Cell-permeable peptide-based disruption of endogenous PKA-AKAP complexes: a tool for studying the molecular roles of AKAP-mediated PKA subcellular anchoring

Using the Proteomics Toolbox to Resolve Topology and Dynamics of Compartmentalized cAMP Signaling

cAMP is a second messenger that regulates a myriad of cellular functions in response to multiple extracellular stimuli. New developments in the field have provided exciting insights into how cAMP utilizes compartmentalization to ensure specificity when the message conveyed to the cell by an extracellular stimulus is translated into the appropriate functional outcome. cAMP compartmentalization relies on the formation of local signaling domains where the subset of cAMP signaling effectors, regulators and targets involved in a specific cellular response cluster together. These domains are dynamic in nature and underpin the exacting spatiotemporal regulation of cAMP signaling. In this review, we focus on how the proteomics toolbox can be utilized to identify the molecular components of these domains and to define the dynamic cellular cAMP signaling landscape. From a therapeutic perspective, compiling data on compartmentalized cAMP signaling in physiological and pathological conditions will help define the signaling events underlying disease and may reveal domain-specific targets for the development of precision medicine interventions.

Biochemical Analysis of AKAP-Anchored PKA Signaling Complexes

Generation of the prototypic second messenger cAMP instigates numerous signaling events. A major intracellular target of cAMP is Protein kinase A (PKA), a Ser/Thr protein kinase. Where and when this enzyme is activated inside the cell has profound implications on the functional impact of PKA. It is now well established that PKA signaling is focused locally into subcellular signaling "islands" or "signalosomes." The A-Kinase Anchoring Proteins (AKAPs) play a critical role in this process by dictating spatial and temporal aspects of PKA action. Genetically encoded biosensors, small molecule and peptide-based disruptors of PKA signaling are valuable tools for rigorous investigation of local PKA action at the biochemical level. This chapter focuses on approaches to evaluate PKA signaling islands, including a simple assay for monitoring the interaction of an AKAP with a tunable PKA holoenzyme. The latter approach evaluates the composition of PKA holoenzymes, in which regulatory subunits and catalytic subunits can be visualized in the presence of test compounds and small-molecule inhibitors.

AKAP9 supports spermatogenesis through its effects on microtubule and actin cytoskeletons in the rat testis

In mammalian testes, extensive remodeling of the microtubule (MT) and actin cytoskeletons takes place in Sertoli cells across the seminiferous epithelium to support spermatogenesis. However, the mechanism(s) involving regulatory and signaling proteins remains poorly understood. Herein, A-kinase anchoring protein 9 (AKAP9, a member of the AKAP multivalent scaffold protein family) was shown to be one of these crucial regulatory proteins in the rat testis. Earlier studies have shown that AKAP9 serves as a signaling platform by recruiting multiple signaling and regulatory proteins to create a large protein complex that binds to the Golgi and centrosome to facilitate the assembly of the MT-nucleating γ-tubulin ring complex to initiate MT polymerization. We further expanded our earlier studies based on a Sertoli cell-specific AKAP9 knockout mouse model to probe the function of AKAP9 by using the techniques of immunofluorescence analysis, RNA interference (RNAi), and biochemical assays on an in vitro primary Sertoli cell culture model, and an adjudin-based animal model. AKAP9 robustly expressed across the seminiferous epithelium in adult rat testes, colocalizing with MT-based tracks, and laid perpendicular across the seminiferous epithelium, and prominently expressed at the Sertoli-spermatid cell-cell anchoring junction (called apical ectoplasmic specialization [ES]) and at the Sertoli cell-cell interface (called basal ES, which together with tight junction [TJ] created the blood-testis barrier [BTB]) stage specifically. AKAP9 knockdown in Sertoli cells by RNAi was found to perturb the TJ-permeability barrier through disruptive changes in the distribution of BTB-associated proteins at the Sertoli cell cortical zone, mediated by a considerable loss of ability to induce both MT polymerization and actin filament bundling. A considerable decline in AKAP9 expression and a disruptive distribution of AKAP9 across the seminiferous tubules was also noted during adjudin-induced germ cell (GC) exfoliation in this animal model, illustrating AKAP9 is essential to maintain the homeostasis of cytoskeletons to maintain Sertoli and GC adhesion in the testis.

Potential therapeutic applications of AKAP disrupting peptides

The 3'-5'-cyclic adenosine monophosphate (cAMP)/PKA pathway represents a major target for pharmacological intervention in multiple disease conditions. Although the last decade saw the concept of highly compartmentalized cAMP/PKA signaling consolidating, current means for the manipulation of this pathway still do not allow to specifically intervene on discrete cAMP/PKA microdomains. Since compartmentalization is crucial for action specificity, identifying new tools that allow local modulation of cAMP/PKA responses is an urgent need. Among key players of cAMP/PKA signaling compartmentalization, a major role is played by A-kinase anchoring proteins (AKAPs) that, by definition, anchor PKA, its substrates and its regulators within multiprotein complexes in well-confined subcellular compartments. Different tools have been conceived to interfere with AKAP-based protein-protein interactions (PPIs), and these primarily include peptides and peptidomimetics that disrupt AKAP-directed multiprotein complexes. While these molecules have been extensively used to understand the molecular mechanisms behind AKAP function in pathophysiological processes, less attention has been devoted to their potential application for therapy. In this review, we will discuss how AKAP-based PPIs can be pharmacologically targeted by synthetic peptides and peptidomimetics.

Drugs That Regulate Local Cell Signaling: AKAP Targeting as a Therapeutic Option

Cells respond to environmental cues by mobilizing signal transduction cascades that engage protein kinases and phosphoprotein phosphatases. Correct organization of these enzymes in space and time enables the efficient and precise transmission of chemical signals. The cyclic AMP-dependent protein kinase A is compartmentalized through its association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of multivalent scaffolds that constrain signaling enzymes and effectors at subcellular locations to drive essential physiological events. More recently, it has been recognized that defective signaling in certain endocrine disorders and cancers proceeds through pathological AKAP complexes. Consequently, pharmacologically targeting these macromolecular complexes unlocks new therapeutic opportunities for a growing number of clinical indications. This review highlights recent findings on AKAP signaling in disease, particularly in certain cancers, and offers an overview of peptides and small molecules that locally regulate AKAP-binding partners.

New Insights into Beta-Cell GLP-1 Receptor and cAMP Signaling

Harnessing the translational potential of the GLP-1/GLP-1R system in pancreatic beta cells has led to the development of established GLP-1R-based therapies for the long-term preservation of beta cell function. In this review, we discuss recent advances in the current research on the GLP-1/GLP-1R system in beta cells, including the regulation of signaling by endocytic trafficking as well as the application of concepts such as signal bias, allosteric modulation, dual agonism, polymorphic receptor variants, spatial compartmentalization of cAMP signaling and new downstream signaling targets involved in the control of beta cell function.

N-Methyl D-aspartate receptor subunit signaling in fear extinction

N-Methyl D-aspartate receptors (NMDAR) are central mediators of glutamate actions underlying learning and memory processes including those required for extinction of fear and fear-related behaviors. Consistent with this view, in animal models, antagonists of NMDAR typically impair fear extinction, whereas partial agonists have facilitating effects. Promoting NMDAR function has thus been recognized as a promising strategy towards reduction of fear symptoms in patients suffering from anxiety disorders and post-traumatic disorder (PTSD). Nevertheless, application of these drugs in clinical trials has proved of limited utility. Here we summarize recent advances in our knowledge of NMDAR pharmacology relevant for fear extinction, focusing on molecular, cellular, and circuit aspects of NMDAR function as they relate to fear extinction at the level of behavior and cognition. We also discuss how these advances from animal models might help to understand and overcome the limitations of existing approaches in human anxiety disorders and how novel, more specific, and personalized approaches might help advance future therapeutic strategies.

cAMP signalling in insulin and glucagon secretion

The "second messenger" archetype cAMP is one of the most important cellular signalling molecules with central functions including the regulation of insulin and glucagon secretion from the pancreatic β- and α-cells, respectively. cAMP is generally considered as an amplifier of insulin secretion triggered by Ca²⁺ elevation in the β-cells. Both messengers are also positive modulators of glucagon release from α-cells, but in this case cAMP may be the important regulator and Ca²⁺ have a more permissive role. The actions of cAMP are mediated by protein kinase A (PKA) and the guanine nucleotide exchange factor Epac. The present review focuses on how cAMP is regulated by nutrients, hormones and neural factors in β- and α-cells via adenylyl cyclase-catalysed generation and phosphodiesterase-mediated degradation. We will also discuss how PKA and Epac affect ion fluxes and the secretory machinery to transduce the stimulatory effects on insulin and glucagon secretion. Finally, we will briefly describe disturbances of the cAMP system associated with diabetes and how cAMP signalling can be targeted to normalize hypo- and hypersecretion of insulin and glucagon, respectively, in diabetic patients.

Menin and PRMT5 suppress GLP1 receptor transcript and PKA-mediated phosphorylation of FOXO1 and CREB

Menin is a scaffold protein that interacts with several epigenetic mediators to regulate gene transcription, and suppresses pancreatic β-cell proliferation. Tamoxifen-inducible deletion of multiple endocrine neoplasia type 1 (MEN1) gene, which encodes the protein menin, increases β-cell mass in multiple murine models of diabetes and ameliorates diabetes. Glucagon-like-peptide-1 (GLP1) is another key physiological modulator of β-cell mass and glucose homeostasis. However, it is not clearly understood whether menin crosstalks with GLP1 signaling. Here, we show that menin and protein arginine methyltransferase 5 (PRMT5) suppress GLP1 receptor (GLP1R) transcript levels. Notably, a GLP1R agonist induces phosphorylation of forkhead box protein O1 (FOXO1) at S253, and the phosphorylation is mediated by PKA. Interestingly, menin suppresses GLP1-induced and PKA-mediated phosphorylation of both FOXO1 and cAMP response element binding protein (CREB), likely through a protein arginine methyltransferase. Menin-mediated suppression of FOXO1 and CREB phosphorylation increases FOXO1 levels and suppresses CREB target genes, respectively. A small-molecule menin inhibitor reverses menin-mediated suppression of both FOXO1 and CREB phosphorylation. In addition, ex vivo treatment of both mouse and human pancreatic islets with a menin inhibitor increases levels of proliferation marker Ki67. In conclusion, our results suggest that menin and PRMT5 suppress GLP1R transcript levels and PKA-mediated phosphorylation of FOXO1 and CREB, and a menin inhibitor may reverse this suppression to induce β-cell proliferation.

Sphingosine kinase 1-interacting protein is a novel regulator of glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) is essential in keeping blood glucose levels within normal range. GSIS is impaired in type 2 diabetes, and its recovery is crucial in treatment of the disease. We find here that sphingosine kinase 1-interacting protein (SKIP, also called Sphkap) is highly expressed in pancreatic β-cells but not in α-cells. Intraperitoneal glucose tolerance test showed that plasma glucose levels were decreased and insulin levels were increased in SKIP^−/− mice compared to SKIP^+/+ mice, but exendin-4-enhanced insulin secretion was masked. GSIS was amplified more in SKIP^−/− but exendin-4-enhanced insulin secretion was masked compared to that in SKIP^+/+ islets. The ATP and cAMP content were similarly increased in SKIP^+/+ and SKIP^−/− islets; depolarization-evoked, PKA and cAMP-mediated insulin secretion were not affected. Inhibition of PDE activity equally augmented GSIS in SKIP^+/+ and SKIP^−/− islets. These results indicate that SKIP modulates GSIS by a pathway distinct from that of cAMP-, PDE- and sphingosine kinase-dependent pathways.

Molecular Mechanisms for cAMP-Mediated Immunoregulation in T cells - Role of Anchored Protein Kinase A Signaling Units

The cyclic AMP/protein kinase A (cAMP/PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells. A-kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP/PKA pathway. In the immune system, cAMP is a potent negative regulator of T cell receptor-mediated activation of effector T cells (Teff) acting through a proximal PKA/Csk/Lck pathway anchored via a scaffold consisting of the AKAP Ezrin holding PKA, the linker protein EBP50, and the anchoring protein phosphoprotein associated with glycosphingolipid-enriched microdomains holding Csk. As PKA activates Csk and Csk inhibits Lck, this pathway in response to cAMP shuts down proximal T cell activation. This immunomodulating pathway in Teff mediates clinically important responses to regulatory T cell (Treg) suppression and inflammatory mediators, such as prostaglandins (PGs), adrenergic stimuli, adenosine, and a number of other ligands. A major inducer of T cell cAMP levels is PG E2 (PGE2) acting through EP2 and EP4 prostanoid receptors. PGE2 plays a crucial role in the normal physiological control of immune homeostasis as well as in inflammation and cancer immune evasion. Peripherally induced Tregs express cyclooxygenase-2, secrete PGE2, and elicit the immunosuppressive cAMP pathway in Teff as one tumor immune evasion mechanism. Moreover, a cAMP increase can also be induced by indirect mechanisms, such as intercellular transfer between T cells. Indeed, Treg, known to have elevated levels of intracellular cAMP, may mediate their suppressive function by transferring cAMP to Teff through gap junctions, which we speculate could also be regulated by PKA/AKAP complexes. In this review, we present an updated overview on the influence of cAMP-mediated immunoregulatory mechanisms acting through localized cAMP signaling and the therapeutical increasing prospects of AKAPs disruptors in T-cell immune function.

Type II PKAs are anchored to mature insulin secretory granules in INS-1 β-cells and required for cAMP-dependent potentiation of exocytosis

Specificity of the cAMP-dependent protein kinase (PKA) pathway relies on an extremely sophisticated compartmentalization mechanism of the kinase within a given cell, based on high-affinity binding of PKA tetramer pools to different A-Kinase Anchoring Proteins (AKAPs). We and others have previously shown that AKAPs-dependent PKA subcellular targeting is a requisite for optimal cAMP-dependent potentiation of insulin exocytosis. We thus hypothesized that a PKA pool may directly anchor to the secretory compartment to potentiate insulin exocytosis. Here, using immunofluorescence analyses combined to subcellular fractionations and purification of insulin secretory granules (ISGs), we identified discrete subpools of type II PKAs, RIIα and RIIβ PKAs, along with the catalytic subunit, physically associated with ISGs within pancreatic insulin-secreting β-cells. Ultrastructural analysis of native rodent β-cells confirmed in vivo the occurrence of PKA on dense-core ISGs. Isoform-selective disruption of binding of PKAs to AKAPs reinforced the requirement of type II PKA isoforms for cAMP potentiation of insulin exocytosis. This granular localization of PKA was of critical importance since siRNA-mediated depletion of either RIIα or RIIβ PKAs resulted in a significant reduction of cAMP-dependent potentiation of insulin release. The present work provides evidence for a previously unrecognized pool of type II PKAs physically anchored to the β-cell ISGs compartment and supports a non-redundant function for type II PKAs during cAMP potentiation of exocytosis.

Selective disruption of the AKAP signaling complexes

Synthesis of the second messenger cAMP activates a variety of signaling pathways critical for all facets of intracellular regulation. Protein kinase A (PKA) is the major cAMP-responsive effector. Where and when this enzyme is activated has profound implications on the cellular role of PKA. A-Kinase Anchoring Proteins (AKAPs) play a critical role in this process by orchestrating spatial and temporal aspects of PKA action. A popular means of evaluating the impact of these anchored signaling events is to biochemically interfere with the PKA-AKAP interface. Hence, peptide disruptors of PKA anchoring are valuable tools in the investigation of local PKA action. This article outlines the development of PKA isoform-selective disruptor peptides, documents the optimization of cell-soluble peptide derivatives, and introduces alternative cell-based approaches that interrogate other aspects of the PKA-AKAP interface.

Regulation of fear extinction versus other affective behaviors by discrete cortical scaffolding complexes associated with NR2B and PKA signaling

In patients suffering from post-traumatic stress disorder (PTSD), fear evoked by trauma-related memories lasts long past the traumatic event and it is often complicated by general anxiety and depressed mood. This poses a treatment challenge, as drugs beneficial for some symptoms might exacerbate others. For example, in preclinical studies, antagonists of the NR2B subunit of N-methyl-d-aspartate receptors and activators of cAMP-dependent protein kinase (PKA) act as potent antidepressants and anxiolytics, but they block fear extinction. Using mice, we attempted to overcome this problem by interfering with individual NR2B and PKA signaling complexes organized by scaffolding proteins. We infused cell-permeable Tat peptides that displaced either NR2B from receptor for activated C kinase 1 (RACK1), or PKA from A-kinase anchor proteins (AKAPs) or microtubule-associated proteins (MAPs). The infusions were targeted to the retrosplenial cortex, an area involved in both fear extinction of remotely acquired memories and in mood regulation. Tat-RACK1 and Tat-AKAP enhanced fear extinction, all peptides reduced anxiety and none affected baseline depression-like behavior. However, disruption of PKA complexes distinctively interfered with the rapid antidepressant actions of the N-methyl-D-aspartate receptors antagonist MK-801 in that Tat-MAP2 blocked, whereas Tat-AKAP completely inverted the effect of MK-801 from antidepressant to depressant. These effects were unrelated to the MK-801-induced changes of brain-derived neurotrophic factor messenger RNA levels. Together, the findings suggest that NR2B-RACK1 complexes specifically contribute to fear extinction, and may provide a target for the treatment of PTSD. AKAP-PKA, on the other hand, appears to modulate fear extinction and antidepressant responses in opposite directions.

Targeting protein-protein interactions in complexes organized by A kinase anchoring proteins

Cyclic AMP is a ubiquitous intracellular second messenger involved in the regulation of a wide variety of cellular processes, a majority of which act through the cAMP – protein kinase A (PKA) signaling pathway and involve PKA phosphorylation of specific substrates. PKA phosphorylation events are typically spatially restricted and temporally well controlled. A-kinase anchoring proteins (AKAPs) directly bind PKA and recruit it to specific subcellular loci targeting the kinase activity toward particular substrates, and thereby provide discrete spatiotemporal control of downstream phosphorylation events. AKAPs also scaffold other signaling molecules into multi-protein complexes that function as crossroads between different signaling pathways. Targeting AKAP coordinated protein complexes with high-affinity peptidomimetics or small molecules to tease apart distinct protein–protein interactions (PPIs) therefore offers important means to disrupt binding of specific components of the complex to better understand the molecular mechanisms involved in the function of individual signalosomes and their pathophysiological role. Furthermore, development of novel classes of small molecules involved in displacement of AKAP-bound signal molecules is now emerging. Here, we will focus on mechanisms for targeting PPI, disruptors that modulate downstream cAMP signaling and their role, especially in the heart.

Targeting of type I protein kinase A to lipid rafts is required for platelet inhibition by the 3',5'-cyclic adenosine monophosphate-signaling pathway

BACKGROUND

Platelet adhesion to von Willebrand factor (VWF) is modulated by 3',5'-cyclic adenosine monophosphate (cAMP) signaling through protein kinase A (PKA)-mediated phosphorylation of glycoprotein (GP)Ibβ. A-kinase anchoring proteins (AKAPs) are proposed to control the localization and substrate specificity of individual PKA isoforms. However, the role of PKA isoforms in regulating the phosphorylation of GPIbβ and platelet response to VWF is unknown.

OBJECTIVES

We wished to determine the role of PKA isoforms in the phosphorylation of GPIbβ and platelet activation by VWF as a model for exploring the selective partitioning of cAMP signaling in platelets.

RESULTS

The two isoforms of PKA in platelets, type I (PKA-I) and type II (PKA-II), were differentially localized, with a small pool of PKA-I found in lipid rafts. Using a combination of Far Western blotting, immunoprecipitation, proximity ligation assay and cAMP pull-down we identified moesin as an AKAP that potentially localizes PKA-I to rafts. Introduction of cell-permeable anchoring disruptor peptide, RI anchoring disruptor (RIAD-Arg11 ), to block PKA-I/AKAP interactions, uncoupled PKA-RI from moesin, displaced PKA-RI from rafts and reduced kinase activity in rafts. Examination of GPIbβ demonstrated that it was phosphorylated in response to low concentrations of PGI2 in a PKA-dependent manner and occurred primarily in lipid raft fractions. RIAD-Arg11 caused a significant reduction in raft-localized phosphoGPIbβ and diminished the ability of PGI2 to regulate VWF-mediated aggregation and thrombus formation in vitro.

CONCLUSION

We propose that PKA-I-specific AKAPs in platelets, including moesin, organize a selective localization of PKA-I required for phosphorylation of GPIbβ and contribute to inhibition of platelet VWF interactions.

PKA compartmentalization via AKAP220 and AKAP12 contributes to endothelial barrier regulation

cAMP-mediated PKA signaling is the main known pathway involved in maintenance of the endothelial barrier. Tight regulation of PKA function can be achieved by discrete compartmentalization of the enzyme via physical interaction with A-kinase anchoring proteins (AKAPs). Here, we investigated the role of AKAPs 220 and 12 in endothelial barrier regulation. Analysis of human and mouse microvascular endothelial cells as well as isolated rat mesenteric microvessels was performed using TAT-Ahx-AKAPis peptide, designed to competitively inhibit PKA-AKAP interaction. In vivo microvessel hydraulic conductivity and in vitro transendothelial electrical resistance measurements showed that this peptide destabilized endothelial barrier properties, and dampened the cAMP-mediated endothelial barrier stabilization induced by forskolin and rolipram. Immunofluorescence analysis revealed that TAT-Ahx-AKAPis led to both adherens junctions and actin cytoskeleton reorganization. Those effects were paralleled by redistribution of PKA and Rac1 from endothelial junctions and by Rac1 inactivation. Similarly, membrane localization of AKAP220 was also reduced. In addition, depletion of either AKAP12 or AKAP220 significantly impaired endothelial barrier function and AKAP12 was also shown to interfere with cAMP-mediated barrier enhancement. Furthermore, immunoprecipitation analysis demonstrated that AKAP220 interacts not only with PKA but also with VE-cadherin and ß-catenin. Taken together, these results indicate that AKAP-mediated PKA subcellular compartmentalization is involved in endothelial barrier regulation. More specifically, AKAP220 and AKAP12 contribute to endothelial barrier function and AKAP12 is required for cAMP-mediated barrier stabilization.

Targeting protein-protein interactions within the cyclic AMP signaling system as a therapeutic strategy for cardiovascular disease

The cAMP signaling system can trigger precise physiological cellular responses that depend on the fidelity of many protein-protein interactions, which act to bring together signaling intermediates at defined locations within cells. In the heart, cAMP participates in the fine control of excitation-contraction coupling, hence, any disregulation of this signaling cascade can lead to cardiac disease. Due to the ubiquitous nature of the cAMP pathway, general inhibitors of cAMP signaling proteins such as PKA, EPAC and PDEs would act non-specifically and universally, increasing the likelihood of serious 'off target' effects. Recent advances in the discovery of peptides and small molecules that disrupt the protein-protein interactions that underpin cellular targeting of cAMP signaling proteins are described and discussed.

Cyclic AMP dynamics in the pancreatic β-cell

Insulin secretion from pancreatic β-cells is tightly regulated by glucose and other nutrients, hormones, and neural factors. The exocytosis of insulin granules is triggered by an elevation of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and is further amplified by cyclic AMP (cAMP). Cyclic AMP is formed primarily in response to glucoincretin hormones and other G(s)-coupled receptor agonists, but generation of the nucleotide is critical also for an optimal insulin secretory response to glucose. Nutrient and receptor stimuli trigger oscillations of the cAMP concentration in β-cells. The oscillations arise from variations in adenylyl cyclase-mediated cAMP production and phosphodiesterase-mediated degradation, processes controlled by factors like cell metabolism and [Ca(2+)](i). Protein kinase A and the guanine nucleotide exchange factor Epac2 mediate the actions of cAMP in β-cells and operate at multiple levels to promote exocytosis and pulsatile insulin secretion. The cAMP signaling system contains important targets for pharmacological improvement of insulin secretion in type 2 diabetes.

Setting sail for glucose homeostasis with the AKAP150-PP2B-anchor

Glucose-stimulated insulin secretion, controlled by multiple protein phosphorylation events, is critical for the regulation of glucose homeostasis. Protein kinase A (PKA) is known to play a role in β cell physiology, but the role of its anchoring protein is not fully understood. Hinke et al (2012) illustrate the significance of A-kinase anchoring protein 150 in tethering protein phosphatase 2B to mediate nutrient-stimulated insulin secretion and thus modulate glucose homeostasis.

A-kinase anchoring proteins as potential drug targets

A-kinase anchoring proteins (AKAPs) crucially contribute to the spatial and temporal control of cellular signalling. They directly interact with a variety of protein binding partners and cellular constituents, thereby directing pools of signalling components to defined locales. In particular, AKAPs mediate compartmentalization of cAMP signalling. Alterations in AKAP expression and their interactions are associated with or cause diseases including chronic heart failure, various cancers and disorders of the immune system such as HIV. A number of cellular dysfunctions result from mutations of specific AKAPs. The link between malfunctions of single AKAP complexes and a disease makes AKAPs and their interactions interesting targets for the development of novel drugs. 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.

Molecular partners for interaction and cell internalization of cell-penetrating peptides: how identical are they?

Cell-penetrating peptides are short basic peptide sequences that might display amphipathic properties. These positively charged peptides internalize into all cell types, albeit with different efficiency. Cell-penetrating peptides use all routes of pinocytosis to internalize, in addition to direct membrane translocation that requires interaction with lipid membrane domains. These differences in internalization efficiency according to the peptide sequence and cell type suggest that the cell-penetrating peptides interact with different molecular partners at the cell surface. This review will first report on data that describe the molecular interaction of the most popular cell-penetrating peptides (penetratin, Tat and oligoarginine) with carbohydrates and lipids. The second part of the review will be dedicated to cell studies that have reported how cell surface composition influences cell internalization. Discussion will focus on the gap between in vitro and in cellulo studies, and more specifically to which extent the interaction with molecules found in membranes reflect the internalization efficiency of the peptides.

Palmitoylation targets AKAP79 protein to lipid rafts and promotes its regulation of calcium-sensitive adenylyl cyclase type 8

PKA anchoring proteins (AKAPs) optimize the efficiency of cAMP signaling by clustering interacting partners. Recently, AKAP79 has been reported to directly bind to adenylyl cyclase type 8 (AC8) and to regulate its responsiveness to store-operated Ca²⁺ entry (SOCE). Although AKAP79 is well targeted to the plasma membrane via phospholipid associations with three N-terminal polybasic regions, recent studies suggest that AKAP79 also has the potential to be palmitoylated, which may specifically allow it to target the lipid rafts where AC8 resides and is regulated by SOCE. In this study, we have addressed the role of palmitoylation of AKAP79 using a combination of pharmacological, mutagenesis, and cell biological approaches. We reveal that AKAP79 is palmitoylated via two cysteines in its N-terminal region. This palmitoylation plays a key role in targeting the AKAP to lipid rafts in HEK-293 cells. Mutation of the two critical cysteines results in exclusion of AKAP79 from lipid rafts and alterations in its membrane diffusion behavior. This is accompanied by a loss of the ability of AKAP79 to regulate SOCE-dependent AC8 activity in intact cells and decreased PKA-dependent phosphorylation of raft proteins, including AC8. We conclude that palmitoylation plays a key role in the targeting and action of AKAP79. This novel property of AKAP79 adds an unexpected regulatory and targeting option for AKAPs, which may be exploited in the cellular context.

Adenylyl cyclase 8 is central to glucagon-like peptide 1 signalling and effects of chronically elevated glucose in rat and human pancreatic beta cells

AIMS/HYPOTHESIS

Glucose and incretins regulate beta cell function, gene expression and insulin exocytosis via calcium and cAMP. Prolonged exposure to elevated glucose (also termed glucotoxicity) disturbs calcium homeostasis, but little is known about cAMP signalling. We therefore investigated long-term effects of glucose on this pathway with special regard to the incretin glucagon-like peptide 1 (GLP-1).

METHODS

We exposed INS-1E cells and rat or human islets to different levels of glucose for 3 days and determined functional responses in terms of second messengers (cAMP, Ca(2+)), transcription profiles, activation of cAMP-responsive element (CRE) and secretion by measuring membrane capacitance. Moreover, we modulated directly the abundance of a calcium-sensitive adenylyl cyclase (ADCY8) and GLP-1 receptor (GLP1R).

RESULTS

GLP-1- or forskolin-mediated increases in cytosolic calcium, cAMP-levels or insulin secretion were largely reduced in INS-1E cells cultured at elevated glucose (>5.5 mmol/l). Statistical analysis of transcription profiles identified cAMP pathways as major targets regulated by glucose. Quantitative PCR confirmed these findings and unravelled marked downregulation of the calcium-sensitive adenylyl cyclase ADCY8 also in rat and in human islets. Re-expression of ADCY8, but not of the GLP1R, recovered GLP-1 signalling in glucotoxicity in INS-1E cells and in rat islets. Moreover, knockdown of this adenylyl cyclase showed that GLP-1-induced cAMP generation, calcium signalling, activation of the downstream target CRE and direct amplification of exocytosis by cAMP-raising agents (evaluated by capacitance measurement) proceeds via ADCY8.

CONCLUSIONS/INTERPRETATION

cAMP-mediated pathways are modelled by glucose, and downregulation of the calcium-sensitive ADCY8 plays a central role herein, including signalling via the GLP1R.

Modulation of cell adhesion and migration by the histone methyltransferase subunit mDpy-30 and its interacting proteins

We have previously shown that a subset of mDpy-30, an accessory subunit of the nuclear histone H3 lysine 4 methyltransferase (H3K4MT) complex, also localizes at the trans-Golgi network (TGN), where its recruitment is mediated by the TGN-localized ARF guanine nucleotide exchange factor (ArfGEF) BIG1. Depletion of mDpy-30 inhibits the endosome-to-TGN transport of internalized CIMPR receptors and concurrently promotes their accumulation at the cell protrusion. These observations suggest mDpy-30 may play a novel role at the crossroads of endosomal trafficking, nuclear transcription and adhesion/migration. Here we provide novel mechanistic and functional insight into this association. First, we demonstrate a direct interaction between mDpy-30 and BIG1 and locate the binding region in the N-terminus of BIG1. Second, we provide evidence that the depletion or overexpression of mDpy-30 enhances or inhibits cellular adhesion/migration of glioma cells in vitro, respectively. A similar increase in cell adhesion/migration is observed in cells with reduced levels of BIG1 or other H3K4MT subunits. Third, knockdown of mDpy-30, BIG1, or the RbBP5 H3K4MT subunit increases the targeting of beta1 integrin to cell protrusions, and suppression of H3K4MT activity by depleting mDpy-30 or RbBP5 leads to increased protein and mRNA levels of beta1 integrin. Moreover, stimulation of cell adhesion/migration via mDpy-30 knockdown is abolished after treating cells with a function-blocking antibody to beta1 integrin. Taken together, these data indicate that mDpy-30 and its interacting proteins function as a novel class of cellular adhesion/migration modulators partially by affecting the subcellular distribution of endosomal compartments as well as the expression of key adhesion/migration proteins such as beta1 integrin.

Cyclic AMP signaling in pancreatic islets

Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.

Networking with AKAPs: context-dependent regulation of anchored enzymes

A-Kinase Anchoring Proteins (AKAPs) orchestrate and synchronize cellular events by tethering the cAMP-dependent protein kinase (PKA) and other signaling enzymes to organelles and membranes. The control of kinases and phosphatases that are held in proximity to activators, effectors, and substrates favors the rapid dissemination of information from one cellular location to the next. This article charts the inception of the PKA-anchoring hypothesis, the characterization of AKAPs and their nomenclature, and the physiological roles of context-specific AKAP signaling complexes.

The contribution of AKAP5 in amylase secretion from mouse parotid acini

A-kinase (PKA) anchoring proteins (AKAPs) are essential for targeting type II PKA to specific locales in the cell to control function. In the present study, AKAP5 (formerly AKAP150) and AKAP6 were identified in mouse parotid acini by type II PKA regulatory subunit (RII) overlay assay and Western blot analysis of mouse parotid cellular fractions, and the role of AKAP5 in mouse parotid acinar cell secretion was determined. Mice were euthanized with CO(2). Immunofluorescence staining of acinar cells localized AKAP5 to the basolateral membrane, whereas AKAP6 was associated with the perinuclear region. In functional studies, amylase secretion from acinar cells of AKAP5 mutant [knockout (KO)] mice treated with the beta-adrenergic agonist, isoproterenol, was reduced overall by 30-40% compared with wild-type (WT) mice. In contrast, amylase secretion in response to the adenylyl cyclase (AC) activator, forskolin, and the cAMP-dependent protein kinase (PKA) activator, N(6)-phenyl-cAMP, was not statistically different in acini from WT and AKAP5 KO mice. Treatment of acini with isoproterenol mimicked the effect of the Epac activator, 8-(4-methoxyphenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8-pMeOPT-2'-O-Me-cAMP), in stimulating Rap1. However, in contrast to isoproterenol, treatment of acini with 8-pMeOPT-2'-O-Me-cAMP resulted in stimulation of amylase secretion from both AKAP5 KO and WT acinar cells. As a scaffolding protein, AKAP5 was found to coimmunoprecipitate with AC6, but not AC8. Data suggest that isoproterenol-stimulated amylase secretion occurs via both an AKAP5/AC6/PKA complex and a PKA-independent, Epac pathway in mouse parotid acini.

AKAP 18 alpha and gamma have opposing effects on insulin release in INS-1E cells

A-kinase anchoring proteins (AKAPs) are known to compartmentalise protein kinase(s) to discrete cellular locations. Here we show that silencing of AKAP 18 alpha or gamma expression results in decreased or increased glucose-stimulated insulin secretion in INS-1E cells. Glucose stimulates AKAP 18 alpha and inhibits AKAP 18 gamma mRNA expressions while palmitate markedly reduces AKAP 18 alpha expression. Human growth hormone (GH) stimulates AKAP 18 alpha expression and attenuates palmitate-induced suppression of AKAP 18 alpha mRNA level. The roles of AKAP 18 alpha and gamma in mediating insulin release are consistent with their respective regulations by glucose.

Pleiotropic actions of the incretin hormones

The insulin secretory response to a meal results largely from glucose stimulation of the pancreatic islets and both direct and indirect (autonomic) glucose-dependent stimulation by incretin hormones released from the gastrointestinal tract. Two incretins, Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), have so far been identified. Localization of the cognate G protein-coupled receptors for GIP and GLP-1 revealed that they are present in numerous tissues in addition to the endocrine pancreas, including the gastrointestinal, cardiovascular, central nervous and autonomic nervous systems (ANSs), adipose tissue, and bone. At these sites, the incretin hormones exert a range of pleiotropic effects, many of which contribute to the integration of processes involved in the regulation of food intake, and nutrient and mineral processing and storage. From detailed studies at the cellular and molecular level, it is also evident that both incretin hormones act via multiple signal transduction pathways that regulate both acute and long-term cell function. Here, we provide an overview of current knowledge relating to the physiological roles of GIP and GLP-1, with specific emphasis on their modes of action on islet hormone secretion, β-cell proliferation and survival, central and autonomic neuronal function, gastrointestinal motility, and glucose and lipid metabolism. However, it is emphasized that despite intensive research on the various body systems, in many cases there is uncertainty as to the pathways by which the incretins mediate their pleiotropic effects and only a rudimentary understanding of the underlying cellular mechanisms involved, and these are challenges for the future.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Design of proteolytically stable RI-anchoring disruptor peptidomimetics for in vivo studies of anchored type I protein kinase A-mediated signalling

We have reported previously the design of a RIAD (RI-anchoring disruptor) peptide that specifically displaces PKA (protein kinase A) type I from the AKAP (A-kinase-anchoring protein) ezrin, which is present in the immunological synapse of T-cells. This increases immune reactivity by reducing the threshold for activation and may prove a feasible approach for improving immune function in patients with cAMP-mediated T-cell dysfunction. However, the use of RIAD in biological systems is restricted by its susceptibility to enzymatic cleavage and, consequently, its short half-life in presence of the ubiquitous serum peptidases. In the present study, carefully selected non-natural amino acids were employed in the design of RIAD analogues with improved stability. The resulting peptidomimetics demonstrated up to 50-fold increased half-lives in serum compared with RIAD, while maintaining similar or improved specificity and potency with respect to disruption of PKA type I-AKAP interactions.