Role of receptor-attached phosphates in binding of visual and non-visual arrestins to G protein-coupled receptors

GPCR-dependent and -independent arrestin signaling

Biological activity of free arrestins is often overlooked. Based on available data, we compare arrestin-mediated signaling that requires and does not require binding to G-protein-coupled receptors (GPCRs). Receptor-bound arrestins activate ERK1/2, Src, and focal adhesion kinase (FAK). Yet, arrestin-3 regulation of Src family member Fgr does not appear to involve receptors. Free arrestin-3 facilitates the activation of JNK family kinases, preferentially binds E3 ubiquitin ligases Mdm2 and parkin, and facilitates parkin-dependent mitophagy. The binding of arrestins to microtubules and calmodulin and their function in focal adhesion disassembly and apoptosis also do not involve receptors. Biased GPCR ligands and the phosphorylation barcode can only affect receptor-dependent arrestin signaling. Thus, elucidation of receptor dependence or independence of arrestin functions has important scientific and therapeutic implications.

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

ArreSTick motif controls β-arrestin-binding stability and extends phosphorylation-dependent β-arrestin interactions to non-receptor proteins

The binding and function of β-arrestins are regulated by specific phosphorylation motifs present in G protein-coupled receptors (GPCRs). However, the exact arrangement of phosphorylated amino acids responsible for establishing a stable interaction remains unclear. We employ a 1D sequence convolution model trained on GPCRs with established β-arrestin-binding properties. With this approach, amino acid motifs characteristic of GPCRs that form stable interactions with β-arrestins can be identified, a pattern that we name "arreSTick." Intriguingly, the arreSTick pattern is also present in numerous non-receptor proteins. Using proximity biotinylation assay and mass spectrometry analysis, we demonstrate that the arreSTick motif controls the interaction between many non-receptor proteins and β-arrestin2. The HIV-1 Tat-specific factor 1 (HTSF1 or HTATSF1), a nuclear transcription factor, contains the arreSTick pattern, and its subcellular localization is influenced by β-arrestin2. Our findings unveil a broader role for β-arrestins in phosphorylation-dependent interactions, extending beyond GPCRs to encompass non-receptor proteins as well.

In-Cell Arrestin-Receptor Interaction Assays

G protein-coupled receptors (GPCRs) represent ∼30% of current drug targets. Ligand binding to these receptors activates G proteins and arrestins, which function in different signaling pathways. Given that functionally selective or biased ligands preferentially activate one of these two groups of pathways, they may be superior medications for certain disease states. The identification of such ligands requires robust drug screening assays for both G protein and arrestin activity. This unit describes protocols for assays that monitor reversible arrestin recruitment to GPCRs in living cells using either bioluminescence resonance energy transfer (BRET) or nanoluciferase complementation (NanoLuc). Two types of assays can be used: one configuration directly measures arrestin recruitment to a GPCR fused to a protein tag at its intracellular C-terminus, whereas the other configuration detects arrestin translocation to the plasma membrane in response to activation of an unmodified GPCR. Together, these assays are powerful tools for studying dynamic interactions between GPCRs and arrestins. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Receptor-arrestin BRET assay to measure ligand-induced recruitment of arrestin to receptors Basic Protocol 2: Receptor-arrestin NANOBIT assay to measure ligand-induced recruitment of arrestin to receptors Alternative Protocol 1: BRET assay to measure ligand-induced recruitment of arrestin to the plasma membrane Alternative Protocol 2: NANOBIT assay to measure ligand-induced recruitment of arrestin to the plasma membrane Support Protocol 1: Optimization of polyethylenimine (PEI) concentration for transfection.

Plasma membrane preassociation drives β-arrestin coupling to receptors and activation

β-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-β-arrestin interactions at the plasma membrane of living cells remain elusive. Here, we combine single-molecule microscopy with molecular dynamics simulations to dissect the complex sequence of events involved in β-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that β-arrestin spontaneously inserts into the lipid bilayer and transiently interacts with receptors via lateral diffusion on the plasma membrane. Moreover, they indicate that, following receptor interaction, the plasma membrane stabilizes β-arrestin in a longer-lived, membrane-bound state, allowing it to diffuse to clathrin-coated pits separately from the activating receptor. These results expand our current understanding of β-arrestin function at the plasma membrane, revealing a critical role for β-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

The Effect of Β-Arrestin2 Overexpression Regarding Viability and Temozolomide Treatment in High-Grade Glioma Cells

The β-arrestins (β-arr) family are proteins that regulate the signaling and trafficking of various G protein-coupled receptors. Out of the four members, β-arr 1 and 2 have been proven as essential actors behind different processes that lead to the progression of cancer as cell proliferation, migration, invasion and metastasis. In addition to this, these proteins are also capable of transmitting anti-apoptotic signals, influence tumor growth rate and drug resistance. Several studies have demonstrated that β-arr 2 overexpression corelates with an impaired overall survival and also showed that it may mediate multidrug resistance in certain types of cancer. In the current study we analyzed the effect of β-arr 2 overexpression on proliferation and how it affects Temozolomide (TMZ) response on the CL2:6 High Grade Glioma (HGG) cell line. We observed contradictory results after transfection, with β-arr 2 overexpressing cells having a superior proliferation rate after 24 and 48h, when compared to untransfected cells, while the opposite was noted after 72h. In terms of response to TMZ, we observed a similar contradictory pattern with modest differences between doses being observed at 24h, while the smallest and largest doses in our experiment produced opposite effects after 48h and 72h. This further underscores the scarcity of information regarding the exact roles and the importance of β-arrs in the intrinsic mechanisms which govern cancer cells.

Solo vs. Chorus: Monomers and Oligomers of Arrestin Proteins

Three out of four subtypes of arrestin proteins expressed in mammals self-associate, each forming oligomers of a distinct kind. Monomers and oligomers have different subcellular localization and distinct biological functions. Here we summarize existing evidence regarding arrestin oligomerization and discuss specific functions of monomeric and oligomeric forms, although too few of the latter are known. The data on arrestins highlight biological importance of oligomerization of signaling proteins. Distinct modes of oligomerization might be an important contributing factor to the functional differences among highly homologous members of the arrestin protein family.

Structural Basis of Arrestin Selectivity for Active Phosphorylated G Protein-Coupled Receptors

Arrestins are a small family of proteins that bind G protein-coupled receptors (GPCRs). Arrestin binds to active phosphorylated GPCRs with higher affinity than to all other functional forms of the receptor, including inactive phosphorylated and active unphosphorylated. The selectivity of arrestins suggests that they must have two sensors, which detect receptor-attached phosphates and the active receptor conformation independently. Simultaneous engagement of both sensors enables arrestin transition into a high-affinity receptor-binding state. This transition involves a global conformational rearrangement that brings additional elements of the arrestin molecule, including the middle loop, in contact with a GPCR, thereby stabilizing the complex. Here, we review structural and mutagenesis data that identify these two sensors and additional receptor-binding elements within the arrestin molecule. While most data were obtained with the arrestin-1-rhodopsin pair, the evidence suggests that all arrestins use similar mechanisms to achieve preferential binding to active phosphorylated GPCRs.

Receptor-Arrestin Interactions: The GPCR Perspective

Arrestins are a small family of four proteins in most vertebrates that bind hundreds of different G protein-coupled receptors (GPCRs). Arrestin binding to a GPCR has at least three functions: precluding further receptor coupling to G proteins, facilitating receptor internalization, and initiating distinct arrestin-mediated signaling. The molecular mechanism of arrestin–GPCR interactions has been extensively studied and discussed from the “arrestin perspective”, focusing on the roles of arrestin elements in receptor binding. Here, we discuss this phenomenon from the “receptor perspective”, focusing on the receptor elements involved in arrestin binding and emphasizing existing gaps in our knowledge that need to be filled. It is vitally important to understand the role of receptor elements in arrestin activation and how the interaction of each of these elements with arrestin contributes to the latter’s transition to the high-affinity binding state. A more precise knowledge of the molecular mechanisms of arrestin activation is needed to enable the construction of arrestin mutants with desired functional characteristics.

The single nucleotide β -arrestin2 variant, A248T, resembles dynamical properties of activated arrestin

β -arrestins are responsible for termination of G protein-coupled receptor (GPCR)-mediated signaling. Association of single nucleotide variants with onset of crucial diseases has made this protein family hot targets in the field of GPCR-mediated pharmacology. However, impact of these mutations on function of these variants has remained elusive. In this study, structural and dynamical properties of one of β -arrestin2 (arrestin 3) variants, A248T, which has been identified in some cancer tissue samples, were investigated via molecular dynamics simulations. The results showed that the variant underwent structural rearrangements which are seen in crystal structures of active arrestin. Specifically, the "short helix" unravels and the "gate loop" swings forward as seen in crystal structures of receptor-bound and GPCR phosphopeptide-bound arrestin. Moreover, the "finger loop" samples upward position in the variant. Importantly, these regions harbor crucial residues that are involved in receptor binding interfaces. Cumulatively, these local structural rearrangements help the variant adopt active-like domain angle without perturbing the "polar core". Considering that phosphorylation of the receptor is required for activation of arrestin, A248T might serve as a model system to understand phosphorylation-independent activation mechanism, thus enabling modulation of function of arrestin variants which are activated independent of receptor phosphorylation as seen in cancer.

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.

Allosteric activation of proto-oncogene kinase Src by GPCR-beta-arrestin complexes

G protein-coupled receptors (GPCRs) initiate signaling cascades via G-proteins and beta-arrestins (βarr). βarr-dependent actions begin with recruitment of βarr to the phosphorylated receptor tail and are followed by engagement with the receptor core. βarrs are known to act as adaptor proteins binding receptors and various effectors, but it is unclear whether in addition to the scaffolding role βarrs can allosterically activate their downstream targets. Here we demonstrate the direct allosteric activation of proto-oncogene kinase Src by GPCR-βarr complexes in vitro and establish the conformational basis of the activation. Whereas free βarr1 had no effect on Src activity, βarr1 in complex with M2 muscarinic or β2-adrenergic receptors reconstituted in lipid nanodiscs activate Src by reducing the lag phase in Src autophosphorylation. Interestingly, receptor-βarr1 complexes formed with a βarr1 mutant, in which the finger-loop, required to interact with the receptor core, has been deleted, fully retain the ability to activate Src. Similarly, βarr1 in complex with only a phosphorylated C-terminal tail of the vasopressin 2 receptor activates Src as efficiently as GPCR-βarr complexes. In contrast, βarr1 and chimeric M2 receptor with nonphosphorylated C-terminal tail failed to activate Src. Taken together, these data demonstrate that the phosphorylated GPCR tail interaction with βarr1 is necessary and sufficient to empower it to allosterically activate Src. Our findings may have implications for understanding more broadly the mechanisms of allosteric activation of downstream targets by βarrs.

PAR4 activation involves extracellular loop 3 and transmembrane residue Thr153

Protease-activated receptor 4 (PAR4) mediates sustained thrombin signaling in platelets and is required for a stable thrombus. PAR4 is activated by proteolysis of the N terminus to expose a tethered ligand. The structural basis for PAR4 activation and the location of its ligand binding site (LBS) are unknown. Using hydrogen/deuterium exchange (H/D exchange), computational modeling, and signaling studies, we determined the molecular mechanism for tethered ligand-mediated PAR4 activation. H/D exchange identified that the LBS is composed of transmembrane 3 (TM3) domain and TM7. Unbiased computational modeling further predicted an interaction between Gly48 from the tethered ligand and Thr153 from the LBS. Mutating Thr153 significantly decreased PAR4 signaling. H/D exchange and modeling also showed that extracellular loop 3 (ECL3) serves as a gatekeeper for the interaction between the tethered ligand and LBS. A naturally occurring sequence variant (P310L, rs2227376) and 2 experimental mutations (S311A and P312L) determined that the rigidity conferred by prolines in ECL3 are essential for PAR4 activation. Finally, we examined the role of the polymorphism at position 310 in venous thromboembolism (VTE) using the International Network Against Venous Thrombosis (INVENT) consortium multi-ancestry genome-wide association study (GWAS) meta-analysis. Individuals with the PAR4 Leu310 allele had a 15% reduction in relative risk for VTE (odds ratio, 0.85; 95% confidence interval, 0.77-0.94) compared with the Pro310 allele. These data are consistent with our H/D exchange, molecular modeling, and signaling studies. In conclusion, we have uncovered the structural basis for PAR4 activation and identified a previously unrecognized role for PAR4 in VTE.

Arrestin Recruitment to C-C Chemokine Receptor 5: Potent C-C Chemokine Ligand 5 Analogs Reveal Differences in Dependence on Receptor Phosphorylation and Isoform-Specific Recruitment Bias

C-C chemokine receptor 5 (CCR5) is a chemokine receptor belonging to the G protein-coupled receptor (GPCR) superfamily. An established anti-human immunodeficiency virus drug target, CCR5 is attracting significant additional interest in both cancer and neuroinflammation. Several N-terminally engineered analogs of C-C chemokine ligand 5 (CCL5), a natural ligand of CCR5, are highly potent CCR5 inhibitors. The inhibitory mechanisms of certain analogs relate to modulation of receptor desensitization, but the cellular and molecular mechanisms have not been fully elucidated. Here we made use of a collection of CCR5 phosphorylation mutants and arrestin variants to investigate how CCL5 analogs differ from CCL5 in their capacity to elicit both CCR5 phosphorylation and arrestin recruitment, with reference to the current "core" and "tail" interaction model for arrestin-GPCR interaction. We showed that CCL5 recruits both arrestin 2 and arrestin 3 to CCR5 with recruitment, particularly of arrestin 2, strongly dependent on the arrestin tail interaction. 5P12-RANTES does not elicit receptor phosphorylation or arrestin recruitment. In contrast, PSC-RANTES induces CCR5 hyperphosphorylation, driving enhanced arrestin recruitment with lower dependence on the arrestin tail interaction. 5P14-RANTES induces comparable levels of receptor phosphorylation to CCL5, but arrestin recruitment is absolutely dependent on the arrestin tail interaction, and in one of the cellular backgrounds used, recruitment showed isoform bias toward arrestin 3 versus arrestin 2. No evidence for ligand-specific differences in receptor phosphorylation patterns across the four implicated serine residues was observed. Our results improve understanding of the molecular pharmacology of CCR5 and help further elucidate the inhibitory mechanisms of a group of potent inhibitors. SIGNIFICANCE STATEMENT: C-C chemokine receptor 5 (CCR5) is a key drug target for human immunodeficiency virus, cancer, and inflammation. Highly potent chemokine analog inhibitors act via the modulation of receptor desensitization, a process initiated by the recruitment of arrestin proteins. This study shows that potent C-C chemokine ligand 5 analogs differ from each other and from the parent chemokine in the extent and quality of CCR5-arrestin association that they elicit, providing valuable insights into CCR5 pharmacology and cell biology that will facilitate the development of new medicines targeting this important receptor.

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.

Crystal Structure of β-Arrestin 2 in Complex with CXCR7 Phosphopeptide

β-Arrestins (βarrs) critically regulate G-protein-coupled receptor (GPCR) signaling and trafficking. βarrs have two isoforms, βarr1 and βarr2. Receptor phosphorylation is a key determinant for the binding of βarrs, and understanding the intricate details of receptor-βarr interaction is the next frontier in GPCR structural biology. The high-resolution structure of active βarr1 in complex with a phosphopeptide derived from GPCR has been revealed, but that of βarr2 remains elusive. Here, we present a 2.3-Å crystal structure of βarr2 in complex with a phosphopeptide (C7pp) derived from the carboxyl terminus of CXCR7. The structural analysis of C7pp-bound βarr2 reveals key differences from the previously determined active conformation of βarr1. One of the key differences is that C7pp-bound βarr2 shows a relatively small inter-domain rotation. Antibody-fragment-based conformational sensor and hydrogen/deuterium exchange experiments further corroborated the structural features of βarr2 and suggested that βarr2 adopts a range of inter-domain rotations.

Functional Selectivity and Antinociceptive Effects of a Novel KOPr Agonist

Kappa opioid receptor (KOPr) agonists represent alternative analgesics for their low abuse potential, although relevant adverse effects have limited their clinical use. Functionally selective KOPr agonists may activate, in a pathway-specific manner, G protein-mediated signaling, that produces antinociception, over β-arrestin 2-dependent induction of p38MAPK, which preferentially contributes to adverse effects. Thus, functionally selective KOPr agonists biased toward G protein-coupled intracellular signaling over β-arrestin-2-mediated pathways may be considered candidate therapeutics possibly devoid of many of the typical adverse effects elicited by classic KOPr agonists. Nonetheless, the potential utility of functionally selective agonists at opioid receptors is still highly debated; therefore, further studies are necessary to fully understand whether it will be possible to develop more effective and safer analgesics by exploiting functional selectivity at KOPr. In the present study we investigated in vitro functional selectivity and in vivo antinociceptive effects of LOR17, a novel KOPr selective peptidic agonist that we synthesized. LOR17-mediated effects on adenylyl cyclase inhibition, ERK1/2, p38MAPK phosphorylation, and astrocyte cell proliferation were studied in HEK-293 cells expressing hKOPr, U87-MG glioblastoma cells, and primary human astrocytes; biased agonism was investigated via cAMP ELISA and β-arrestin 2 recruitment assays. Antinociception and antihypersensitivity were assessed in mice via warm-water tail-withdrawal test, intraperitoneal acid-induced writhing, and a model of oxaliplatin-induced neuropathic cold hypersensitivity. Effects of LOR17 on locomotor activity, exploratory activity, and forced-swim behavior were also assayed. We found that LOR17 is a selective, G protein biased KOPr agonist that inhibits adenylyl cyclase and activates early-phase ERK1/2 phosphorylation. Conversely to classic KOPr agonists as U50,488, LOR17 neither induces p38MAPK phosphorylation nor increases KOPr-dependent, p38MAPK-mediated cell proliferation in astrocytes. Moreover, LOR17 counteracts, in a concentration-dependent manner, U50,488-induced p38MAPK phosphorylation and astrocyte cell proliferation. Both U50,488 and LOR17 display potent antinociception in models of acute nociception, whereas LOR17 counteracts oxaliplatin-induced thermal hypersensitivity better than U50,488, and it is effective after single or repeated s.c. administration. LOR17 administered at a dose that fully alleviated oxaliplatin-induced thermal hypersensitivity did not alter motor coordination, locomotor and exploratory activities nor induced pro-depressant-like behavior. LOR17, therefore, may emerge as a novel KOPr agonist displaying functional selectivity toward G protein signaling and eliciting antinociceptive/antihypersensitivity effects in different animal models, including oxaliplatin-induced neuropathy.

Revealing Conformational Transitions in G-Protein-Coupled Receptor Rhodopsin upon Phosphorylation

G-protein-coupled receptors (GPCRs) have evolved as highly specialized cellular machinery that can dictate biological outcomes in response to diverse stimuli. Specifically, they induce multiple pathway responses upon structural perturbations induced at local protein sites. GPCRs utilize a concurrent strategy involving a central transmembrane topology and biochemical modifications for precise functional implementation. However, the specific role of the latter is not known due to the lack of precise probing techniques that can characterize receptor dynamics upon biochemical modifications. Phosphorylation is known to be one of the critical biochemical modifications in GPCRs that aids in receptor desensitization via arrestin binding. Here, we carry out all-atom molecular dynamics simulations of rhodopsin in a membrane environment to study its conformational dynamics induced upon phosphorylation. Interestingly, our comparative analysis of non-phosphorylated and phosphorylated rhodopsin structure demonstrated enhanced receptor stability upon phosphorylation at the C-terminal region that leads to the opening of the extracellular part of the transmembrane helices. In addition, monitoring the distinct number of phosphorylation states showed that having fewer phosphorylated residues does not bring about appropriate conformational changes in the extracellular region. Since phosphorylation results in receptor desensitization and recycling of the ligand, our findings provide significant insights into the conformational dynamics of the mechanism of ligand exit from the receptor.

Targeting arrestin interactions with its partners for therapeutic purposes

Most vertebrates express four arrestin subtypes: two visual ones in photoreceptor cells and two non-visuals expressed ubiquitously. The latter two interact with hundreds of G protein-coupled receptors, certain receptors of other types, and numerous non-receptor partners. Arrestins have no enzymatic activity and work by interacting with other proteins, often assembling multi-protein signaling complexes. Arrestin binding to every partner affects cell signaling, including pathways regulating cell survival, proliferation, and death. Thus, targeting individual arrestin interactions has therapeutic potential. This requires precise identification of protein-protein interaction sites of both participants and the choice of the side of each interaction which would be most advantageous to target. The interfaces involved in each interaction can be disrupted by small molecule therapeutics, as well as by carefully selected peptides of the other partner that do not participate in the interactions that should not be targeted.

Structure-function analysis of β-arrestin Kurtz reveals a critical role of receptor interactions in downregulation of GPCR signaling in vivo

Arrestins control signaling via the G protein coupled receptors (GPCRs), serving as both signal terminators and transducers. Previous studies identified several structural elements in arrestins that contribute to their functions as GPCR regulators. However, the importance of these elements in vivo is unclear, and the developmental roles of arrestins are not well understood. We carried out an in vivo structure-function analysis of Kurtz (Krz), the single ortholog of mammalian β-arrestins in the Drosophila genome. A combination of Krz mutations affecting the GPCR-phosphosensing and receptor core-binding ("finger loop") functions (Krz-KKVL/A) resulted in a complete loss of Krz activity during development. Endosome recruitment and bioluminescence resonance energy transfer (BRET) assays revealed that the KKVL/A mutations abolished the GPCR-binding ability of Krz. We found that the isolated "finger loop" mutation (Krz-VL/A), while having a negligible effect on GPCR internalization, severely affected Krz function, suggesting that tight receptor interactions are necessary for proper termination of signaling in vivo. Genetic analysis as well as live imaging demonstrated that mutations in Krz led to hyperactivity of the GPCR Mist (also known as Mthl1), which is activated by its ligand Folded gastrulation (Fog) and is responsible for cellular contractility and epithelial morphogenesis. Krz mutations affected two developmental events that are under the control of Fog-Mist signaling: gastrulation and morphogenesis of the wing. Overall, our data reveal the functional importance in vivo of direct β-arrestin/GPCR binding, which is mediated by the recognition of the phosphorylated receptor tail and receptor core interaction. These Krz-GPCR interactions are critical for setting the correct level of Fog-Mist signaling during epithelial morphogenesis.

Roles of M3 receptor in the effect of penehyclidine hydrochloride upregulated beta-arrestin-1 expression in LPS-stimulated HPMVEC

Background: This study is to investigate the roles of muscarinic receptor 3 (M₃ receptor) in the effect of penehyclidine hydrochloride (PHC) upregulated beta-arrestin-1 expression in lipopolysaccharide (LPS)-stimulated human pulmonary microvascular endothelial cell (HPMVEC). Methods: HPMVECs were transfected with a shRNA-containing plasmid that specifically targets M₃ receptor mRNA. Cells were collected to measure F-actin contents, levels of intercellular cell adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1), as well as changes of F-actin cytoskeleton arrangement by Laser scanning confocal. Beta-arrestin-1 protein expressions were determined by Western blot and beta-arrestin-1 mRNA expressions were measured by Real-time PCR. Results: Similar to normal cells, PHC could also increase F-actin contents and beta-arrestin-1 expressions, reduce ICAM-1 and VCAM-1 expressions, and inhibit LPS-stimulated reorganization of F-actin and formation of stress fiber in M₃ receptor shRNA group. Compared with normal cells, F-actin cytoskeleton was neat, ICAM-1 and VCAM-1 expressions were decreased, as well as F-actin contents were increased in M₃ receptor shRNA group. However, there were no differences in beta-arrestin-1 expressions between normal cell groups and M₃ receptor shRNA groups. Conclusion: These results indicate that M₃ receptor plays an important role in pulmonary microvascular endothelial barrier function, and knock-out of M₃ receptor could attenuate LPS-induced pulmonary microvascular endothelial injury. However, upregulative effect of PHC on beta-arrestin-1 expression is independent with presence of M₃ receptor.

Utilization of Biased G Protein-Coupled ReceptorSignaling towards Development of Safer andPersonalized Therapeutics

G protein-coupled receptors (GPCRs) are involved in a wide variety of physiological processes. Therefore, approximately 40% of currently prescribed drugs have targeted this receptor family. Discovery of β-arrestin mediated signaling and also separability of G protein and β-arrestin signaling pathways have switched the research focus in the GPCR field towards development of biased ligands, which provide engagement of the receptor with a certain effector, thus enriching a specific signaling pathway. In this review, we summarize possible factors that impact signaling profiles of GPCRs such as oligomerization, drug treatment, disease conditions, genetic background, etc. along with relevant molecules that can be used to modulate signaling properties of GPCRs such as allosteric or bitopic ligands, ions, aptamers and pepducins. Moreover, we also discuss the importance of inclusion of pharmacogenomics and molecular dynamics simulations to achieve a holistic understanding of the relation between genetic background and structure and function of GPCRs and GPCR-related proteins. Consequently, specific downstream signaling pathways can be enriched while those that bring unwanted side effects can be prevented on a patient-specific basis. This will improve studies that centered on development of safer and personalized therapeutics, thus alleviating the burden on economy and public health.

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.

GPCR Signaling Regulation: The Role of GRKs and Arrestins

Every animal species expresses hundreds of different G protein-coupled receptors (GPCRs) that respond to a wide variety of external stimuli. GPCRs-driven signaling pathways are involved in pretty much every physiological function and in many pathologies. Therefore, GPCRs are targeted by about a third of clinically used drugs. The signaling of most GPCRs via G proteins is terminated by the phosphorylation of active receptor by specific kinases (GPCR kinases, or GRKs) and subsequent binding of arrestin proteins, that selectively recognize active phosphorylated receptors. In addition, GRKs and arrestins play a role in multiple signaling pathways in the cell, both GPCR-initiated and receptor-independent. Here we focus on the mechanisms of GRK- and arrestin-mediated regulation of GPCR signaling, which includes homologous desensitization and redirection of signaling to additional pathways by bound arrestins.

The Role of β-Arrestin Proteins in Organization of Signaling and Regulation of the AT1 Angiotensin Receptor

AT1 angiotensin receptor plays important physiological and pathophysiological roles in the cardiovascular system. Renin-angiotensin system represents a target system for drugs acting at different levels. The main effects of ATR1 stimulation involve activation of Gq proteins and subsequent IP3, DAG, and calcium signaling. It has become evident in recent years that besides the well-known G protein pathways, AT1R also activates a parallel signaling pathway through β-arrestins. β-arrestins were originally described as proteins that desensitize G protein-coupled receptors, but they can also mediate receptor internalization and G protein-independent signaling. AT1R is one of the most studied receptors, which was used to unravel the newly recognized β-arrestin-mediated pathways. β-arrestin-mediated signaling has become one of the most studied topics in recent years in molecular pharmacology and the modulation of these pathways of the AT1R might offer new therapeutic opportunities in the near future. In this paper, we review the recent advances in the field of β-arrestin signaling of the AT1R, emphasizing its role in cardiovascular regulation and heart failure.

A Novel Polar Core and Weakly Fixed C-Tail in Squid Arrestin Provide New Insight into Interaction with Rhodopsin

Photoreceptors of the squid Loligo pealei contain a G-protein-coupled receptor (GPCR) signaling system that activates phospholipase C in response to light. Analogous to the mammalian visual system, signaling of the photoactivated GPCR rhodopsin is terminated by binding of squid arrestin (sArr). sArr forms a light-dependent, high-affinity complex with squid rhodopsin, which does not require prior receptor phosphorylation for interaction. This is at odds with classical mammalian GPCR desensitization where an agonist-bound phosphorylated receptor is needed to break stabilizing constraints within arrestins, the so-called "three-element interaction" and "polar core" network, before a stable receptor-arrestin complex can be established. Biophysical and mass spectrometric analysis of the squid rhodopsin-arrestin complex indicates that in contrast to mammalian arrestins, the sArr C-tail is not involved in a stable three-element interaction. We determined the crystal structure of C-terminally truncated sArr that adopts a basal conformation common to arrestins and is stabilized by a series of weak but novel polar core interactions. Unlike mammalian arrestin-1, deletion of the sArr C-tail does not influence kinetic properties of complex formation of sArr with the receptor. Hydrogen-deuterium exchange studies revealed the footprint of the light-activated rhodopsin on sArr. Furthermore, double electron-electron resonance spectroscopy experiments provide evidence that receptor-bound sArr adopts a conformation different from the one known for arrestin-1 and molecular dynamics simulations reveal the residues that account for the weak three-element interaction. Insights gleaned from studying this system add to our general understanding of GPCR-arrestin interaction.

Multisite phosphorylation is required for sustained interaction with GRKs and arrestins during rapid μ-opioid receptor desensitization

G protein receptor kinases (GRKs) and β-arrestins are key regulators of μ-opioid receptor (MOR) signaling and trafficking. We have previously shown that high-efficacy opioids such as DAMGO stimulate a GRK2/3-mediated multisite phosphorylation of conserved C-terminal tail serine and threonine residues, which facilitates internalization of the receptor. In contrast, morphine-induced phosphorylation of MOR is limited to Ser³⁷⁵ and is not sufficient to drive substantial receptor internalization. We report how specific multisite phosphorylation controlled the dynamics of GRK and β-arrestin interactions with MOR and show how such phosphorylation mediated receptor desensitization. We showed that GRK2/3 was recruited more quickly than was β-arrestin to a DAMGO-activated MOR. β-Arrestin recruitment required GRK2 activity and MOR phosphorylation, but GRK recruitment also depended on the phosphorylation sites in the C-terminal tail, specifically four serine and threonine residues within the ³⁷⁰TREHPSTANT³⁷⁹ motif. Our results also suggested that other residues outside this motif participated in the initial and transient recruitment of GRK and β-arrestins. We identified two components of high-efficacy agonist desensitization of MOR: a sustained component, which required GRK2-mediated phosphorylation and a potential soluble factor, and a rapid component, which was likely mediated by GRK2 but independent of receptor phosphorylation. Elucidating these complex receptor-effector interactions represents an important step toward a mechanistic understanding of MOR desensitization that leads to the development of tolerance and dependence.

Structural Basis of Arrestin-Dependent Signal Transduction

Arrestins are a small family of proteins with four isoforms in humans. Remarkably, two arrestins regulate signaling from >800 G protein-coupled receptors (GPCRs) or nonreceptor activators by simultaneously binding an activator and one out of hundreds of other signaling proteins. When arrestins are bound to GPCRs or other activators, the affinity for these signaling partners changes. Thus, it is proposed that an activator alters arrestin's ability to transduce a signal. The comparison of all available arrestin structures identifies several common conformational rearrangements associated with activation. In particular, it identifies elements that are directly involved in binding to GPCRs or other activators, elements that likely engage distinct downstream effectors, and elements that likely link the activator-binding sites with the effector-binding sites.

Molecular mechanism of GPCR-mediated arrestin activation

Despite intense interest in discovering drugs that cause G-protein-coupled receptors (GPCRs) to selectively stimulate or block arrestin signalling, the structural mechanism of receptor-mediated arrestin activation remains unclear1,2. Here we reveal this mechanism through extensive atomic-level simulations of arrestin. We find that the receptor's transmembrane core and cytoplasmic tail-which bind distinct surfaces on arrestin-can each independently stimulate arrestin activation. We confirm this unanticipated role of the receptor core, and the allosteric coupling between these distant surfaces of arrestin, using site-directed fluorescence spectroscopy. The effect of the receptor core on arrestin conformation is mediated primarily by interactions of the intracellular loops of the receptor with the arrestin body, rather than the marked finger-loop rearrangement that is observed upon receptor binding. In the absence of a receptor, arrestin frequently adopts active conformations when its own C-terminal tail is disengaged, which may explain why certain arrestins remain active long after receptor dissociation. Our results, which suggest that diverse receptor binding modes can activate arrestin, provide a structural foundation for the design of functionally selective ('biased') GPCR-targeted ligands with desired effects on arrestin signalling.

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

The role and mechanism of β‑arrestins in cancer invasion and metastasis (Review)

β-arrestins are a family of adaptor proteins that regulate the signaling and trafficking of various G protein-coupled receptors (GPCRs). They consist of β-arrestin1 and β-arrestin2 and are considered to be scaffolding proteins. β-arrestins regulate cell proliferation, promote cell invasion and migration, transmit anti-apoptotic survival signals and affect other characteristics of tumors, including tumor growth rate, angiogenesis, drug resistance, invasion and metastatic potential. It has been demonstrated that β-arrestins serve roles in various physiological and pathological processes and exhibit a similar function to GPCRs. β-arrestins serve primary roles in cancer invasion and metastasis via various signaling pathways. The present review assessed the function and mechanism of β-arrestins in cancer invasion and metastasis via multiple signaling pathways, including mitogen-activated protein kinase/extracellular signal regulated kinase, Wnt/β-catenin, nuclear factor-κB and phosphoinositide-3 kinase/Akt.

Heterologous phosphorylation-induced formation of a stability lock permits regulation of inactive receptors by β-arrestins

β-Arrestins are key regulators and signal transducers of G protein-coupled receptors (GPCRs). The interaction between receptors and β-arrestins is generally believed to require both receptor activity and phosphorylation by GPCR kinases. In this study, we investigated whether β-arrestins are able to bind second messenger kinase-phosphorylated, but inactive receptors as well. Because heterologous phosphorylation is a common phenomenon among GPCRs, this mode of β-arrestin activation may represent a novel mechanism of signal transduction and receptor cross-talk. Here we demonstrate that activation of protein kinase C (PKC) by phorbol myristate acetate, G_q/11-coupled GPCR, or epidermal growth factor receptor stimulation promotes β-arrestin2 recruitment to unliganded AT₁ angiotensin receptor (AT₁R). We found that this interaction depends on the stability lock, a structure responsible for the sustained binding between GPCRs and β-arrestins, formed by phosphorylated serine-threonine clusters in the receptor's C terminus and two conserved phosphate-binding lysines in the β-arrestin2 N-domain. Using improved FlAsH-based serine-threonine clusters β-arrestin2 conformational biosensors, we also show that the stability lock not only stabilizes the receptor-β-arrestin interaction, but also governs the structural rearrangements within β-arrestins. Furthermore, we found that β-arrestin2 binds to PKC-phosphorylated AT₁R in a distinct active conformation, which triggers MAPK recruitment and receptor internalization. Our results provide new insights into the activation of β-arrestins and reveal their novel role in receptor cross-talk.

Structural basis of arrestin-3 activation and signaling

A unique aspect of arrestin-3 is its ability to support both receptor-dependent and receptor-independent signaling. Here, we show that inositol hexakisphosphate (IP₆) is a non-receptor activator of arrestin-3 and report the structure of IP₆-activated arrestin-3 at 2.4-Å resolution. IP₆-activated arrestin-3 exhibits an inter-domain twist and a displaced C-tail, hallmarks of active arrestin. IP₆ binds to the arrestin phosphate sensor, and is stabilized by trimerization. Analysis of the trimerization surface, which is also the receptor-binding surface, suggests a feature called the finger loop as a key region of the activation sensor. We show that finger loop helicity and flexibility may underlie coupling to hundreds of diverse receptors and also promote arrestin-3 activation by IP₆. Importantly, we show that effector-binding sites on arrestins have distinct conformations in the basal and activated states, acting as switch regions. These switch regions may work with the inter-domain twist to initiate and direct arrestin-mediated signaling.

While arrestins are mainly associated with GPCR signaling, arrestin-3 can signal independently of receptor interaction. Here the authors present the structure of arrestin-3 bound to inositol hexakisphosphate (IP₆) and propose a model for arrestin-3 activation.

Identification of Phosphorylation Codes for Arrestin Recruitment by G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) mediate diverse signaling in part through interaction with arrestins, whose binding promotes receptor internalization and signaling through G protein-independent pathways. High-affinity arrestin binding requires receptor phosphorylation, often at the receptor's C-terminal tail. Here, we report an X-ray free electron laser (XFEL) crystal structure of the rhodopsin-arrestin complex, in which the phosphorylated C terminus of rhodopsin forms an extended intermolecular β sheet with the N-terminal β strands of arrestin. Phosphorylation was detected at rhodopsin C-terminal tail residues T336 and S338. These two phospho-residues, together with E341, form an extensive network of electrostatic interactions with three positively charged pockets in arrestin in a mode that resembles binding of the phosphorylated vasopressin-2 receptor tail to β-arrestin-1. Based on these observations, we derived and validated a set of phosphorylation codes that serve as a common mechanism for phosphorylation-dependent recruitment of arrestins by GPCRs.

Differential manipulation of arrestin-3 binding to basal and agonist-activated G protein-coupled receptors

Non-visual arrestins interact with hundreds of different G protein-coupled receptors (GPCRs). Here we show that by introducing mutations into elements that directly bind receptors, the specificity of arrestin-3 can be altered. Several mutations in the two parts of the central "crest" of the arrestin molecule, middle-loop and C-loop, enhanced or reduced arrestin-3 interactions with several GPCRs in receptor subtype and functional state-specific manner. For example, the Lys139Ile substitution in the middle-loop dramatically enhanced the binding to inactive M₂ muscarinic receptor, so that agonist activation of the M₂ did not further increase arrestin-3 binding. Thus, the Lys139Ile mutation made arrestin-3 essentially an activation-independent binding partner of M₂, whereas its interactions with other receptors, including the β₂-adrenergic receptor and the D₁ and D₂ dopamine receptors, retained normal activation dependence. In contrast, the Ala248Val mutation enhanced agonist-induced arrestin-3 binding to the β₂-adrenergic and D₂ dopamine receptors, while reducing its interaction with the D₁ dopamine receptor. These mutations represent the first example of altering arrestin specificity via enhancement of the arrestin-receptor interactions rather than selective reduction of the binding to certain subtypes.

Understanding the Differential Selectivity of Arrestins toward the Phosphorylation State of the Receptor

Proteins in the arrestin family exhibit a conserved structural fold that nevertheless allows for significant differences in their selectivity for G-protein coupled receptors (GPCRs) and their phosphorylation states. To reveal the mechanism of activation that prepares arrestin for selective interaction with GPCRs, and to understand the basis for these differences, we used unbiased molecular dynamics simulations to compare the structural and dynamic properties of wild type Arr1 (Arr1-WT), Arr3 (Arr3-WT), and a constitutively active Arr1 mutant, Arr1-R175E, characterized by a perturbation of the phosphate recognition region called "polar core". We find that in our simulations the mutant evolves toward a conformation that resembles the known preactivated structures of an Arr1 splice-variant, and the structurally similar phosphopeptide-bound Arr2-WT, while this does not happen for Arr1-WT. Hence, we propose an activation allosteric mechanism connecting the perturbation of the polar core to a global conformational change, including the relative reorientation of N- and C-domains, and the emergence of electrostatic properties of putative binding surfaces. The underlying local structural changes are interpreted as markers of the evolution of an arrestin structure toward an active-like conformation. Similar activation related changes occur in Arr3-WT in the absence of any perturbation of the polar core, suggesting that this system could spontaneously visit preactivated states in solution. This hypothesis is proposed to explain the lower selectivity of Arr3 toward nonphosphorylated receptors. Moreover, by elucidating the allosteric mechanism underlying activation, we identify functionally critical regions on arrestin structure that can be targeted with drugs or chemical tools for functional modulation.

Identification of key phosphorylation sites in PTH1R that determine arrestin3 binding and fine-tune receptor signaling

The parathyroid hormone receptor 1 (PTH1R) is a member of family B of G-protein-coupled receptors (GPCRs), predominantly expressed in bone and kidney where it modulates extracellular Ca²⁺ homeostasis and bone turnover. It is well established that phosphorylation of GPCRs constitutes a key event in regulating receptor function by promoting arrestin recruitment and coupling to G-protein-independent signaling pathways. Mapping phosphorylation sites on PTH1R would provide insights into how phosphorylation at specific sites regulates cell signaling responses and also open the possibility of developing therapeutic agents that could target specific receptor functions. Here, we have used mass spectrometry to identify nine sites of phosphorylation in the C-terminal tail of PTH1R. Mutational analysis revealed identified two clusters of serine and threonine residues (Ser489–Ser495 and Ser501–Thr506) specifically responsible for the majority of PTH(1–34)-induced receptor phosphorylation. Mutation of these residues to alanine did not affect negatively on the ability of the receptor to couple to G-proteins or activate extracellular-signal-regulated kinase 1/2. Using fluorescence resonance energy transfer and bioluminescence resonance energy transfer to monitor PTH(1–34)-induced interaction of PTH1R with arrestin3, we show that the first cluster Ser489–Ser495 and the second cluster Ser501–Thr506 operated in concert to mediate both the efficacy and potency of ligand-induced arrestin3 recruitment. We further demonstrate that Ser503 and Thr504 in the second cluster are responsible for 70% of arrestin3 recruitment and are key determinants for interaction of arrestin with the receptor. Our data are consistent with the hypothesis that the pattern of C-terminal tail phosphorylation on PTH1R may determine the signaling outcome following receptor activation.

Analyzing the roles of multi-functional proteins in cells: The case of arrestins and GRKs

Most proteins have multiple functions. Obviously, conventional methods of manipulating the level of the protein of interest in the cell, such as over-expression, knockout or knockdown, affect all of its functions simultaneously. The key advantage of these methods is that over-expression, knockout or knockdown does not require any knowledge of the molecular mechanisms of the function(s) of the protein of interest. The disadvantage is that these approaches are inadequate to elucidate the role of an individual function of the protein in a particular cellular process. An alternative is the use of re-engineered proteins, in which a single function is eliminated or enhanced. The use of mono-functional elements of a multi-functional protein can also yield cleaner answers. This approach requires detailed knowledge of the structural basis of each function of the protein in question. Thus, a lot of preliminary structure-function work is necessary to make it possible. However, when this information is available, replacing the protein of interest with a mutant in which individual functions are modified can shed light on the biological role of those particular functions. Here, we illustrate this point using the example of protein kinases, most of which have additional non-enzymatic functions, as well as arrestins, known multi-functional signaling regulators in the cell.

A G Protein-biased Designer G Protein-coupled Receptor Useful for Studying the Physiological Relevance of Gq/11-dependent Signaling Pathways

Designerreceptorsexclusivelyactivated by adesignerdrug (DREADDs) are clozapine-N-oxide-sensitive designer G protein-coupled receptors (GPCRs) that have emerged as powerful novel chemogenetic tools to study the physiological relevance of GPCR signaling pathways in specific cell types or tissues. Like endogenous GPCRs, clozapine-N-oxide-activated DREADDs do not only activate heterotrimeric G proteins but can also trigger β-arrestin-dependent (G protein-independent) signaling. To dissect the relative physiological relevance of G protein-mediatedversusβ-arrestin-mediated signaling in different cell types or physiological processes, the availability of G protein- and β-arrestin-biased DREADDs would be highly desirable. In this study, we report the development of a mutationally modified version of a non-biased DREADD derived from the M3muscarinic receptor that can activate Gq/11with high efficacy but lacks the ability to interact with β-arrestins. We also demonstrate that this novel DREADD is activein vivoand that cell type-selective expression of this new designer receptor can provide novel insights into the physiological roles of G protein (Gq/11)-dependentversusβ-arrestin-dependent signaling in hepatocytes. Thus, this novel Gq/11-biased DREADD represents a powerful new tool to study the physiological relevance of Gq/11-dependent signaling in distinct tissues and cell types, in the absence of β-arrestin-mediated cellular effects. Such studies should guide the development of novel classes of functionally biased ligands that show high efficacy in various pathophysiological conditions but display a reduced incidence of side effects.

The effect of phosphorylation on arrestin-rhodopsin interaction in the squid visual system

Invertebrate visual opsins are G protein-coupled receptors coupled to retinoid chromophores that isomerize reversibly between inactive rhodopsin and active metarhodopsin upon absorption of photons of light. The squid visual system has an arrestin protein that binds to metarhodopsin to block signaling to Gq and activation of phospholipase C. Squid rhodopsin kinase (SQRK) can phosphorylate both metarhodopsin and arrestin, a dual role that is unique among the G protein-coupled receptor kinases. The sites and role of arrestin phosphorylation by SQRK were investigated here using recombinant proteins. Arrestin was phosphorylated on serine 392 and serine 397 in the C-terminus. Unphosphorylated arrestin bound to metarhodopsin and phosphorylated metarhodopsin with similar high affinities (Kd 33 and 21 nM respectively), while phosphorylation of arrestin reduced the affinity 3- to 5-fold (Kd 104 nM). Phosphorylation of metarhodopsin slightly increased the dissociation of arrestin observed during a 1 hour incubation. Together these studies suggest a unique role for SQRK in phosphorylating both receptor and arrestin and inhibiting the binding of these two proteins in the squid visual system. Invertebrate visual systems are inactivated by arrestin binding to metarhodopsin that does not require receptor phosphorylation. Here we show that squid rhodopsin kinase phosphorylates arrestin on two serines (S392,S397) in the C-terminus and phosphorylation decreases the affinity of arrestin for squid metarhodopsin. Metarhodopsin phosphorylation has very little effect on arrestin binding but does increase arrestin dissociation.

Using Bioluminescence Resonance Energy Transfer (BRET) to Characterize Agonist-Induced Arrestin Recruitment to Modified and Unmodified G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) represent ∼25% of current drug targets. Ligand binding to these receptors activates G proteins and arrestins, which are involved in differential signaling pathways. Because functionally selective or biased ligands activate one of these two pathways, they may be superior medications for certain diseases states. The identification of such ligands requires robust drug screening assays for both G protein and arrestin activity. This unit describes protocols for two bioluminescence resonance energy transfer (BRET)-based assays used to monitor arrestin recruitment to GPCRs. One assay requires modification of GPCRs by fusion to a BRET donor or acceptor moiety, whereas the other can detect arrestin recruitment to unmodified GPCRs.

Receptor sequestration in response to β-arrestin-2 phosphorylation by ERK1/2 governs steady-state levels of GPCR cell-surface expression

MAPKs are activated in response to G protein-coupled receptor (GPCR) stimulation and play essential roles in regulating cellular processes downstream of these receptors. However, very little is known about the reciprocal effect of MAPK activation on GPCRs. To investigate possible crosstalk between the MAPK and GPCRs, we assessed the effect of ERK1/2 on the activity of several GPCR family members. We found that ERK1/2 activation leads to a reduction in the steady-state cell-surface expression of many GPCRs because of their intracellular sequestration. This subcellular redistribution resulted in a global dampening of cell responsiveness, as illustrated by reduced ligand-mediated G-protein activation and second-messenger generation as well as blunted GPCR kinases and β-arrestin recruitment. This ERK1/2-mediated regulatory process was observed for GPCRs that can interact with β-arrestins, such as type-2 vasopressin, type-1 angiotensin, and CXC type-4 chemokine receptors, but not for the prostaglandin F receptor that cannot interact with β-arrestin, implicating this scaffolding protein in the receptor's subcellular redistribution. Complementation experiments in mouse embryonic fibroblasts lacking β-arrestins combined with in vitro kinase assays revealed that β-arrestin-2 phosphorylation on Ser14 and Thr276 is essential for the ERK1/2-promoted GPCR sequestration. This previously unidentified regulatory mechanism was observed after constitutive activation as well as after receptor tyrosine kinase- or GPCR-mediated activation of ERK1/2, suggesting that it is a central node in the tonic regulation of cell responsiveness to GPCR stimulation, acting both as an effector and a negative regulator.

Quantitative Signaling and Structure-Activity Analyses Demonstrate Functional Selectivity at the Nociceptin/Orphanin FQ Opioid Receptor

Comprehensive studies that consolidate selective ligands, quantitative comparisons of G protein versus arrestin-2/3 coupling, together with structure-activity relationship models for G protein-coupled receptor (GPCR) systems are less commonly employed. Here we examine biased signaling at the nociceptin/orphanin FQ opioid receptor (NOPR), the most recently identified member of the opioid receptor family. Using real-time, live-cell assays, we identified the signaling profiles of several NOPR-selective ligands in upstream GPCR signaling (G protein and arrestin pathways) to determine their relative transduction coefficients and signaling bias. Complementing this analysis, we designed novel ligands on the basis of NOPR antagonist J-113,397 [(±)-1-[(3R*,4R*)-1-(cyclooctylmethyl)-3-(hydroxymethyl)-4-piperidinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one] to explore structure-activity relationships. Our study shows that NOPR is capable of biased signaling, and further, the NOPR selective ligands MCOPPB [1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-(3R)-3-piperidinyl-1H-benzimidazole trihydrochloride] and NNC 63-0532 [8-(1-naphthalenylmethyl)-4-oxo-1-phenyl-1,3,8-triazaspiro[4.5]decane-3-acetic acid, methyl ester] are G protein-biased agonists. Additionally, minor structural modification of J-113,397 can dramatically shift signaling from antagonist to partial agonist activity. We explore these findings with in silico modeling of binding poses. This work is the first to demonstrate functional selectivity and identification of biased ligands at the nociceptin opioid receptor.

GRK3 suppresses L-DOPA-induced dyskinesia in the rat model of Parkinson's disease via its RGS homology domain

Degeneration of dopaminergic neurons causes Parkinson’s disease. Dopamine replacement therapy with L-DOPA is the best available treatment. However, patients develop L-DOPA-induced dyskinesia (LID). In the hemiparkinsonian rat, chronic L-DOPA increases rotations and abnormal involuntary movements modeling LID, via supersensitive dopamine receptors. Dopamine receptors are controlled by G protein-coupled receptor kinases (GRKs). Here we demonstrate that LID is attenuated by overexpression of GRK3 in the striatum, whereas knockdown of GRK3 by microRNA exacerbated it. Kinase-dead GRK3 and its separated RGS homology domain (RH) suppressed sensitization to L-DOPA, whereas GRK3 with disabled RH did not. RH alleviated LID without compromising anti-akinetic effect of L-DOPA. RH binds striatal Gq. GRK3, kinase-dead GRK3, and RH inhibited accumulation of ∆FosB, a marker of LID. RH-dead mutant was ineffective, whereas GRK3 knockdown exacerbated ∆FosB accumulation. Our findings reveal a novel mechanism of GRK3 control of the dopamine receptor signaling and the role of Gq in LID.

Unraveling the molecular architecture of a G protein-coupled receptor/β-arrestin/Erk module complex

β-arrestins serve as signaling scaffolds downstream of G protein-coupled receptors, and thus play a crucial role in a plethora of cellular processes. Although it is largely accepted that the ability of β-arrestins to interact simultaneously with many protein partners is key in G protein-independent signaling of GPCRs, only the precise knowledge of these multimeric arrangements will allow a full understanding of the dynamics of these interactions and their functional consequences. However, current experimental procedures for the determination of the three-dimensional structures of protein-protein complexes are not well adapted to analyze these short-lived, multi-component assemblies. We propose a model of the receptor/β-arrestin/Erk1 signaling module, which is consistent with most of the available experimental data. Moreover, for the β-arrestin/Raf1 and the β-arrestin/ERK interactions, we have used the model to design interfering peptides and shown that they compete with both partners, hereby demonstrating the validity of the predicted interaction regions.

The Viral G Protein-Coupled Receptor ORF74 Hijacks β-Arrestins for Endocytic Trafficking in Response to Human Chemokines

Kaposi’s sarcoma-associated herpesvirus-infected cells express the virally encoded G protein-coupled receptor ORF74. Although ORF74 is constitutively active, it binds human CXC chemokines that modulate this basal activity. ORF74-induced signaling has been demonstrated to underlie the development of the angioproliferative tumor Kaposi’s sarcoma. Whereas G protein-dependent signaling of ORF74 has been the subject of several studies, the interaction of this viral GPCR with β-arrestins has hitherto not been investigated. Bioluminescence resonance energy transfer experiments demonstrate that ORF74 recruits β-arrestins and subsequently internalizes in response to human CXCL1 and CXCL8, but not CXCL10. Internalized ORF74 traffics via early endosomes to recycling and late endosomes. Site-directed mutagenesis and homology modeling identified four serine and threonine residues at the distal end of the intracellular carboxyl-terminal of ORF74 that are required for β-arrestin recruitment and subsequent endocytic trafficking. Hijacking of the human endocytic trafficking machinery is a previously unrecognized action of ORF74.

Intracellular Trafficking of Neuropeptide Y Receptors

The multireceptor multiligand system of neuropeptide Y receptors and their ligands is involved in the regulation of a multitude of physiological and pathophysiological processes. Specific expression patterns, ligand-binding modes, and signaling properties contribute to the complex network regulating distinct cellular responses. Intracellular trafficking processes are important key steps that are regulated in context with accessory proteins. These proteins exert their influence by interacting directly or indirectly with the receptors, causing modification of the receptors, or operating as scaffolds for the assembly of larger signaling complexes. On the intracellular receptor faces, sequence-specific motifs have been identified that play an important role in this process. Interestingly, it is also possible to influence the receptor internalization by modification of the peptide ligand.

G Protein-coupled Receptor Kinases of the GRK4 Protein Subfamily Phosphorylate Inactive G Protein-coupled Receptors (GPCRs)

G protein-coupled receptor (GPCR) kinases (GRKs) play a key role in homologous desensitization of GPCRs. It is widely assumed that most GRKs selectively phosphorylate only active GPCRs. Here, we show that although this seems to be the case for the GRK2/3 subfamily, GRK5/6 effectively phosphorylate inactive forms of several GPCRs, including β2-adrenergic and M2 muscarinic receptors, which are commonly used as representative models for GPCRs. Agonist-independent GPCR phosphorylation cannot be explained by constitutive activity of the receptor or membrane association of the GRK, suggesting that it is an inherent ability of GRK5/6. Importantly, phosphorylation of the inactive β2-adrenergic receptor enhanced its interactions with arrestins. Arrestin-3 was able to discriminate between phosphorylation of the same receptor by GRK2 and GRK5, demonstrating preference for the latter. Arrestin recruitment to inactive phosphorylated GPCRs suggests that not only agonist activation but also the complement of GRKs in the cell regulate formation of the arrestin-receptor complex and thereby G protein-independent signaling.

Bioluminescence resonance energy transfer (BRET) to detect the interactions between kappa opioid receptor and non visual arrestins

Bioluminescence resonance energy transfer (BRET) is a very sensitive technique employed to study protein-protein interactions, including G-protein-coupled receptors (GPCRs) hetero- and homo-dimerization. Recently, BRET has also been used to investigate the interaction between GPCRs (e.g., β2 adrenergic receptor, muscarinic M2 receptor, dopaminergic D2 receptor) and non-visual arrestins. Here a BRET protocol is described to investigate interactions between the kappa opioid receptor (KOR) and non visual arrestins (arrestin-2 and arrestin-3) in HEK-293 cells, both under basal conditions and after exposure to KOR ligands.