beta 1-adrenergic receptor association with the synaptic scaffolding protein membrane-associated guanylate kinase inverted-2 (MAGI-2). Differential regulation of receptor internalization by MAGI-2 and PSD-95

Regulation of tissue factor activity by interaction with the first PDZ domain of MAGI1

BACKGROUND

Tissue factor (TF) activity is stringently regulated through processes termed encryption. Post-translational modification of TF and its interactions with various protein and lipid moieties allows for a multi-step de-encryption of TF and procoagulant activation. Membrane-associated guanylate kinase-with inverted configuration (MAGI) proteins are known to regulate the localisation and activity of a number of proteins including cell-surface receptors.

METHODS

The interaction of TF with MAGI1 protein was examined as a means of regulating TF activity. MDA-MB-231 cell line was used which express TF and MAGI1, and respond well to protease activated receptor (PAR)2 activation. Proximity ligation assay (PLA), co-immunoprecipitation and pull-down experiments were used to examine the interaction of TF with MAGI1-3 proteins and to investigate the influence of PAR2 activation. Furthermore, by cloning and expressing the PDZ domains from MAGI1, the TF-binding domain was identified. The ability of the recombinant PDZ domains to act as competitors for MAGI1, allowing the induction of TF procoagulant and signalling activity was then examined.

RESULTS

PLA and fluorescence microscopic analysis indicated that TF predominantly associates with MAGI1 and less with MAGI2 and MAGI3 proteins. The interaction of TF with MAGI1 was also demonstrated by both co-immunoprecipitation of TF with MAGI1, and co-immunoprecipitation of MAGI1 with TF. Moreover, activation of PAR2 resulted in reduction in the association of these two proteins. Pull-down assays using TF-cytoplasmic domain peptides indicated that the phosphorylation of Ser253 within TF prevents its association with MAGI1. Additionally, the five HA-tagged PDZ domains of MAGI1 were overexpressed separately, and the putative TF-binding domain was identified as PDZ1 domain. Expression of this PDZ domain in cells significantly augmented the TF activity measured both as thrombin-generation and also TF-mediated proliferative signalling.

CONCLUSIONS

Our data indicate a stabilising interaction between TF and the PDZ-1 domain of MAGI1 and demonstrate that the activation of PAR2 disrupts this interaction. The release of TF from MAGI1 appears to be an initial step in TF de-encryption, associated with increased TF-mediated procoagulant and signalling activities. This mechanism is also likely to lead to further interactions and modifications leading to further enhancement of procoagulant activity, or the release of TF.

Proteomic Changes in the Hippocampus after Repeated Explosive-Driven Blasts

Repeated blast-traumatic brain injury (blast-TBI) has been hypothesized to cause persistent and unusual neurological and psychiatric symptoms in service members returning from war zones. Blast-wave primary effects have been supposed to induce damage and molecular alterations in the brain. However, the mechanisms through which the primary effect of an explosive-driven blast wave generate brain lesions and induce brain consequences are incompletely known. Prior findings from rat brains exposed to two consecutive explosive-driven blasts showed molecular changes (hyperphosphorylated-Tau, AQP4, S100β, PDGF, and DNA-polymerase-β) that varied in magnitude and direction across different brain regions. We aimed to compare, in an unbiased manner, the proteomic profile in the hippocampus of double blast vs sham rats using mass spectrometry (MS). Data showed differences in up- and down-regulation for protein abundances in the hippocampus of double blast vs sham rats. Tandem mass tag (TMT)-MS results showed 136 up-regulated and 94 down-regulated proteins between the two groups (10.25345/C52B8VP0X). These TMT-MS findings revealed changes never described before in blast studies, such as increases in MAGI3, a scaffolding protein at cell-cell junctions, which were confirmed by Western blotting analyses. Due to the absence of behavioral and obvious histopathological changes as described in our previous publications, these proteomic data further support the existence of an asymptomatic blast-induced molecular altered status (ABIMAS) associated with specific protein changes in the hippocampus of rats repeatedly expsosed to blast waves generated by explosive-driven detonations.

A New Story of the Three Magi: Scaffolding Proteins and lncRNA Suppressors of Cancer

Scaffolding molecules exert a critical role in orchestrating cellular response through the spatiotemporal assembly of effector proteins as signalosomes. By increasing the efficiency and selectivity of intracellular signaling, these molecules can exert (anti/pro)oncogenic activities. As an archetype of scaffolding proteins with tumor suppressor property, the present review focuses on MAGI1, 2, and 3 (membrane-associated guanylate kinase inverted), a subgroup of the MAGUK protein family, that mediate networks involving receptors, junctional complexes, signaling molecules, and the cytoskeleton. MAGI1, 2, and 3 are comprised of 6 PDZ domains, 2 WW domains, and 1 GUK domain. These 9 protein binding modules allow selective interactions with a wide range of effectors, including the PTEN tumor suppressor, the β-catenin and YAP1 proto-oncogenes, and the regulation of the PI3K/AKT, the Wnt, and the Hippo signaling pathways. The frequent downmodulation of MAGIs in various human malignancies makes these scaffolding molecules and their ligands putative therapeutic targets. Interestingly, MAGI1 and MAGI2 genetic loci generate a series of long non-coding RNAs that act as a tumor promoter or suppressor in a tissue-dependent manner, by selectively sponging some miRNAs or by regulating epigenetic processes. Here, we discuss the different paths followed by the three MAGIs to control carcinogenesis.

Scribble co-operatively binds multiple α1D-adrenergic receptor C-terminal PDZ ligands

Many G protein-coupled receptors (GPCRs) are organized as dynamic macromolecular complexes in human cells. Unraveling the structural determinants of unique GPCR complexes may identify unique protein:protein interfaces to be exploited for drug development. We previously reported α_1D-adrenergic receptors (α_1D-ARs) – key regulators of cardiovascular and central nervous system function – form homodimeric, modular PDZ protein complexes with cell-type specificity. Towards mapping α_1D-AR complex architecture, biolayer interferometry (BLI) revealed the α_1D-AR C-terminal PDZ ligand selectively binds the PDZ protein scribble (SCRIB) with >8x higher affinity than known interactors syntrophin, CASK and DLG1. Complementary in situ and in vitro assays revealed SCRIB PDZ domains 1 and 4 to be high affinity α_1D-AR PDZ ligand interaction sites. SNAP-GST pull-down assays demonstrate SCRIB binds multiple α_1D-AR PDZ ligands via a co-operative mechanism. Structure-function analyses pinpoint R1110^PDZ4 as a unique, critical residue dictating SCRIB:α_1D-AR binding specificity. The crystal structure of SCRIB PDZ4 R1110G predicts spatial shifts in the SCRIB PDZ4 carboxylate binding loop dictate α_1D-AR binding specificity. Thus, the findings herein identify SCRIB PDZ domains 1 and 4 as high affinity α_1D-AR interaction sites, and potential drug targets to treat diseases associated with aberrant α_1D-AR signaling.

Scaffold Proteins: From Coordinating Signaling Pathways to Metabolic Regulation

Among their pleiotropic functions, scaffold proteins are required for the accurate coordination of signaling pathways. It has only been within the past 10 years that their roles in glucose homeostasis and metabolism have emerged. It is well appreciated that changes in the expression or function of signaling effectors, such as receptors or kinases, can influence the development of chronic diseases such as diabetes and obesity. However, little is known regarding whether scaffolds have similar roles in the pathogenesis of metabolic diseases. In general, scaffolds are often underappreciated in the context of metabolism or metabolic diseases. In the present review, we discuss various scaffold proteins and their involvement in signaling pathways related to metabolism and metabolic diseases. The aims of the present review were to highlight the importance of scaffold proteins and to raise awareness of their physiological contributions. A thorough understanding of how scaffolds influence metabolism could aid in the discovery of novel therapeutic approaches to treat chronic conditions, such as diabetes, obesity, and cardiovascular disease, for which the incidence of all continue to increase at alarming rates.

G Protein-Coupled Receptor Resensitization Paradigms

Cellular responses to extracellular milieu/environment are driven by cell surface receptors that transmit the signal into the cells resulting in a synchronized and measured response. The ability to provide such exquisite responses to changes in external environment is mediated by the tight and yet, deliberate regulation of cell surface receptor function. In this regard, the seven transmembrane G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that regulate responses like cardiac contractility, vision, and olfaction including platelet activation. GPCRs regulate these plethora of events through GPCR-activation, -desensitization, and -resensitization. External stimuli (ligands or agonists) activate GPCR initiating downstream signals. The activated GPCR undergoes inactivation or desensitization by phosphorylation and binding of β-arrestin resulting in diminution of downstream signals. The desensitized GPCRs are internalized into endosomes, wherein they undergo dephosphorylation or resensitization by protein phosphatase to be recycled back to the cell membrane as naïve GPCR ready for the next wave of stimuli. Despite the knowledge that activation, desensitization, and resensitization shoulder an equal role in maintaining GPCR function, major advances have been made in understanding activation and desensitization compared to resensitization. However, increasing evidence shows that resensitization is exquisitely regulated process, thereby contributing to the dynamic regulation of GPCR function. In recognition of these observations, in this chapter we discuss the key advances on the mechanistic underpinning that drive and regulate GPCR function with a focus on resensitization.

MAGI proteins can differentially regulate the signaling pathways of 5-HT2AR by enhancing receptor trafficking and PLC recruitment

MAGI proteins are Membrane-Associated Guanylate Kinase Inverted proteins that belong to the MAGUK family. They are scaffolding proteins that were shown to mediate the trafficking and signaling of various G protein-coupled receptors (GPCRs). They contain PDZ domains in their structure and many GPCRs interact with these proteins via the PDZ motifs on the carboxyl terminal end of a receptor. In a PDZ overlay assay performed with the carboxyl terminal tail of 5-HT_2AR, we were able to detect all three members of the MAGI subfamily, MAGI-1, MAGI-2 and MAGI-3 as interacting PDZ proteins. The PDZ motif of 5-HT_2AR consists of three amino acids; serine (S), cysteine (C) and valine (V). In this study, we characterize these 5-HT_2AR interactions with MAGI proteins. We first confirm the interaction using co-immunopricipitation and illustrate that the interaction is PDZ motif-dependent in human embryonic kidney (HEK 293) cells. We then assess the effects of overexpression and knockdown of the MAGI proteins on the internalization, trafficking and signaling of 5-HT_2AR. We find that knockdown of either MAGI-1 or MAGI-3 using siRNA results in a significant reduction in the internalization of 5-HT_2AR. As for signaling, we report here that MAGI proteins can modulate the signaling via the two transduction pathways that 5-HT_2AR can activate. We illustrate a significant effect of modulating MAGI proteins expression on 5-HT-stimulated IP formation. We demonstrate an enhancement in 5-HT_2AR-stimulated IP formation upon MAGI proteins overexpression. In addition, we show that knockdown of MAGI proteins with siRNA leads to a significant reduction in 5-HT_2AR-stimulated IP formation. Furthermore, we illustrate a significant increase in 5-HT-stimulated ERK1/2 phosphorylation upon MAGI proteins knockdown. Interestingly, this effect on ERK1/2 activation is PDZ motif-independent. We also suggest two possible mechanisms of regulation for the effect of MAGI proteins on 5-HT_2AR function. One mechanism involves the regulation of cell surface expression since we show that both MAGI-2 and MAGI-3 can enhance receptor trafficking to the plasma membrane when overexpressed in HEK 293 cells. The other mechanism points to regulation of second messengers in the signaling pathways. Specifically, we show that overexpression of any of the three MAGI proteins can enhance the recruitment of PLCβ3 to 5-HT_2AR. In addition, we report a negative effect for knocking down MAGI-3 on β-arrestin recruitment to the receptor and this effect is PDZ motif-independent. Taken together, our findings document distinct roles for the three MAGI proteins in regulating 5-HT_2AR trafficking and signaling and emphasize the importance of studying PDZ proteins and their interactions with GPCRs to regulate their function.

Barcoding of GPCR trafficking and signaling through the various trafficking roadmaps by compartmentalized signaling networks

Proper signaling by G protein coupled receptors (GPCR) is dependent on the specific repertoire of transducing, enzymatic and regulatory kinases and phosphatases that shape its signaling output. Activation and signaling of the GPCR through its cognate G protein is impacted by G protein-coupled receptor kinase (GRK)-imprinted "barcodes" that recruit β-arrestins to regulate subsequent desensitization, biased signaling and endocytosis of the GPCR. The outcome of agonist-internalized GPCR in endosomes is also regulated by sequence motifs or "barcodes" within the GPCR that mediate its recycling to the plasma membrane or retention and eventual degradation as well as its subsequent signaling in endosomes. Given the vast number of diverse sequences in GPCR, several trafficking mechanisms for endosomal GPCR have been described. The majority of recycling GPCR, are sorted out of endosomes in a "sequence-dependent pathway" anchored around a type-1 PDZ-binding module found in their C-tails. For a subset of these GPCR, a second "barcode" imprinted onto specific GPCR serine/threonine residues by compartmentalized kinase networks was required for their efficient recycling through the "sequence-dependent pathway". Mutating the serine/threonine residues involved, produced dramatic effects on GPCR trafficking, indicating that they played a major role in setting the trafficking itinerary of these GPCR. While endosomal SNX27, retromer/WASH complexes and actin were required for efficient sorting and budding of all these GPCR, additional proteins were required for GPCR sorting via the second "barcode". Here we will review recent developments in GPCR trafficking in general and the human β1-adrenergic receptor in particular across the various trafficking roadmaps. In addition, we will discuss the role of GPCR trafficking in regulating endosomal GPCR signaling, which promote biochemical and physiological effects that are distinct from those generated by the GPCR signal transduction pathway in membranes.

MAGI Proteins Regulate the Trafficking and Signaling of Corticotropin-Releasing Factor Receptor 1 via a Compensatory Mechanism

Corticotropin-releasing factor (CRF) receptor1 (CRFR1) is associated with psychiatric illness and is a proposed target for the treatment of anxiety and depression. Similar to many G protein-coupled receptors (GPCRs), CRFR1 harbors a PDZ (PSD-95/Disc Large/Zona Occludens)-binding motif at the end of its carboxyl-terminal tail. The interactions of PDZ proteins with GPCRs are crucial for the regulation of receptor function. In the present study, we characterize the interaction of all members of the membrane-associated guanylate kinase with inverted orientation PDZ (MAGI) proteins with CRFR1. We show using co-immunoprecipitation that CRFR1 interacts with MAGI-1 and MAGI-3 in human embryonic kidney (HEK293) cells in a PDZ motif-dependent manner. We find that overexpression as well as knockdown of MAGI proteins result in a significant reduction in CRFR1 endocytosis. This effect is dependent on an intact PDZ binding motif for MAGI-2 and MAGI-3 but not MAGI-1. We show that the alteration in expression levels of MAGI-1, MAGI-2 or MAGI-3 can interfere with β-arrestin recruitment to CRFR1. This could explain the effects observed with receptor internalization. We also find that knockdown of endogenous MAGI-1, MAGI-2 or MAGI-3 in HEK293 cells can lead to an enhancement in ERK1/2 signaling but has no effect on cAMP formation. Interestingly, we observe a compensation effect between MAGI-1 and MAGI-3. Taken together, our data suggest that the MAGI proteins, MAGI-1, MAGI-2 and MAGI-3 can regulate β-arrestin-mediated internalization of CRFR1 as well as its signaling and that there is a compensatory mechanism involved in regulating the function of the MAGI subfamily.

PSD-95 regulates CRFR1 localization, trafficking and β-arrestin2 recruitment

Corticotropin-releasing factor (CRF) is a neuropeptide commonly associated with the hypothalamic-pituitary adrenal axis stress response. Upon release, CRF activates two G protein-coupled receptors (GPCRs): CRF receptor 1 (CRFR1) and CRF receptor 2 (CRFR2). Although both receptors contribute to mood regulation, CRFR1 antagonists have demonstrated anxiolytic and antidepressant-like properties that may be exploited in the generation of new pharmacological interventions for mental illnesses. Previous studies have demonstrated CRFR1 capable of heterologously sensitizing serotonin 2A receptor (5-HT2AR) signaling: another GPCR implicated in psychiatric disease. Interestingly, this phenomenon was dependent on Postsynaptic density 95 (PSD-95)/Disc Large/Zona Occludens (PDZ) interactions on the distal carboxyl termini of both receptors. In the current study, we demonstrate that endogenous PSD-95 can be co-immunoprecipitated with CRFR1 from cortical brain homogenate, and this interaction appears to be primarily via the PDZ-binding motif. Additionally, PSD-95 colocalizes with CRFR1 within the dendritic projections of cultured mouse neurons in a PDZ-binding motif-dependent manner. In HEK 293 cells, PSD-95 overexpression inhibited CRFR1 endocytosis, whereas PSD-95 shRNA knockdown enhanced CRFR1 endocytosis. Although PSD-95 does not appear to play a significant role in CRF-mediated cAMP or ERK1/2 signaling, PSD-95 was demonstrated to suppress β-arrestin2 recruitment: providing a potential mechanism for PSD-95's inhibition of endocytosis. In revisiting previously documented heterologous sensitization, PSD-95 shRNA knockdown did not prevent CRFR1-mediated enhancement of 5-HT2AR signaling. In conclusion, we have identified and characterized a novel functional relationship between CRFR1 and PSD-95 that may have implications in the design of new treatment strategies for mental illness.

Dynamic mass redistribution reveals diverging importance of PDZ-ligands for G protein-coupled receptor pharmacodynamics

G protein-coupled receptors (GPCRs) are essential membrane proteins that facilitate cell-to-cell communication and co-ordinate physiological processes. At least 30 human GPCRs contain a Type I PSD-95/DLG/Zo-1 (PDZ) ligand in their distal C-terminal domain; this four amino acid motif of X-[S/T]-X-[φ] sequence facilitates interactions with PDZ domain-containing proteins. Because PDZ protein interactions have profound effects on GPCR ligand pharmacology, cellular localization, signal-transduction effector coupling and duration of activity, we analyzed the importance of Type I PDZ ligands for the function of 23 full-length and PDZ-ligand truncated (ΔPDZ) human GPCRs in cultured human cells. SNAP-epitope tag polyacrylamide gel electrophoresis revealed most Type I PDZ GPCRs exist as both monomers and multimers; removal of the PDZ ligand played minimal role in multimer formation. Additionally, SNAP-cell surface staining indicated removal of the PDZ ligand had minimal effects on plasma membrane localization for most GPCRs examined. Label-free dynamic mass redistribution functional responses, however, revealed diverging effects of the PDZ ligand. While no clear trend was observed across all GPCRs tested or even within receptor families, a subset of GPCRs displayed diminished agonist efficacy in the absence of a PDZ ligand (i.e. HT2RB, ADRB1), whereas others demonstrated enhanced agonist efficacies (i.e. LPAR2, SSTR5). These results demonstrate the utility of label-free functional assays to tease apart the contributions of conserved protein interaction domains for GPCR signal-transduction coupling in cultured cells.

Role of cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) in regulating the trafficking and signaling of corticotropin-releasing factor receptor 1

Corticotropin releasing factor (CRF) receptor1 (CRFR1) is associated with psychiatric illness and is a proposed target for the treatment of anxiety and depression. Like many G protein-coupled receptors (GPCRs), CRFR1 harbors a PDZ (PSD95/Disc Large/Zona Occludens 1)-binding motif at the end of its carboxyl terminal tail. The interactions of PDZ proteins with GPCRs are crucial for the regulation of their receptor function. In the present study, we characterize the interaction of the cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) with CRFR1. We show using co-immunoprecipitation that the two proteins interact in human embryonic kidney (HEK293) cells in a PDZ motif-dependent manner. We find that the interaction occurs at the Golgi apparatus and that overexpression of CAL retains a proportion of CRFR1 in the intracellular compartment and prevents trafficking to the cell surface. We also demonstrate a significant reduction in the levels of receptor at the plasma membrane upon CAL overexpression, as well as a reduction in internalization. We find that the overexpression of CAL in HEK293 cells resulted in a significant decrease in CRF-stimulated extracellular-regulated protein kinase 1/2 (ERK1/2) phosphorylation, but has no effect on cAMP signaling mediated by the receptor. This effect was dependent on an intact PDZ motif and knockdown of CAL expression using CAL siRNA results in a significant enhancement in ERK1/2 signaling. We show that CAL contributes to the regulation of CRFR1 glycosylation and utilize glycosylation-deficient CRFR1 mutants to further examine the role of glycosylation in the cell surface trafficking of CRFR1. We find that the mutation of Asn residues 90 and 98 results in a reduction in cell surface CRFR1 that is comparable to the effect of CAL overexpression and that these mutants are retained in the Golgi apparatus. Mutation of Asn residues 90 and 98 also results in a decrease in the efficacy for CRF-stimulated cAMP formation mediated by CRFR1. Taken together, our data suggest that CAL can regulate the anterograde trafficking, the internalization as well as the signaling of CRFR1 via modulating the post-translational modifications that the receptor undergoes at the Golgi apparatus.

Using Gene Ontology to describe the role of the neurexin-neuroligin-SHANK complex in human, mouse and rat and its relevance to autism

Background

People with an autistic spectrum disorder (ASD) display a variety of characteristic behavioral traits, including impaired social interaction, communication difficulties and repetitive behavior. This complex neurodevelopment disorder is known to be associated with a combination of genetic and environmental factors. Neurexins and neuroligins play a key role in synaptogenesis and neurexin-neuroligin adhesion is one of several processes that have been implicated in autism spectrum disorders.

Results

In this report we describe the manual annotation of a selection of gene products known to be associated with autism and/or the neurexin-neuroligin-SHANK complex and demonstrate how a focused annotation approach leads to the creation of more descriptive Gene Ontology (GO) terms, as well as an increase in both the number of gene product annotations and their granularity, thus improving the data available in the GO database.

Conclusions

The manual annotations we describe will impact on the functional analysis of a variety of future autism-relevant datasets. Comprehensive gene annotation is an essential aspect of genomic and proteomic studies, as the quality of gene annotations incorporated into statistical analysis tools affects the effective interpretation of data obtained through genome wide association studies, next generation sequencing, proteomic and transcriptomic datasets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0622-0) contains supplementary material, which is available to authorized users.

Minireview: Role of intracellular scaffolding proteins in the regulation of endocrine G protein-coupled receptor signaling

The majority of hormones stimulates and mediates their signal transduction via G protein-coupled receptors (GPCRs). The signal is transmitted into the cell due to the association of the GPCRs with heterotrimeric G proteins, which in turn activates an extensive array of signaling pathways to regulate cell physiology. However, GPCRs also function as scaffolds for the recruitment of a variety of cytoplasmic protein-interacting proteins that bind to both the intracellular face and protein interaction motifs encoded by GPCRs. The structural scaffolding of these proteins allows GPCRs to recruit large functional complexes that serve to modulate both G protein-dependent and -independent cellular signaling pathways and modulate GPCR intracellular trafficking. This review focuses on GPCR interacting PSD95-disc large-zona occludens domain containing scaffolds in the regulation of endocrine receptor signaling as well as their potential role as therapeutic targets for the treatment of endocrinopathies.

Regulation of β2-adrenergic receptor cell surface expression by interaction with cystic fibrosis transmembrane conductance regulator-associated ligand (CAL)

The beta-2 adrenergic receptor (β2AR), a member of GPCR, can activate multiple signaling pathways and is an important treatment target for cardiac failure. However, the molecular mechanism about β2AR signaling regulation is not fully understood. In this study, we found that cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) overexpression reduced β2AR-mediated extracellular signal-regulated kinase-1/2 (ERK1/2) activation. Further study identified CAL as a novel binding partner of β2AR. CAL is associated with β2AR mainly via the third intracellular loop (ICL3) of receptor and the coiled-coil domains of CAL, which is distinct from CAL/β1AR interaction mediated by the carboxyl terminal (CT) of β1AR and PDZ domain of CAL. CAL overexpression retarded β2AR expression in Golgi apparatus and reduced the receptor expression in plasma membrane.

PDZK1/NHERF3 differentially regulates corticotropin-releasing factor receptor 1 and serotonin 2A receptor signaling and endocytosis

The corticotropin-releasing factor receptor 1 (CRFR1) and serotonin 2A receptor (5-HT2AR) are linked to cellular mechanisms underlying stress anxiety and depression. Both receptors are members of the G protein-coupled receptor (GPCR) superfamily and encode class I PSD-95/DiscsLarge/Zona Occludens 1 (PDZ) binding motifs (-S/T-x-V/I/L) at the end of their carboxyl-terminal tails. We have identified PDZK1, also referred to as Na(+)/H(+) exchange regulatory cofactor 3 (NHERF3) as both a CRFR1- and 5-HT2AR-interacting protein. We have examined whether PDZK1 plays a role in regulating both CRFR1 and 5-HT2AR activity. We find that while PDZK1 interactions with CRFR1 are PDZ binding motif-dependent, PDZK1 associates with 5-HT2AR in a PDZ binding motif-independent manner and CRFR1 expression, but not 5-HT2AR expression, redistributes PDZK1 to the plasma membrane in PDZ binding motif-dependent manner. PDZK1, negatively regulates 5-HT2AR endocytosis and has no effect upon 5-HT2AR-mediated ERK1/2 phosphorylation. In contrast, PDZK1 overexpression does not affect CRFR1 endocytosis, but selectively increases CRFR1-stimulated ERK1/2 phosphorylation. Similar to what has been previously reported for PSD-95 and SAP97, PDZK1 positively influences 5-HT2AR-stimulated inositol phosphate formation, but does not contribute to the regulation of CRFR1-mediated cAMP signaling. Taken together, these results indicate that PDZK1 differentially regulates the signaling and trafficking of CRFR1 and 5-HT2AR via PDZ-dependent and -independent mechanisms, respectively.

Homo- and hetero-oligomerization of β2-adrenergic receptor in receptor trafficking, signaling pathways and receptor pharmacology

The β2-adrenergic receptor (β2AR) is the prototypic member of G protein-coupled receptors (GPCRs) involved in the production of physiological responses to adrenaline and noradrenaline. Research done in the past few years vastly demonstrated that β2AR can form homo- and hetero-oligomers. Despite the fact that currently this phenomenon is widely accepted, the spread and relevance of β2AR oligomerization are still a matter of debate. This review considers the progress achieved in the field of β2AR oligomerization with focus on the implications of the receptor-receptor interactions to β2AR trafficking, pharmacology and downstream signal transduction pathways.

Phosphorylation of the Usher syndrome 1G protein SANS controls Magi2-mediated endocytosis

The human Usher syndrome (USH) is a complex ciliopathy with at least 12 chromosomal loci assigned to three clinical subtypes, USH1-3. The heterogeneous USH proteins are organized into protein networks. Here, we identified Magi2 (membrane-associated guanylate kinase inverted-2) as a new component of the USH protein interactome, binding to the multifunctional scaffold protein SANS (USH1G). We showed that the SANS-Magi2 complex assembly is regulated by the phosphorylation of an internal PDZ-binding motif in the sterile alpha motif domain of SANS by the protein kinase CK2. We affirmed Magi2's role in receptor-mediated, clathrin-dependent endocytosis and showed that phosphorylated SANS tightly regulates Magi2-mediated endocytosis. Specific depletions by RNAi revealed that SANS and Magi2-mediated endocytosis regulates aspects of ciliogenesis. Furthermore, we demonstrated the localization of the SANS-Magi2 complex in the periciliary membrane complex facing the ciliary pocket of retinal photoreceptor cells in situ. Our data suggest that endocytotic processes may not only contribute to photoreceptor cell homeostasis but also counterbalance the periciliary membrane delivery accompanying the exocytosis processes for the cargo vesicle delivery. In USH1G patients, mutations in SANS eliminate Magi2 binding and thereby deregulate endocytosis, lead to defective ciliary transport modules and ultimately disrupt photoreceptor cell function inducing retinal degeneration.

MiR-134/487b/655 cluster regulates TGF-β-induced epithelial-mesenchymal transition and drug resistance to gefitinib by targeting MAGI2 in lung adenocarcinoma cells

Epithelial-mesenchymal transition (EMT) has recently been recognized as a key element of cell invasion, migration, metastasis, and drug resistance in several types of cancer, including non-small cell lung cancer (NSCLC). Our aim was to clarify microRNA (miRNA)-related mechanisms underlying EMT followed by acquired resistance to epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) in NSCLC. miRNA expression profiles were examined before and after transforming growth factor β1 (TGF-β1) exposure in four human adenocarcinoma cell lines with or without EMT. Correlation between expressions of EMT-related miRNAs and resistance to EGFR-TKI gefitinib was evaluated. miRNA array and real-time quantitative reverse transcription PCR (qRT-PCR) revealed that TGF-β1 significantly induced overexpression of miR-134, miR-487b, and miR-655, which belong to the same cluster located on chromosome 14q32, in lung adenocarcinoma cells with EMT. MAGI2 (membrane-associated guanylate kinase, WW, and PDZ domain-containing protein 2), a predicted target of these miRNAs and a scaffold protein required for PTEN, was diminished in A549 cells with EMT after the TGF-β1 stimulation. Overexpression of miR-134 and miR-487b promoted the EMT phenomenon and affected the drug resistance to gefitinib, whereas knockdown of these miRNAs inhibited the EMT process and reversed TGF-β1-induced resistance to gefitinib. Our study demonstrated that the miR-134/487b/655 cluster contributed to the TGF-β1-induced EMT phenomenon and affected the resistance to gefitinib by directly targeting MAGI2, in which suppression subsequently caused loss of PTEN stability in lung cancer cells. The miR-134/miR-487b/miR-655 cluster may be a new therapeutic target in patients with advanced lung adenocarcinoma, depending on the EMT phenomenon.

β1-adrenergic receptor recycles via a membranous organelle, recycling endosome, by binding with sorting nexin27

In cardiomyocytes, β1-adrenergic receptor (β1-AR) plays an important role in regulating cardiac functions. Upon continuous ligand stimulation, β1-AR is internalized and mostly recycled back to the plasma membrane (PM). The recycling endosome (RE) is one of the membranous organelles involved in the protein recycling pathway. To determine whether RE is involved in the internalization of β1-AR upon ligand stimulation, we evaluated the localization of β1-AR after stimulation with a β-agonist, isoproterenol (Iso), in β1-AR-transfected COS-1 cells. After 30 min of Iso treatment and cell surface labeling with the appropriate antibodies, β1-AR was internalized from PM and translocated into the perinuclear region, the same location as the transferrin receptor, an RE marker. We then evaluated whether sorting nexin 27 (SNX27) participated in the β1-AR recycling pathway. When β1-AR and SNX27 were coexpressed, β1-AR coimmunoprecipitated with SNX27. In addition, shRNA-mediated silencing of SNX27 compromised β1-AR recycling and enhanced its delivery into lysosome. Overall, β1-AR on PM was internalized into RE upon Iso stimulation and recycled by RE through binding with SNX27 in COS-1 cells.

MAGI2 enhances the sensitivity of BEL-7404 human hepatocellular carcinoma cells to staurosporine-induced apoptosis by increasing PTEN stability

Adaptor proteins are involved in the assembly of various intracellular complexes and the regulation of cellular functions. Membrane-associated guanylate kinase inverted 2 (MAGI2), also known as synaptic scaffolding molecule (S-SCAM), plays a critical role in signal transduction by assembling and anchoring its ligands. However, the role of MAGI2 in mediating apoptosis remains largely unknown. In the present study, BEL-7404 human hepatocellular carcinoma cells were transfected with a plasmid containing myc-MAGI2 or an empty plasmid and cell viability was then determined using the Cell Counting kit-8. Apoptosis was also detected using an Annexin V apoptosis assay. The cells were then treated with various doses of staurosporine (STS) for different periods of time. The overexpression of myc-MAGI2 was found to sensitize the BEL-7404 cells to apoptosis in response to STS in a time- and dose-dependent manner. Our results demonstrated that MAGI2 enhanced STS-induced apoptosis by increasing the protein expression of cytoplasmic phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and decreasing its protein degradation. The apoptotic sensitivity of the cells caused by the overexpression of myc-MAGI2 was reversed by the silencing of PTEN expression by PTEN siRNA, thus revealing a momentous role of PTEN in the enhancement of the sensitivity of cancer cells to STS-induced apoptosis by MAGI2. Finally, we observed that the MAGI-PTEN complex triggered by MAGI2 overexpression reduced the phosphorylation levels of AKT. These results suggest that MAGI2 overexpression enhances the sensitivity of cancer cells harboring ectopic PTEN to STS-induced apoptosis.

Role of SAP97 protein in the regulation of corticotropin-releasing factor receptor 1 endocytosis and extracellular signal-regulated kinase 1/2 signaling

The corticotropin-releasing factor (CRF) receptor 1 (CRFR1) is a target for the treatment of psychiatric diseases such as depression, schizophrenia, anxiety disorder, and bipolar disorder. The carboxyl-terminal tail of the CRFR1 terminates in a PDZ-binding motif that provides a potential site for the interaction of PSD-95/Discs Large/Zona Occludens 1 (PDZ) domain-containing proteins. In this study, we found that CRFR1 interacts with synapse-associated protein 97 (SAP97; also known as DLG1) by co-immunoprecipitation in human embryonic 293 (HEK 293) cells and cortical brain lysates and that this interaction is dependent upon an intact PDZ-binding motif at the end of the CRFR1 carboxyl-terminal tail. Similarly, we demonstrated that SAP97 is recruited to the plasma membrane in HEK 293 cells expressing CRFR1 and that mutation of the CRFR1 PDZ-binding motif results in the redistribution of SAP97 into the cytoplasm. Overexpression of SAP97 antagonized agonist-stimulated CRFR1 internalization, whereas single hairpin (shRNA) knockdown of endogenous SAP97 in HEK 293 cells resulted in increased agonist-stimulated CRFR1 endocytosis. CRFR1 was internalized as a complex with SAP97 resulting in the redistribution of SAP97 to endocytic vesicles. Overexpression or shRNA knockdown of SAP97 did not significantly affect CRFR1-mediated cAMP formation, but SAP97 knockdown did significantly attenuate CRFR1-stimulated ERK1/2 phosphorylation in a PDZ interaction-independent manner. Taken together, our studies show that SAP97 interactions with CRFR1 attenuate CRFR1 endocytosis and that SAP97 is involved in coupling G protein-coupled receptors to the activation of the ERK1/2 signaling pathway.

Identification of PDZ Domain Containing Proteins Interacting with 1.2 and PMCA4b

PDZ (PSD-95/Disc large/Zonula occludens-1) protein interaction domains bind to cytoplasmic protein C-termini of transmembrane proteins. In order to identify new interaction partners of the voltage-gated L-type Ca2+ channel 1.2 and the plasma membrane Ca2+ ATPase 4b (PMCA4b), we used PDZ domain arrays probing for 124 PDZ domains. We confirmed this by GST pull-downs and immunoprecipitations. In PDZ arrays, strongest interactions with 1.2 and PMCA4b were found for the PDZ domains of SAP-102, MAST-205, MAGI-1, MAGI-2, MAGI-3, and ZO-1. We observed binding of the 1.2 C-terminus to PDZ domains of NHERF1/2, Mint-2, and CASK. PMCA4b was observed to interact with Mint-2 and its known interactions with Chapsyn-110 and CASK were confirmed. Furthermore, we validated interaction of 1.2 and PMCA4b with NHERF1/2, CASK, MAST-205 and MAGI-3 via immunoprecipitation. We also verified the interaction of 1.2 and nNOS and hypothesized that nNOS overexpression might reduce Ca2+ influx through 1.2. To address this, we measured Ca2+ currents in HEK 293 cells co-expressing 1.2 and nNOS and observed reduced voltage-dependent 1.2 activation. Taken together, we conclude that 1.2 and PMCA4b bind promiscuously to various PDZ domains, and that our data provides the basis for further investigation of the physiological consequences of these interactions.

Agonist-induced receptor internalization in Chinese hamster ovary cells stably co-expressing β(1)- and β(2)-adrenergic receptors

β(1)- and β(2)-Adrenergic receptors (β(1)-AR and β(2)-AR) are co-expressed in numerous tissues, for example, heart and bladder. They play a very important role in the responses of a variety of organs to sympathetic nerve stimulation. Recent studies suggest that many G protein-coupled receptors, such as β(1)-AR, β(2)-AR, μ opioid receptor and δ opioid receptor, can form homo- and heterooligomers. Previous studies demonstrated that the β(1)-AR and β(2)-AR formed dimers in living HEK 293 cells. The aim of the present study is to investigate whether such heterooligomerization affect the agonist-induced receptor internalization in the CHO-K1 cells stably co-expressing β(1)-AR and β(2)-AR. Using co-immunoprecipitation, we confirmed that β(1)-AR and β(2)-AR formed heterooligomers in the CHO-K1 cells. In cells co-expressing β(1)-AR and β(2)-AR, 30% of β(1)-AR was internalized by isoproterenol, whereas only 20% of β(1)-AR was internalized in cells expressing the β(1)-AR alone. Heterooligomerization did not affect the ratio of internalized β(2)-AR. Salmeterol, a specific β(2)-AR agonist, broke β(1)-AR/β(2)-AR heterooligomers, and induced β(2)-AR-specific internalization in cells co-expressing β(1)-AR and β(2)-AR. The present study demonstrated that heterooligomerization between β(1)-AR and β(2)-AR accelerates the isoproterenol-promoted internalization of the β(1)-AR, and that salmeterol induces β(2)-AR-specific internalization in Chinese hamster ovary (CHO) cells stably co-expressing β(1)-AR and β(2)-AR.

Mammalian α arrestins link activated seven transmembrane receptors to Nedd4 family e3 ubiquitin ligases and interact with β arrestins

The complement of fungal cell surface proteins is widely regulated by ubiquitination of membrane proteins, which results in their endocytosis and vacuolar degradation. For diverse fungal transporters, the specificity of ubiquitination is conferred by alpha arrestin adaptors, which recruit the Nedd4 family E3 ubiquitin ligase Rsp5. A recent study showed that one mammalian alpha arrestin also mediates ubiquitination and lysosomal trafficking of an activated plasma membrane receptor. Here we first screen all five widely-expressed human alpha arrestins for subcellular localization in ligand-stimulated and -unstimulated cells overexpressing the seven transmembrane receptor vasopressin 2. We then characterize the effects of alpha arrestins ARRDC3 and ARRDC4 upon activation of the seven transmembrane receptors vasopressin 2 and beta adrenergic 2. Using biochemical and imaging approaches, we show that ligand-activated receptors interact with alpha arrestins, and this results in recruitment of Nedd4 family E3 ubiquitin ligases and receptor ubiquitination - which are known to result in lysosomal trafficking. Our time course studies show these effects occur in the first 1-5 minutes after ligand activation, the same time that beta arrestins are known to have roles in receptor endocytic trafficking and kinase signaling. We tested the possibility that alpha and beta arrestins function coordinately and found co-immunoprecipitation and colocalization evidence to support this. Others recently reported that Arrdc3 knockout mice are lean and resistant to obesity. In the course of breeding our own Arrdc3-deficient mice, we observed two novel phenotypes in homozygotes: skin abnormalities, and embryonic lethality on normal chow diet, but not on high fat diet. Our findings suggest that alpha and beta arrestins function coordinately to maintain the optimal complement and function of cell surface proteins according to cellular physiological context and external signals. We discuss the implications of the alpha arrestin functions in fungi having evolved into coordinated alpha/beta arrestin functions in animals.

Protein-protein interactions at the adrenergic receptors

The adrenergic receptors are among the best characterized G protein-coupled receptors (GPCRs) and knowledge on this receptor family has provided several important paradigms about GPCR function and regulation. One of the most recent paradigms initially supported by studies on adrenergic receptors is that both βarrestins and G protein-coupled receptors themselves can act as scaffolds binding a variety of proteins and this can result in growing complexity of the receptor-mediated cellular effects. In this review we will briefly summarize the main features of βarrestin binding to the adrenergic receptor subtypes and we will review more in detail the main proteins found to selectively interact with distinct AR subtype. At the end, we will review the main findings on oligomerization of the AR subtypes.

Regulation of GPCR activity, trafficking and localization by GPCR-interacting proteins

GPCRs represent the largest family of integral membrane proteins and were first identified as receptor proteins that couple via heterotrimeric G-proteins to regulate a vast variety of effector proteins to modulate cellular function. It is now recognized that GPCRs interact with a myriad of proteins that not only function to attenuate their signalling but also function to couple these receptors to heterotrimeric G-protein-independent signalling pathways. In addition, intracellular and transmembrane proteins associate with GPCRs and regulate their processing in the endoplasmic reticulum, trafficking to the cell surface, compartmentalization to plasma membrane microdomains, endocytosis and trafficking between intracellular membrane compartments. The present review will overview the functional consequence of β-arrestin, receptor activity-modifying proteins (RAMPS), regulators of G-protein signalling (RGS), GPCR-associated sorting proteins (GASPs), Homer, small GTPases, PSD95/Disc Large/Zona Occludens (PDZ), spinophilin, protein phosphatases, calmodulin, optineurin and Src homology 3 (SH3) containing protein interactions with GPCRs.

Compartmentalization of beta-adrenergic signals in cardiomyocytes

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

Modulation of monoamine receptors by adaptor proteins and lipid rafts: role in some effects of centrally acting drugs and therapeutic agents

The monoamines and their cognate receptors are widespread in the central nervous system and are vital for normal brain function. Dysfunction in these systems underlies several psychiatric and neurological disease states, and consequently monoamines are targets of a host of pharmacotherapies. This review provides an overview on how monoamine receptors are regulated by adaptor proteins and lipid rafts with emphasis on interactions in nerve cells. Monoamine receptors have prominent intracellular loops that provide binding sites for adaptor proteins. Receptor function is further modulated by cholesterol and submembranous microdomains termed lipid rafts. These interactions determine several facets of G protein-coupled receptor (GPCR) function including trafficking, localization, and signaling. Possible roles of adaptor proteins and lipid rafts in disease states and in mediating actions of drugs and therapeutic agents are also discussed.

Receptor specific crosstalk and modulation of signaling upon heterodimerization between β1-adrenergic receptor and somatostatin receptor-5

In the present study we describe heterodimerization, trafficking, coupling to adenylyl cyclase and signaling in HEK-293 cells cotransfected with human-somatostatin receptor 5 (hSSTR5) and β(1)-adrenergic receptor (β(1)AR). hSSTR5/β(1)AR exists as heterodimers in basal conditions which was further enhanced upon synergistic activation of both receptors. Activation of either β(1)AR or hSSTR5 displayed dissociation of heterodimerization. In cotransfectants, β(1)AR effect on cAMP was predominant; however, blocking β(1)AR with antagonist resulted in 60% inhibition of forskolin-stimulated cAMP in the presence of hSSTR5 agonists. cAMP/PKA pathway in cotransfected cells was regulated in receptor-specific manner, in contrast, the status of pERK1/2 and pPI3K/AKT was predominantly regulated by hSSTR5. The expression levels of phosphorylated NFAT remained unchanged indicating blockade of calcineurin-mediated dephosphorylation and nuclear translocation of NFAT, the process predominantly regulated by pJNK in SSTR5 dependent manner. Taken together, the functional consequences of results described here might have relevance in the cardiovascular system where SSTR and AR subtypes play important roles.

Regulation of cardiac inward rectifier potassium current (I(K1)) by synapse-associated protein-97

Synapse-associated protein-97 (SAP97) is a membrane-associated guanylate kinase scaffolding protein expressed in cardiomyocytes. SAP97 has been shown to associate and modulate voltage-gated potassium (Kv) channel function. In contrast to Kv channels, little information is available on interactions involving SAP97 and inward rectifier potassium (Kir2.x) channels that underlie the classical inward rectifier current, I(K1). To investigate the functional effects of silencing SAP97 on I(K1) in adult rat ventricular myocytes, SAP97 was silenced using an adenoviral short hairpin RNA vector. Western blot analysis showed that SAP97 was silenced by approximately 85% on day 3 post-infection. Immunostaining showed that Kir2.1 and Kir2.2 co-localize with SAP97. Co-immunoprecipitation (co-IP) results demonstrated that Kir2.x channels associate with SAP97. Voltage clamp experiments showed that silencing SAP97 reduced I(K1) whole cell density by approximately 55%. I(K1) density at -100 mV was -1.45 +/- 0.15 pA/picofarads (n = 6) in SAP97-silenced cells as compared with -3.03 +/- 0.37 pA/picofarads (n = 5) in control cells. Unitary conductance properties of I(K1) were unaffected by SAP97 silencing. The major mechanism for the reduction of I(K1) density appears to be a decrease in Kir2.x channel abundance. Furthermore, SAP97 silencing impaired I(K1) regulation by beta(1)-adrenergic receptor (beta1-AR) stimulation. In control, isoproterenol reduced I(K1) amplitude by approximately 75%, an effect that was blunted following SAP97 silencing. Our co-IP data show that beta1-AR associates with SAP97 and Kir2.1 and also that Kir2.1 co-IPs with protein kinase A and beta1-AR. SAP97 immunolocalizes with protein kinase A and beta1-AR in the cardiac myocytes. Our results suggest that in cardiac myocytes SAP97 regulates surface expression of channels underlying I(K1), as well as assembles a signaling complex involved in beta1-AR regulation of I(K1).

Beta-2 adrenergic receptor mediated ERK activation is regulated by interaction with MAGI-3

The beta-2 adrenergic receptor (beta2AR) has a carboxyl terminus motif that can interact with PSD-95/discs-large/ZO1 homology (PDZ) domain-containing proteins. In this paper, we identified membrane-associated guanylate kinase inverted-3 (MAGI-3) as a novel binding partner of beta2AR. The carboxyl terminus of beta2AR binds with high affinity to the fifth PDZ domain of MAGI-3, with the last four amino acids (D-S-L-L) of the receptor being the key determinants of the interaction. In cells, the association of full-length beta2AR with MAGI-3 occurs constitutively and is enhanced by agonist stimulation of the receptor. Our data also demonstrated that beta2AR-stimulated extracellular signal-regulated kinase-1/2 (ERK1/2) activation was substantially retarded by MAGI-3 expression. These data suggest that MAGI-3 regulates beta2AR-mediated ERK activation through the physical interaction between beta2AR and MAGI-3.

Ligand-directed signalling at beta-adrenoceptors

beta-Adrenoceptors (ARs) classically mediate responses to the endogenous ligands adrenaline and noradrenaline by coupling to Gsalpha and stimulating cAMP production; however, drugs designed as beta-AR agonists or antagonists can activate alternative cell signalling pathways, with the potential to influence clinical efficacy. Furthermore, drugs acting at beta-ARs have differential capacity for pathway activation, described as stimulus trafficking, biased agonism, functional selectivity or ligand-directed signalling. These terms refer to responses where drug A has higher efficacy than drug B for one signalling pathway, but a lower efficacy than drug B for a second pathway. The accepted explanation for such responses is that drugs A and B have the capacity to induce or stabilize distinct active conformations of the receptor that in turn display altered coupling efficiency to different effectors. This is consistent with biophysical studies showing that drugs can indeed promote distinct conformational states. Agonists acting at beta-ARs display ligand-directed signalling, but many drugs acting as cAMP antagonists are also able to activate signalling pathways central to cell survival and proliferation or cell death. The observed complexity of drug activity at beta-ARs, prototypical G protein-coupled receptors, necessitates rethinking of the approaches used for screening and characterization of novel therapeutic agents. Most studies of ligand-directed signalling employ recombinant cell systems with high receptor abundance. While such systems are valid for examining upstream signalling events, such as receptor conformational changes and G protein activation, they are less robust when comparing downstream signalling outputs as these are likely to be affected by complex pathway interactions.

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.

Assembly of a beta2-adrenergic receptor--GluR1 signalling complex for localized cAMP signalling

Central noradrenergic signalling mediates arousal and facilitates learning through unknown molecular mechanisms. Here, we show that the beta(2)-adrenergic receptor (beta(2)AR), the trimeric G(s) protein, adenylyl cyclase, and PKA form a signalling complex with the AMPA-type glutamate receptor subunit GluR1, which is linked to the beta(2)AR through stargazin and PSD-95 and their homologues. Only GluR1 associated with the beta(2)AR is phosphorylated by PKA on beta(2)AR stimulation. Peptides that interfere with the beta(2)AR-GluR1 association prevent this phosphorylation of GluR1. This phosphorylation increases GluR1 surface expression at postsynaptic sites and amplitudes of EPSCs and mEPSCs in prefrontal cortex slices. Assembly of all proteins involved in the classic beta(2)AR-cAMP cascade into a supramolecular signalling complex and thus allows highly localized and selective regulation of one of its major target proteins.

Synaptic scaffolding molecule binds to and regulates vasoactive intestinal polypeptide type-1 receptor in epithelial cells

BACKGROUND & AIMS

Vasoactive intestinal polypeptide (VIP) is a principal regulator of fluid and electrolyte secretion in the gastrointestinal system. The VIP type-1 receptor (VPAC1), a class II G-protein-coupled receptor, contains a putative C-terminal PDZ-binding motif. A yeast 2-hybrid screen indicated that the C-terminus of VPAC1 bound to the PDZ domain of synaptic scaffolding molecule (S-SCAM, also known as membrane-associated guanylate kinase inverted-2 [MAGI-2]). We analyzed the association between S-SCAM and VPAC1.

METHODS

The biochemical properties and physiologic significance of the interaction between VPAC1 and S-SCAM were examined in heterologous expression systems, T84 colonic epithelial cells, and human pancreas and colon tissues using an integrated molecular and physiologic approach.

RESULTS

The physical interaction between VPAC1 and S-SCAM was confirmed by immunoprecipitation in HEK 293 mammalian cells and human pancreatic and colonic tissues. Immunocytochemical analysis indicated that S-SCAM recruited VPAC1 to the junctional area near the apical end of the lateral membrane in T84 cells. Several lines of evidence revealed that S-SCAM inhibits VPAC1 activation. Overexpression of S-SCAM inhibited VPAC1-mediated cAMP production and agonist-induced VPAC1 internalization in HEK 293 and HeLa cells. In addition, S-SCAM decreased the VPAC1-mediated current through the cystic fibrosis transmembrane conductance regulator in Xenopus oocytes, especially at low concentrations of VIP. Importantly, loss of S-SCAM increased VIP-induced short-circuit currents in T84 monolayers, which endogenously express VPAC1 and S-SCAM.

CONCLUSIONS

S-SCAM/MAGI-2 interacts with and regulates VPAC1 intracellular localization in epithelial cells and inhibits VPAC1 agonist-induced activation and internalization.

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.

MAGI-2 Inhibits cell migration and proliferation via PTEN in human hepatocarcinoma cells

MAGI-2, a multidomain scaffolding protein, contains nine potential protein-protein interaction modules, including a GuK domain, two WW domains and six PDZ domains. In this study, we examined eight human hepatocarcinoma cell lines (HHCCs) and found that MAGI-2 was expressed only in 7721 cells. After 7721, 7404 and 97H cells were transfected with myc-MAGI-2 plasmid, their migration and proliferation was significantly inhibited, which was associated with downregulation of p-FAK and p-Akt. It is known that p-FAK is a substrate of PTEN and p-Akt can be regulated by PTEN via PIP(3). We demonstrated that PTEN was upregulated after myc-MAGI-2 transfection, which was due to the enhancement of PTEN protein stability rather than mRNA levels. Furthermore, MAGI-2-induced inhibition of cell migration and proliferation was attenuated in 7721 cells with PTEN silence or in PTEN-null cell line U87MG, and PTEN transfection could restore the effect of MAGI-2 in U87MG cells. Finally, the molecular association between PTEN and MAGI-2 was confirmed. Our results suggested that PTEN played a critical role in MAGI-2-induced inhibition of cell migration and proliferation in HHCCs.

Anti-beta1-adrenergic receptor autoantibodies in patients with chronic Chagas heart disease

Chronic Chagas heart disease (cChHD), a chronic manifestation of the Trypanosoma cruzi infection, is characterized by high antibody levels against the C-terminal region of the ribosomal P proteins (i.e. peptide R13, EEEDDDMGFGLFD) which bears similarity with the second extracellular loop of beta1-adrenergic receptor (beta1-AR, peptide H26R HWWRAESDEARRCYNDPKCCDFVTNR). Because it has not been demonstrated clearly that IgGs from cChHD patients bind to native human beta1-AR, the aim of this study was to investigate further the physical interaction between cChHD IgGs and the human beta1-AR. Immunofluorescence assays demonstrated the binding of these antibodies to the receptor expressed on stably transfected cells, together with a beta1-AR agonist-like effect. In addition, immunoadsorption of the serum samples from cChHD patients with a commercially available matrix, containing peptides representing the first and the second extracellular loop of the beta1-AR, completely abolished reactivity against the H26R peptide and the physiological response to the receptor. The follow-up of this specificity after in vitro immunoadsorption procedures suggests that this treatment might be used to diminish significantly the serum levels of anti-beta1-AR antibodies in patients with Chagas heart disease.

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.

Golgin-160 Promotes Cell Surface Expression of the Beta-1 Adrenergic Receptor

Golgin-160 is a ubiquitously expressed peripheral Golgi membrane protein that is important for transduction of certain pro-apoptotic signals at the Golgi complex. However, the role of golgin-160 in normal Golgi structure and function is unknown. Here, we show that depletion of golgin-160 using RNA interference (RNAi) does not affect Golgi morphology or constitutive membrane traffic in HeLa cells. However, depletion of golgin-160 leads to significantly decreased cell surface levels of exogenously expressed beta1-adrenergic receptor (beta1AR), which can be rescued by expression of RNAi-resistant forms of golgin-160. Furthermore, overexpression of golgin-160 leads to higher surface levels of beta1AR. Golgin-160 is localized mostly in the cis and medial regions of the Golgi stack by immunoelectron microscopy, suggesting that it does not directly promote incorporation of beta1AR into transport vesicles at the trans Golgi network. Golgin-160 interacts with beta1AR in vitro, and we mapped the interaction to a region between residues 140 and 257 in the head of golgin-160 and the third intracellular loop of beta1AR. Our results support the idea that golgin-160 may promote efficient surface delivery of a subset of cargo molecules.

Trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) play an integral role in the signal transduction of an enormous array of biological phenomena, thereby serving to modulate at a molecular level almost all components of human biology. This role is nowhere more evident than in cardiovascular biology, where GPCRs regulate such core measures of cardiovascular function as heart rate, contractility, and vascular tone. GPCR/ligand interaction initiates signal transduction cascades, and requires the presence of the receptor at the plasma membrane. Plasma membrane localization is in turn a function of the delivery of a receptor to and removal from the cell surface, a concept defined most broadly as receptor trafficking. This review illuminates our current view of GPCR trafficking, particularly within the cardiovascular system, as well as highlights the recent and provocative finding that components of the GPCR trafficking machinery can facilitate GPCR signaling independent of G protein activation.