPKAPK2). Phosphorylation by MK2, although not altering PDE4A5 activity, markedly attenuates PDE4A5 activation through phosphorylation by protein kinase A. This modification confers the amplification of intracellular cAMP accumulation in response to adenylate cyclase activation by attenuating a major desensitization system to cAMP. Such reprogramming of cAMP accumulation is recapitulated in wild-type primary macrophages, but not MK2/3-null macrophages. Phosphorylation by MK2 also triggers a conformational change in PDE4A5 that attenuates PDE4A5 interaction with proteins whose binding involves UCR2, such as DISC1 (disrupted in schizophrenia 1) and AIP (aryl hydrocarbon receptor-interacting protein), but not the UCR2-independent interacting scaffold protein β-arrestin. Long PDE4 isoforms thus provide a novel node for cross-talk between the cAMP and p38 MAPK signalling systems at the level of MK2.

Interaction between LIS1 and PDE4, and its role in cytoplasmic dynein function

LIS1, a WD40 repeat scaffold protein, interacts with components of the cytoplasmic dynein motor complex to regulate dynein-dependent cell motility. Here, we reveal that cAMP-specific phosphodiesterases (PDE4s) directly bind PAFAH1B1 (also known as LIS1). Dissociation of LIS1-dynein complexes is coupled with loss of dynein function, as determined in assays of both microtubule transport and directed cell migration in wounded monolayers. Such loss in dynein functioning can be achieved by upregulation of PDE4, which sequesters LIS1 away from dynein, thereby uncovering PDE4 as a regulator of dynein functioning. This process is facilitated by increased intracellular cAMP levels, which selectively augment the interaction of long PDE4 isoforms with LIS1 when they become phosphorylated within their regulatory UCR1 domain by protein kinase A (PKA). We propose that PDE4 and dynein have overlapping interaction sites for LIS1, which allows PDE4 to compete with dynein for LIS1 association in a process enhanced by the PKA phosphorylation of PDE4 long isoforms. This provides a further example to the growing notion that PDE4 itself may provide a signalling role independent of its catalytic activity, exemplified here by its modulation of dynein motor function.

Mammalian Cyclic Nucleotide Phosphodiesterases: Molecular Mechanisms and Physiological Functions

The superfamily of cyclic nucleotide (cN) phosphodiesterases (PDEs) is comprised of 11 families of enzymes. PDEs break down cAMP and/or cGMP and are major determinants of cellular cN levels and, consequently, the actions of cN-signaling pathways. PDEs exhibit a range of catalytic efficiencies for breakdown of cAMP and/or cGMP and are regulated by myriad processes including phosphorylation, cN binding to allosteric GAF domains, changes in expression levels, interaction with regulatory or anchoring proteins, and reversible translocation among subcellular compartments. Selective PDE inhibitors are currently in clinical use for treatment of erectile dysfunction, pulmonary hypertension, intermittent claudication, and chronic pulmonary obstructive disease; many new inhibitors are being developed for treatment of these and other maladies. Recently reported x-ray crystallographic structures have defined features that provide for specificity for cAMP or cGMP in PDE catalytic sites or their GAF domains, as well as mechanisms involved in catalysis, oligomerization, autoinhibition, and interactions with inhibitors. In addition, major advances have been made in understanding the physiological impact and the biochemical basis for selective localization and/or recruitment of specific PDE isoenzymes to particular subcellular compartments. The many recent advances in understanding PDE structures, functions, and physiological actions are discussed in this review.

A phosphodiesterase 3B-based signaling complex integrates exchange protein activated by cAMP 1 and phosphatidylinositol 3-kinase signals in human arterial endothelial cells

Enzymes of the phosphodiesterase 3 (PDE3) and PDE4 families each regulate the activities of both protein kinases A (PKAs) and exchange proteins activated by cAMP (EPACs) in cells of the cardiovascular system. At present, the mechanisms that allow selected PDEs to individually regulate the activities of these two effectors are ill understood. The objective of this study was to determine how a specific PDE3 variant, namely PDE3B, interacts with and regulates EPAC1-based signaling in human arterial endothelial cells (HAECs). Using several biochemical approaches, we show that PDE3B and EPAC1 bind directly through protein-protein interactions. By knocking down PDE3B expression or by antagonizing EPAC1 binding with PDE3B, we show that PDE3B regulates cAMP binding by its tethered EPAC1. Interestingly, we also show that PDE3B binds directly to p84, a PI3Kγ regulatory subunit, and that this interaction allows PI3Kγ recruitment to the PDE3B-EPAC1 complex. Of potential cardiovascular importance, we demonstrate that PDE3B-tethered EPAC1 regulates HAEC PI3Kγ activity and that this allows dynamic cAMP-dependent regulation of HAEC adhesion, spreading, and tubule formation. We identify and molecularly characterize a PDE3B-based "signalosome" that integrates cAMP- and PI3Kγ-encoded signals and show how this signal integration regulates HAEC functions of importance in angiogenesis.

Disruption of the cyclic AMP phosphodiesterase-4 (PDE4)-HSP20 complex attenuates the β-agonist induced hypertrophic response in cardiac myocytes

The small heat shock protein HSP20 is known to be cardioprotective during times of stress and the mechanism underlying its protective abilities depends on its phosphorylation on Ser16 by PKA (protein kinase A). Although the external stimuli that trigger Ser16 phosphorylation have been well studied, the events that modulate spatial and temporal control of this modification remain to be clarified. Here, we report that inhibition of cAMP phosphodiesterase-4 (PDE4) induces the phosphorylation of HSP20 in resting cardiac myocytes and augments its phosphorylation by PKA following β-adrenergic stimulation. Moreover, using peptide array technology, in vitro binding studies, co-immunoprecipitation techniques and immunocytochemistry, we show that HSP20 binds directly to PDE4 within a region of the conserved catalytic domain. We also show that FRET-based, genetically-encoded cAMP reporters anchored to HSP20 exhibit a larger response to PDE4 inhibition compared to free cytosolic cAMP reporters, suggesting that the interaction with PDE4 is crucial in modulating the highly localised pool of cAMP to which HSP20 is exposed. Using information gleaned from peptide array analyses, we developed a cell-permeable peptide that serves to inhibit the interaction of PDE4 with HSP20. Disruption of the HSP20-PDE4 complex, using this peptide, suffices to induce phosphorylation of HSP20 by PKA and to protect against the hypertrophic response measured in neonatal cardiac myocytes following chronic β-adrenergic stimulation.

PDE4D and PDE4B function in distinct subcellular compartments in mouse embryonic fibroblasts

Signaling through cAMP regulates most cellular functions. The spatiotemporal control of cAMP is, therefore, crucial for differential regulation of specific cellular targets. Here we investigated the consequences of PDE4B or PDE4D gene ablation on cAMP signaling at a subcellular level using mouse embryonic fibroblasts. PDE4B ablation had no effect on the global or bulk cytosol accumulation of cAMP but increased both basal and hormone-dependent cAMP in a near-membrane pool. Conversely, PDE4D ablation enhanced agonist-induced cAMP accumulation in the bulk cytosol as well as at the plasma membrane. Both PDE4B and PDE4D ablation significantly modified the time course and the level of isoproterenol-induced phosphorylation of vasodilator-stimulated phosphoprotein, a membrane cytoskeletal component. A second membrane response through Toll-like receptor signaling, however, was only affected by PDE4B ablation. PDE4D but not PDE4B ablation significantly prolonged cAMP-response element-binding protein-mediated transcription. These findings demonstrate that PDE4D and PDE4B have specialized functions in mouse embryonic fibroblasts with PDE4B controlling cAMP in a discrete subdomain near the plasma membrane.

Small molecule allosteric modulators of phosphodiesterase 4

Phosphodiesterase 4 (PDE4) inhibitors have shown benefit in human clinical trials but dosing is limited by tolerability, particularly because of emesis. Novel cocrystal structures of PDE4 catalytic units with their regulatory domains together with bound inhibitors have revealed three different PDE4 conformers that can be exploited in the design of novel therapeutic agents. The first is an open conformer, which has been employed in the traditional approach to the design of competitive PDE4 inhibitors. The second is an asymmetric dimer in which a UCR2 regulatory helix from one monomer is placed in a closed conformation over the opposite active site in the PDE4 dimer (trans-capping). Only one active site can be closed by an inhibitor at a time with the consequence that compounds exploiting this conformer only partially inhibit PDE4 enzymatic activity while retaining potency in cellular and in vivo models. By placing an intrinsic ceiling on the magnitude of PDE4 inhibition, such compounds may better maintain spatial and temporal patterning of signaling in cAMP microdomains with consequent improved tolerability. The third is a symmetric PDE4 conformer in which helices from the C-terminal portion of the catalytic unit cap both active sites (cis-capping). We propose that dual-gating of PDE4 activity may be further fine tuned by accessory proteins that recognize open or closed conformers of PDE4 regulatory helices.

β-Arrestin 1 inhibits the GTPase-activating protein function of ARHGAP21, promoting activation of RhoA following angiotensin II type 1A receptor stimulation

Activation of the small GTPase RhoA following angiotensin II stimulation is known to result in actin reorganization and stress fiber formation. Full activation of RhoA, by angiotensin II, depends on the scaffolding protein β-arrestin 1, although the mechanism behind its involvement remains elusive. Here we uncover a novel partner and function for β-arrestin 1, namely, in binding to ARHGAP21 (also known as ARHGAP10), a known effector of RhoA activity, whose GTPase-activating protein (GAP) function it inhibits. Using yeast two-hybrid screening, a peptide array, in vitro binding studies, truncation analyses, and coimmunoprecipitation techniques, we show that β-arrestin 1 binds directly to ARHGAP21 in a region that transects the RhoA effector GAP domain. Moreover, we show that the level of a complex containing β-arrestin 1 and ARHGAP21 is dynamically increased following angiotensin stimulation and that the kinetics of this interaction modulates the temporal activation of RhoA. Using information gleaned from a peptide array, we developed a cell-permeant peptide that serves to inhibit the interaction of these proteins. Using this peptide, we demonstrate that disruption of the β-arrestin 1/ARHGAP21 complex results in a more active ARHGAP21, leading to less-efficient signaling via the angiotensin II type 1A receptor and, thereby, attenuation of stimulated stress fiber formation.

Interaction with receptor for activated C-kinase 1 (RACK1) sensitizes the phosphodiesterase PDE4D5 towards hydrolysis of cAMP and activation by protein kinase C

We have previously identified the PKC (protein kinase C)-anchoring protein RACK1 (receptor for activated C-kinase 1), as a specific binding partner for the cAMP-specific phosphodiesterase PDE4D5, suggesting a potential site for cross-talk between the PKC and cAMP signalling pathways. In the present study we found that elevation of intracellular cAMP, with the β₂-adrenoceptor agonist isoproterenol (isoprenaline), led to activation of PDE4 enzymes in the particulate and soluble fractions of HEK (human embryonic kidney)-293 cells. In contrast activation of PDE4D5, with isoproterenol and the PKC activator PMA, was restricted to the particulate fraction, where it interacts with RACK1; however, RACK1 is dispensable for anchoring PDE4D5 to the particulate fraction. Kinetic studies demonstrated that RACK1 alters the conformation of particulate-associated PDE4D5 so that it more readily interacts with its substrate cAMP and with rolipram, a PDE4 inhibitor that specifically targets the active site of the enzyme. Interaction with RACK1 was also essential for PKC-dependent and ERK (extracellular-signal-regulated kinase)-independent phosphorylation (on Ser¹²⁶), and activation of PDE4D5 in response to PMA and isoproterenol, both of which trigger the recruitment of PKCα to RACK1. Together these results reveal novel signalling cross-talk, whereby RACK1 mediates PKC-dependent activation of PDE4D5 in the particulate fraction of HEK-293 cells in response to elevations in intracellular cAMP.

RACK1 associates with muscarinic receptors and regulates M(2) receptor trafficking

Receptor internalization from the cell surface occurs through several mechanisms. Some of these mechanisms, such as clathrin coated pits, are well understood. The M₂ muscarinic acetylcholine receptor undergoes internalization via a poorly-defined clathrin-independent mechanism. We used isotope coded affinity tagging and mass spectrometry to identify the scaffolding protein, receptor for activated C kinase (RACK1) as a protein enriched in M₂-immunoprecipitates from M₂-expressing cells over those of non-M₂ expressing cells. Treatment of cells with the agonist carbachol disrupted the interaction of RACK1 with M₂. We further found that RACK1 overexpression inhibits the internalization and subsequent down regulation of the M₂ receptor in a receptor subtype-specific manner. Decreased RACK1 expression increases the rate of agonist internalization of the M₂ receptor, but decreases the extent of subsequent down-regulation. These results suggest that RACK1 may both interfere with agonist-induced sequestration and be required for subsequent targeting of internalized M₂ receptors to the degradative pathway.

Beta-arrestins as regulators of signal termination and transduction: how do they determine what to scaffold?

Over the last decade β-arrestins have emerged as pleiotropic scaffold proteins, capable of mediating numerous diverse responses to multiple agonists. Most well characterized are the G-protein-coupled receptor (GPCR) stimulated β-arrestin signals, which are sometimes synergistic with, and sometimes independent of, heterotrimeric G-protein signals. β-arrestin signaling involves the recruitment of downstream signaling moieties to β-arrestins; in many cases specific sites of interaction between β-arrestins and the downstream target have been identified. As more information unfolds about the nature of β-arrestin scaffolding interactions, it is evident that these proteins are capable of adopting multiple conformations which in turn reveal a specific set of interacting domains. Recruitment of β-arrestin to a specific GPCR can promote formation of a specific subset of available β-arrestin scaffolds, allowing for a higher level of specificity to given agonists. This review discusses recent advances in β-arrestin signaling, discussing the molecular details of a subset of known β-arrestin scaffolds and the significance of specific binding interactions on the ultimate cellular response.

Cyclic AMP controls mTOR through regulation of the dynamic interaction between Rheb and phosphodiesterase 4D

The mammalian target of rapamycin complex 1 (mTORC1) is a molecular hub that regulates protein synthesis in response to a number of extracellular stimuli. Cyclic AMP (cAMP) is considered to be an important second messenger that controls mTOR; however, the signaling components of this pathway have not yet been elucidated. Here, we identify cAMP phosphodiesterase 4D (PDE4D) as a binding partner of Rheb that acts as a cAMP-specific negative regulator of mTORC1. Under basal conditions, PDE4D binds Rheb in a noncatalytic manner that does not require its cAMP-hydrolyzing activity and thereby inhibits the ability of Rheb to activate mTORC1. However, elevated cAMP levels disrupt the interaction of PDE4D with Rheb and increase the interaction between Rheb and mTOR. This enhanced Rheb-mTOR interaction induces the activation of mTORC1 and cap-dependent translation, a cellular function of mTORC1. Taken together, our results suggest a novel regulatory mechanism for mTORC1 in which the cAMP-determined dynamic interaction between Rheb and PDE4D provides a key, unique regulatory event. We also propose a new role for PDE4 as a molecular transducer for cAMP signaling.

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.

The interaction of Epac1 and Ran promotes Rap1 activation at the nuclear envelope

Epac1 (exchange protein directly activated by cyclic AMP [cAMP]) couples intracellular cAMP to the activation of Rap1, a Ras family GTPase that regulates cell adhesion, proliferation, and differentiation. Using mass spectrometry, we identified the small G protein Ran and Ran binding protein 2 (RanBP2) as potential binding partners of Epac1. Ran is a small G protein best known for its role in nuclear transport and can be found at the nuclear pore through its interaction with RanBP2. Here we demonstrate that Ran-GTP and Epac1 interact with each other in vivo and in vitro. This binding requires a previously uncharacterized Ras association (RA) domain in Epac1. Surprisingly, the interaction of Epac1 with Ran is necessary for the efficient activation of Rap1 by Epac1. We propose that Ran and RanBP2 anchor Epac1 to the nuclear pore, permitting cAMP signals to activate Rap1 at the nuclear envelope.

A complex between FAK, RACK1, and PDE4D5 controls spreading initiation and cancer cell polarity

A fundamental question in cell biology concerns how cells respond to their environment by polarizing after sensing directional cues. This requires the differential localization of protein complexes in cells, and it is important to identify and understand how these complexes function. Here we describe a novel "direction-sensing" pathway that links the integrin effector focal adhesion kinase (FAK), the molecular scaffold protein RACK1, and activity of one of its client proteins, PDE4D5, a cAMP-degrading phosphodiesterase. The complex is recruited to nascent adhesions and promotes cell polarity. We identify FAK FERM domain residues whose mutation impairs RACK1 binding. When re-expressed in cancer cells in which endogenous fak is deleted by Cre-lox-mediated recombination, the RACK1-binding-impaired FAK mutant protein does not support formation of nascent actin adhesion structures as cells spread. These cancer cells, like FAK-deficient cells, cannot undergo directional responses, including wound-induced polarization or chemotactic invasion into three-dimensional matrix gels. We show that RACK1 serves as the molecular bridge linking FAK to the recruitment of PDE4D5. FAK/RACK1/PDE4D5 is a novel 'direction-sensing' complex that acts to recruit specific components of the cAMP second-messenger system to nascent integrin adhesions and to the leading edge of polarizing cells.

Viagra for your synapses: Enhancement of hippocampal long-term potentiation by activation of beta-adrenergic receptors

Beta-adrenergic receptors (beta-ARs) critically modulate long-lasting synaptic plasticity and long-term memory storage in the mammalian brain. Synaptic plasticity is widely believed to mediate memory storage at the cellular level. Long-term potentiation (LTP) is one type of synaptic plasticity that has been linked to memory storage. Activation of beta-ARs can enhance LTP and facilitate long-term memory storage. Interestingly, many of the molecular signaling pathways that are critical for beta-adrenergic modulation of LTP mirror those required for the persistence of memory. In this article, we review the roles of signaling cascades and translation regulation in enabling beta-ARs to control expression of long-lasting LTP in the rodent hippocampus. These include the cyclic-AMP/protein kinase-A (cAMP-PKA) and extracellular signal-regulated protein kinase cascades, two key pathways known to link transmitter receptors with translation regulation. Future research directions are discussed, with emphasis on defining the roles of signaling complexes (e.g. PSD-95) and glutamatergic receptors in controlling the efficacy of beta-AR modulation of LTP.

Selective SUMO modification of cAMP-specific phosphodiesterase-4D5 (PDE4D5) regulates the functional consequences of phosphorylation by PKA and ERK

Enzymes from the PDE (phosphodiesterase) 4 cAMP-specific PDE family are crucial for the maintenance of compartmentalized cAMP responses in many cell types. Regulation of PDE activity can be achieved via post-translational modification such as phosphorylation by ERK (extracellular-signal-regulated kinase) MAPKs (mitogen-activated protein kinases) and PKA (protein kinase A). In the present paper, we report for the first time that PDE4 isoforms from the PDE4A and PDE4D subfamilies can be selectively modified by SUMO (small ubiquitin-related modifier). We have identified a single SUMO site within a consensus tetrapeptide motif, ΨKXE (where Ψ represents a hydrophobic residue), which lies in the catalytic unit of these enzymes. SUMO modification of PDE4 at this site was observed upon overexpression of the SUMO E3 ligase PIASy [protein inhibitor of activated STAT (signal transducer and activator of transcription) Y] in HEK (human embryonic kidney)-293 cells and we identify PIASy as a novel binding partner for long PDE4 isoforms. Site-directed mutagenesis of the acceptor lysine residue ablated conjugation of PDE4 with SUMO, suggesting the presence of a single SUMO site in the first subdomain of the conserved PDE4 catalytic unit. This observation was supported by both cell-free in vitro SUMOylation assays and analysis of SUMOylated spot-immobilized peptide arrays. SUMO modification of long PDE4 isoforms serves to augment their activation by PKA phosphorylation and repress their inhibition by ERK phosphorylation. Following ligation of β-adrenergic receptors, SUMOylation of PDE4 isoforms sufficiently amplified PKA-stimulated PDE4 activity to reduce markedly the PKA phosphorylation status of the β2-adrenergic receptor. These results highlight a new means whereby cells might achieve the selective regulation of the activity of cAMP-specific PDE4 enyzmes.

Disrupted-in-Schizophrenia 1 (DISC1) regulates spines of the glutamate synapse via Rac1

Synaptic spines are dynamic structures that regulate neuronal responsiveness and plasticity. We examined the role of the schizophrenia risk factor DISC1 in the maintenance of spine morphology and function. We found that DISC1 anchored Kalirin-7 (Kal-7), regulating access of Kal-7 to Rac1 and controlling the duration and intensity of Rac1 activation in response to NMDA receptor activation in both cortical cultures and rat brain in vivo. These results explain why Rac1 and its activator (Kal-7) serve as important mediators of spine enlargement and why constitutive Rac1 activation decreases spine size. This mechanism likely underlies disturbances in glutamatergic neurotransmission that have been frequently reported in schizophrenia that can lead to alteration of dendritic spines with consequential major pathological changes in brain function. Furthermore, the concept of a signalosome involving disease-associated factors, such as DISC1 and glutamate, may well contribute to the multifactorial and polygenetic characteristics of schizophrenia.

Cross talk between phosphatidylinositol 3-kinase and cyclic AMP (cAMP)-protein kinase a signaling pathways at the level of a protein kinase B/beta-arrestin/cAMP phosphodiesterase 4 complex

Engagement of the T-cell receptor (TCR) in human primary T cells activates a cyclic AMP (cAMP)-protein kinase A (PKA)-Csk inhibitory pathway that prevents full T-cell activation in the absence of a coreceptor stimulus. Here, we demonstrate that stimulation of CD28 leads to recruitment to lipid rafts of a beta-arrestin/phosphodiesterase 4 (PDE4) complex that serves to degrade cAMP locally. Redistribution of the complex from the cytosol depends on Lck and phosphatidylinositol 3-kinase (PI3K) activity. Protein kinase B (PKB) interacts directly with beta-arrestin to form part of the supramolecular complex together with sequestered PDE4. Translocation is mediated by the PKB plextrin homology (PH) domain, thus revealing a new role for PKB as an adaptor coupling PI3K and cAMP signaling. Functionally, PI3K activation and phosphatidylinositol-(3,4,5)-triphosphate (PIP3) production, leading to recruitment of the supramolecular PKB/beta-arrestin/PDE4 complex to the membrane via the PKB PH domain, results in degradation of the TCR-induced cAMP pool located in lipid rafts, thereby allowing full T-cell activation to proceed.

A scanning peptide array approach uncovers association sites within the JNK/beta arrestin signalling complex

Beta arrestins are molecular scaffolds that can bring together three-component mitogen-activated protein kinase signalling modules to promote signal compartmentalisation. We use peptide array technology to define novel interfaces between components within the c-Jun N-terminal kinase (JNK)/beta arrestin signalling complex. We show that beta arrestin 1 and beta arrestin 2 associate with JNK3 via the kinase N-terminal domain in a region that, surprisingly, does not harbour a known 'common docking' motif. In the N-domain and C-terminus of beta arrestin 1 and beta arrestin 2 we identify two novel apoptosis signal-regulating kinase 1 binding sites and in the N-domain of the beta arrestin 1 and beta arrestin 2 we identify a novel MKK4 docking site.

The role of the PDE4D cAMP phosphodiesterase in the regulation of glucagon-like peptide-1 release

BACKGROUND AND PURPOSE

Increases in intracellular cyclic AMP (cAMP) augment the release/secretion of glucagon-like peptide-1 (GLP-1). As cAMP is hydrolysed by cAMP phosphodiesterases (PDEs), we determined the role of PDEs and particularly PDE4 in regulating GLP-1 release.

EXPERIMENTAL APPROACH

GLP-1 release, PDE expression and activity were investigated using rats and GLUTag cells, a GLP-1-releasing cell line. The effects of rolipram, a selective PDE4 inhibitor both in vivo and in vitro and stably overexpressed catalytically inactive PDE4D5 (D556A-PDE4D5) mutant in vitro on GLP-1 release were investigated.

KEY RESULTS

Rolipram (1.5 mg x kg(-1) i.v.) increased plasma GLP-1 concentrations approximately twofold above controls in anaesthetized rats and enhanced glucose-induced GLP-1 release in GLUTag cells (EC(50) approximately 1.2 nmol x L(-1)). PDE4D mRNA transcript and protein were detected in GLUTag cells using RT-PCR with gene-specific primers and Western blotting with a specific PDE4D antibody respectively. Moreover, significant PDE activity was inhibited by rolipram in GLUTag cells. A GLUTag cell clone (C1) stably overexpressing the D556A-PDE4D5 mutant, exhibited elevated intracellular cAMP levels and increased basal and glucose-induced GLP-1 release compared with vector-transfected control cells. A role for intracellular cAMP/PKA in enhancing GLP-1 release in response to overexpression of D556A-PDE4D5 mutant was demonstrated by the finding that the PKA inhibitor H89 reduced both basal and glucose-induced GLP-1 release by 37% and 39%, respectively, from C1 GLUTag cells.

CONCLUSIONS AND IMPLICATIONS

PDE4D may play an important role in regulating intracellular cAMP linked to the regulation of GLP-1 release.

Phosphorylation of RACK1 on tyrosine 52 by c-Abl is required for insulin-like growth factor I-mediated regulation of focal adhesion kinase

Focal Adhesion Kinase (FAK) activity is controlled by growth factors and adhesion signals in tumor cells. The scaffolding protein RACK1 (receptor for activated C kinases) integrates insulin-like growth factor I (IGF-I) and integrin signaling, but whether RACK1 is required for FAK function is unknown. Here we show that association of FAK with RACK1 is required for both FAK phosphorylation and dephosphorylation in response to IGF-I. Suppression of RACK1 by small interfering RNA ablates FAK phosphorylation and reduces cell adhesion, cell spreading, and clonogenic growth. Peptide array and mutagenesis studies localize the FAK binding interface to blades I-III of the RACK1 beta-propeller and specifically identify a set of basic and hydrophobic amino acids (Arg-47, Tyr-52, Arg-57, Arg-60, Phe-65, Lys-127, and Lys-130) as key determinants for association with FAK. Mutation of tyrosine 52 alone is sufficient to disrupt interaction of RACK1 with FAK in cells where endogenous RACK1 is suppressed by small interfering RNA. Cells expressing a Y52F mutant RACK1 are impaired in adhesion, growth, and foci formation. Comparative analyses of homology models and crystal structures for RACK1 orthologues suggest a role for Tyr-52 as a site for phosphorylation that induces conformational change in RACK1, switching the protein into a FAK binding state. Tyrosine 52 is further shown to be phosphorylated by c-Abl kinase, and the c-Abl inhibitor STI571 disrupts FAK interaction with RACK1. We conclude that FAK association with RACK1 is regulated by phosphorylation of Tyr-52. Our data reveal a novel mechanism whereby IGF-I and c-Abl control RACK1 association with FAK to facilitate adhesion signaling.

MEK1 binds directly to betaarrestin1, influencing both its phosphorylation by ERK and the timing of its isoprenaline-stimulated internalization

betaArrestin is a multifunctional signal scaffold protein. Using SPOT immobilized peptide arrays, coupled with scanning alanine substitution and mutagenesis, we show that the MAPK kinase, MEK1, interacts directly with betaarrestin1. Asp(26) and Asp(29) in the N-terminal domain of betaarrestin1 are critical for its binding to MEK1, whereas Arg(47) and Arg(49) in the N-terminal domain of MEK1 are critical for its binding to betaarrestin1. Wild-type FLAG-tagged betaarrestin1 co-immunopurifies with MEK1 in HEKB2 cells, whereas the D26A/D29A mutant does not. ERK-dependent phosphorylation at Ser(412) was compromised in the D26A/D29A-betaarrestin1 mutant. A cell-permeable, 25-mer N-stearoylated betaarrestin1 peptide that encompassed the N-domain MEK1 binding site blocked betaarrestin1/MEK1 association in HEK cells and recapitulated the altered phenotype seen with the D26A/D29A-betaarrestin1 in compromising the ERK-dependent phosphorylation of betaarrestin1. In addition, the MEK disruptor peptide promoted the ability of betaarrestin1 to co-immunoprecipitate with endogenous c-Src and clathrin, facilitating the isoprenaline-stimulated internalization of the beta(2)-adrenergic receptor.

Mdm2 directs the ubiquitination of beta-arrestin-sequestered cAMP phosphodiesterase-4D5

Beta-arrestin plays a key role in regulating beta2-adrenoreceptor signaling by interdicting activation of adenylyl cyclase and selectively sequestering cAMP phosphodiesterase-4D5 (PDE4D5) for delivery of an active cAMP degrading system to the site of cAMP synthesis. Here we show that the beta-agonist, isoprenaline, triggers the rapid and transient ubiquitination of PDE4D5 in primary cardiomyocytes, mouse embryo fibroblasts, and HEK293B2 cells constitutively expressing beta2-adrenoceptors. Reconstitution analyses in beta-arrestin1/2 double knockout cells plus small interference RNA knockdown studies indicate that a beta-arrestin-scaffolded pool of the E3-ubiquitin ligase, Mdm2, mediates PDE4D5 ubiquitination. Critical for this is the ubiquitin-interacting motif located in the extreme C terminus of PDE4D5, which is specific to the PDE4D sub-family. In vitro ubiquitination [corrected] of a PDE4D5 spot-immobilized peptide array, followed by a mutagenesis strategy, showed that PDE4D5 ubiquitination occurs at Lys-48, Lys-53, and Lys-78, which are located within its isoform-specific N-terminal region, as well as at Lys-140 located within its regulatory UCR1 module. We suggest that mono-ubiquitination at Lys-140 primes PDE4D5 for a subsequent cascade of polyubiquitination occurring within its isoform-specific N-terminal region at Lys-48, Lys-53, and Lys-78. PDE4D5 interacts with a non-ubiquitinated beta-arrestin sub-population that is likely to be protected from Mdm2-mediated ubiquitination due to steric hindrance caused by sequestered PDE4D5. Ubiquitination of PDE4D5 elicits an increase in the fraction of PDE4D5 sequestered by beta-arrestin in cells, thereby contributing to the fidelity of PDE4D5-beta-arrestin interaction, as well as decreasing the fraction of PDE4D5 sequestered by the scaffolding protein, RACK1.

The cardiac IKs potassium channel macromolecular complex includes the phosphodiesterase PDE4D3

The cardiac I(Ks) potassium channel is a macromolecular complex consisting of alpha-(KCNQ1) and beta-subunits (KCNE1) and the A kinase-anchoring protein (AKAP) Yotiao (AKAP-9), which recruits protein kinase A) and protein phosphatase 1 to the channel. Here, we have tested the hypothesis that specific cAMP phosphodiesterase (PDE) isoforms of the PDE4D family that are expressed in the heart are also part of the I(Ks) signaling complex and contribute to its regulation by cAMP. PDE4D isoforms co-immunoprecipitated with I(Ks) channels in hearts of mice expressing the I(Ks) channel. In myocytes isolated from these mice, I(Ks) was increased by pharmacological PDE inhibition. PDE4D3, but not PDE4D5, co-immunoprecipitated with the I(Ks) channel only in Chinese hamster ovary cells co-expressing AKAP-9, and PDE4D3, but not PDE4D5, co-immunoprecipitated with AKAP-9. Functional experiments in Chinese hamster ovary cells expressing AKAP-9 and either PDE4D3 or PDE4D5 isoforms revealed modulation of the I(Ks) response to cAMP by PDE4D3 but not PDE4D5. We conclude that PDE4D3, like protein kinase A and protein phosphatase 1, is recruited to the I(Ks) channel via AKAP-9 and contributes to its critical regulation by cAMP.

Antimicrobial peptide arrays for detection of inactivated biothreat agents

Arrays of immobilized antimicrobial peptides are used to detect bacterial, viral, and rickettsial pathogens, including inactivated biothreat agents. These arrays differ from the many combinatorial peptide arrays described in the literature in that the peptides used here have naturally evolved to interact with and disrupt microbial membranes with high affinity but broad specificity. The interaction of these naturally occurring peptides with membranes of pathogens has been harnessed for the purpose of detection, with immobilized antimicrobial peptides acting as "capture" molecules in detection assays. Methods are presented for immobilizing the antimicrobial peptides in planar arrays, performing direct and sandwich assays, and detecting bound targets.

The spot technique: synthesis and screening of peptide macroarrays on cellulose membranes

Peptide arrays are a widely used tool for drug development. For peptide-based drug design it is necessary to screen a large number of peptides. However, there are often difficulties with this approach. Most common peptide synthesis techniques are able to simultaneously synthesize only up to a few hundred single peptides. Spot synthesis is a positionally addressable, multiple synthesis technique offering the possibility of synthesizing and screening up to 10,000 peptides or peptide mixtures on cellulose or other membrane surfaces. In this chapter we present the basic procedures and screening methods related to spot synthesis and outline protocols for easy-to-use detection methods on these peptide arrays.

Regulation of a Drosophila melanogaster cGMP-specific phosphodiesterase by prenylation and interaction with a prenyl-binding protein

Post-translational modification by isoprenylation is a pivotal process for the correct functioning of many signalling proteins. The Drosophila melanogaster cGMP-PDE (cGMP-specific phosphodiesterase) DmPDE5/6 possesses a CaaX-box prenylation signal motif, as do several novel cGMP-PDEs from insect and echinoid species (in CaaX, C is cysteine, a is an aliphatic amino acid and X is 'any' amino acid). DmPDE5/6 is prenylated in vivo at Cys(1128) and is localized to the plasma membrane when expressed in Drosophila S2 cells. Site-directed mutagenesis of the prenylated cysteine residue (C1128S-DmPDE5/6), pharmacological inhibition of prenylation or co-expression of DmPrBP (Drosophila prenyl-binding protein)/delta each alters the subcellular localization of DmPDE5/6. Thus prenylation constitutes a critical post-translational modification of DmPDE5/6 for membrane targeting. Co-immunoprecipitation and subcellular-fractionation experiments have shown that DmPDE5/6 interacts with DmPrBP/delta in Drosophila S2 cells. Transgenic lines allow targeted expression of tagged prenylation-deficient C1128S-DmPDE5/6 in Type I (principal) cells in Drosophila Malpighian tubules, an in vivo model for DmPDE5/6 function. In contrast with wild-type DmPDE5/6, which was exclusively associated with the apical membrane, the C1128S-DmPDE5/6 mutant form was located primarily in the cytosol, although some residual association occurred at the apical membrane. Despite the profound change in intracellular localization of C1128S-DmPDE5/6, active transport of cGMP is affected in the same way as it is by DmPDE5/6. This suggests that, in addition to prenylation and interaction with DmPrBP/delta, further functional membrane-targeting signals exist within DmPDE5/6.