Complementary roles of the DRY motif and C-terminus tail of GPCRS for G protein coupling and β-arrestin interaction

Discovery of 3-((4-Benzylpyridin-2-yl)amino)benzamides as Potent GPR52 G Protein-Biased Agonists

Orphan GPR52 is emerging as a promising neurotherapeutic target. Optimization of previously reported lead 4a employing an iterative drug design strategy led to the identification of a series of unique GPR52 agonists, such as 10a (PW0677), 15b (PW0729), and 24f (PW0866), with improved potency and efficacy. Intriguingly, compounds 10a and 24f showed greater bias for G protein/cAMP signaling and induced significantly less in vitro desensitization than parent compound 4a, indicating that reducing GPR52 β-arrestin activity with biased agonism results in sustained GPR52 activation. Further exploration of compounds 15b and 24f indicated improved potency and efficacy, and excellent target selectivity, but limited brain exposure warranting further optimization. These balanced and biased GPR52 agonists provide important pharmacological tools to study GPR52 activation, signaling bias, and therapeutic potential for neuropsychiatric and neurological diseases.

Structural and pharmacological characterization of a medium-chain fatty acid receptor GPR84 in common carp (Cyprinus carpio)

The medium-chain fatty acid receptor GPR84, a member of the G protein-coupled receptor family, is mainly expressed in macrophages and microglia, and is involved in the regulation of inflammatory responses and retinal development in mammals and amphibians. However, structure, tissue distribution, and pharmacology of this receptor have rarely been reported in fish. In this study, we cloned the coding sequence (CDS) of common carp GPR84 (ccGPR84), examined its tissue distribution, and explored its cellular signaling function. The results showed that the CDS of ccGPR84 is 1191 bp and encodes a putative protein with 396 amino acids. Phylogenetic and chromosomal synteny analyses revealed that ccGPR84 was evolutionarily conserved with Cyprinids. Real-time quantitative PCR (qPCR) indicated that ccGPR84 was predominantly expressed in the intestine and spleen. Luciferase reporter assay demonstrated that nonanoic acid, capric acid (decanoic acid), undecanoic acid and lauric acid could inhibit cAMP signaling pathway and activate MAPK/ERK signaling pathway, while the potencies of these four fatty acids on the two signaling pathways were different. Lauric acid has the highest inhibitory potency on cAMP signaling pathway, followed by undecanoic acid, nonanoic acid, and capric acid. While for MAPK/ERK signaling pathway, nonanoic acid has the highest activation potency, followed by undecanoic acid, capric acid, and lauric acid. These findings lay the foundation for revealing the roles of different medium-chain fatty acids in the inflammatory response of common carp.

Unveiling the Differences in Signaling and Regulatory Mechanisms between Dopamine D2 and D3 Receptors and Their Impact on Behavioral Sensitization

Dopamine receptors are classified into five subtypes, with D2R and D3R playing a crucial role in regulating mood, motivation, reward, and movement. Whereas D2R are distributed widely across the brain, including regions responsible for motor functions, D3R are primarily found in specific areas related to cognitive and emotional functions, such as the nucleus accumbens, limbic system, and prefrontal cortex. Despite their high sequence homology and similar signaling pathways, D2R and D3R have distinct regulatory properties involving desensitization, endocytosis, posttranslational modification, and interactions with other cellular components. In vivo, D3R is closely associated with behavioral sensitization, which leads to increased dopaminergic responses. Behavioral sensitization is believed to result from D3R desensitization, which removes the inhibitory effect of D3R on related behaviors. Whereas D2R maintains continuous signal transduction through agonist-induced receptor phosphorylation, arrestin recruitment, and endocytosis, which recycle and resensitize desensitized receptors, D3R rarely undergoes agonist-induced endocytosis and instead is desensitized after repeated agonist exposure. In addition, D3R undergoes more extensive posttranslational modifications, such as glycosylation and palmitoylation, which are needed for its desensitization. Overall, a series of biochemical settings more closely related to D3R could be linked to D3R-mediated behavioral sensitization.

Pan-cancer functional analysis of somatic mutations in G protein-coupled receptors

G Protein-coupled receptors (GPCRs) are the most frequently exploited drug target family, moreover they are often found mutated in cancer. Here we used a dataset of mutations found in patient samples derived from the Genomic Data Commons and compared it to the natural human variance as exemplified by data from the 1000 genomes project. We explored cancer-related mutation patterns in all GPCR classes combined and individually. While the location of the mutations across the protein domains did not differ significantly in the two datasets, a mutation enrichment in cancer patients was observed among class-specific conserved motifs in GPCRs such as the Class A "DRY" motif. A Two-Entropy Analysis confirmed the correlation between residue conservation and cancer-related mutation frequency. We subsequently created a ranking of high scoring GPCRs, using a multi-objective approach (Pareto Front Ranking). Our approach was confirmed by re-discovery of established cancer targets such as the LPA and mGlu receptor families, but also discovered novel GPCRs which had not been linked to cancer before such as the P2Y Receptor 10 (P2RY10). Overall, this study presents a list of GPCRs that are amenable to experimental follow up to elucidate their role in cancer.

Recent Progress in Assays for GPCR Drug Discovery

G-protein coupled receptors (GPCRs), also known as 7 transmembrane receptors, are the largest family of cell surface receptors in eukaryotes. There are ~800 GPCRs in human, regulating diverse physiological processes. GPCRs are the most intensively studied drug targets. Drugs that target GPCRs account for about a quarter of the global market share of therapeutic drugs. Therefore, to develop physiologically relevant and robust assays to search new GPCR ligands or modulators remain the major focus of drug discovery research worldwide. Early functional GPCR assays are mainly depend on the measurement of G protein-mediated second messenger generation. Recent development in GPCR biology indicate the signaling of these receptors is much more complex than the oversimplified classical view. GPCRs have been found to activate multiple G proteins simultaneously and induce b-arrestin-mediated signaling. GPCRs have also been found to interacte with other cytosolic scaffolding proteins and form dimer or heteromer with GPCRs or other transmembrane proteins. Here we mainly discuss technologies focused on detecting protein-protein interactions, such as FRET/BRET, NanoBiT, Tango, etc, and their applications in measuring GPCRs interacting with various signaling partners. In the final part, we also discuss the species differences in GPCRs when using animal models to study the in vivofunctions of GPCR ligands, and possible ways to solve this problem with modern genetic tools.

A Split Luciferase Complementation Assay for the Quantification of β-Arrestin2 Recruitment to Dopamine D2-Like Receptors

Investigations on functional selectivity of GPCR ligands have become increasingly important to identify compounds with a potentially more beneficial side effect profile. In order to discriminate between individual signaling pathways, the determination of β-arrestin2 recruitment, in addition to G-protein activation, is of great value. In this study, we established a sensitive split luciferase-based assay with the ability to quantify β-arrestin2 recruitment to D_2long and D₃ receptors and measure time-resolved β-arrestin2 recruitment to the D_2long receptor after agonist stimulation. We were able to characterize several standard (inverse) agonists as well as antagonists at the D_2longR and D₃R subtypes, whereas for the D_4.4R, no β-arrestin2 recruitment was detected, confirming previous reports. Extensive radioligand binding studies and comparisons with the respective wild-type receptors confirm that the attachment of the Emerald luciferase fragment to the receptors does not affect the integrity of the receptor proteins. Studies on the involvement of GRK2/3 and PKC on the β-arrestin recruitment to the D_2longR and D₃R, as well as at the D₁R using different kinase inhibitors, showed that the assay could also contribute to the elucidation of signaling mechanisms. Its broad applicability, which provides concentration-dependent and kinetic information on receptor/β-arrestin2 interactions, renders this homogeneous assay a valuable method for the identification of biased agonists.

Dynamics and structural communication in the ternary complex of fully phosphorylated V2 vasopressin receptor, vasopressin, and β-arrestin 1

G protein-coupled receptors (GPCRs) are critically regulated by arrestins, which not only desensitize G-protein signaling but also initiate a G protein-independent wave of signaling. The information from structure determination was herein exploited to build a structural model of the ternary complex, comprising fully phosphorylated V2 vasopressin receptor (V2R), the agonist arginine vasopressin (AVP), and β-arrestin 1 (β-arr1). Molecular simulations served to explore dynamics and structural communication in the ternary complex. Flexibility and mechanical profiles reflect fold of V2R and β-arr1. Highly conserved amino acids tend to behave as hubs in the structure network and contribute the most to the mechanical rigidity of V2R seven-helix bundle and of β-arr1. Two structurally and dynamically distinct receptor-arrestin interfaces assist the twist of the N- and C-terminal domains (ND and CD, respectively) of β-arr1 with respect to each other, which is linked to arrestin activation. While motion of the ND is essentially assisted by the fully phosphorylated C-tail of V2R (V2RCt), that of CD is assisted by the second and third intracellular loops and the cytosolic extensions of helices 5 and 6. In the presence of the receptor, the β-arr1 inter-domain twist angle correlates with the modes describing the essential subspace of the ternary complex. β-arr1 motions are also influenced by the anchoring to the membrane of the C-edge-loops in the β-arr1-CD. Overall fluctuations reveal a coupling between motions of the agonist binding site and of β-arr1-ND, which are in allosteric communication between each other. Mechanical rigidity points, often acting as hubs in the structure network and distributed along the main axis of the receptor helix bundle, contribute to establish a preferential communication pathway between agonist ligand and the ND of arrestin. Such communication, mediated by highly conserved amino acids, involves also the first amino acid in the arrestin C-tail, which is highly dynamic and is involved in clathrin-mediated GPCR internalization.

Biased signaling agonist of dopamine D3 receptor induces receptor internalization independent of β-arrestin recruitment

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is a significant step in receptor kinetics and is known to be involved in receptor down-regulation. However, the dopamine D3 receptor (D3R) has been an exception wherein agonist induces D3Rs to undergo desensitization followed by pharmacological sequestration - which is defined as the sequestration of cell surface receptors into a more hydrophobic fraction within the plasma membrane without undergoing the process of receptor internalization. Pharmacological sequestration renders the receptor in an inactive state on the membrane. In our previous study we demonstrated that a novel class of D3R agonists exemplified by SK608 have biased signaling properties via the G-protein dependent pathway and do not induce D3R desensitization. In this study, using radioligand binding assay, immunoblot or immunocytochemistry methods, we observed that SK608 induced internalization of human D3R stably expressed in CHO, HEK and SH-SY5Y cells which are derived from neuroblastoma cells, suggesting that it is not a cell-type specific event. Further, we have evaluated the potential mechanism of D3R internalization induced by these biased signaling agonists. SK608-induced D3R internalization was time- and concentration-dependent. In comparison, dopamine induced D3R upregulation and pharmacological sequestration in the same assays. GRK2 and clathrin/dynamin I/II are the key molecular players in the SK608-induced D3R internalization process, while β-arrestin 1/2 and GRK-interacting protein 1(GIT1) are not involved. These results suggest that SK608-promoted D3R internalization is similar to the type II internalization observed among peptide binding GPCRs.

Current Concepts and Treatments of Schizophrenia

Schizophrenia is a debilitating mental illness which involves three groups of symptoms, i.e., positive, negative and cognitive, and has major public health implications. According to various sources, it affects up to 1% of the population. The pathomechanism of schizophrenia is not fully understood and current antipsychotics are characterized by severe limitations. Firstly, these treatments are efficient for about half of patients only. Secondly, they ameliorate mainly positive symptoms (e.g., hallucinations and thought disorders which are the core of the disease) but negative (e.g., flat affect and social withdrawal) and cognitive (e.g., learning and attention disorders) symptoms remain untreated. Thirdly, they involve severe neurological and metabolic side effects and may lead to sexual dysfunction or agranulocytosis (clozapine). It is generally agreed that the interactions of antipsychotics with various neurotransmitter receptors are responsible for their effects to treat schizophrenia symptoms. In particular, several G protein-coupled receptors (GPCRs), mainly dopamine, serotonin and adrenaline receptors, are traditional molecular targets for antipsychotics. Comprehensive research on GPCRs resulted in the exploration of novel important signaling mechanisms of GPCRs which are crucial for drug discovery: intentionally non-selective multi-target compounds, allosteric modulators, functionally selective compounds and receptor oligomerization. In this review, we cover current hypotheses of schizophrenia, involving different neurotransmitter systems, discuss available treatments and present novel concepts in schizophrenia and its treatment, involving mainly novel mechanisms of GPCRs signaling.

S-Nitrosylation of β-Arrestins Biases Receptor Signaling and Confers Ligand Independence

Most G protein-coupled receptors (GPCRs) signal through both heterotrimeric G proteins and β-arrestins (βarr1 and βarr2). Although synthetic ligands can elicit biased signaling by G protein- vis-à-vis βarr-mediated transduction, endogenous mechanisms for biasing signaling remain elusive. Here we report that S-nitrosylation of a novel site within βarr1/2 provides a general mechanism to bias ligand-induced signaling through GPCRs by selectively inhibiting βarr-mediated transduction. Concomitantly, S-nitrosylation endows cytosolic βarrs with receptor-independent function. Enhanced βarr S-nitrosylation characterizes inflammation and aging as well as human and murine heart failure. In genetically engineered mice lacking βarr2-Cys253 S-nitrosylation, heart failure is exacerbated in association with greatly compromised β-adrenergic chronotropy and inotropy, reflecting βarr-biased transduction and β-adrenergic receptor downregulation. Thus, S-nitrosylation regulates βarr function and, thereby, biases transduction through GPCRs, demonstrating a novel role for nitric oxide in cellular signaling with potentially broad implications for patho/physiological GPCR function, including a previously unrecognized role in heart failure.

A calcium-sensing receptor mutation causing hypocalcemia disrupts a transmembrane salt bridge to activate β-arrestin-biased signaling

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor (GPCR) that signals through Gq/11 and Gi/o to stimulate cytosolic calcium (Ca2+i) and mitogen-activated protein kinase (MAPK) signaling to control extracellular calcium homeostasis. Studies of loss- and gain-of-function CASR mutations, which cause familial hypocalciuric hypercalcemia type 1 (FHH1) and autosomal dominant hypocalcemia type 1 (ADH1), respectively, have revealed that the CaSR signals in a biased manner. Thus, some mutations associated with FHH1 lead to signaling predominantly through the MAPK pathway, whereas mutations associated with ADH1 preferentially enhance Ca2+i responses. We report a previously unidentified ADH1-associated R680G CaSR mutation, which led to the identification of a CaSR structural motif that mediates biased signaling. Expressing CaSRR680G in HEK 293 cells showed that this mutation increased MAPK signaling without altering Ca2+i responses. Moreover, this gain of function in MAPK activity occurred independently of Gq/11 and Gi/o and was mediated instead by a noncanonical pathway involving β-arrestin proteins. Homology modeling and mutagenesis studies showed that the R680G CaSR mutation selectively enhanced β-arrestin signaling by disrupting a salt bridge formed between Arg680 and Glu767, which are located in CaSR transmembrane domain 3 and extracellular loop 2, respectively. Thus, our results demonstrate CaSR signaling through β-arrestin and the importance of the Arg680-Glu767 salt bridge in mediating signaling bias.

Molecular dissection of the human A3 adenosine receptor coupling with β-arrestin2

Besides classical G protein coupling, G protein-coupled receptors (GPCRs) are nowadays well known to show significant signalling via other adaptor proteins, such as β-arrestin2 (βarr2). The elucidation of the molecular mechanism of the GPCR-βarr2 interaction is a prerequisite for the structure-activity based design of biased ligands, which introduces a new chapter in drug discovery. The general mechanism of the interaction is believed to rely on phosphorylation sites, exposed upon agonist binding. However, it is not known whether this mechanism is universal throughout the GPCR family or if GPCR-specific patterns are involved. In recent years, promising orally active agonists for the human A₃ adenosine receptor (A₃AR), a GPCR highly expressed in inflammatory and cancer cells, have been evaluated in clinical trials for the treatment of rheumatoid arthritis, psoriasis, and hepatocellular carcinoma. In this study, the effect of cytoplasmic modifications of the A₃AR on βarr2 recruitment was evaluated in transiently transfected HEK293T cells, using a live-cell split-reporter system (NanoBit®, Promega), based on the structural complementation of NanoLuc luciferase, allowing real-time βarr2 monitoring. The A₃AR-selective reference agonist 2-Cl-IB-MECA yielded a robust, concentration dependent (5 nM-1 µM) recruitment of βarr2 (logEC50: -7.798 ± 0.076). The role of putative phosphorylation sites, located in the C-terminal part and cytoplasmic loops, and the role of the 'DRY' motif was evaluated. It was shown that the A₃AR C-terminus was dispensable for βarr2 recruitment. This contrasts with studies in the past for the rat A₃AR, which pointed at crucial C-terminal phosphorylation sites. When combining truncation of the A₃AR with modification of the 'DRY' motif to 'AAY', the βarr2 recruitment was drastically reduced. Recruitment could be partly rescued by back-mutation to 'NQY', or by extending the C-terminus again. In conclusion, other parts of the human A₃AR, either cytosolic or exposed upon receptor activation, rather than the C-terminus alone, are responsible for βarr2 recruitment in a complementary or synergistic way.

Novel Structural Insights into GPCR-β-Arrestin Interaction and Signaling

G protein-coupled receptors (GPCRs) are major signal recognition and transmission units in the plasma membrane. The interaction of activated and phosphorylated GPCRs with the multifunctional adaptor proteins β-arrestins (βarrs) is crucial for regulation of their signaling and functional outcomes. Over the past few years, a range of structural, biochemical, and cellular studies have revealed novel insights into GPCR-βarr interaction and signaling. Some of these findings have come as a surprise and therefore have the potential to significantly refine the conceptual framework of the GPCR-βarr system. Here we discuss these recent advances with particular emphasis on biphasic GPCR-βarr interaction, the formation of GPCR-G-protein-βarr supercomplexes, and receptor-specific conformational signatures in βarrs. We also underline the emerging research areas that are likely to be at the center stage of investigations in the coming years.

Differential phosphorylation signals control endocytosis of GPR15

GPR15 undergoes a ligand-independent endocytosis, which requires phosphorylation of a distal C-terminal Ser-357 mediated by multiple basophilic kinases. The functional role of Ser-357 in endocytosis is distinct from that of a conserved Ser/Thr cluster, which is more responsible for the use of GRKs and β-arrestin.

GPR15 is an orphan G protein–coupled receptor (GPCR) that serves for an HIV coreceptor and was also recently found as a novel homing receptor for T-cells implicated in colitis. We show that GPR15 undergoes a constitutive endocytosis in the absence of ligand. The endocytosis was clathrin dependent and partially dependent on β-arrestin in HEK293 cells, and nearly half of the internalized GPR15 receptors were recycled to the plasma membrane. An Ala mutation of the distal C-terminal Arg-354 or Ser-357, which forms a consensus phosphorylation site for basophilic kinases, markedly reduced the endocytosis, whereas phosphomimetic mutation of Ser-357 to Asp did not. Ser-357 was phosphorylated in vitro by multiple kinases, including PKA and PKC, and pharmacological activation of these kinases enhanced both phosphorylation of Ser-357 and endocytosis of GPR15. These results suggested that Ser-357 phosphorylation critically controls the ligand-independent endocytosis of GPR15. The functional role of Ser-357 in endocytosis was distinct from that of a conserved Ser/Thr cluster in the more proximal C-terminus, which was responsible for the β-arrestin– and GPCR kinase–dependent endocytosis of GPR15. Thus phosphorylation signals may differentially control cell surface density of GPR15 through endocytosis.

Functional competence of a partially engaged GPCR-β-arrestin complex

G Protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors and drug targets. GPCR signalling and desensitization is critically regulated by β-arrestins (βarr). GPCR–βarr interaction is biphasic where the phosphorylated carboxyl terminus of GPCRs docks to the N-domain of βarr first and then seven transmembrane core of the receptor engages with βarr. It is currently unknown whether fully engaged GPCR–βarr complex is essential for functional outcomes or partially engaged complex can also be functionally competent. Here we assemble partially and fully engaged complexes of a chimeric β₂V₂R with βarr1, and discover that the core interaction is dispensable for receptor endocytosis, ERK MAP kinase binding and activation. Furthermore, we observe that carvedilol, a βarr biased ligand, does not promote detectable engagement between βarr1 and the receptor core. These findings uncover a previously unknown aspect of GPCR-βarr interaction and provide novel insights into GPCR signalling and regulatory paradigms.

β-arrestins initially contact with the phosphorylated carboxyl-terminus of GPCRs before engaging with the GPCR core. Here, the authors use a chimeric GPCR partially and fully engaged with β-arrestin1 and show that the core interaction is dispensable for receptor endocytosis and signalling.

RalA employs GRK2 and β-arrestins for the filamin A-mediated regulation of trafficking and signaling of dopamine D2 and D3 receptor

Filamin A (FLNA) is known to act as platform for the signaling and intracellular trafficking of various GPCRs including dopamine D2 and D3 receptors (D2R, D3R). To understand molecular mechanisms involved in the FLNA-mediated regulation of D2R and D3R, comparative studies were conducted on the signaling and intracellular trafficking of the D2R and D3R in FLNA-knockdown cells, with a specific focus on the roles of the proteins that interact with FLNA and the D2R and D3R. Lowering the level of cellular FLNA caused an elevation in RalA activity and resulted in selective interference with the normal intracellular trafficking and signaling of the D2R and D3R, through GRK2 and β-arrestins, respectively. Knockdown of FLNA or coexpression of active RalA interfered with the recycling of the internalized D2R and resulted in the development of receptor tolerance. Active RalA was found to interact with GRK2 to sequester it from D2R. Knockdown of FLNA or coexpression of active RalA prevented D3R from coupling with G protein. The selective involvement of GRK2- and β-arrestins in the RalA-mediated cellular processes of the D2R and D3R was achieved via their different modes of interactions with the receptor and their distinct functional roles in receptor regulation. Our results show that FLNA is a multi-functional protein that acts as a platform on which D2R and D3R can interact with various proteins, through which selective regulation of these receptors occurs in combination with GRK2 and β-arrestins.

Biased signalling: the instinctive skill of the cell in the selection of appropriate signalling pathways

GPCRs (G-protein-coupled receptors) are members of a family of proteins which are generally regarded as the largest group of therapeutic drug targets. Ligands of GPCRs do not usually activate all cellular signalling pathways linked to a particular seven-transmembrane receptor in a uniform manner. The fundamental idea behind this concept is that each ligand has its own ability, while interacting with the receptor, to activate different signalling pathways (or a particular set of signalling pathways) and it is this concept which is known as biased signalling. The importance of biased signalling is that it may selectively activate biological responses to favour therapeutically beneficial signalling pathways and to avoid adverse effects. There are two levels of biased signalling. First, bias can arise from the ability of GPCRs to couple to a subset of the available G-protein subtypes: Gαs, Gαq/11, Gαi/o or Gα12/13. These subtypes produce the diverse effects of GPCRs by targeting different effectors. Secondly, biased GPCRs may differentially activate G-proteins or β-arrestins. β-Arrestins are ubiquitously expressed and function to terminate or inhibit classic G-protein signalling and initiate distinct β-arrestin-mediated signalling processes. The interplay of G-protein and β-arrestin signalling largely determines the cellular consequences of the administration of GPCR-targeted drugs. In the present review, we highlight the particular functionalities of biased signalling and discuss its biological effects subsequent to GPCR activation. We consider that biased signalling is potentially allowing a choice between signalling through ‘beneficial’ pathways and the avoidance of ‘harmful’ ones.

PRESTO-Tango as an open-source resource for interrogation of the druggable human GPCRome

G protein-coupled receptors (GPCRs) are essential mediators of cellular signaling and are important targets of drug action. Of the approximately 350 nonolfactory human GPCRs, more than 100 are still considered to be 'orphans' because their endogenous ligands remain unknown. Here, we describe a unique open-source resource that allows interrogation of the druggable human GPCRome via a G protein-independent β-arrestin-recruitment assay. We validate this unique platform at more than 120 nonorphan human GPCR targets, demonstrate its utility for discovering new ligands for orphan human GPCRs and describe a method (parallel receptorome expression and screening via transcriptional output, with transcriptional activation following arrestin translocation (PRESTO-Tango)) for the simultaneous and parallel interrogation of the entire human nonolfactory GPCRome.

Mutations in the 'DRY' motif of the CB1 cannabinoid receptor result in biased receptor variants

The role of the highly conserved 'DRY' motif in the signaling of the CB1 cannabinoid receptor (CB1R) was investigated by inducing single-, double-, and triple-alanine mutations into this site of the receptor. We found that the CB1R-R3.50A mutant displays a partial decrease in its ability to activate heterotrimeric Go proteins (∼80% of WT CB1R (CB1R-WT)). Moreover, this mutant showed an enhanced basal β-arrestin2 (β-arr2) recruitment. More strikingly, the double-mutant CB1R-D3.49A/R3.50A was biased toward β-arrs, as it gained a robustly increased β-arr1 and β-arr2 recruitment ability compared with the WT receptor, while its G-protein activation was decreased. In contrast, the double-mutant CB1R-R3.50A/Y3.51A proved to be G-protein-biased, as it was practically unable to recruit β-arrs in response to agonist stimulus, while still activating G-proteins, although at a reduced level (∼70% of CB1R-WT). Agonist-induced ERK1/2 activation of the CB1R mutants showed a good correlation with their β-arr recruitment ability but not with their G-protein activation or inhibition of cAMP accumulation. Our results suggest that G-protein activation and β-arr binding of the CB1R are mediated by distinct receptor conformations, and the conserved 'DRY' motif plays different roles in the stabilization of these conformations, thus mediating both G-protein- and β-arr-mediated functions of CB1R.

Phenotypic regulation of the sphingosine 1-phosphate receptor miles apart by G protein-coupled receptor kinase 2

The evolutionarily conserved DRY motif at the end of the third helix of rhodopsin-like, class-A G protein-coupled receptors (GPCRs) is a major regulator of receptor stability, signaling activity, and β-arrestin-mediated internalization. Substitution of the DRY arginine with histidine in the human vasopressin receptor results in a loss-of-function phenotype associated with diabetes insipidus. The analogous R150H substitution of the DRY motif in zebrafish sphingosine-1 phosphate receptor 2 (S1p2) produces a mutation, miles apart m⁹³ (mil^m93), that not only disrupts signaling but also impairs heart field migration. We hypothesized that constitutive S1p2 desensitization is the underlying cause of this strong zebrafish developmental defect. We observed in cell assays that the wild-type S1p2 receptor is at the cell surface whereas in distinct contrast the S1p2 R150H receptor is found in intracellular vesicles, blocking G protein but not arrestin signaling activity. Surface S1p2 R150H expression could be restored by inhibition of G protein-coupled receptor kinase 2 (GRK2). Moreover, we observed that β-arrestin 2 and GRK2 colocalize with S1p2 in developing zebrafish embryos and depletion of GRK2 in the S1p2 R150H miles apart zebrafish partially rescued cardia bifida. The ability of reduced GRK2 activity to reverse a developmental phenotype associated with constitutive desensitization supports efforts to genetically or pharmacologically target this kinase in diseases involving biased GPCR signaling.

Novel roles for β-arrestins in the regulation of pharmacological sequestration to predict agonist-induced desensitization of dopamine D3 receptors

BACKGROUND AND PURPOSE

In addition to typical GPCR kinase (GRK)-/β-arrestin-dependent internalization, dopamine D3 receptor employed an additional GRK-independent sequestration pathway. In this study, we investigated the molecular mechanism of this novel sequestration pathway.

EXPERIMENTAL APPROACH

Radioligand binding, flow cytometry and cell surface biotinylation assay were used to characterize trafficking properties of D2 and D3 receptors. Serine/threonine and N-linked glycosylation mutants of the D3 receptor were utilized to locate receptor regions involved in pharmacological sequestration and desensitization. Various point mutants of the D2 and D3 receptors, whose sequestration and desensitization properties were altered, were combined with knockdown cells of GRKs or β-arrestins to functionally correlate pharmacological sequestration and desensitization.

KEY RESULTS

The D3 receptor, but not the D2 receptor, showed characteristic trafficking behaviour in which receptors were shifted towards the more hydrophobic domains within the plasma membrane without translocation into other intracellular compartments. Among various amino acid residues tested, S145/S146, C147 and N12/19 were involved in pharmacological sequestration and receptor desensitization. Both pharmacological sequestration and desensitization of D3 receptor required β-arrestins, and functional relationship was observed between two processes when it was tested for D3 receptor variants and agonists.

CONCLUSIONS AND IMPLICATIONS

Pharmacological sequestration of D3 receptor accompanies movement of cell surface receptors into a more hydrophobic fraction within the plasma membrane and renders D3 receptor inaccessible to hydrophilic ligands. Pharmacological sequestration is correlated with desensitization of the D3 receptor in a Gβγ- and β-arrestin-dependent manner. This study provides new insights into molecular mechanism governing GPCR trafficking and desensitization.

Visualization of arrestin recruitment by a G-protein-coupled receptor

G Protein Coupled Receptors (GPCRs) are critically regulated by β-arrestins (βarrs), which not only desensitize G protein signaling but also initiate a G protein independent wave of signaling^1-5. A recent surge of structural data on a number of GPCRs, including the β₂ adrenergic receptor (β₂AR)-G protein complex, has provided novel insights into the structural basis of receptor activation^6-11. Lacking however has been complementary information on recruitment of βarrs to activated GPCRs primarily due to challenges in obtaining stable receptor-βarr complexes for structural studies. Here, we devised a strategy for forming and purifying a functional β₂AR-βarr1 complex that allowed us to visualize its architecture by single particle negative stain electron microscopy (EM) and to characterize the interactions between β₂AR and βarr1 using hydrogen-deuterium exchange mass spectrometry (HDXMS) and chemical cross-linking. EM 2D averages and 3D reconstructions reveal bimodal binding of βarr1 to the β₂AR, involving two separate sets of interactions, one with the phosphorylated carboxy-terminus of the receptor and the other with its seven-transmembrane core. Areas of reduced HDX together with identification of cross-linked residues suggest engagement of the finger loop of βarr1 with the seven-transmembrane core of the receptor. In contrast, focal areas of increased HDX indicate regions of increased dynamics in both N and C domains of βarr1 when coupled to the β₂AR. A molecular model of the β₂AR-βarr signaling complex was made by docking activated βarr1 and β₂AR crystal structures into the EM map densities with constraints provided by HDXMS and cross-linking, allowing us to obtain valuable insights into the overall architecture of a receptor-arrestin complex. The dynamic and structural information presented herein provides a framework for better understanding the basis of GPCR regulation by arrestins.

Computational analysis of the 3-D structure of human GPR87 protein: implications for structure-based drug design

The G-protein coupled receptor 87 (GPR87) is a recently discovered orphan GPCR which means that the search of their endogenous ligands has been a novel challenge. GPR87 has been shown to be overexpressed in squamous cell carcinomas (SCCs) or adenocarcinomas in lungs and bladder. The 3D structure of GPR87 was here modeled using two templates (2VT4 and 2ZIY) by a threading method. Functional assignment of GPR87 by SVM revealed that along with transporter activity, various novel functions were predicted. The 3D structure was further validated by comparison with structural features of the templates through Verify-3D, ProSA and ERRAT for determining correct stereochemical parameters. The resulting model was evaluated by Ramachandran plot and good 3D structure compatibility was evidenced by DOPE score. Molecular dynamics simulation and solvation of protein were studied through explicit spherical boundaries with a harmonic restraint membrane water system. A DRY-motif (Asp-Arg-Tyr sequence) was found at the end of transmembrane helix3, where GPCR binds and thus activation of signals is transduced. In a search for better inhibitors of GPR87, in silico modification of some substrate ligands was carried out to form polar interactions with Arg115 and Lys296. Thus, this study provides early insights into the structure of a major drug target for SCCs.

Post-synaptic density-95 (PSD-95) binding capacity of G-protein-coupled receptor 30 (GPR30), an estrogen receptor that can be identified in hippocampal dendritic spines

The estrogen 17β-estradiol (E2) modulates dendritic spine plasticity in the cornu ammonis 1 (CA1) region of the hippocampus, and GPR30 (G-protein coupled estrogen receptor 1 (GPER1)) is an estrogen-sensitive G-protein-coupled receptor (GPCR) that is expressed in the mammalian brain and in specific subregions that are responsive to E2, including the hippocampus. The subcellular localization of hippocampal GPR30, however, remains unclear. Here, we demonstrate that GPR30 immunoreactivity is detected in dendritic spines of rat CA1 hippocampal neurons in vivo and that GPR30 protein can be found in rat brain synaptosomes. GPR30 immunoreactivity is identified at the post-synaptic density (PSD) and in the adjacent peri-synaptic zone, and GPR30 can associate with the spine scaffolding protein PSD-95 both in vitro and in vivo. This PSD-95 binding capacity of GPR30 is specific and determined by the receptor C-terminal tail that is both necessary and sufficient for PSD-95 interaction. The interaction with PSD-95 functions to increase GPR30 protein levels residing at the plasma membrane surface. GPR30 associates with the N-terminal tandem pair of PDZ domains in PSD-95, suggesting that PSD-95 may be involved in clustering GPR30 with other receptors in the hippocampus. We demonstrate that GPR30 has the potential to associate with additional post-synaptic GPCRs, including the membrane progestin receptor, the corticotropin releasing hormone receptor, and the 5HT1a serotonin receptor. These data demonstrate that GPR30 is well positioned in the dendritic spine compartment to integrate E2 sensitivity directly onto multiple inputs on synaptic activity and might begin to provide a molecular explanation as to how E2 modulates dendritic spine plasticity.

Multiple signaling routes involved in the regulation of adenylyl cyclase and extracellular regulated kinase by dopamine D(2) and D(3) receptors

Most G protein coupled receptors (GPCR) regulate multiple cellular processes by coupling to more than one kind of G protein. Furthermore, recent studies have reported G protein-independent/β-arrestin-dependent signaling pathway for some GPCRs. Dopamine D(2) and D(3) receptors (D(2)R, D(3)R), the major targets of currently used antipsychotic drugs, are co-expressed in some of the same dopaminergic neurons and regulate the same overlapping effectors. However, the specific subunits of G proteins that regulate each signaling pathway are not clearly identified. In addition, the existence of β-arrestin-dependent/G protein-independent signaling is not clear for these receptors. In this study, we determined the G protein subtypes and β-arrestin dependency involved in the signaling of D(2)R and D(3)R, which was measured by inhibition of adenylyl cyclase and extracellular signal-regulated kinase (ERK) activation. For the inhibition of cAMP production in HEK-293 cells, D(2)R used the Gαo subunit but D(3)R used the βγ subunit of Gi family proteins. For the regulation of ERK activation, D(2)R used the α subunits of Gi/o proteins both in HEK-293 cells and COS-7 cells, but D(3)R used Gαo and Gβγ in HEK-293 cells and COS-7 cells, respectively. β-Arrestin-dependent/G protein-independent ERK activation was not observed for both D(2)R and D(3)R. Agonist-induced β-arrestin translocation was observed with D(2)R but not with D(3)R, and β-arrestins exerted inhibitory influences on G protein-dependent ERK activation by D(2)R, but not D(3)R. These results show that the D(2)R and D(3)R, which have overlapping cellular expressions and functional roles, employ distinct G protein subunits depending on the cell types and the effectors they control.

Minireview: the intimate link between calcium sensing receptor trafficking and signaling: implications for disorders of calcium homeostasis

The calcium-sensing receptor (CaSR) regulates organismal Ca(2+) homeostasis. Dysregulation of CaSR expression or mutations in the CASR gene cause disorders of Ca(2+) homeostasis and contribute to the progression or severity of cancers and cardiovascular disease. This brief review highlights recent findings that define the CaSR life cycle, which controls the cellular abundance of CaSR and CaSR signaling. A novel mechanism, termed agonist-driven insertional signaling (ADIS), contributes to the unique hallmarks of CaSR signaling, including the high degree of cooperativity and the lack of functional desensitization. Agonist-mediated activation of plasma membrane-localized CaSR increases the rate of insertion of CaSR at the plasma membrane without altering the constitutive endocytosis rate, thereby acutely increasing the maximum signaling response. Prolonged CaSR signaling requires a large intracellular ADIS-mobilizable pool of CaSR, which is maintained by signaling-mediated increases in biosynthesis. This model provides a rational framework for characterizing the defects caused by CaSR mutations and the altered functional expression of wild-type CaSR in disease states. Mechanistic dissection of ADIS of CaSR should lead to optimized pharmacological approaches to normalize CaSR signaling in disorders of Ca(2+) homeostasis.

Novel dopamine D2 receptor signaling through proteins interacting with the third cytoplasmic loop

The diverse activities of dopamine D2-like receptors, including D2, D3, and D4 receptors, are mediated by proteins that interact with the third cytoplasmic loop and regulate receptor signaling, receptor trafficking, and apoptosis. Such interacting proteins include calmodulin, the N-methyl-D: -aspartate receptor 2B subunit, calcium/calmodulin-dependent protein kinase II, prostate apoptosis response-4, and β-arrestins, which regulate receptor signaling and the pharmacological action through D2 receptor. The gene encoding the D2 receptor gives rise to two isoforms, termed the dopamine D2 receptor long isoform (D2L) and the dopamine D2 receptor short isoform; the latter lacks 29 amino acids of the D2L receptor within the third cytoplasmic loop. In this review, we first focus on novel functions of the hetero-oligomeric D1/D2 and D2/adenosine A(2A) receptors. We next discuss novel signaling through proteins interacting with the D2 receptor third cytoplasmic loop and define the function of a novel binding protein, heart-type fatty acid binding protein, which interacts with the D2L third cytoplasmic loop.

β-arrestin2 plays permissive roles in the inhibitory activities of RGS9-2 on G protein-coupled receptors by maintaining RGS9-2 in the open conformation

Together with G protein-coupled receptor (GPCR) kinases (GRKs) and β-arrestins, RGS proteins are the major family of molecules that control the signaling of GPCRs. The expression pattern of one of these RGS family members, RGS9-2, coincides with that of the dopamine D(3) receptor (D(3)R) in the brain, and in vivo studies have shown that RGS9-2 regulates the signaling of D2-like receptors. In this study, β-arrestin2 was found to be required for scaffolding of the intricate interactions among the dishevelled-EGL10-pleckstrin (DEP) domain of RGS9-2, Gβ5, R7-binding protein (R7BP), and D(3)R. The DEP domain of RGS9-2, under the permission of β-arrestin2, inhibited the signaling of D(3)R in collaboration with Gβ5. β-Arrestin2 competed with R7BP and Gβ5 so that RGS9-2 is placed in the cytosolic region in an open conformation which is able to inhibit the signaling of GPCRs. The affinity of the receptor protein for β-arrestin2 was a critical factor that determined the selectivity of RGS9-2 for the receptor it regulates. These results show that β-arrestins function not only as mediators of receptor-G protein uncoupling and initiators of receptor endocytosis but also as scaffolding proteins that control and coordinate the inhibitory effects of RGS proteins on the signaling of certain GPCRs.

Asymmetry of GPCR oligomers supports their functional relevance

G protein-coupled receptors (GPCRs) can exist as dimers or as larger oligomeric clusters that enable intercommunication between different receptor protomers within the same complex. This phenomenon is observed at three distinct levels: (i) at the level of ligand binding where the activation of one protomer can allosterically inhibit or facilitate ligand binding to the second protomer; (ii) at the level of ligand-induced conformational switches, which occur between transmembrane domains of the two protomers; and (iii) within GPCR-associated protein complexes, either directly at the level of GPCR-interacting proteins or at further downstream levels of the complex. Intercommunication at these different levels introduces asymmetry within GPCR dimers wherein each protomer fulfills its specific task. In this review, we discuss how the asymmetric behavior of GPCRs highlights the advantage of oligomeric receptor organization and supports the functional relevance of GPCR dimerization.

Novel regulatory mechanism of canonical Wnt signaling by dopamine D2 receptor through direct interaction with beta-catenin

Classical G protein-coupled receptors (GPCRs) and canonical Wnt pathways were believed to use distinct signaling pathways. However, recent studies have shown that these two pathways interact each other by sharing several intermediate signaling components. Recent in vivo studies showed that antipsychotic drugs, which block dopamine D2-like receptors, increase the cellular levels of downstream signaling components of canonical Wnt pathways, such as dishevelled (Dvl), glycogen synthase kinase 3β (GSK3β), and β-catenin. These results suggest that some functional interactions might exist between Wnt pathway and D2-like receptors. In this study, we show that among five different dopamine receptor subtypes, D(2) receptor (D(2)R) selectively inhibited the Wnt signaling, which was measured by lymphoid enhancing factor-1 (LEF-1)-dependent transcriptional activities. D(2)R-mediated inhibition of Wnt signaling was agonist- and G protein-independent and did not require receptor phosphorylation or endocytosis. D(2)R inhibited the LEF-1-dependent transcriptional activities, and this inhibitory activity was not affected by the inhibition of GSK-3β, suggesting that D(2)R inhibited the Wnt signaling by acting on the downstream of GSK3β. D(2)R directly interacted with β-catenin through the second and third loops, leading to a reduction of β-catenin distribution in the nucleus, resulting in an inhibition of LEF-1-dependent transcription. This is a novel mechanism for the regulation of canonical Wnt signaling by GPCRs, in which receptor proteins recruit β-catenin from cytosol to the plasma membrane, resulting in the decrement of the β-catenin/LEF-1-dependent transcription in the nucleus.

Soft X-ray Laser Microscopy of Lipid Rafts towards GPCR-Based Drug Discovery Using Time-Resolved FRET Spectroscopy

Many signaling molecules involved in G protein-mediated signal transduction, which are present in the lipid rafts and believed to be controlled spatially and temporally, influence the potency and efficacy of neurotransmitter receptors and transporters. This has focus interest on lipid rafts and the notion that these microdomains acts as a kind of signaling platform and thus have an important role in the expression of membrane receptor-mediated signal transduction, cancer, immune responses, neurotransmission, viral infections and various other phenomena due to specific and efficient signaling according to extracellular stimuli. However, the real structure of lipid rafts has not been observed so far due to its small size and a lack of sufficiently sophisticated observation systems. A soft X-ray microscope using a coherent soft X-ray laser in the water window region (2.3–4.4 nm) should prove to be a most powerful tool to observe the dynamic structure of lipid rafts of several tens of nanometers in size in living cells. We have developed for the X-ray microscope a new compact soft X-ray laser using strongly induced plasma high harmonic resonance. We have also developed a time-resolved highly sensitive fluorescence resonance energy transfer (FRET) system and confirmed protein-protein interactions coupled with ligands. The simultaneous use of these new tools for observation of localization of G-protein coupled receptors (GPCRs) in rafts has become an important and optimum tool system to analyze the dynamics of signal transduction through rafts as signaling platform. New technology to visualize rafts is expected to lead to the understanding of those dynamics and innovative development of drug discovery that targets GPCRs localized in lipid rafts.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

An intracellular loop 2 amino acid residue determines differential binding of arrestin to the dopamine D2 and D3 receptors

Dopamine D(2) and D(3) receptors are similar subtypes with distinct interactions with arrestins; the D(3) receptor mediates less agonist-induced translocation of arrestins than the D(2) receptor. The goals of this study were to compare nonphosphorylated arrestin-binding determinants in the second intracellular domain (IC2) of the D(2) and D(3) receptors to identify residues that contribute to the differential binding of arrestin to the subtypes. Arrestin 3 bound to glutathione transferase (GST) fusion proteins of the D(2) receptor IC2 more avidly than to the D(3) receptor IC2. Mutagenesis of the fusion proteins identified a residue at the C terminus of IC2, Lys149, that was important for the preferential binding of arrestin 3 to D(2)-IC2; arrestin binding to D(2)-IC2-K149C was greatly decreased compared with wild-type D(2)-IC2, whereas binding to the reciprocal mutant D(3)-IC2-C147K was enhanced compared with wild-type D(3)-IC2. Mutating this lysine in the full-length D(2) receptor to cysteine decreased the ability of the D(2) receptor to mediate agonist-induced arrestin 3 translocation to the membrane and decreased agonist-induced receptor internalization in human embryonic kidney 293 cells. The reciprocal mutation in the D(3) receptor increased receptor-mediated translocation of arrestin 3 without affecting agonist-induced receptor internalization. G protein-coupled receptor crystal structures suggest that Lys149, at the junction of IC2 and the fourth membrane-spanning helix, has intramolecular interactions that contribute to maintaining an inactive receptor state. It is suggested that the preferential agonist-induced binding of arrestin3 to the D(2) receptor over the D(3) receptor is due in part to Lys149, which could be exposed as a result of receptor activation.

Characterization of functional roles of DRY motif in the 2nd intracellular loop of dopamine D2 and D3 receptors

Dopamine D(2)R and D(3)R (D(2)R, D(3)R) show very high sequence homology and employ virtually identical signaling pathways even though D(2)R is 2 approximately 5 times more active. Among the structural motifs identified, a triplet sequence, Asp-Arg-Tyr (DRY motif), plays critical roles in the determination of receptor conformations for signaling and intracellular trafficking of G protein-coupled receptors by forming intramolecular interactions. Thus, it is possible that different signaling efficiencies of D(2)R and D(3)R might be caused by the receptor activation levels stabilized by their own DRY motifs. In this study, the Arg and Asp residues of D(2)R and D(3)R were mutated, and resulting changes in their signaling and intracellular trafficking properties were comparatively studied. Mutation of the Arg residues of D(2)R and D(3)R abolished their signaling but differently affected their intracellular localizations. The wildtype and R132H-D(2)R were expressed mainly on the plasma membrane. On the other hand, compared with the wildtype D(3)R, a substantial amount of R128H-D(3)R was localized intracellularly. The expression of receptor proteins on the plasma membrane and their signaling efficiencies were more drastically affected by the mutation of the Asp residue of D(3)R than D(2)R. Therefore, it was concluded that the different levels of conformational strain exerted by the DRY motif might partly determine the quantitative differences in the signaling efficiencies between D(2)R and D(3)R.