G protein-coupled receptor kinase-mediated phosphorylation regulates post-endocytic trafficking of the D2 dopamine receptor

G Protein-Coupled Receptor Kinase 2 Selectively Enhances β-Arrestin Recruitment to the D2 Dopamine Receptor through Mechanisms That Are Independent of Receptor Phosphorylation

The D2 dopamine receptor (D2R) signals through both G proteins and β-arrestins to regulate important physiological processes, such as movement, reward circuitry, emotion, and cognition. β-arrestins are believed to interact with G protein-coupled receptors (GPCRs) at the phosphorylated C-terminal tail or intracellular loops. GPCR kinases (GRKs) are the primary drivers of GPCR phosphorylation, and for many receptors, receptor phosphorylation is indispensable for β-arrestin recruitment. However, GRK-mediated receptor phosphorylation is not required for β-arrestin recruitment to the D2R, and the role of GRKs in D2R-β-arrestin interactions remains largely unexplored. In this study, we used GRK knockout cells engineered using CRISPR-Cas9 technology to determine the extent to which β-arrestin recruitment to the D2R is GRK-dependent. Genetic elimination of all GRK expression decreased, but did not eliminate, agonist-stimulated β-arrestin recruitment to the D2R or its subsequent internalization. However, these processes were rescued upon the re-introduction of various GRK isoforms in the cells with GRK2/3 also enhancing dopamine potency. Further, treatment with compound 101, a pharmacological inhibitor of GRK2/3 isoforms, decreased β-arrestin recruitment and receptor internalization, highlighting the importance of this GRK subfamily for D2R-β-arrestin interactions. These results were recapitulated using a phosphorylation-deficient D2R mutant, emphasizing that GRKs can enhance β-arrestin recruitment and activation independently of receptor phosphorylation.

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.

Endocytosis of dopamine receptor: Signaling in brain

This chapter describes the physiological significance of dopamine receptor endocytosis and the consequence of the receptor signaling. Endocytosis of dopamine receptors is regulated by many components such as clathrin, β-arrestin, caveolin, and Rab family proteins. The dopamine receptors escape from lysosomal digestion, and their recycling occurs rapidly, reinforcing the dopaminergic signal transduction. In addition, the pathological impact of the receptors interacting with specific proteins has been the focus of much attention. Based on this background, this chapter provides an in-depth understanding of the mechanisms of molecules interacting with dopamine receptors and discusses the potential pharmacotherapeutic targets for α-synucleinopathies and neuropsychiatric disorders.

Double life: How GRK2 and β-arrestin signaling participate in diseases

G-protein coupled receptor (GPCR) kinases (GRKs) and β-arrestins play key roles in GPCR and non-GPCR cellular responses. In fact, GRKs and arrestins are involved in a plethora of pathways vital for physiological maintenance of inter- and intracellular communication. Here we review decades of research literature spanning from the discovery, identification of key structural elements, and findings supporting the diverse roles of these proteins in GPCR-mediated pathways. We then describe how GRK2 and β-arrestins partake in non-GPCR signaling and briefly summarize their involvement in various pathologies. We conclude by presenting gaps in knowledge and our prospective on the promising pharmacological potential in targeting these proteins and/or downstream signaling. Future research is warranted and paramount for untangling these novel and promising roles for GRK2 and arrestins in metabolism and disease progression.

Dopamine Reduces SARS-CoV-2 Replication In Vitro through Downregulation of D2 Receptors and Upregulation of Type-I Interferons

Recent evidence suggests that SARS-CoV-2 hinders immune responses via dopamine (DA)-related mechanisms. Nonetheless, studies addressing the specific role of DA in the frame of SARS-CoV-2 infection are still missing. In the present study, we investigate the role of DA in SARS-CoV-2 replication along with potential links with innate immune pathways in CaLu-3 human epithelial lung cells. We document here for the first time that, besides DA synthetic pathways, SARS-CoV-2 alters the expression of D1 and D2 DA receptors (D1DR, D2DR), while DA administration reduces viral replication. Such an effect occurs at non-toxic, micromolar-range DA doses, which are known to induce receptor desensitization and downregulation. Indeed, the antiviral effects of DA were associated with a robust downregulation of D2DRs both at mRNA and protein levels, while the amount of D1DRs was not significantly affected. While halting SARS-CoV-2 replication, DA, similar to the D2DR agonist quinpirole, upregulates the expression of ISGs and Type-I IFNs, which goes along with the downregulation of various pro-inflammatory mediators. In turn, administration of Type-I IFNs, while dramatically reducing SARS-CoV-2 replication, converges in downregulating D2DRs expression. Besides configuring the CaLu-3 cell line as a suitable model to study SARS-CoV-2-induced alterations at the level of the DA system in the periphery, our findings disclose a previously unappreciated correlation between DA pathways and Type-I IFN response, which may be disrupted by SARS-CoV-2 for host cell invasion and replication.

Mutation of putative phosphorylation sites in the free fatty acid receptor 1: Effects on signaling, receptor phosphorylation, and internalization

Free fatty acid receptor 1 phosphorylation sites were studied using mutants, including a) a mutant with T215V in the third intracellular loop (3IL), b) another with changes in the carboxyl terminus (C-term): T287V, T293V, S298A, and c) a mutant with all of these changes (3IL/C-term). Agonist-induced increases in intracellular calcium were similar between cells expressing wild-type or mutant receptors. In contrast, agonist-induced FFA1 receptor phosphorylation was reduced in mutants compared to wild type. Phorbol ester-induced FFA1 receptor phosphorylation was rapid and robust in cells expressing the wild-type receptor and essentially abolished in the mutants. Agonist-induced ERK 1/2 phosphorylation and receptor internalization were decreased in cells expressing the mutant receptors compared to those expressing the wild-type receptor. Our data suggest that the identified sites might participate in receptor phosphorylation, signaling, and internalization.

QR code model: a new possibility for GPCR phosphorylation recognition

G protein-coupled receptors (GPCRs) are the largest family of membrane proteins in the human body and are responsible for accurately transmitting extracellular information to cells. Arrestin is an important member of the GPCR signaling pathway. The main function of arrestin is to assist receptor desensitization, endocytosis and signal transduction. In these processes, the recognition and binding of arrestin to phosphorylated GPCRs is fundamental. However, the mechanism by which arrestin recognizes phosphorylated GPCRs is not fully understood. The GPCR phosphorylation recognition "bar code model" and "flute" model describe the basic process of receptor phosphorylation recognition in terms of receptor phosphorylation sites, arrestin structural changes and downstream signaling. These two models suggest that GPCR phosphorylation recognition is a process involving multiple factors. This process can be described by a "QR code" model in which ligands, GPCRs, G protein-coupled receptor kinase, arrestin, and phosphorylation sites work together to determine the biological functions of phosphorylated receptors. Video Abstract.

Delineating the interactions between the cannabinoid CB2 receptor and its regulatory effectors; β-arrestins and G protein-coupled receptor kinases

BACKGROUND AND PURPOSE

The cannabinoid CB₂ receptor (CB₂ ) is a promising therapeutic target for modulating inflammation. However, little is known surrounding the mechanisms underpinning CB₂ desensitisation and regulation, particularly the role of G protein-coupled receptor kinases (GRKs). Here, we evaluated the role of six GRK isoforms in β-arrestin recruitment to CB₂ . Mutagenesis of several distal C-terminal aspartic acid residues was also performed in an attempt to delineate additional structural elements involved in the regulation of CB₂ .

EXPERIMENTAL APPROACH

In CB₂ -expressing HEK 293 cells, β-arrestin translocation was measured using real-time BRET assays. G protein dissociation BRET assays were performed to assess the activation and desensitisation of CB₂ in the presence of β-arrestin 2.

KEY RESULTS

Overexpression of GRK isoforms 1-6 failed to considerably improve translocation of either β-arrestin 1 or β-arrestin 2 to CB₂ . Consistent with this, inhibition of endogenous GRK2/3 did not substantially reduce β-arrestin 2 translocation. Mutagenesis of C-terminal aspartic acid residues resulted in attenuation of β-arrestin 2 translocation, which translated to a reduction in desensitisation of G protein activation.

CONCLUSION AND IMPLICATIONS

Our findings suggest that CB₂ does not adhere to the classical GPCR regulatory paradigm, entailing GRK- and β-arrestin-mediated desensitisation. Instead, C-terminal aspartic acid residues may act as phospho-mimics to induce β-arrestin activation. This study provides novel insights into the regulatory mechanisms of CB₂ , which may aid in our understanding of drug tolerance and dependence.

Signaling-Biased and Constitutively Active Dopamine D2 Receptor Variant

A dopamine D2 receptor mutation was recently identified in a family with a novel hyperkinetic movement disorder. Compared to the wild type D2 receptor, the novel allelic variant D2-I²¹²F activates a Gα_i1β₁γ₂ heterotrimer with higher potency and modestly enhanced basal activity in human embryonic kidney (HEK) 293 cells and has decreased capacity to recruit arrestin3. We now report that omitting overexpressed G protein-coupled receptor kinase-2 (GRK2) decreased the potency and efficacy of quinpirole for arrestin recruitment. The relative efficacy of quinpirole for arrestin recruitment to D2-I²¹²F compared to D2-WT was considerably lower without overexpressed GRK2 than with added GRK2. D2-I²¹²F exhibited higher basal activation of Gα_oA than Gα_i1 but little or no increase in the potency of quinpirole relative to D2-WT. Other signs of D2-I²¹²F constitutive activity for G protein-mediated signaling, in addition to basal activation of Gα_i/o, were enhanced basal inhibition of forskolin-stimulated cyclic AMP accumulation that was reversed by the inverse agonists sulpiride and spiperone and a ∼4-fold increase in the apparent affinity of D2-I²¹²F for quinpirole, determined from competition binding assays. In mouse midbrain slices, inhibition of tonic current by the inverse agonist sulpiride in dopamine neurons expressing D2-I²¹²F was consistent with our hypothesis of enhanced constitutive activity and sensitivity to dopamine relative to D2-WT. Molecular dynamics simulations with D2 receptor models suggested that an ionic lock between the cytoplasmic ends of the third and sixth α-helices that constrains many G protein-coupled receptors in an inactive conformation spontaneously breaks in D2-I²¹²F. Overall, these results confirm that D2-I²¹²F is a constitutively active and signaling-biased D2 receptor mutant and also suggest that the effect of the likely pathogenic variant in a given brain region will depend on the nature of G protein and GRK expression.

New phosphosite-specific antibodies to unravel the role of GRK phosphorylation in dopamine D2 receptor regulation and signaling

The dopamine D₂ receptor (D₂R) is the target of drugs used to treat the symptoms of Parkinson’s disease and schizophrenia. The D₂R is regulated through its interaction with and phosphorylation by G protein receptor kinases (GRKs) and interaction with arrestins. More recently, D₂R arrestin-mediated signaling has been shown to have distinct physiological functions to those of G protein signalling. Relatively little is known regarding the patterns of D₂R phosphorylation that might control these processes. We aimed to generate antibodies specific for intracellular D₂R phosphorylation sites to facilitate the investigation of these mechanisms. We synthesised double phosphorylated peptides corresponding to regions within intracellular loop 3 of the hD₂R and used them to raise phosphosite-specific antibodies to capture a broad screen of GRK-mediated phosphorylation. We identify an antibody specific to a GRK2/3 phosphorylation site in intracellular loop 3 of the D₂R. We compared measurements of D₂R phosphorylation with other measurements of D₂R signalling to profile selected D₂R agonists including previously described biased agonists. These studies demonstrate the utility of novel phosphosite-specific antibodies to investigate D₂R regulation and signalling.

The short third intracellular loop and cytoplasmic tail of bitter taste receptors provide functionally relevant GRK phosphorylation sites in TAS2R14

For most G protein–coupled receptors, the third intracellular loop (IL3) and carboxy-terminal tail (CT) are sites for G protein–coupled receptor kinase (GRK)–mediated phosphorylation, leading to β-arrestin binding and agonist-specific desensitization. These regions of bitter taste receptors (TAS2Rs) are extremely short compared with the superfamily, and their function in desensitization is unknown. TAS2R14 expressed on human airway smooth muscle cells relax the cell, suggesting a novel target for bronchodilators. To assess IL3 and CT in agonist-promoted TAS2R14 desensitization (tachyphylaxis), we generated fusion proteins of both the WT sequence and Ala substituted for Ser/Thr in the IL3 and CT sequences. In vitro, activated GRK2 phosphorylated WT IL3 and WT CT proteins but not Ala-substituted forms. TAS2R14s with mutations in IL3 (IL-5A), CT (CT-5A), and in both regions (IL/CT-10A) were expressed in human embryonic kidney 293T cells. IL/CT-10A and CT-5A failed to undergo desensitization of the intracellular calcium response compared with WT, indicating that functional desensitization by GRK phosphorylation is at residues in the CT. Desensitization of TAS2R14 was blocked by GRK2 knockdown in human airway smooth muscle cells. Receptor:β-arrestin binding was absent in IL/CT-10A and CT-5A and reduced in IL-5A, indicating a role for IL3 phosphorylation in the β-arrestin interaction for this function. Agonist-promoted internalization of IL-5A and CT-5A receptors was impaired, and they failed to colocalize with early endosomes. Thus, agonist-promoted functional desensitization of TAS2R14 occurs by GRK phosphorylation of CT residues and β-arrestin binding. However, β-arrestin function in the internalization and trafficking of the receptor also requires GRK phosphorylation of IL3 residues.

Resistance to Dopamine Agonists in Pituitary Tumors: Molecular Mechanisms

Pituitary neuroendocrine tumors (PitNET) are commonly benign tumors accounting for 10-25% of intracranial tumors. Prolactin-secreting adenomas represent the most predominant type of all PitNET and for this subtype of tumors, the medical therapy relies on the use of dopamine agonists (DAs). DAs yield an excellent therapeutic response in reducing tumor size and hormonal secretion targeting the dopamine receptor type 2 (D2DR) whose higher expression in prolactin-secreting adenomas compared to other PitNET is now well established. Moreover, although DAs therapy does not represent the first-line therapy for other PitNET, off-label use of DAs is considered in PitNET expressing D2DR. Nevertheless, DAs primary or secondary resistance, occurring in a subset of patients, may involve several molecular mechanisms, presently not fully elucidated. Dopamine receptors (DRs) expression is a prerequisite for a proper DA function in PitNET and several molecular events may negatively modify DR membrane expression, through the DRs down-regulation and intracellular trafficking, and DR signal transduction pathway. The current mini-review will summarise the presently known molecular events that underpin the unsuccessful therapy with DAs.

GRKs as Modulators of Neurotransmitter Receptors

Many receptors for neurotransmitters, such as dopamine, norepinephrine, acetylcholine, and neuropeptides, belong to the superfamily of G protein-coupled receptors (GPCRs). A general model posits that GPCRs undergo two-step homologous desensitization: the active receptor is phosphorylated by kinases of the G protein-coupled receptor kinase (GRK) family, whereupon arrestin proteins specifically bind active phosphorylated receptors, shutting down G protein-mediated signaling, facilitating receptor internalization, and initiating distinct signaling pathways via arrestin-based scaffolding. Here, we review the mechanisms of GRK-dependent regulation of neurotransmitter receptors, focusing on the diverse modes of GRK-mediated phosphorylation of receptor subtypes. The immediate signaling consequences of GRK-mediated receptor phosphorylation, such as arrestin recruitment, desensitization, and internalization/resensitization, are equally diverse, depending not only on the receptor subtype but also on phosphorylation by GRKs of select receptor residues. We discuss the signaling outcome as well as the biological and behavioral consequences of the GRK-dependent phosphorylation of neurotransmitter receptors where known.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

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.

Phosphorylation barcode-dependent signal bias of the dopamine D1 receptor

Agonist-activated G protein-coupled receptors (GPCRs) must correctly select from hundreds of potential downstream signaling cascades and effectors. To accomplish this, GPCRs first bind to an intermediary signaling protein, such as G protein or arrestin. These intermediaries initiate signaling cascades that promote the activity of different effectors, including several protein kinases. The relative roles of G proteins versus arrestins in initiating and directing signaling is hotly debated, and it remains unclear how the correct final signaling pathway is chosen given the ready availability of protein partners. Here, we begin to deconvolute the process of signal bias from the dopamine D1 receptor (D1R) by exploring factors that promote the activation of ERK1/2 or Src, the kinases that lead to cell growth and proliferation. We found that ERK1/2 activation involves both arrestin and Gαs, while Src activation depends solely on arrestin. Interestingly, we found that the phosphorylation pattern influences both arrestin and Gαs coupling, suggesting an additional way the cells regulate G protein signaling. The phosphorylation sites in the D1R intracellular loop 3 are particularly important for directing the binding of G protein versus arrestin and for selecting between the activation of ERK1/2 and Src. Collectively, these studies correlate functional outcomes with a physical basis for signaling bias and provide fundamental information on how GPCR signaling is directed.

A structural basis for how ligand binding site changes can allosterically regulate GPCR signaling and engender functional selectivity

Signaling bias is the propensity for some agonists to preferentially stimulate G protein-coupled receptor (GPCR) signaling through one intracellular pathway versus another. We previously identified a G protein-biased agonist of the D₂ dopamine receptor (D2R) that results in impaired β-arrestin recruitment. This signaling bias was predicted to arise from unique interactions of the ligand with a hydrophobic pocket at the interface of the second extracellular loop and fifth transmembrane segment of the D2R. Here, we showed that residue Phe189 within this pocket (position 5.38 using Ballesteros-Weinstein numbering) functions as a microswitch for regulating receptor interactions with β-arrestin. This residue is relatively conserved among class A GPCRs, and analogous mutations within other GPCRs similarly impaired β-arrestin recruitment while maintaining G protein signaling. To investigate the mechanism of this signaling bias, we used an active-state structure of the β₂-adrenergic receptor (β2R) to build β2R-WT and β2R-Y199^5.38A models in complex with the full β2R agonist BI-167107 for molecular dynamics simulations. These analyses identified conformational rearrangements in β2R-Y199^5.38A that propagated from the extracellular ligand binding site to the intracellular surface, resulting in a modified orientation of the second intracellular loop in β2R-Y199^5.38A, which is predicted to affect its interactions with β-arrestin. Our findings provide a structural basis for how ligand binding site alterations can allosterically affect GPCR-transducer interactions and result in biased signaling.

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.

Role of Helix 8 in Dopamine Receptor Signaling

G protein-coupled receptors (GPCRs) are membrane receptors whose agonist-induced dynamic conformational changes trigger heterotrimeric G protein activation, followed by GRK-mediated phosphorylation and arrestin-mediated desensitization. Cytosolic regions of GPCRs have been studied extensively because they are direct contact sites with G proteins, GRKs, and arrestins. Among various cytosolic regions, the role of helix 8 is least understood, although a few studies have suggested that it is involved in G protein activation, receptor localization, and/or internalization. In the present study, we investigated the role of helix 8 in dopamine receptor signaling focusing on dopamine D1 receptor (D1R) and dopamine D2 receptor (D2R). D1R couples exclusively to Gs, whereas D2R couples exclusively to Gi. Bioinformatic analysis implied that the sequences of helix 8 may affect GPCR-G protein coupling selectivity; therefore, we evaluated if swapping helix 8 between D1R and D2R changed G protein selectivity. Our results suggest that helix 8 is not involved in D1R-Gs or D2R-Gi coupling selectivity. Instead, we observed that D1R with D2R helix 8 or D1R with an increased number of hydrophobic residues in helix 8 relative to wild-type showed diminished β-arrestin-mediated desensitization, resulting in increased Gs signaling.

Critical role of the finger loop in arrestin binding to the receptors

We tested the interactions with four different G protein-coupled receptors (GPCRs) of arrestin-3 mutants with substitutions in the four loops, three of which contact the receptor in the structure of the arrestin-1-rhodopsin complex. Point mutations in the loop at the distal tip of the N-domain (Glu157Ala), in the C-loop (Phe255Ala), back loop (Lys313Ala), and one of the mutations in the finger loop (Gly65Pro) had mild variable effects on receptor binding. In contrast, the deletion of Gly65 at the beginning of the finger loop reduced the binding to all GPCRs tested, with the binding to dopamine D2 receptor being affected most dramatically. Thus, the presence of a glycine at the beginning of the finger loop appears to be critical for the arrestin-receptor interaction.

Assessing the real-time activation of the cannabinoid CB1 receptor and the associated structural changes using a FRET biosensor

The cannabinoid receptor 1 (CB1) is mainly expressed in the nervous system and regulates learning, memory processes, pain and energy metabolism. However, there is no way to directly measure its activation. In this study, we constructed a CB1 intramolecular fluorescence resonance energy transfer (FRET) sensor, which could measure CB1 activation by monitoring structural changes between the third intracellular loop and the C-terminal tail. CB1 agonists induced a time- and concentration-dependent increase in the FRET signal, corresponding to a reduction in the distance between the third intracellular loop and the C-terminal tail. This, in turn, mobilized intracellular Ca²⁺, inhibited cAMP accumulation, and increased phosphorylation of the ERK1/2 MAP kinases. The activation kinetics detected using this method were consistent with those from previous reports. Moreover, the increased FRET signal was markedly inhibited by the CB1 antagonist rimonabant, which also reduced phosphorylation of the ERK1/2 MAP kinases. We mutated a single cysteine residue in the sensor (at position 257 or 264) to alanine. Both mutation reduced the agonist-induced increase in FRET signal and structural changes in the CB1 receptor, which attenuated phosphorylation of the ERK1/2 MAP kinases. In summary, our sensor directly assesses the kinetics of CB1 activation in real-time and can be used to monitor CB1 structure and function.

Role of G protein-regulated inducer of neurite outgrowth 3 (GRIN3) in β-arrestin 2-Akt signaling and dopaminergic behaviors

The G protein-regulated inducer of neurite growth (GRIN) family has three isoforms (GRIN1-3), which bind to the Gαi/o subfamily of G protein that mediate signal processing via G protein-coupled receptors (GPCRs). Here, we show that GRIN3 is involved in regulation of dopamine-dependent behaviors and is essential for activation of the dopamine receptors (DAR)-β-arrestin signaling cascade. Analysis of functional regions of GRIN3 showed that a di-cysteine motif (Cys751/752) is required for plasma membrane localization. GRIN3 was co-immunoprecipitated with GPCR kinases 2/6 and β-arrestins 1/2. Among GRINs, only GRIN3, which is highly expressed in striatum, strongly interacted with β-arrestin 2. We also generated GRIN3-knockout mice (GRIN3KO). GRIN3KO exhibited reduced locomotor activity and increased anxiety-like behavior in the elevated maze test, as well as a reduced locomoter response to dopamine stimulation. We also examined the phosphorylation of Akt at threonine 308 (phospho308-Akt), which is dephosphorylated via a β-arrestin 2-mediated pathway. Dephosphorylation of phospho308-Akt via the D2R-β-arrestin 2 signaling pathway was completely abolished in striatum of GRIN3KO. Our results suggest that GRIN3 has a role in recruitment and assembly of proteins involved in β-arrestin-dependent, G protein-independent signaling.

Dopamine negatively modulates the NCA ion channels in C. elegans

The NALCN/NCA ion channel is a cation channel related to voltage-gated sodium and calcium channels. NALCN has been reported to be a sodium leak channel with a conserved role in establishing neuronal resting membrane potential, but its precise cellular role and regulation are unclear. The Caenorhabditis elegans orthologs of NALCN, NCA-1 and NCA-2, act in premotor interneurons to regulate motor circuit activity that sustains locomotion. Recently we found that NCA-1 and NCA-2 are activated by a signal transduction pathway acting downstream of the heterotrimeric G protein Gq and the small GTPase Rho. Through a forward genetic screen, here we identify the GPCR kinase GRK-2 as a new player affecting signaling through the Gq-Rho-NCA pathway. Using structure-function analysis, we find that the GPCR phosphorylation and membrane association domains of GRK-2 are required for its function. Genetic epistasis experiments suggest that GRK-2 acts on the D2-like dopamine receptor DOP-3 to inhibit Go signaling and positively modulate NCA-1 and NCA-2 activity. Through cell-specific rescuing experiments, we find that GRK-2 and DOP-3 act in premotor interneurons to modulate NCA channel function. Finally, we demonstrate that dopamine, through DOP-3, negatively regulates NCA activity. Thus, this study identifies a pathway by which dopamine modulates the activity of the NCA channels.

Desensitized D2 autoreceptors are resistant to trafficking

Dendritic release of dopamine activates dopamine D2 autoreceptors, which are inhibitory G protein-coupled receptors (GPCRs), to decrease the excitability of dopamine neurons. This study used tagged D2 receptors to identify the localization and distribution of these receptors in living midbrain dopamine neurons. GFP-tagged D2 receptors were found to be unevenly clustered on the soma and dendrites of dopamine neurons within the substantia nigra pars compacta (SNc). Physiological signaling and desensitization of the tagged receptors were not different from wild type receptors. Unexpectedly, upon desensitization the tagged D2 receptors were not internalized. When tagged D2 receptors were expressed in locus coeruleus neurons, a desensitizing protocol induced significant internalization. Likewise, when tagged µ-opioid receptors were expressed in dopamine neurons they too were internalized. The distribution and lack of agonist-induced internalization of D2 receptors on dopamine neurons indicate a purposefully regulated localization of these receptors.

Novel Bivalent Ligands Based on the Sumanirole Pharmacophore Reveal Dopamine D2 Receptor (D2R) Biased Agonism

The development of bivalent ligands has attracted interest as a way to potentially improve the selectivity and/or affinity for a specific receptor subtype. The ability to bind two distinct receptor binding sites simultaneously can allow the selective activation of specific G-protein dependent or β-arrestin-mediated cascade pathways. Herein, we developed an extended SAR study using sumanirole (1) as the primary pharmacophore. We found that substitutions in the N-1- and/or N-5-positions, physiochemical properties of those substituents, and secondary aromatic pharmacophores can enhance agonist efficacy for the cAMP inhibition mediated by G_i/o-proteins, while reducing or suppressing potency and efficacy toward β-arrestin recruitment. Compound 19 was identified as a new lead for its selective D₂ G-protein biased agonism with an EC₅₀ in the subnanomolar range. Structure-activity correlations were observed between substitutions in positions N-1 and/or N-5 of 1 and the capacity of the new bivalent compounds to selectively activate G-proteins versus β-arrestin recruitment in D₂R-BRET functional assays.

RGS2 modulates the activity and internalization of dopamine D2 receptors in neuroblastoma N2A cells

Dysregulated expression and function of dopamine D2 receptors (D2Rs) are implicated in drug addiction, Parkinson's disease and schizophrenia. In the current study, we examined whether D2Rs are modulated by regulator of G protein signaling 2 (RGS2), a member of the RGS family that regulates G protein signaling via acceleration of GTPase activity. Using neuroblastoma 2a (N2A) cells, we found that RGS2 was immunoprecipitated by aluminum fluoride-activated Gαi2 proteins. RGS2 siRNA knockdown enhanced membrane [(35)S] GTPγS binding to activated Gαi/o proteins, augmented inhibition of cAMP accumulation and increased ERK phosphorylation in the presence of a D2/D3R agonist quinpirole when compared to scrambled siRNA treatment. These data suggest that RGS2 is a negative modulator of D2R-mediated Gαi/o signaling. Moreover, RGS2 knockdown slightly increased constitutive D2R internalization and markedly abolished quinpirole-induced D2R internalization assessed by immunocytochemistry. RGS2 knockdown did not compromise agonist-induced β-arrestin membrane recruitment; however, it prevents β-arrestin dissociation from the membrane after prolonged quinpirole treatment during which time β-arrestin moved away from the membrane in control cells. Additionally, confocal microscopy analysis of β-arrestin post-endocytic fate revealed that quinpirole treatment caused β-arrestin to translocate to the early and the recycling endosome in a time-dependent manner in control cells whereas translocation of β-arrestin to these endosomes did not occur in RGS2 knockdown cells. The impaired β-arrestin translocation likely contributed to the abolishment of quinpirole-stimulated D2R internalization in RGS2 knockdown cells. Thus, RGS2 is integral for β-arrestin-mediated D2R internalization. The current study revealed a novel regulation of D2R signaling and internalization by RGS2 proteins.

Heterologous, PKC-Mediated Desensitization of Human Histamine H3 Receptors Expressed in CHO-K1 Cells

Desensitization is a major mechanism to regulate the functional response of G protein-coupled receptors. In this work we studied whether the human histamine H3 receptor of 445 amino acids (hH3R445) experiences heterologous desensitization mediated by PKC activation. Bioinformatic analysis indicated the presence of Serine and Threonine residues susceptible of PKC-mediated phosphorylation on the third intracellular loop and the carboxyl terminus of the hH3R445. In CHO-K1 cells stably transfected with the hH3R445 direct PKC activation by phorbol 12-myristate 13-acetate (TPA, 200 nM) abolished H3R-mediated inhibition of forskolin-stimulated cAMP accumulation. Activation of endogenous purinergic receptors by ATP (adenosine 5'-triphosphate, 10 μM) increased the free calcium intracellular concentration ([Ca(2+)]i) confirming their coupling to phospholipase C stimulation. Incubation with ATP also abolished H3R-mediated inhibition of forskolin-induced cAMP accumulation, and this effect was prevented by the PKC inhibitors Ro-31-8220 and Gö-6976. Pre-incubation with TPA or ATP reduced H3R-mediated stimulation of [(35)S]-GTPγS binding to membranes from CHO-K1-hH3R445 cells by 39.7 and 54.2 %, respectively, with no change in the agonist potency, and the effect was prevented by either Ro-31-8220 or Gö-6976. Exposure to ATP or TPA also resulted in the loss of cell surface H3Rs (-30.4 and -45.1 %) as evaluated by [(3)H]-NMHA binding to intact cells. These results indicate that the hH3R445 undergoes heterologous desensitization upon activation of receptors coupled to PKC stimulation.

G protein-coupled receptor kinases as regulators of dopamine receptor functions

Actions of the neurotransmitter dopamine in the brain are mediated by dopamine receptors that belong to the superfamily of G protein-coupled receptors (GPCRs). Mammals have five dopamine receptor subtypes, D1 through D5. D1 and D5 couple to Gs/olf and activate adenylyl cyclase, whereas D2, D3, and D4 couple to Gi/o and inhibit it. Most GPCRs upon activation by an agonist are phosphorylated by GPCR kinases (GRKs). The GRK phosphorylation makes receptors high-affinity binding partners for arrestin proteins. Arrestin binding to active phosphorylated receptors stops further G protein activation and promotes receptor internalization, recycling or degradation, thereby regulating their signaling and trafficking. Four non- visual GRKs are expressed in striatal neurons. Here we describe known effects of individual GRKs on dopamine receptors in cell culture and in the two in vivo models of dopamine-mediated signaling: behavioral response to psychostimulants and L-DOPA- induced dyskinesia. Dyskinesia, associated with dopamine super-sensitivity of striatal neurons, is a debilitating side effect of L-DOPA therapy in Parkinson's disease. In vivo, GRK subtypes show greater receptor specificity than in vitro or in cultured cells. Overexpression, knockdown, and knockout of individual GRKs, particularly GRK2 and GRK6, have differential effects on signaling of dopamine receptor subtypes in the brain. Furthermore, deletion of GRK isoforms in select striatal neuronal types differentially affects psychostimulant-induced behaviors. In addition, anti-dyskinetic effect of GRK3 does not require its kinase activity: it is mediated by the binding of its RGS-like domain to Gαq/11, which suppresses Gq/11 signaling. The data demonstrate that the dopamine signaling in defined neuronal types in vivo is regulated by specific and finely orchestrated actions of GRK isoforms.

Evidence for Noncanonical Neurotransmitter Activation: Norepinephrine as a Dopamine D2-Like Receptor Agonist

The Gαi/o-coupled dopamine D2-like receptor family comprises three subtypes: the D2 receptor (D2R), with short and long isoform variants (D2SR and D2LR), D3 receptor (D3R), and D4 receptor (D4R), with several polymorphic variants. The common overlap of norepinephrine innervation and D2-like receptor expression patterns prompts the question of a possible noncanonical action by norepinephrine. In fact, previous studies have suggested that norepinephrine can functionally interact with D4R. To our knowledge, significant interactions between norepinephrine and D2R or D3R receptors have not been demonstrated. By using radioligand binding and bioluminescent resonance energy transfer (BRET) assays in transfected cells, the present study attempted a careful comparison between dopamine and norepinephrine in their possible activation of all D2-like receptors, including the two D2R isoforms and the most common D4R polymorphic variants. Functional BRET assays included activation of G proteins with all Gαi/o subunits, adenylyl cyclase inhibition, and β arrestin recruitment. Norepinephrine acted as a potent agonist for all D2-like receptor subtypes, with the general rank order of potency of D3R > D4R ≥ D2SR ≥ D2L. However, for both dopamine and norepinephrine, differences depended on the Gαi/o protein subunit involved. The most striking differences were observed with Gαi2, where the rank order of potencies for both dopamine and norepinephrine were D4R > D2SR = D2LR >> D3R. Furthermore the results do not support the existence of differences in the ability of dopamine and norepinephrine to activate different human D4R variants. The potency of norepinephrine for adrenergic α2A receptor was only about 20-fold higher compared with D3R and D4R across the three functional assays.

Dysbindin-1 modifies signaling and cellular localization of recombinant, human D₃ and D₂ receptors

Dystrobrevin binding protein-1 (dysbindin-1), a candidate gene for schizophrenia, modulates cognition, synaptic plasticity and frontocortical circuitry and interacts with glutamatergic and dopaminergic transmission. Loss of dysbindin-1 modifies cellular trafficking of dopamine (DA) D2 receptors to increase cell surface expression, but its influence upon signaling has never been characterized. Further, the effects of dysbindin-1 upon closely related D3 receptors remain unexplored. Hence, we examined the impact of dysbindin-1 (isoform A) co-expression on the localization and coupling of human D2L and D3 receptors stably expressed in Chinese hamster ovary or SH-SY5Y cells lacking endogenous dysbindin-1. Dysbindin-1 co-transfection decreased cell surface expression of both D3 and D2L receptors. Further, while their affinity for DA was unchanged, dysbindin-1 reduced the magnitude and potency of DA-induced adenylate cylase recruitment/cAMP production. Dysbindin-1 also blunted the amplitude of DA-induced phosphorylation of ERK1/2 and Akt at both D2L and D3 receptors without, in contrast to cAMP, affecting the potency of DA. Interference with calveolin/clathrin-mediated processes of internalization prevented the modification by dysbindin-1 of ERK1/2 and adenylyl cyclase stimulation at D2L and D3 receptors. Finally, underpinning the specificity of the influence of dysbindin-1 on D2L and D3 receptors, dysbindin-1 did not modify recruitment of adenylyl cyclase by D1 receptors. These observations demonstrate that dysbindin-1 influences cell surface expression of D3 in addition to D2L receptors, and that it modulates activation of their signaling pathways. Accordingly, both a deficiency and an excess of dysbindin-1 may be disruptive for dopaminergic transmission, supporting its link to schizophrenia and other CNS disorders. Dysbindin-1, a candidate gene for schizophrenia, alters D2 receptors cell surface expression. We demonstrate that dysbindin-1 expression also influences cell surface levels of D3 receptors. Further, Dysbindin-1 reduces DA-induced adenylate cylase recruitment/cAMP production and modifies major signaling pathways (Akt and extracellular signal-regulated kinases1/2 (ERK1/2)) of both D2 and D3 receptors. Dysbindin-1 modulates thus D2 and D3 receptor signaling, supporting a link to schizophrenia.

Elucidation of G-protein and β-arrestin functional selectivity at the dopamine D2 receptor

The neuromodulator dopamine signals through the dopamine D2 receptor (D2R) to modulate central nervous system functions through diverse signal transduction pathways. D2R is a prominent target for drug treatments in disorders where dopamine function is aberrant, such as schizophrenia. D2R signals through distinct G-protein and β-arrestin pathways, and drugs that are functionally selective for these pathways could have improved therapeutic potential. How D2R signals through the two pathways is still not well defined, and efforts to elucidate these pathways have been hampered by the lack of adequate tools for assessing the contribution of each pathway independently. To address this, Evolutionary Trace was used to produce D2R mutants with strongly biased signal transduction for either the G-protein or β-arrestin interactions. These mutants were used to resolve the role of G proteins and β-arrestins in D2R signaling assays. The results show that D2R interactions with the two downstream effectors are dissociable and that G-protein signaling accounts for D2R canonical MAP kinase signaling cascade activation, whereas β-arrestin only activates elements of this cascade under certain conditions. Nevertheless, when expressed in mice in GABAergic medium spiny neurons of the striatum, the β-arrestin-biased D2R caused a significant potentiation of amphetamine-induced locomotion, whereas the G protein-biased D2R had minimal effects. The mutant receptors generated here provide a molecular tool set that should enable a better definition of the individual roles of G-protein and β-arrestin signaling pathways in D2R pharmacology, neurobiology, and associated pathologies.

Mutation of three residues in the third intracellular loop of the dopamine D2 receptor creates an internalization-defective receptor

Arrestins mediate desensitization and internalization of G protein-coupled receptors and also direct receptor signaling toward heterotrimeric G protein-independent signaling pathways. We previously identified a four-residue segment (residues 212-215) of the dopamine D2 receptor that is necessary for arrestin binding in an in vitro heterologous expression system but that also impairs receptor expression. We now describe the characterization of additional mutations at that arrestin binding site in the third intracellular loop. Mutating two (residues 214 and 215) or three (residues 213-215) of the four residues to alanine partially decreased agonist-induced recruitment of arrestin3 without altering activation of a G protein. Arrestin-dependent receptor internalization, which requires arrestin binding to β2-adaptin (the β2 subunit of the clathrin-associated adaptor protein AP2) and clathrin, was disproportionately affected by the three-residue mutation, with no agonist-induced internalization observed even in the presence of overexpressed arrestin or G protein-coupled receptor kinase 2. The disjunction between arrestin recruitment and internalization could not be explained by alterations in the time course of the receptor-arrestin interaction, the recruitment of G protein-coupled receptor kinase 2, or the receptor-induced interaction between arrestin and β2-adaptin, suggesting that the mutation impairs a property of the internalization complex that has not yet been identified.

Identification of serine 348 on the apelin receptor as a novel regulatory phosphorylation site in apelin-13-induced G protein-independent biased signaling

Phosphorylation plays vital roles in the regulation of G protein-coupled receptor (GPCR) functions. The apelin and apelin receptor (APJ) system is involved in the regulation of cardiovascular function and central control of body homeostasis. Here, using tandem mass spectrometry, we first identified phosphorylated serine residues in the C terminus of APJ. To determine the role of phosphorylation sites in APJ-mediated G protein-dependent and -independent signaling and function, we induced a mutation in the C-terminal serine residues and examined their effects on the interaction between APJ with G protein or GRK/β-arrestin and their downstream signaling. Mutation of serine 348 led to an elimination of both GRK and β-arrestin recruitment to APJ induced by apelin-13. Moreover, APJ internalization and G protein-independent ERK signaling were also abolished by point mutation at serine 348. In contrast, this mutant at serine residues had no demonstrable impact on apelin-13-induced G protein activation and its intracellular signaling. These findings suggest that mutation of serine 348 resulted in inactive GRK/β-arrestin. However, there was no change in the active G protein thus, APJ conformation was biased. These results provide important information on the molecular interplay and impact of the APJ function, which may be extrapolated to design novel drugs for cardiac hypertrophy based on this biased signal pathway.

Selective deletion of GRK2 alters psychostimulant-induced behaviors and dopamine neurotransmission

GRK2 is a G protein-coupled receptor kinase (GRK) that is broadly expressed and is known to regulate diverse types of receptors. GRK2 null animals exhibit embryonic lethality due to a severe developmental heart defect, which has precluded the study of this kinase in the adult brain. To elucidate the specific role of GRK2 in the brain dopamine (DA) system, we used a conditional gene knockout approach to selectively delete GRK2 in DA D1 receptor (D1R)-, DA D2 receptor (D2R)-, adenosine 2A receptor (A2AR)-, or DA transporter (DAT)-expressing neurons. Here we show that select GRK2-deficient mice display hyperactivity, hyposensitivity, or hypersensitivity to the psychomotor effects of cocaine, altered striatal signaling, and DA release and uptake. Mice with GRK2 deficiency in D2R-expressing neurons also exhibited increased D2 autoreceptor activity. These findings reveal a cell-type-specific role for GRK2 in the regulation of normal motor behavior, sensitivity to psychostimulants, dopamine neurotransmission, and D2 autoreceptor function.

G protein beta 5 is targeted to D2-dopamine receptor-containing biochemical compartments and blocks dopamine-dependent receptor internalization

G beta 5 (Gbeta5, Gβ5) is a unique G protein β subunit that is thought to be expressed as an obligate heterodimer with R7 regulator of G protein signaling (RGS) proteins instead of with G gamma (Gγ) subunits. We found that D2-dopamine receptor (D2R) coexpression enhances the expression of Gβ5, but not that of the G beta 1 (Gβ1) subunit, in HEK293 cells, and that the enhancement of expression occurs through a stabilization of Gβ5 protein. We had previously demonstrated that the vast majority of D2R either expressed endogenously in the brain or exogenously in cell lines segregates into detergent-resistant biochemical fractions. We report that when expressed alone in HEK293 cells, Gβ5 is highly soluble, but is retargeted to the detergent-resistant fraction after D2R coexpression. Furthermore, an in-cell biotin transfer proximity assay indicated that D2R and Gβ5 segregating into the detergent-resistant fraction specifically interacted in intact living cell membranes. Dopamine-induced D2R internalization was blocked by coexpression of Gβ5, but not Gβ1. However, the same Gβ5 coexpression levels had no effect on agonist-induced internalization of the mu opioid receptor (MOR), cell surface D2R levels, dopamine-mediated recruitment of β-arrestin to D2R, the amplitude of D2R-G protein coupling, or the deactivation kinetics of D2R-activated G protein signals. The latter data suggest that the interactions between D2R and Gβ5 are not mediated by endogenously expressed R7 RGS proteins.

Reciprocal regulation of endocytosis and metabolism

The cellular uptake of many nutrients and micronutrients governs both their cellular availability and their systemic homeostasis. The cellular rate of nutrient or ion uptake (e.g., glucose, Fe(3+), K(+)) or efflux (e.g., Na(+)) is governed by a complement of membrane transporters and receptors that show dynamic localization at both the plasma membrane and defined intracellular membrane compartments. Regulation of the rate and mechanism of endocytosis controls the amounts of these proteins on the cell surface, which in many cases determines nutrient uptake or secretion. Moreover, the metabolic action of diverse hormones is initiated upon binding to surface receptors that then undergo regulated endocytosis and show distinct signaling patterns once internalized. Here, we examine how the endocytosis of nutrient transporters and carriers as well as signaling receptors governs cellular metabolism and thereby systemic (whole-body) metabolite homeostasis.

Single and binge methamphetamine administrations have different effects on the levels of dopamine D2 autoreceptor and dopamine transporter in rat striatum

Methamphetamine (METH) is a central nervous system psychostimulant with a high potential for abuse. At high doses, METH causes a selective degeneration of dopaminergic terminals in the striatum. Dopamine D2 receptor antagonists and dopamine transporter (DAT) inhibitors protect against neurotoxicity of the drug by decreasing intracellular dopamine content and, consequently, dopamine autoxidation and production of reactive oxygen species. In vitro, amphetamines regulate D2 receptor and DAT functions via regulation of their intracellular trafficking. No data exists on axonal transport of both proteins and there is limited data on their interactions in vivo. The aim of the present investigation was to examine synaptosomal levels of presynaptic D2 autoreceptor and DAT after two different regimens of METH and to determine whether METH affects the D2 autoreceptor-DAT interaction in the rat striatum. We found that, as compared to saline controls, administration of single high-dose METH decreased D2 autoreceptor immunoreactivity and increased DAT immunoreactivity in rat striatal synaptosomes whereas binge high-dose METH increased immunoreactivity of D2 autoreceptor and had no effect on DAT immunoreactivity. Single METH had no effect on D2 autoreceptor-DAT interaction whereas binge METH increased the interaction between the two proteins in the striatum. Our results suggest that METH can affect axonal transport of both the D2 autoreceptor and DAT in an interaction-dependent and -independent manner.

Targeting individual GPCRs with redesigned nonvisual arrestins

Numerous human diseases are caused by excessive signaling of mutant G protein-coupled receptors (GPCRs) or receptors that are overstimulated due to upstream signaling imbalances. The feasibility of functional compensation by arrestins with enhanced ability to quench receptor signaling was recently tested in the visual system. The results showed that even in this extremely demanding situation of rods that have no ability to phosphorylate rhodopsin, enhanced arrestin improved rod morphology, light sensitivity, survival, and accelerated photoresponse recovery. Structurally distinct enhanced mutants of arrestins that bind phosphorylated and non-phosphorylated active GPCRs with much higher affinity than parental wild-type (WT) proteins have been constructed. These "super-arrestins" are likely to have the power to dampen the signaling by hyperactive GPCRs. However, most cells express 5-20 GPCR subtypes, only one of which would be overactive, while nonvisual arrestins are remarkably promiscuous, binding hundreds of different GPCRs. Thus, to be therapeutically useful, enhanced versions of nonvisual arrestins must be made fairly specific for particular receptors. Recent identification of very few arrestin residues as key receptor discriminators paves the way to the construction of receptor subtype-specific nonvisual arrestins.

Are dopamine D2 receptors out of control in psychosis?

It is known that schizophrenia patients are behaviorally supersensitive to dopamine-like drugs (amphetamine, methylphenidate). There is evidence for an increased release of dopamine, a slight increase of dopamine D2 receptors and an increase of dopamine D2High receptors in schizophrenia, all possibly explaining the clinical supersensitivity to dopamine. The elevation in apparent D2High receptors in vivo in schizophrenia matches the elevation in D2High receptors in many animal models of psychosis. The increased amounts of D2High receptors in psychotic-like behavior in animals may result from a loss of control of D2 by various factors. These factors include the rate of phosphorylation and desensitization of D2 receptors by kinases, the attachment of arrestin to D2 receptors, internalization of D2 receptors, the rate of receptor de-phosphorylation, formation of D2 receptor dimers, and GTP regulation by various GTPases. While at present there are no statistically significant associations of any of these controlling factors and their genes with schizophrenia, investigation of D2High receptors in schizophrenia will require a new radioligand in order to selectively label D2High receptors in vivo in patients. Finally, haloperidol reduces the number of D2High receptors that are elevated by amphetamine, indicating that this therapeutic effect may occur clinically.

Effects of the dopamine D2 allosteric modulator, PAOPA, on the expression of GRK2, arrestin-3, ERK1/2, and on receptor internalization

The activity of G protein-coupled receptors (GPCRs) is intricately regulated by a range of intracellular proteins, including G protein-coupled kinases (GRKs) and arrestins. Understanding the effects of ligands on these signaling pathways could provide insights into disease pathophysiologies and treatment. The dopamine D2 receptor is a GPCR strongly implicated in the pathophysiology of a range of neurological and neuropsychiatric disorders, particularly schizophrenia. Previous studies from our lab have shown the preclinical efficacy of a novel allosteric drug, 3(R)- [(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA), in attenuating schizophrenia-like behavioural abnormalities in rodent models of the disease. As an allosteric modulator, PAOPA binds to a site on the D2 receptor, which is distinct from the endogenous ligand-binding site, in order to modulate the binding of the D2 receptor ligand, dopamine. The exact signaling pathways affected by this allosteric modulator are currently unknown. The objectives of this study were to decipher the in vivo effects, in rats, of chronic PAOPA administration on D2 receptor regulatory and downstream molecules, including GRK2, arrestin-3 and extracellular receptor kinase (ERK) 1/2. Additionally, an in vitro cellular model was also used to study PAOPA’s effects on D2 receptor internalization. Results from western immunoblots showed that chronic PAOPA treatment increased the striatal expression of GRK2 by 41%, arrestin-3 by 34%, phospho-ERK1 by 51% and phospho-ERK2 by 36%. Results also showed that the addition of PAOPA to agonist treatment in cells increased D2 receptor internalization by 33%. This study provides the foundational evidence of putative signaling pathways, and changes in receptor localization, affected by treatment with PAOPA. It improves our understanding on the diverse mechanisms of action of allosteric modulators, while advancing PAOPA’s development into a novel drug for the improved treatment of schizophrenia.

Arrestin-dependent but G-protein coupled receptor kinase-independent uncoupling of D2-dopamine receptors

We reconstituted D2 like dopamine receptor (D2R) and the delta opioid receptor (DOR) coupling to G-protein gated inwardly rectifying potassium channels (K(ir)3) and directly compared the effects of co-expression of G-protein coupled receptor kinase (GRK) and arrestin on agonist-dependent desensitization of the receptor response. We found, as described previously, that co-expression of a GRK and an arrestin synergistically increased the rate of agonist-dependent desensitization of DOR. In contrast, only arrestin expression was required to produce desensitization of D2R responses. Furthermore, arrestin-dependent GRK-independent desensitization of D2R-K(ir)3 coupling could be transferred to DOR by substituting the third cytoplasmic loop of DOR with that of D2R. The arrestin-dependent GRK-independent desensitization of D2R desensitization was inhibited by staurosporine treatment, and blocked by alanine substitution of putative protein kinase C phosphorylation sites in the third cytoplasmic loop of D2R. Finally, the D2R construct in which putative protein kinase C phosphorylation sites were mutated did not undergo significant agonist-dependent desensitization even after GRK co-expression, suggesting that GRK phosphorylation of D2R does not play an important role in uncoupling of the receptor.

D1-D2 dopamine receptor synergy promotes calcium signaling via multiple mechanisms

The D(1) dopamine receptor (D(1)R) has been proposed to form a hetero-oligomer with the D(2) dopamine receptor (D(2)R), which in turn results in a complex that couples to phospholipase C-mediated intracellular calcium release. We have sought to elucidate the pharmacology and mechanism of action of this putative signaling pathway. Dopamine dose-response curves assaying intracellular calcium mobilization in cells heterologously expressing the D(1) and D(2) subtypes, either alone or in combination, and using subtype selective ligands revealed that concurrent stimulation is required for coupling. Surprisingly, characterization of a putative D(1)-D(2) heteromer-selective ligand, 6-chloro-2,3,4,5-tetrahydro-3-methyl-1-(3-methylphenyl)-1H-3-benzazepine-7,8-diol (SKF83959), found no stimulation of calcium release, but it did find a broad range of cross-reactivity with other G protein-coupled receptors. In contrast, SKF83959 appeared to be an antagonist of calcium mobilization. Overexpression of G(qα) with the D(1) and D(2) dopamine receptors enhanced the dopamine-stimulated calcium response. However, this was also observed in cells expressing G(qα) with only the D1R. Inactivation of Gi or Gs with pertussis or cholera toxin, respectively, largely, but not entirely, reduced the calcium response in D(1)R and D(2)R cotransfected cells. Moreover, sequestration of G(βγ) subunits through overexpression of G protein receptor kinase 2 mutants either completely or largely eliminated dopamine-stimulated calcium mobilization. Our data suggest that the mechanism of D(1)R/D(2)R-mediated calcium signaling involves more than receptor-mediated G(q) protein activation, may largely involve downstream signaling pathways, and may not be completely heteromer-specific. In addition, SKF83959 may not exhibit selective activation of D(1)-D(2) heteromers, and its significant cross-reactivity to other receptors warrants careful interpretation of its use in vivo.

Plasma membrane compartmentalization of D2 dopamine receptors

Plasma membrane microcompartments could allow different signaling pathways to operate more efficiently and prevent cross-talk. We utilized a novel in-cell biotin transfer assay to demonstrate that the majority of plasma membrane-expressed D2 dopamine receptor (D2R) is microcompartmentalized within detergent-resistant structures. Conversely, a minority of D2R existed in a detergent-soluble form and interacted in a relatively unrestricted manner with other cellular proteins. The microcompartmentalization of D2R had functional consequences because dopamine-induced internalization of D2R was largely restricted to the compartmentalized receptor. The D2R-containing microcompartments did not correspond to putative detergent-resistant lipid raft structures. First, the detergent-insoluble D2R structures were significantly denser than detergent-resistant membrane fragments containing flotillin, a widely utilized lipid raft marker protein. Second, the detergent solubility of D2R was unaffected by treatment of cells with the cholesterol chelating agent, methyl-β-cyclodextrin, that is thought to disrupt lipid rafts. Finally, the in-cell biotinylation assay did not provide any evidence for the membrane compartmentalization of peptide motifs thought to target to lipid rafts. Thus, our observations form one of the first demonstrations, in living cells, of plasma membrane microcompartments defined by the ability of the compartment structure to broadly restrict the interaction of resident molecules with other cellular proteins.

ARF6 and GASP-1 are post-endocytic sorting proteins selectively involved in the intracellular trafficking of dopamine D₂ receptors mediated by GRK and PKC in transfected cells

BACKGROUND AND PURPOSE

GPCRs undergo both homologous and heterologous regulatory processes in which receptor phosphorylation plays a critical role. The protein kinases responsible for each pathway are well established; however, other molecular details that characterize each pathway remain unclear. In this study, the molecular mechanisms that determine the differences in the functional roles and intracellular trafficking between homologous and PKC-mediated heterologous internalization pathways for the dopamine D₂ receptor were investigated.

EXPERIMENTAL APPROACH

All of the S/T residues located within the intracellular loops of D₂ receptor were mutated, and the residues responsible for GRK- and PKC-mediated internalization were determined in HEK-293 cells and SH-SY5Y cells. The functional role of receptor internalization and the cellular components that determine the post-endocytic fate of internalized D₂ receptors were investigated in the transfected cells.

KEY RESULTS

T134, T225/S228/S229 and S325 were involved in PKC-mediated D₂ receptor desensitization. S229 and adjacent S/T residues mediated the PKC-dependent internalization of D₂ receptors, which induced down-regulation and desensitization. S/T residues within the second intracellular loop and T225 were the major residues involved in GRK-mediated internalization of D₂ receptors, which induced receptor resensitization. ARF6 mediated the recycling of D₂ receptors internalized in response to agonist stimulation. In contrast, GASP-1 mediated the down-regulation of D₂ receptors internalized in a PKC-dependent manner.

CONCLUSIONS AND IMPLICATIONS

GRK- and PKC-mediated internalizations of D₂ receptors occur through different intracellular trafficking pathways and mediate distinct functional roles. Distinct S/T residues within D₂ receptors and different sorting proteins are involved in the dissimilar regulation of D₂ receptors by GRK2 and PKC.

Approaches for probing allosteric interactions at 7 transmembrane spanning receptors

In recent years, allosteric modulation of 7 transmembrane spanning receptors (7TMRs) has become a highly productive and exciting field of receptor pharmacology and drug discovery efforts. Positive and negative allosteric modulators (PAMs and NAMs, respectively) present a number of pharmacological and therapeutic advantages over conventional orthosteric ligands, including improved receptor-subtype selectivity, a lower propensity to induce receptor desensitization, the preservation of endogenous temporal and spatial activation of receptors, greater chemical flexibility for optimization of drug metabolism and pharmacokinetic parameters, and saturability of effect at target receptors, thus improving safety concerns and risk of overdose. Additionally, the relatively new concept of allosteric modulator-mediated receptor signal bias opens up a number of intriguing possibilities for PAMs, NAMs, and allosteric agonists, including the potential to selectively activate therapeutically beneficial signaling cascades, which could yield a superior tissue selectivity and side effect profile of allosteric modulators. However, there are a number of considerations and caveats that must be addressed when screening for and characterizing the properties of 7TMR allosteric modulators. Mode of pharmacology, methodology used to monitor receptor activity, detection of appropriate downstream analytes, selection of orthosteric probe, and assay time-course must all be considered when implementing any high-throughput screening campaign or when characterizing the properties of active compounds. Yet compared to conventional agonist/antagonist drug discovery programs, these elements of assay design are often a great deal more complicated when working with 7TMRs allosteric modulators. Moreover, for classical pharmacological methodologies and analyses, like radioligand binding and the assessment of compound affinity, the properties of allosteric modulators yield data that are more nuanced than orthosteric ligand-receptor interactions. In this review, we discuss the current methodologies being used to identify and characterize allosteric modulators, lending insight into the approaches that have been most successful in accurately and robustly identifying hit compounds. New label-free technologies capable of detecting phenotypic cellular changes in response to receptor activation are powerful tools well suited for assessing subtle or potentially masked cellular responses to allosteric modulation of 7TMRs. Allosteric modulator-induced receptor signal bias and the assay systems available to probe the various downstream signaling outcomes of receptor activation are also discussed.