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

Arrestin-dependent nuclear export of phosphodiesterase 4D promotes GPCR-induced nuclear cAMP signaling required for learning and memory

G protein-coupled receptors (GPCRs) promote the expression of immediate early genes required for learning and memory. Here, we showed that β2-adrenergic receptor (β2AR) stimulation induced the nuclear export of phosphodiesterase 4D5 (PDE4D5), an enzyme that degrades the second messenger cAMP, to enable memory consolidation. We demonstrated that the endocytosis of β2AR phosphorylated by GPCR kinases (GRKs) mediated arrestin3-dependent nuclear export of PDE4D5, which was critical for promoting nuclear cAMP signaling and gene expression in hippocampal neurons for memory consolidation. Inhibition of the arrestin3-PDE4D5 association prevented β2AR-induced nuclear cAMP signaling without affecting receptor endocytosis. Direct PDE4 inhibition rescued β2AR-induced nuclear cAMP signaling and ameliorated memory deficits in mice expressing a form of the β2AR that could not be phosphorylated by GRKs. These data reveal how β2AR phosphorylated by endosomal GRK promotes the nuclear export of PDE4D5, leading to nuclear cAMP signaling, changes in gene expression, and memory consolidation. This study also highlights the translocation of PDEs as a mechanism to promote cAMP signaling in specific subcellular locations downstream of GPCR activation.

Short Arrestin-3-Derived Peptides Activate JNK3 in Cells

Arrestins were first discovered as suppressors of G protein-mediated signaling by G protein-coupled receptors. It was later demonstrated that arrestins also initiate several signaling branches, including mitogen-activated protein kinase cascades. Arrestin-3-dependent activation of the JNK family can be recapitulated with peptide fragments, which are monofunctional elements distilled from this multi-functional arrestin protein. Here, we use maltose-binding protein fusions of arrestin-3-derived peptides to identify arrestin elements that bind kinases of the ASK1-MKK4/7-JNK3 cascade and the shortest peptide facilitating JNK signaling. We identified a 16-residue arrestin-3-derived peptide expressed as a Venus fusion that leads to activation of JNK3α2 in cells. The strength of the binding to the kinases does not correlate with peptide activity. The ASK1-MKK4/7-JNK3 cascade has been implicated in neuronal apoptosis. While inhibitors of MAP kinases exist, short peptides are the first small molecule tools that can activate MAP kinases.

Phosphodiesterase type 4 anchoring regulates cAMP signaling to Popeye domain-containing proteins

Cyclic AMP is a ubiquitous second messenger used to transduce intracellular signals from a variety of Gs-coupled receptors. Compartmentalisation of protein intermediates within the cAMP signaling pathway underpins receptor-specific responses. The cAMP effector proteins protein-kinase A and EPAC are found in complexes that also contain phosphodiesterases whose presence ensures a coordinated cellular response to receptor activation events. Popeye domain containing (POPDC) proteins are the most recent class of cAMP effectors to be identified and have crucial roles in cardiac pacemaking and conduction. We report the first observation that POPDC proteins exist in complexes with members of the PDE4 family in cardiac myocytes. We show that POPDC1 preferentially binds the PDE4A sub-family via a specificity motif in the PDE4 UCR1 region and that PDE4s bind to the Popeye domain of POPDC1 in a region known to be susceptible to a mutation that causes human disease. Using a cell-permeable disruptor peptide that displaces the POPDC1-PDE4 complex we show that PDE4 activity localized to POPDC1 modulates cycle length of spontaneous Ca²⁺ transients firing in intact mouse sinoatrial nodes.

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POPDC1 forms a complex with type 4 phosphodiesterases (PDE4s) in cardiac myocytes.

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POPDC1 binds PDE4 enzymes in the Upstream Conserved Region 1 (UCR1) domain.

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The PDE4 binding motif within the Popeye domain lies in a region that harbours a mutation, which underpins human disease.

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Disruption of the POPDC1-PDE4 complex modulates the cycle length of spontaneous Ca2+ transients in the sinoatrial node.

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Disruption of the POPDC1-PDE4 complex causes a significant prolongation of the action potential repolarization phase.

β2-Adrenoceptor Agonists Promote ERK1/2 Dephosphorylation in Human Airway Epithelial Cells by Canonical, cAMP-driven Signaling Independently of β-Arrestin 2

Chronic use of β₂-adrenoceptor agonists as a monotherapy in asthma is associated with a loss of disease control and an increased risk of mortality. Herein, we tested the hypothesis that β₂-adrenoceptor agonists, including formoterol, promote biased, β-arrestin 2 (βArr2)-dependent activation of the mitogen-activated protein (MAP) kinases, ERK1/2, in human airway epithelial cells and, thereby, effect changes in gene expression that could contribute to their adverse clinical outcomes. Three airway epithelial cell models were used: the BEAS-2B cell line, human primary bronchial epithelial cells (HBEC) grown in submersion culture and HBEC that were highly differentiated at an air-liquid interface. Unexpectedly, treatment of all epithelial cell models with formoterol decreased basal ERK1/2 phosphorylation. This was mediated by cAMP-dependent protein kinase and involved the inactivation of C-rapidly-activated fibrosarcoma, which attenuated down-stream ERK1/2 activity, and the induction of dual-specificity phosphatase-1. Formoterol also inhibited the basal expression of early growth response-1, an ERK1/2-regulated gene that controls cell growth and repair in the airways. Neither carvedilol, a β₂-adrenoceptor agonist biased towards βArr2, nor formoterol promoted ERK1/2 phosphorylation in BEAS-2B cells although β₂-adrenoceptor desensitization was compromised in ARRB2-deficient cells. Collectively, these results contest the hypothesis that formoterol activates ERK1/2 in airway epithelia by nucleating a βArr2 signaling complex; instead, they indicate that β₂-adrenoceptor agonists inhibit constitutive ERK1/2 activity in a cAMP-dependent manner. These findings are the antithesis of results obtained using acutely challenged native and engineered HEK293 cells, which have been used extensively to study mechanisms of ERK1/2 activation, and highlight the cell-type-dependence of β₂-adrenoceptor-mediated signaling. Significance Statement It has been proposed that the adverse-effects of β₂-adrenoceptor agonist monotherapy in asthma are mediated by genomic mechanisms that occur principally in airway epithelial cells and are the result of β-arrestin 2-dependent activation of ERK1/2. This study shows that β₂-adrenoceptor agonists, paradoxically, reduced ERK1/2 phosphorylation in airway epithelia by disrupting upstream Ras-C-Raf complex formation and inducing DUSP1. Moreover, these effects were PKA-dependent suggesting that β₂-adrenoceptor agonists were not biased toward β-arrestin 2 and acted via canonical, cAMP-dependent signaling.

The Molecular Biology of Phosphodiesterase 4 Enzymes as Pharmacological Targets: An Interplay of Isoforms, Conformational States, and Inhibitors

The phosphodiesterase 4 (PDE4) enzyme family plays a pivotal role in regulating levels of the second messenger cAMP. Consequently, PDE4 inhibitors have been investigated as a therapeutic strategy to enhance cAMP signaling in a broad range of diseases, including several types of cancers, as well as in various neurologic, dermatological, and inflammatory diseases. Despite their widespread therapeutic potential, the progression of PDE4 inhibitors into the clinic has been hampered because of their related relatively small therapeutic window, which increases the chance of producing adverse side effects. Interestingly, the PDE4 enzyme family consists of several subtypes and isoforms that can be modified post-translationally or can engage in specific protein-protein interactions to yield a variety of conformational states. Inhibition of specific PDE4 subtypes, isoforms, or conformational states may lead to more precise effects and hence improve the safety profile of PDE4 inhibition. In this review, we provide an overview of the variety of PDE4 isoforms and how their activity and inhibition is influenced by post-translational modifications and interactions with partner proteins. Furthermore, we describe the importance of screening potential PDE4 inhibitors in view of different PDE4 subtypes, isoforms, and conformational states rather than testing compounds directed toward a specific PDE4 catalytic domain. Lastly, potential mechanisms underlying PDE4-mediated adverse effects are outlined. In this review, we illustrate that PDE4 inhibitors retain their therapeutic potential in myriad diseases, but target identification should be more precise to establish selective inhibition of disease-affected PDE4 isoforms while avoiding isoforms involved in adverse effects. SIGNIFICANCE STATEMENT: Although the PDE4 enzyme family is a therapeutic target in an extensive range of disorders, clinical use of PDE4 inhibitors has been hindered because of the adverse side effects. This review elaborately shows that safer and more effective PDE4 targeting is possible by characterizing 1) which PDE4 subtypes and isoforms exist, 2) how PDE4 isoforms can adopt specific conformations upon post-translational modifications and protein-protein interactions, and 3) which PDE4 inhibitors can selectively bind specific PDE4 subtypes, isoforms, and/or conformations.

Increased isoform-specific phosphodiesterase 4D expression is associated with pathology and cognitive impairment in Alzheimer's disease

Pharmacological phosphodiesterase 4D (PDE4D) inhibition shows therapeutic potential to restore memory function in Alzheimer's disease (AD), but will likely evoke adverse side effects. As PDE4D encodes multiple isoforms, targeting specific isoforms may improve treatment efficacy and safety. Here, we investigated whether PDE4D isoform expression and PDE4D DNA methylation is affected in AD and whether expression changes are associated with severity of pathology and cognitive impairment. In post-mortem temporal lobe brain material from AD patients (n = 42) and age-matched controls (n = 40), we measured PDE4D isoform expression and PDE4D DNA (hydroxy)methylation using quantitative polymerase chain reaction and Illumina 450k Beadarrays, respectively. Linear regression revealed increased PDE4D1, -D3, -D5, and -D8 expression in AD with concurrent (hydroxy)methylation changes in associated promoter regions. Moreover, increased PDE4D1 and -D3 expression was associated with higherplaque and tau pathology levels, higher Braak stages, and progressed cognitive impairment. Future studies should indicate functional roles of specific PDE4D isoforms and the efficacy and safety of their selective inhibition to restore memory function in AD.

Dominant-Negative Attenuation of cAMP-Selective Phosphodiesterase PDE4D Action Affects Learning and Behavior

PDE4 cyclic nucleotide phosphodiesterases reduce 3′, 5′ cAMP levels in the CNS and thereby regulate PKA activity and the phosphorylation of CREB, fundamental to depression, cognition, and learning and memory. The PDE4 isoform PDE4D5 interacts with the signaling proteins β-arrestin2 and RACK1, regulators of β₂-adrenergic and other signal transduction pathways. Mutations in PDE4D in humans predispose to acrodysostosis, associated with cognitive and behavioral deficits. To target PDE4D5, we developed mice that express a PDE4D5-D556A dominant-negative transgene in the brain. Male transgenic mice demonstrated significant deficits in hippocampus-dependent spatial learning, as assayed in the Morris water maze. In contrast, associative learning, as assayed in a fear conditioning assay, appeared to be unaffected. Male transgenic mice showed augmented activity in prolonged (2 h) open field testing, while female transgenic mice showed reduced activity in the same assay. Transgenic mice showed no demonstrable abnormalities in prepulse inhibition. There was also no detectable difference in anxiety-like behavior, as measured in the elevated plus-maze. These data support the use of a dominant-negative approach to the study of PDE4D5 function in the CNS and specifically in learning and memory.

New Insights into Arrestin Recruitment to GPCRs

G protein-coupled receptors (GPCRs) are cellular master regulators that translate extracellular stimuli such as light, small molecules or peptides into a cellular response. Upon ligand binding, they bind intracellular proteins such as G proteins or arrestins, modulating intracellular signaling cascades. Here, we use a protein-fragment complementation approach based on nanoluciferase (split luciferase assay) to assess interaction of all four known human arrestins with four different GPCRs (two class A and two class B receptors) in live cells. Besides directly tagging the 11S split-luciferase subunit to the receptor, we also could demonstrate that membrane localization of the 11S subunit with a CAAX-tag allowed us to probe arrestin recruitment by endogenously expressed GPCRs. Varying the expression levels of our reporter constructs changed the dynamic behavior of our assay, which we addressed with an advanced baculovirus-based multigene expression system. Our detection assay allowed us to probe the relevance of each of the two arrestin binding sites in the different GPCRs for arrestin binding. We observed remarkable differences between the roles of each arresting binding site in the tested GPCRs and propose that the distinct advantages of our system for probing receptor interaction with effector proteins will help elucidate the molecular basis of GPCR signaling efficacy and specificity in different cell types.

The structural basis of the arrestin binding to GPCRs

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins targeted by more clinically used drugs than any other protein family. GPCR signaling via G proteins is quenched (desensitized) by the phosphorylation of the active receptor by specific GPCR kinases (GRKs) followed by tight binding of arrestins to active phosphorylated receptors. Thus, arrestins engage two types of receptor elements: those that contain GRK-added phosphates and those that change conformation upon activation. GRKs attach phosphates to serines and threonines in the GPCR C-terminus or any one of the cytoplasmic loops. In addition to these phosphates, arrestins engage the cavity that appears between trans-membrane helices upon receptor activation and several other non-phosphorylated elements. The residues that bind GPCRs are localized on the concave side of both arrestin domains. Arrestins undergo a global conformational change upon receptor binding (become activated). Arrestins serve as important hubs of cellular signaling, emanating from activated GPCRs and receptor-independent.

β-Arrestins: Multitask Scaffolds Orchestrating the Where and When in Cell Signalling

The β-arrestins (β-arrs) were initially appreciated for the roles they play in the desensitization and endocytosis of G protein-coupled receptors (GPCRs). They are now also known to act as multifunctional adaptor proteins binding many non-receptor protein partners to control multiple signalling pathways. β-arrs therefore act as key regulatory hubs at the crossroads of external cell inputs and functional outputs in cellular processes ranging from gene transcription to cell growth, survival, cytoskeletal regulation, polarity, and migration. An increasing number of studies have also highlighted the scaffolding roles β-arrs play in vivo in both physiological and pathological conditions, which opens up therapeutic avenues to explore. In this introductory review chapter, we discuss the functional roles that β-arrs exert to control GPCR function, their dynamic scaffolding roles and how this impacts signal transduction events, compartmentalization of β-arrs, how β-arrs are regulated themselves, and how the combination of these events culminates in cellular regulation.

Identification of Integrin β1 as a Novel PAG1-Interacting Protein Involved in the Inherent Radioresistance of Human Laryngeal Carcinoma

Inherent radioresistance plays a crucial role in the failure of radiotherapy. Using the inherent radioresistant (Hep-2max) and radiosensitive (Hep-2min) cell lines established from the parental cell line Hep-2, we previously reported that phosphoprotein associated with glycosphingolipid-enriched microdomains 1(PAG1) overexpression in laryngeal carcinoma cells was correlated with inherent radioresistant phenotypes. However, the underlying mechanisms of this effect remain unknown. In the present study, we performed a proteomic screen to investigate the interactome of PAG1 in Hep-2max cells resulting in the identification of several interaction partners. Bioinformatic analysis and immunofluorescence experiments indicated the integrin β1 to be a crucial interaction partner of PAG1. PAG1 was also highly expressed in laryngeal carcinoma radioresistant tissues and showed co-localization with integrin β1. In addition, we demonstrated that integrin β1's binding to PAG1 could be interrupted by MβCD, an inhibitor of lipid rafts formation. Moreover, knockdown of integrin β1 by RNA interference sensitized radioresistant cells to irradiation. Importantly, we identified 2 potential interaction sites (Pro²¹⁶-Arg²³² and Asn³⁵⁶-Gly³⁷⁷) in the cytoplasmic domain of PAG1 using high throughput peptide arrays. Taken together, these results suggest that the binding of PAG1 to integrin β1 in lipid rafts is essential for inherent radioresistance of human laryngeal carcinoma.

Arrestins: Introducing Signaling Bias Into Multifunctional Proteins

Arrestins were discovered as proteins that bind active phosphorylated G protein-coupled receptors (GPCRs) and block their interactions with G proteins, i.e., for their role in homologous desensitization of GPCRs. Mammals express only four arrestin subtypes, two of which are largely restricted to the retina. Two nonvisual arrestins are ubiquitous and interact with hundreds of different GPCRs and dozens of other binding partners. Changes of just a few residues on the receptor-binding surface were shown to dramatically affect GPCR preference of inherently promiscuous nonvisual arrestins. Mutations on the cytosol-facing side of arrestins modulate their interactions with individual downstream signaling molecules. Thus, it appears feasible to construct arrestin mutants specifically linking particular GPCRs with signaling pathways of choice or mutants that sever the links between selected GPCRs and unwanted pathways. Signaling-biased "designer arrestins" have the potential to become valuable molecular tools for research and therapy.

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

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

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

Altered phosphorylation, electrophysiology, and behavior on attenuation of PDE4B action in hippocampus

Background

PDE4 cyclic nucleotide phosphodiesterases regulate 3′, 5′ cAMP abundance in the CNS and thereby regulate PKA activity and phosphorylation of CREB, which has been implicated in learning and memory, depression and other functions. The PDE4 isoform PDE4B1 also interacts with the DISC1 protein, implicated in neural development and behavioral disorders. The cellular functions of PDE4B1 have been investigated extensively, but its function(s) in the intact organism remained unexplored.

Results

To specifically disrupt PDE4B1, we developed mice that express a PDE4B1-D564A transgene in the hippocampus and forebrain. The transgenic mice showed enhanced phosphorylation of CREB and ERK1/2 in hippocampus. Hippocampal neurogenesis was increased in the transgenic mice. Hippocampal electrophysiological studies showed increased baseline synaptic transmission and enhanced LTP in male transgenic mice. Behaviorally, male transgenic mice showed increased activity in prolonged open field testing, but neither male nor female transgenic mice showed detectable anxiety-like behavior or antidepressant effects in the elevated plus-maze, tail-suspension or forced-swim tests. Neither sex showed any significant differences in associative fear conditioning or showed any demonstrable abnormalities in pre-pulse inhibition.

Conclusions

These data support the use of an isoform-selective approach to the study of PDE4B1 function in the CNS and suggest a probable role of PDE4B1 in synaptic plasticity and behavior. They also provide additional rationale and a refined approach to the development of small-molecule PDE4B1-selective inhibitors, which have potential functions in disorders of cognition, memory, mood and affect.

The RNA-binding protein SERBP1 interacts selectively with the signaling protein RACK1

The RACK1 protein interacts with numerous proteins involved in signal transduction, the cytoskeleton, and mRNA splicing and translation. We used the 2-hybrid system to identify additional proteins interacting with RACK1 and isolated the RNA-binding protein SERBP1. SERPB1 shares amino acid sequence homology with HABP4 (also known as Ki-1/57), a component of the RNA spicing machinery that has been shown previously to interact with RACK1. Several different isoforms of SERBP1, generated by alternative mRNA splicing, interacted with RACK1 with indistinguishable interaction strength, as determined by a 2-hybrid beta-galactosidase assay. Analysis of deletion constructs of SERBP1 showed that the C-terminal third of the SERBP1 protein, which contains one of its two substrate sites for protein arginine N-methyltransferase 1 (PRMT1), is necessary and sufficient for it to interact with RACK1. Analysis of single amino acid substitutions in RACK1, identified in a reverse 2-hybrid screen, showed very substantial overlap with those implicated in the interaction of RACK1 with the cAMP-selective phosphodiesterase PDE4D5. These data are consistent with SERBP1 interacting selectively with RACK1, mediated by an extensive interaction surface on both proteins.

The PDE4 cAMP-Specific Phosphodiesterases: Targets for Drugs with Antidepressant and Memory-Enhancing Action

The PDE4 cyclic nucleotide phosphodiesterases are essential regulators of cAMP abundance in the CNS through their ability to regulate PKA activity, the phosphorylation of CREB, and other important elements of signal transduction. In pre-clinical models and in early-stage clinical trials, PDE4 inhibitors have been shown to have antidepressant and memory-enhancing activity. However, the development of clinically-useful PDE4 inhibitors for CNS disorders has been limited by variable efficacy and significant side effects. Recent structural studies have greatly enhanced our understanding of the molecular configuration of PDE4 enzymes, especially the "long" PDE4 isoforms that are abundant in the CNS. The new structural data provide a rationale for the development of a new generation of PDE4 inhibitors that specifically act on long PDE4 isoforms. These next generation PDE4 inhibitors may also be capable of targeting the interactions of select long forms with their "partner" proteins, such as RACK1, β-arrestin, and DISC1. They would therefore have the ability to affect cAMP levels in specific cellular compartments and target localized cellular functions, such as synaptic plasticity. These new agents might also be able to target PDE4 populations in select regions of the CNS that are implicated in learning and memory, affect, and cognition. Potential therapeutic uses of these agents could include affective disorders, memory enhancement, and neurogenesis.

Identification of a multifunctional docking site on the catalytic unit of phosphodiesterase-4 (PDE4) that is utilised by multiple interaction partners

Cyclic AMP (cAMP)-specific phosphodiesterase-4 (PDE4) enzymes underpin compartmentalised cAMP signalling by localising to distinct signalling complexes. PDE4 long isoforms can be phosphorylated by mitogen-activated protein kinase-activated protein kinase 2 (MK2), which attenuates activation of such enzymes through their phosphorylation by protein kinase A. Here we show that MK2 interacts directly with PDE4 long isoforms and define the sites of interaction. One is a unique site that locates within the regulatory upstream conserved region 1 (UCR1) domain and contains a core Phe141, Leu142 and Tyr143 (FLY) cluster (PDE4A5 numbering). Located with the second site is a critical core Phe693, Glu694, Phe695 (FQF) motif that is also employed in the sequestering of PDE4 long forms by an array of other signalling proteins, including the signalling scaffold β-arrestin, the tyrosyl kinase Lyn, the SUMOylation E2 ligase UBC9, the dynein regulator Lis1 (PAFAH1B1) and the protein kinase Erk. We propose that the FQF motif lies at the heart of a multifunctional docking (MFD) site located within the PDE4 catalytic unit. It is clear from our data that, as well as aiding fidelity of interaction, the MFD site confers exclusivity of binding between PDE4 and a single specific partner protein from the cohort of signalling proteins whose interaction with PDE4 involves the FQF motif.

Analyzing the roles of multi-functional proteins in cells: The case of arrestins and GRKs

Most proteins have multiple functions. Obviously, conventional methods of manipulating the level of the protein of interest in the cell, such as over-expression, knockout or knockdown, affect all of its functions simultaneously. The key advantage of these methods is that over-expression, knockout or knockdown does not require any knowledge of the molecular mechanisms of the function(s) of the protein of interest. The disadvantage is that these approaches are inadequate to elucidate the role of an individual function of the protein in a particular cellular process. An alternative is the use of re-engineered proteins, in which a single function is eliminated or enhanced. The use of mono-functional elements of a multi-functional protein can also yield cleaner answers. This approach requires detailed knowledge of the structural basis of each function of the protein in question. Thus, a lot of preliminary structure-function work is necessary to make it possible. However, when this information is available, replacing the protein of interest with a mutant in which individual functions are modified can shed light on the biological role of those particular functions. Here, we illustrate this point using the example of protein kinases, most of which have additional non-enzymatic functions, as well as arrestins, known multi-functional signaling regulators in the cell.

RACK1 and β-arrestin2 attenuate dimerization of PDE4 cAMP phosphodiesterase PDE4D5

PDE4 family cAMP-selective cyclic nucleotide phosphodiesterases are important in the regulation of cAMP abundance in numerous systems, and thereby play an important role in the regulation of PKA and EPAC activity and the phosphorylation of CREB. We have used the yeast 2-hybrid system to demonstrate recently that long PDE4 isoforms form homodimers, consistent with data obtained recently by structural studies. The long PDE4 isoform PDE4D5 interacts selectively with β-arrestin2, implicated in the regulation of G-protein-coupled receptors and other cell signaling components, and also with the β-propeller protein RACK1. In the present study, we use 2-hybrid approaches to demonstrate that RACK1 and β-arrestin2 inhibit the dimerization of PDE4D5. We also show that serine-to-alanine mutations at PKA and ERK1/2 phosphorylation sites on PDE4D5 detectably ablate dimerization. Conversely, phospho-mimic serine-to-aspartate mutations at the MK2 and oxidative stress kinase sites ablate dimerization. Analysis of PDE4D5 that is locked into the dimeric configuration by the formation of a trans disulfide bond between Ser261 and Ser602 shows that RACK1 interacts strongly with both the monomeric and dimeric forms, but that β-arrestin2 interacts exclusively with the monomeric form. This is consistent with the concept that β-arrestin2 can preferentially recruit the monomeric, or "open," form of PDE4D5 to β2-adrenergic receptors, where it can regulate cAMP signaling.

Uncovering the function of Disrupted in Schizophrenia 1 through interactions with the cAMP phosphodiesterase PDE4: Contributions of the Houslay lab to molecular psychiatry

Nearly 10years ago the laboratory of Miles Houslay was part of a collaboration which identified and characterized the interaction between Disrupted in Schizophrenia 1 and phosphodiesterase type 4. This work has had significant impact on our thinking of psychiatric illness causation and the potential for therapeutics.

Active site coupling in PDE:PKA complexes promotes resetting of mammalian cAMP signaling

Cyclic 3'5' adenosine monophosphate (cAMP)-dependent-protein kinase (PKA) signaling is a fundamental regulatory pathway for mediating cellular responses to hormonal stimuli. The pathway is activated by high-affinity association of cAMP with the regulatory subunit of PKA and signal termination is achieved upon cAMP dissociation from PKA. Although steps in the activation phase are well understood, little is known on how signal termination/resetting occurs. Due to the high affinity of cAMP to PKA (KD ∼ low nM), bound cAMP does not readily dissociate from PKA, thus begging the question of how tightly bound cAMP is released from PKA to reset its signaling state to respond to subsequent stimuli. It has been recently shown that phosphodiesterases (PDEs) can catalyze dissociation of bound cAMP and thereby play an active role in cAMP signal desensitization/termination. This is achieved through direct interactions with the regulatory subunit of PKA, thereby facilitating cAMP dissociation and hydrolysis. In this study, we have mapped direct interactions between a specific cyclic nucleotide phosphodiesterase (PDE8A) and a PKA regulatory subunit (RIα isoform) in mammalian cAMP signaling, by a combination of amide hydrogen/deuterium exchange mass spectrometry, peptide array, and computational docking. The interaction interface of the PDE8A:RIα complex, probed by peptide array and hydrogen/deuterium exchange mass spectrometry, brings together regions spanning the phosphodiesterase active site and cAMP-binding sites of RIα. Computational docking combined with amide hydrogen/deuterium exchange mass spectrometry provided a model for parallel dissociation of bound cAMP from the two tandem cAMP-binding domains of RIα. Active site coupling suggests a role for substrate channeling in the PDE-dependent dissociation and hydrolysis of cAMP bound to PKA. This is the first instance, to our knowledge, of PDEs directly interacting with a cAMP-receptor protein in a mammalian system, and highlights an entirely new class of binding partners for RIα. This study also highlights applications of structural mass spectrometry combined with computational docking for mapping dynamics in transient signaling protein complexes. Together, these results present a novel and critical role for phosphodiesterases in moderating local concentrations of cAMP in microdomains and signal resetting.

Dimerization of cAMP phosphodiesterase-4 (PDE4) in living cells requires interfaces located in both the UCR1 and catalytic unit domains

PDE4 family cAMP phosphodiesterases play a pivotal role in determining compartmentalised cAMP signalling through targeted cAMP breakdown. Expressing the widely found PDE4D5 isoform, as both bait and prey in a yeast 2-hybrid system, we demonstrated interaction consistent with the notion that long PDE4 isoforms form dimers. Four potential dimerization sites were uncovered using a scanning peptide array approach, where a recombinant purified PDE4D5 fusion protein was used to probe a 25-mer library of overlapping peptides covering the entire PDE4D5 sequence. Key residues involved in PDE4D5 dimerization were defined using a site-directed mutagenesis programme directed by an alanine scanning peptide array approach. Critical residues stabilising PDE4D5 dimerization were defined within the regulatory UCR1 region found in long, but not short, PDE4 isoforms, namely the Arg¹⁷³, Asn¹⁷⁴ and Asn¹⁷⁵ (DD1) cluster. Disruption of the DD1 cluster was not sufficient, in itself, to destabilise PDE4D5 homodimers. Instead, disruption of an additional interface, located on the PDE4 catalytic unit, was also required to convert PDE4D5 into a monomeric form. This second dimerization site on the conserved PDE4 catalytic unit is dependent upon a critical ion pair interaction. This involves Asp⁴⁶³ and Arg⁴⁹⁹ in PDE4D5, which interact in a trans fashion involving the two PDE4D5 molecules participating in the homodimer. PDE4 long isoforms adopt a dimeric state in living cells that is underpinned by two key contributory interactions, one involving the UCR modules and one involving an interface on the core catalytic domain. We propose that short forms do not adopt a dimeric configuration because, in the absence of the UCR1 module, residual engagement of the remaining core catalytic domain interface provides insufficient free energy to drive dimerization. The functioning of PDE4 long and short forms is thus poised to be inherently distinct due to this difference in quaternary structure.

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In a yeast 2-hybrid system we show that long PDE4 isoforms dimerize.

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Scanning peptide array and mutagenesis located two dimerization surfaces.

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One surface maps to the regulatory UCR1 region found only in long forms.

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A second locates to the core catalytic domain.

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PDE4 long and short forms differ in quaternary structure.

Phosphodiesterase 4 interacts with the 5-HT4(b) receptor to regulate cAMP signaling

Phosphodiesterase (PDE) 3 and PDE4, which degrade cyclic adenosine monophosphate (cAMP), are important regulators of 5-hydroxytryptamine (5-HT) 4 receptor signaling in cardiac tissue. Therefore, we investigated whether they interact with the 5-HT4(b) receptor, and whether A-kinase anchoring proteins (AKAPs), scaffolding proteins that bind to the regulatory subunit of protein kinase A (PKA) and contribute to the spacial-temporal control of cAMP signaling, are involved in the regulation of 5-HT4(b) receptor signaling. By measuring PKA activity in the absence and presence of PDE3 and PDE4 inhibitiors, we found that constitutive signaling of the overexpressed HA-tagged 5-HT4(b) receptor in HEK293 cells is regulated predominantly by PDE4, with a secondary role for PDE3 that is unmasked in the presence of PDE4 inhibition. Overexpressed PDE4D3 and PDE3A1, and to a smaller extent PDE4D5 co-immunoprecipitate constitutively with the 5-HT4(b) receptor. PDE activity measurements in immunoprecipitates of the 5-HT4(b) receptor confirm the association of PDE4D3 with the receptor and provide evidence that the activity of this PDE may be increased upon receptor stimulation with 5-HT. A possible involvement of AKAPs in 5-HT4(b) receptor signaling was uncovered in experiments using the St-Ht31 inhibitor peptide, which disrupts the interaction of AKAPs with PKA. However, St-Ht31 did not influence 5-HT4(b) receptor-stimulated PKA activity, and endogenous AKAP79 and gravin were not found in immunoprecipitates of the 5-HT4(b) receptor. In conclusion, we found that both PDE3A1 and PDE4D3 are integrated into complexes that contain the 5-HT4(b) receptor and may thereby regulate 5-HT4(b) receptor-mediated signaling.

Arrestin-dependent localization of phosphodiesterases

Many G-protein-coupled receptors trigger the synthesis of cAMP in order to transduce signals from the membrane into the cell cytoplasm. As stimulation of each receptor type results in a specific physiological outcome, compartmentalization of proteins that make, break, and are activated by cAMP underpin receptor-specific responses. Until 2002, it was thought that static compartmentalization of phosphodiesterase 4 (PDE4), conferred by N-terminal targeting sequences, was one way to shape intricate cAMP gradients that formed after receptor activation. Discovery of the PDE4-β-arrestin complex represented a major breakthrough in cAMP signaling, as it spurred the initial realization that PDE4s could be transported to sites of high cAMP to orchestrate destruction of the second messenger at the same time as the receptor's signal to the G-protein is silenced. This chapter charts the scientific process that led to the discovery and characterization of the PDE4-β-arrestin interaction and discusses the known functions of this signaling complex.

Arrestin-3 binds the MAP kinase JNK3α2 via multiple sites on both domains

Although arrestins bind dozens of non-receptor partners, the interaction sites for most signaling proteins remain unknown. Here we report the identification of arrestin-3 elements involved in binding MAP kinase JNK3α2. Using purified JNK3α2 and MBP fusions containing separated arrestin-3 domains and peptides exposed on the non-receptor-binding surface of arrestin-3 we showed that both domains bind JNK3α2 and identified one element on the N-domain and two on the C-domain that directly interact with JNK3α2. Using in vitro competition we confirmed that JNK3α2 engages identified N-domain element and one of the C-domain peptides in the full-length arrestin-3. The 25-amino acid N-domain element has the highest affinity for JNK3α2, suggesting that it is the key site for JNK3α2 docking. The identification of elements involved in protein-protein interactions paves the way to targeted redesign of signaling proteins to modulate cell signaling in desired ways. The tools and methods developed here to elucidate the molecular mechanism of arrestin-3 interactions with JNK3α2 are suitable for mapping of arrestin-3 sites involved in interactions with other partners.

Extensive shape shifting underlies functional versatility of arrestins

Among four vertebrate arrestins, only two are ubiquitously expressed. Arrestins specifically bind active phosphorylated G protein-coupled receptors (GPCRs), thereby precluding further G protein activation. Recent discoveries suggest that the formation of the arrestin-receptor complex initiates the second round of signaling with comparable biological importance. Despite having virtually no recognizable sequence motifs known to mediate protein-protein interactions, arrestins bind a surprising variety of signaling proteins with mind-boggling range of functional consequences. High conformational flexibility allows arrestins to show many distinct 'faces' to the world, which allows these relatively small ∼45kDa proteins to bind various partners under different physiological conditions, organizing multi-protein signaling complexes and localizing them to distinct subcellular compartments.

Phosphodiesterase activity is regulated by CC2D1A that is implicated in non-syndromic intellectual disability

Background

Cyclic adenosine 3^′,5^′-monophosphate (cAMP) is a key regulator of many cellular processes, including in the neuronal system, and its activity is tuned by Phosphodiesterase (PDE) activation. Further, the CC2D1A protein, consisting of N-Terminal containing four DM14 domains and C-terminal containing C2 domain, was shown to regulate the cAMP-PKA pathway. A human deletion mutation lacking the fourth DM14 and the adjacent C2 domain results in Non Syndromic Intellectual Disability (NSID) also referred to as Non Syndromic Mental Retardation (NSMR).

Findings

Here we demonstrate that in Mouse Embryonic Fibroblasts (MEF) CC2D1A co-localizes with PDE4D in the cytosol before cAMP stimulation and on the periphery after stimulation, and that the movement to the periphery requires the full-length CC2D1A. In CC2D1A mouse mutant cells, the absence of three of the four DM14 domains abolishes migration of the complex to the periphery and causes constitutive phosphorylation of PDE4D Serine 126 (S¹²⁶) via the cAMP-dependent protein kinase A (PKA) resulting in PDE4D hyperactivity. Suppressing PDE4D activity with Rolipram in turn restores the down-stream phosphorylation of the “cAMP response element-binding protein” (CREB) that is defective in mouse mutant cells.

Conclusion

Our findings suggest that CC2D1A is a novel regulator of PDE4D. CC2D1A interacts directly with PDE4D regulating its activity and thereby fine-tuning cAMP-dependent downstream signaling. Based on our in vitro evidence we propose a model which links CC2D1A structure and function to cAMP homeostasis thereby affecting CREB phosphorylation. We speculate that CC2D1A and/or PDE4D may be promising targets for therapeutic interventions in many disorders with impaired PDE4D function such as NSID.

Phosphodiesterase-8A binds to and regulates Raf-1 kinase

V-raf-1 murine leukemia viral oncogene homolog 1 (Raf-1) is a key activator of the ERK pathway and is a target for cross-regulation of this pathway by the cAMP signaling system. The cAMP-activated protein kinase, PKA, inhibits Raf-1 by phosphorylation on S259. Here, we show that the cAMP-degrading phosphodiesterase-8A (PDE8A) associates with Raf-1 to protect it from inhibitory phosphorylation by PKA, thereby enhancing Raf-1's ability to stimulate ERK signaling. PDE8A binds to Raf-1 with high (picomolar) affinity. Mapping of the interaction domain on PDE8A using peptide array technology identified amino acids 454-465 as the main binding site, which could be disrupted by mutation. A cell-permeable peptide corresponding to this region disrupted the PDE8A/Raf-1 interaction in cells, thereby reducing ERK activation and the cellular response to EGF. Overexpression of a catalytically inactive PDE8A in cells displayed a dominant negative phenotype on ERK activation. These effects were recapitulated at the organism level in genetically modified (PDE8A(-/-)) mice. Similarly, PDE8 deletion in Drosophila melanogaster reduced basal ERK activation and sensitized flies to stress-induced death. We propose that PDE8A is a physiological regulator of Raf-1 signaling in some cells.

Structure–activity relationship studies using peptide arrays: the example of HIV-1 Rev–integrase interaction

We used peptide arrays to perform structure–activity relationship studies on anti-HIV peptides derived from HIV-1 integrase.

Structural determinants of arrestin functions

Arrestins are a small protein family with only four members in mammals. Arrestins demonstrate an amazing versatility, interacting with hundreds of different G protein-coupled receptor (GPCR) subtypes, numerous nonreceptor signaling proteins, and components of the internalization machinery, as well as cytoskeletal elements, including regular microtubules and centrosomes. Here, we focus on the structural determinants that mediate various arrestin functions. The receptor-binding elements in arrestins were mapped fairly comprehensively, which set the stage for the construction of mutants targeting particular GPCRs. The elements engaged by other binding partners are only now being elucidated and in most cases we have more questions than answers. Interestingly, even very limited and imprecise identification of structural requirements for the interaction with very few other proteins has enabled the development of signaling-biased arrestin mutants. More comprehensive understanding of the structural underpinning of different arrestin functions will pave the way for the construction of arrestins that can link the receptor we want to the signaling pathway of our choosing.

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.

β-Arrestin-2 desensitizes the transient receptor potential vanilloid 1 (TRPV1) channel

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel activated by multiple stimuli and is implicated in a variety of pain disorders. Dynamic sensitization of TRPV1 activity by A-kinase anchoring protein 150 demonstrates a critical role for scaffolding proteins in nociception, yet few studies have investigated scaffolding proteins capable of mediating receptor desensitization. In this study, we identify β-arrestin-2 as a scaffolding protein that regulates TRPV1 receptor activity. We report β-arrestin-2 association with TRPV1 in multiple cell models. Moreover, siRNA-mediated knockdown of β-arrestin-2 in primary cultures resulted in a significant increase in both initial and repeated responses to capsaicin. Electrophysiological analysis further revealed significant deficits in TRPV1 desensitization in primary cultures from β-arrestin-2 knock-out mice compared with wild type. In addition, we found that β-arrestin-2 scaffolding of phosphodiesterase PDE4D5 to the plasma membrane was required for TRPV1 desensitization. Importantly, inhibition of PDE4D5 activity reversed β-arrestin-2 desensitization of TRPV1. Together, these results identify a new endogenous scaffolding mechanism that regulates TRPV1 ligand binding and activation.

β-Adrenergic receptor stimulation increases surface NKCC2 expression in rat thick ascending limbs in a process inhibited by phosphodiesterase 4

The thick ascending limb of the loop of Henle (THAL) reabsorbs ∼30% of the filtered NaCl in a process mediated by the apical Na-K-2Cl cotransporter NKCC2. Stimulation of β-adrenergic receptors in the THAL enhances NaCl reabsorption and increases intracellular cAMP. We found that intracellular cAMP stimulates NKCC2 trafficking to the apical membrane via protein kinase A (PKA). Several cAMP-specific phosphodiesterases (PDE) have been identified in rat THALs, and PDE4 decreases cAMP generated by β-adrenergic stimulation in other cells. However, it is not known whether β-adrenergic receptors activation stimulates NKCC2 trafficking. Thus we hypothesized that β-adrenergic receptor stimulation enhances THAL apical membrane NKCC2 expression via the PKA pathway and PDE4 blunts this effect. THAL suspensions were obtained from Sprague-Dawley rats, and surface NKCC2 expression was measured by surface biotinylation and Western blot. Incubation of THALs with the β-adrenergic receptor agonist isoproterenol at 0.5 and 1.0 μM increased surface NKCC2 by 17 ± 1 and 29 ± 5% respectively (P < 0.05). Preventing cAMP degradation with 3-isobutyl-methylxanthine (IBMX; a nonselective phosphodiesterase inhibitor) enhanced isoproterenol-stimulated surface NKCC2 expression to 51 ± 7% (P < 0.05 vs. isoproterenol). The β-adrenergic receptor antagonist propranolol or the PKA inhibitor H-89 completely blocked isoproterenol + IBMX-induced increase on surface NKCC2, while propranolol or H-89 alone had no effect. Selective inhibition of PDE4 with rolipram (20 μM) potentiated the effect of isoproterenol on surface NKCC2 and increased cAMP levels. We concluded that β-adrenergic receptor stimulation enhances surface NKCC2 expression in the THALs via PKA and PDE4 blunts this effect.

The protein kinase C inhibitor, Ro-31-7459, is a potent activator of ERK and JNK MAP kinases in HUVECs and yet inhibits cyclic AMP-stimulated SOCS-3 gene induction through inactivation of the transcription factor c-Jun

Induction of the suppressor of cytokine signalling 3 (SOCS-3) gene is vital to the normal control of inflammatory signalling. In order to understand these processes we investigated the role of the proto-oncogene component of the AP-1 transcription factor complex, c-Jun, in the regulation of SOCS-3 gene induction. We found that cyclic AMP stimulation of HUVECs promoted phosphorylation and activation of JNK MAP kinase and its substrate c-Jun. The JNK responsive element of the human SOCS-3 promoter mapped to a putative AP-1 site within 1000 bp of the transcription start site. The PKC inhibitors, GF-109203X, Gö-6983 and Ro-317549, were all found to inhibit AP-1 transcriptional activity, transcriptional activation of this minimal SOCS-3 promoter and SOCS-3 gene induction in HUVECs. Interestingly, Ro-317549 treatment was also found to promote PKC-dependent activation of ERK and JNK MAP kinases and promote JNK-dependent hyper-phosphorylation of c-Jun, whereas GF-109203X and Gö-6983 had little effect. Despite this, all three PKC inhibitors were found to be effective inhibitors of c-Jun DNA-binding activity. The JNK-dependent hyper-phosphorylation of c-Jun in response to Ro-317549 treatment of HUVECs does therefore not interfere with its ability to inhibit c-Jun activity and acts as an effective inhibitor of c-Jun-dependent SOCS-3 gene induction.

PDE4: a novel target in the treatment of chronic obstructive pulmonary disease

Phosphodiesterases (PDEs) are important modulators of inflammation and wound healing. In this capacity, specific targeting of PDEs for the treatment of many diseases, including chronic obstructive pulmonary disease (COPD), has been investigated. Currently, treatment of COPD is suboptimal. PDE4 modulates the inflammatory response of the lung, and inhibition of PDE4 may be a novel, COPD-specific approach toward more effective treatment strategies. This review describes the state of PDE4-inhibitor therapy for use in COPD treatment.

Nonvisual arrestins function as simple scaffolds assembling the MKK4-JNK3α2 signaling complex

Arrestins make up a small family of proteins with four mammalian members that play key roles in the regulation of multiple G protein-coupled receptor-dependent and -independent signaling pathways. Although arrestins were reported to serve as scaffolds for MAP kinase cascades, promoting the activation of JNK3, ERK1/2, and p38, the molecular mechanisms involved were not elucidated, and even the direct binding of arrestins with MAP kinases was never demonstrated. Here, using purified proteins, we show that both nonvisual arrestins directly bind JNK3α2 and its upstream activator MKK4, and that the affinity of arrestin-3 for these kinases is higher than that of arrestin-2. Reconstitution of the MKK4-JNK3α2 signaling module from pure proteins in the presence of different arrestin-3 concentrations showed that arrestin-3 acts as a "true" scaffold, facilitating JNK3α2 phosphorylation by bringing the two kinases together. Both the level of JNK3α2 phosphorylation by MKK4 and JNK3α2 activity toward its substrate ATF2 increase at low and then decrease at high arrestin-3 levels, yielding a bell-shaped concentration dependence expected with true scaffolds that do not activate the upstream kinase or its substrate. Thus, direct binding of both kinases and true scaffolding is the molecular mechanism of action of arrestin-3 on the MKK4-JNK3α2 signaling module.

Phosphodiesterases and subcellular compartmentalized cAMP signaling in the cardiovascular system

Phosphodiesterases are key enzymes in the cAMP signaling cascade. They convert cAMP in its inactive form 5'-AMP and critically regulate the intensity and the duration of cAMP-mediated signals. Multiple isoforms exist that possess different intracellular distributions, different affinities for cAMP, and different catalytic and regulatory properties. This complex repertoire of enzymes provides a multiplicity of ways to modulate cAMP levels, to integrate more signaling pathways, and to respond to the specific needs of the cell within distinct subcellular domains. In this review we summarize key findings on phosphodiesterase compartmentalization in the cardiovascular system.

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.

Receptor tyrosine kinase-G-protein-coupled receptor signalling platforms: out of the shadow?

Receptor tyrosine kinases (RTKs) and G-protein-coupled receptors (GPCRs) can form platforms in which protein signalling components specific for each receptor are shared (owing to close proximity) to produce an integrated response upon engagement of ligands. RTK-GPCR signalling platforms respond to growth factors and GPCR agonists to increase gain over and above that which is normally produced by separate receptors. They can also function to change the spatial context of signalling in response to growth factor activation. The function of RTK-GPCR signalling platforms can be modulated with conformational-specific inhibitors that stabilise defined GPCR states to abrogate both GPCR agonist- and growth factor-stimulated cell responses. In this paper, we provide an opinion of the biology and unusual pharmacology of RTK-GPCR signalling platforms and make comparisons with a more traditional model of crosstalk between RTKs and GPCRs.

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.

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.

Peptide arrays for screening cancer specific peptides

In this paper, we describe a novel method to screen peptides for specific recognition by cancer cells. Seventy peptides were synthesized on a cellulose membrane in an array format, and a direct method to study the peptide-whole cell interaction was developed. The relative binding affinity of the cells for different peptides with respect to a lead 12-mer p160 peptide, identified by phage display, was evaluated using the CyQUANT fluorescence of the bound cells. Screening allowed identification of at least five new peptides that displayed higher affinity (up to 3-fold) for MDA-MB-435 and MCF-7 human cancer cells compared to the p160 peptide. These peptides showed very little binding to the control (noncancerous) human umbilical vein endothelial cells (HUVECs). Three of these peptides were synthesized separately and labeled with fluorescein isothiocyanate (FITC) to study their uptake and interaction with the cancer and control cells using confocal laser scanning microscopy and flow cytometry. The results confirmed the high and specific affinity of an 11-mer peptide 11 (RGDPAYQGRFL) and a 10-mer peptide 18 (WXEAAYQRFL) for the cancer cells versus HUVECs. Peptide 11 binds different receptors on target cancer cells as its sequence contains multiple recognition motifs, whereas peptide 18 binds mainly to the putative p160 receptor. The peptide array-whole cell binding assay reported here is a complementary method to phage display for further screening and optimization of cancer targeting peptides for cancer therapy and diagnosis.

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.

Cyclic AMP-mediated immune regulation--overview of mechanisms of action in T cells

The canonical second messenger cAMP is well established as a potent negative regulator of T cell immune function. Through protein kinase A (PKA) it regulates T cell function at the level of transcription factors, members of the mitogen-activated protein kinase pathway, phospholipases (PLs), Ras homolog (Rho)A and proteins involved in the control of cell cycle progression. Type I PKA is the predominant PKA isoform in T cells. Furthermore, whereas type II PKA is located at the centrosome, type I PKA is anchored close to the T cell receptor (TCR) in lipid rafts by the Ezrin-ERM-binding phosphoprotein of 50 kDa (EBP50)-phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG) scaffold complex. The most TCR-proximal target for type I PKA is C-terminal Src kinase (Csk), which upon activation by raft recruitment and phosphorylation inhibits the Src family tyrosine kinases Lck and Fyn and thus functions to maintain T cell homeostasis. Recently, induction of cAMP levels in responder T cells has emerged as one of the mechanisms by which regulatory T (T(R)) cells execute their suppressive action. Thus, the cAMP-type I PKA-Csk pathway emerges as a putative target for therapeutic intervention in autoimmune disorders as well as in cancer, where T(R) cell-mediated suppression contributes to suboptimal local immune responses.

Elucidation of inositol hexaphosphate and heparin interaction sites and conformational changes in arrestin-1 by solution nuclear magnetic resonance

Arrestins specifically bind activated and phosphorylated G protein-coupled receptors and orchestrate both receptor trafficking and channel signaling through G protein-independent pathways via direct interactions with numerous nonreceptor partners. Here we report the first successful use of solution NMR in mapping the binding sites in arrestin-1 (visual arrestin) for two polyanionic compounds that mimic phosphorylated light-activated rhodopsin: inositol hexaphosphate (IP6) and heparin. This yielded an identification of residues involved in the binding with these ligands that was more complete than what has previously been feasible. IP6 and heparin appear to bind to the same site on arrestin-1, centered on a positively charged region in the N-domain. We present the first direct evidence that both IP6 and heparin induced a complete release of the arrestin C-tail. These observations provide novel insight into the nature of the transition of arrestin from the basal to active state and demonstrate the potential of NMR-based methods in the study of protein-protein interactions involving members of the arrestin family.

Cyclic AMP phosphodiesterase 4D (PDE4D) Tethers EPAC1 in a vascular endothelial cadherin (VE-Cad)-based signaling complex and controls cAMP-mediated vascular permeability

Vascular endothelial cell (VEC) permeability is largely dependent on the integrity of vascular endothelial cadherin (VE-cadherin or VE-Cad)-based intercellular adhesions. Activators of protein kinase A (PKA) or of exchange protein activated by cAMP (EPAC) reduce VEC permeability largely by stabilizing VE-Cad-based intercellular adhesions. Currently, little is known concerning the nature and composition of the signaling complexes that allow PKA or EPAC to regulate VE-Cad-based structures and through these actions control permeability. Using pharmacological, biochemical, and cell biological approaches we identified and determined the composition and functionality of a signaling complex that coordinates cAMP-mediated control of VE-Cad-based adhesions and VEC permeability. Thus, we report that PKA, EPAC1, and cyclic nucleotide phosphodiesterase 4D (PDE4D) enzymes integrate into VE-Cad-based signaling complexes in human arterial endothelial cells. Importantly, we show that protein-protein interactions between EPAC1 and PDE4D serve to foster their integration into VE-Cad-based complexes and allow robust local regulation of EPAC1-based stabilization of VE-Cad-based adhesions. Of potential translational importance, we mapped the EPAC1 peptide motif involved in binding PDE4D and show that a cell-permeable variant of this peptide antagonizes EPAC1-PDE4D binding and directly alters VEC permeability. Collectively, our data indicate that PDE4D regulates both the activity and subcellular localization of EPAC1 and identify a novel mechanism for regulated EPAC1 signaling in these cells.