AKAP79-mediated targeting of the cyclic AMP-dependent protein kinase to the beta1-adrenergic receptor promotes recycling and functional resensitization of the receptor

Fine-tuning of GPCR activity by receptor-interacting proteins

G protein-coupled receptors (GPCRs) mediate physiological responses to various ligands, such as hormones, neurotransmitters and sensory stimuli. The signalling and trafficking properties of GPCRs are often highly malleable depending on the cellular context. Such fine-tuning of GPCR function can be attributed in many cases to receptor-interacting proteins that are differentially expressed in distinct cell types. In some cases these GPCR-interacting partners directly mediate receptor signalling, whereas in other cases they act mainly as scaffolds to modulate G protein-mediated signalling. Furthermore, GPCR-interacting proteins can have a big impact on the regulation of GPCR trafficking, localization and/or pharmacological properties.

A-kinase anchoring proteins: from protein complexes to physiology and disease

Protein scaffold complexes are a key mechanism by which a common signaling pathway can serve many different functions. Sequestering a signaling enzyme to a specific subcellular environment not only ensures that the enzyme is near its relevant targets, but also segregates this activity to prevent indiscriminate phosphorylation of other substrates. One family of diverse, well-studied scaffolding proteins are the A-kinase anchoring proteins (AKAPs). These anchoring proteins form multi-protein complexes that integrate cAMP signaling with other pathways and signaling events. In this review, we focus on recent advances in the elucidation of AKAP function.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

cAMP signal transduction in the heart: understanding spatial control for the development of novel therapeutic strategies

3'-5'-Cyclic adenosine monophosphate (cAMP) is a pleiotropic intracellular second messenger generated in response to activation of G(s) protein-coupled receptors. In the heart, cAMP mediates the catecholaminergic control on heart rate and contractility but, at the same time, it is responsible for the functional response to a wide variety of other hormones and neurotransmitters, raising the question of how the myocyte can decode the cAMP signal and generate the appropriate functional output to each individual extracellular stimulus. A growing body of evidence points to the spatial organization of the components of the cAMP signalling pathway in distinct, spatially segregated signalling domains as the key feature underpinning specificity of response and data is emerging, indicating that alteration of spatial control of the cAMP signal cascade associates with heart pathology. Most of the details of the molecular organization and regulation of individual cAMP signalling compartments are still to be elucidated but future research should provide the knowledge necessary to develop and test new therapeutic strategies that, by acting on a limited subset of downstream targets, would improve efficacy and minimize off-target effects.

Arachidonic acid, ARC channels, and Orai proteins

A critical role for arachidonic acid in the regulation of calcium entry during agonist activation of calcium signals has become increasingly apparent in numerous studies over the past 10 years or so. In particular, low concentrations of this fatty acid, generated as a result of physiologically relevant activation of appropriate receptors, induces the activation of a unique, highly calcium-selective conductance now known as the ARC channel. Activation of this channel is specifically dependent on arachidonic acid acting at the intracellular surface of the membrane, and is entirely independent of any depletion of internal calcium stores. Importantly, a specific role of this channel in modulating the frequency of oscillatory calcium signals in various cell types has been described. Recent studies, subsequent to the discovery of STIM1 and the Orai proteins and their role in the store-operated CRAC channels, have revealed that these same proteins are also integral components of the ARC channels and their activation. However, unlike the CRAC channels, activation of the ARC channels depends on the pool of STIM1 that is constitutively resident in the plasma membrane, and the pore of these channels is comprised of both Orai1 and Orai3 subunits. The clear implication is that CRAC channels and ARC channels are closely related, but have evolved to play unique roles in the modulation of calcium signals-largely as a result of their entirely distinct modes of activation. Given this, although the precise details of how arachidonic acid acts to activate the channels remain unclear, it seems likely that the specific molecular features of these channels that distinguish them from the CRAC channels--namely Orai3 and/or plasma membrane STIM1--will be involved.

A-kinase anchoring in dendritic cells is required for antigen presentation

Background

Dendritic cells (DC) are the most potent antigen presenting cells (APC) of the immune system. Prostaglandin E₂, cyclic AMP, and protein kinase A (PKA) have all been shown to regulate DC maturation and activity. In other cells, the ability of these molecules to convey their signals has been shown to be dependent on A-kinase anchoring proteins (AKAPs). Here we present evidence for the existence and functional importance of AKAPs in human DC.

Methodology/Principal Findings

Using immunofluorescence and/or western analyses we identify AKAP79, AKAP149, AKAP95, AKAP LBC and Ezrin. We also demonstrate by western analysis that expression of AKAP79, AKAP149 and RII are upregulated with DC differentiation and maturation. We establish the functional importance of PKA anchoring in multiple aspects of DC biology using the anchoring inhibitor peptides Ht31 and AKAP-IS. Incubation of protein or peptide antigen loaded DC with Ht31 or AKAP-IS results in a 30–50% decrease in antigen presentation as measured by IFN-γ production from antigen specific CD4⁺ T cells. Incubation of LPS treated DC with Ht31 results in 80% inhibition of TNF-α and IL-10 production. Ht31 slightly decreases the expression of CD18 and CD11a and CD11b, slightly increases the basal expression of CD83, dramatically decreases the LPS stimulated expression of CD40, CD80 and CD83, and significantly increases the expression of the chemokine receptor CCR7.

Conclusions

These experiments represent the first evidence for the functional importance of PKA anchoring in multiple aspects of DC biology.

A novel approach to syncopal patients: association analysis of polymorphisms in G-protein genes and tilt outcome

AIMS

G-proteins signal transduction pathways play a basic role in cardiovascular reflexes. We hypothesized that the predisposition to reflex-mediated syncope may be associated with genetic variations in G-protein genes. The aim of this study was to evaluate the effect of three single-nucleotide polymorphisms in G-protein genes on tilting outcome in syncopal patients.

METHODS AND RESULTS

A total of 217 syncopal patients free from any other disease were genotyped and examined related to tilting results. Genotyping was performed by polymerase chain reaction followed by restriction fragment length polymorphism in gene encoding the Gs-protein alpha-subunit (polymorphism C393T), the G-protein beta 3 subunit--GNB3 (polymorphism C825T)--and for the cardiac regulator of G-protein signalling RGS2 (polymorphism C1114G). In multivariate logistic regression analysis, the homozygotes 825TT GNB3 (OR 0.37; 95% CI 0.14-0.97; P < 0.05) and body mass index (OR 0.87; 95% CI 0.78-0.97; P = 0.005) were independently associated with a lower chance of positive tilting results. No relationship was found between Vasovagal Syncope International Study type of syncope and the studied genotypes or the carriage of the polymorphic alleles.

CONCLUSIONS

An association between tilting results and C825T GNB3 polymorphism in syncopal patients was found. The syncopal homozygotes 825TT GNB3 had a significantly lower chance of syncope during tilt testing.

The A-kinase anchoring protein Yotiao binds and regulates adenylyl cyclase in brain

A-kinase anchoring proteins (AKAPs) influence the spatial and temporal regulation of cAMP signaling events. Anchoring of PKA in proximity to certain adenylyl cyclase (AC) isoforms is thought to enhance the phosphorylation dependent termination of cAMP synthesis. Using a combination of immunoprecipitation and enzymological approaches, we show that the plasma membrane targeted anchoring protein AKAP9/Yotiao displays unique specificity for interaction and the regulation of a variety of AC isoforms. Yotiao inhibits AC 2 and 3, but has no effect on AC 1 or 9, serving purely as a scaffold for these latter isoforms. Thus, Yotiao represents an inhibitor of AC2. The N terminus of AC2 (AC2-NT), which binds directly to amino acids 808-957 of Yotiao, mediates this interaction. Additionally, AC2-NT and Yotiao (808-957) are able to effectively inhibit the association of AC2 with Yotiao and, thus, reverse the inhibition of AC2 by Yotiao in membranes. Finally, disruption of Yotiao-AC interactions gives rise to a 40% increase in brain AC activity, indicating that this anchoring protein functions to directly regulate cAMP production in the brain.

Mutation T/C,Ile 131 of the gene encoding the alfa subunit of the human Gs protein and predisposition to vasovagal syncope

BACKGROUND

Mutation T/C inside codon 131 of the gene encoding the alpha subunit of Gs protein (GNAS1) causes the increased activation of adenyl cyclase, which plays an important role in cardiovascular regulation. The aim of the present study was to evaluate GNAS1 T/C,Ile 131 mutation's manifestation in syncopal patients regarding head-up tilt test (HUTT) results.

METHODS AND RESULTS

In 137 syncopal patients (without any other diseases) the silent T/C,Ile 131 mutation within the GNAS1 codon on chromosome 20 q was identified. This mutation consists of the presence (+) or absence (-) of a target site for endonuclease FokI (Promega). Ninety-six patients (70%) with positive HUTT had a higher FokI+ allele frequency compared with those with negative tilting results (49% vs 27%, X(2)=12.05; p<0.001). In positive tilted patients, the studied mutation had significant influence on blood pressure (p<0.05). When comparing positive HUTT with vasodepressore component, cardioinhibition results and negative HUTT, the frequencies of the FokI+ allele were decreased among these groups: 53%, 36% and 27%, respectively.

CONCLUSIONS

An association between positive tilting and mutation C/T,Ile 131 within the GNAS1 codon was found. The predisposition to vasovagal syncope seems to be associated with the GNAS1 FokI+ allele.

Location, location, location...site-specific GPCR phosphorylation offers a mechanism for cell-type-specific signalling

It is now established that most of the approximately 800 G-protein-coupled receptors (GPCRs) are regulated by phosphorylation in a process that results in the recruitment of arrestins, leading to receptor desensitization and the activation of arrestin-dependent processes. This generalized view of GPCR regulation, however, does not provide an adequate mechanism for the control of tissue-specific GPCR signalling. Here, we review the evidence that GPCR phosphorylation is, in fact, a flexible and dynamic regulatory process in which GPCRs are phosphorylated in a unique manner that is associated with the cell type in which the receptor is expressed. In this scenario, phosphorylation offers a mechanism of regulating the signalling outcome of GPCRs that can be tailored to meet a specific physiological role.

Regulation of GPCRs by endocytic membrane trafficking and its potential implications

The endocytic pathway tightly controls the activity of G protein-coupled receptors (GPCRs). Ligand-induced endocytosis can drive receptors into divergent lysosomal and recycling pathways, producing essentially opposite effects on the strength and duration of cellular signaling via heterotrimeric G proteins, and may also promote distinct signaling events from intracellular membranes. This chapter reviews recent developments toward understanding the molecular machinery and functional implications of GPCR sorting in the endocytic pathway, focusing on mammalian GPCRs whose ligand-induced endocytosis is mediated primarily by clathrin-coated pits. Lysosomal sorting of a number of GPCRs occurs via a highly conserved mechanism requiring covalent tagging of receptors with ubiquitin. There is increasing evidence that additional, noncovalent mechanisms control the sorting of endocytosed GPCRs to lysosomes in mammalian cells. Recycling of several GPCRs to the plasma membrane is also specifically sorted, via a mechanism requiring both receptor-specific and shared sorting proteins. The current data reveal an unprecedented degree of specificity and plasticity in the cellular regulation of mammalian GPCRs by endocytic membrane trafficking. These developments have fundamental implications for GPCR pharmacology, and suggest new mechanisms that could be exploited in GPCR-directed pharmacotherapy.

Signaling from beta1- and beta2-adrenergic receptors is defined by differential interactions with PDE4

Beta1- and beta2-adrenergic receptors (betaARs) are highly homologous, yet they play clearly distinct roles in cardiac physiology and pathology. Myocyte contraction, for instance, is readily stimulated by beta1AR but not beta2AR signaling, and chronic stimulation of the two receptors has opposing effects on myocyte apoptosis and cell survival. Differences in the assembly of macromolecular signaling complexes may explain the distinct biological outcomes. Here, we demonstrate that beta1AR forms a signaling complex with a cAMP-specific phosphodiesterase (PDE) in a manner inherently different from a beta2AR/beta-arrestin/PDE complex reported previously. The beta1AR binds a PDE variant, PDE4D8, in a direct manner, and occupancy of the receptor by an agonist causes dissociation of this complex. Conversely, agonist binding to the beta2AR is a prerequisite for the recruitment of a complex consisting of beta-arrestin and the PDE4D variant, PDE4D5, to the receptor. We propose that the distinct modes of interaction with PDEs result in divergent cAMP signals in the vicinity of the two receptors, thus, providing an additional layer of complexity to enforce the specificity of beta1- and beta2-adrenoceptor signaling.

Insulin-like growth factor-I provokes functional antagonism and internalization of beta1-adrenergic receptors

Hormones that activate receptor tyrosine kinases have been shown to regulate G protein-coupled receptors, and herein we investigate the ability of IGF-I to regulate the beta(1)-adrenergic receptor. Treating Chinese hamster ovary cells in culture with IGF-I is shown to functionally antagonize the ability of expressed beta(1)-adrenergic receptors to accumulate intracellular cAMP in response to stimulation by the beta-adrenergic agonist Iso. The attenuation of beta(1)-adrenergic action was accompanied by internalization of beta(1)-adrenergic receptors in response to IGF-I. Inhibiting either phosphatidylinositol 3-kinase or the serine/threonine protein kinase Akt blocks the ability of IGF-I to antagonize and to internalize beta(1)-adrenergic receptors. Mutation of one potential Akt substrate site Ser412Ala, but not another Ser312Ala, of the beta(1)-adrenergic receptor abolishes the ability of IGF-I to functionally antagonize and to sequester the beta(1)-adrenergic receptor. We also tested the ability of IGF-I to regulate beta(1)-adrenergic receptors and their signaling in adult canine cardiac myocytes. IGF-I attenuates the ability of beta(1)-adrenergic receptors to accumulate intracellular cAMP in response to Iso and promotes internalization of beta(1)-adrenergic receptors in these cardiac myocytes.

Gene-trapped mouse embryonic stem cell-derived cardiac myocytes and human genetics implicate AKAP10 in heart rhythm regulation

Sudden cardiac death due to abnormal heart rhythm kills 400,000-460,000 Americans each year. To identify genes that regulate heart rhythm, we are developing a screen that uses mouse embryonic stem cells (mESCs) with gene disruptions that can be differentiated into cardiac cells for phenotyping. Here, we show that the heterozygous disruption of the Akap10 (D-AKAP2) gene that disrupts the final 51 aa increases the contractile response of cultured cardiac cells to cholinergic signals. In both heterozygous and homozygous mutant mice derived from these mESCs, the same Akap10 disruption increases the cardiac response to cholinergic signals, suggesting a dominant interfering effect of the Akap10 mutant allele. The mutant mice have cardiac arrhythmias and die prematurely. We also found that a common variant of AKAP10 in humans (646V, 40% of alleles) was associated with increased basal heart rate and decreased heart rate variability (markers of low cholinergic/vagus nerve sensitivity). These markers predict an increased risk of sudden cardiac death. Although the molecular mechanism remains unknown, our findings in mutant mESCs, mice, and a common human AKAP10 SNP all suggest a role for AKAP10 in heart rhythm control. Our stem cell-based screen may provide a means of identifying other genes that control heart rhythm.

Assembly of an SAP97-AKAP79-cAMP-dependent protein kinase scaffold at the type 1 PSD-95/DLG/ZO1 motif of the human beta(1)-adrenergic receptor generates a receptosome involved in receptor recycling and networking

Appropriate trafficking of the beta(1)-adrenergic receptor (beta(1)-AR) after agonist-promoted internalization is crucial for the resensitization of its signaling pathway. Efficient recycling of the beta(1)-AR required the binding of the protein kinase A anchoring protein-79 (AKAP79) to the carboxyl terminus of the beta(1)-AR (Gardner, L. A., Tavalin, S. A., Goehring, A., Scott, J. D., and Bahouth, S. W. (2006) J. Biol. Chem. 281, 33537-33553). In this study we show that AKAP79 forms a complex with the type 1 PDZ-binding sequence (ESKV) at the extreme carboxyl terminus of the beta(1)-AR, which is mediated by the membrane-associated guanylate kinase (MAGUK) protein SAP97. Thus, the PDZ and its associated SAP97-AKAP79 complex are involved in targeting the cyclic AMP-dependent protein kinase (PKA) to the beta(1)-AR. The PDZ and its scaffold were required for efficient recycling of the beta(1)-AR and for PKA-mediated phosphorylation of the beta(1)-AR at Ser(312). Overexpression of the catalytic subunit of PKA or mutagenesis of Ser(312) to the phosphoserine mimic aspartic acid both rescued the recycling of the trafficking-defective beta(1)-ARDelta PDZ mutant. Thus, trafficking signals transmitted from the PDZ-associated scaffold in the carboxyl terminus of the beta(1)-AR to Ser(312) in the 3rd intracellular loop (3rd IC) were paramount in setting the trafficking itinerary of the beta(1)-AR. The data presented here show that a novel beta(1)-adrenergic receptosome is organized at the beta(1)-AR PDZ to generate a scaffold essential for trafficking and networking of the beta(1)-AR.