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

Endogenous Opioid Peptides and Alternatively Spliced Mu Opioid Receptor Seven Transmembrane Carboxyl-Terminal Variants

There exist three main types of endogenous opioid peptides, enkephalins, dynorphins and β-endorphin, all of which are derived from their precursors. These endogenous opioid peptides act through opioid receptors, including mu opioid receptor (MOR), delta opioid receptor (DOR) and kappa opioid receptor (KOR), and play important roles not only in analgesia, but also many other biological processes such as reward, stress response, feeding and emotion. The MOR gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, generating multiple splice variants or isoforms. One type of these splice variants, the full-length 7 transmembrane (TM) Carboxyl (C)-terminal variants, has the same receptor structures but contains different intracellular C-terminal tails. The pharmacological functions of several endogenous opioid peptides through the mouse, rat and human OPRM1 7TM C-terminal variants have been considerably investigated together with various mu opioid ligands. The current review focuses on the studies of these endogenous opioid peptides and summarizes the results from early pharmacological studies, including receptor binding affinity and G protein activation, and recent studies of β-arrestin2 recruitment and biased signaling, aiming to provide new insights into the mechanisms and functions of endogenous opioid peptides, which are mediated through the OPRM1 7TM C-terminal splice variants.

Inflamed Ulcerative Colitis Regions Associated With MRGPRX2-Mediated Mast Cell Degranulation and Cell Activation Modules, Defining a New Therapeutic Target

BACKGROUND & AIMS

Recent literature has implicated a key role for mast cells in murine models of colonic inflammation, but their role in human ulcerative colitis (UC) is not well established. A major advance has been the identification of mrgprb2 (human orthologue, MRGPX2) as mediating IgE-independent mast cell activation. We sought to define mechanisms of mast cell activation and MRGPRX2 in human UC.

METHODS

Colon tissues were collected from patients with UC for bulk RNA sequencing and lamina propria cells were isolated for MRGPRX2 activation studies and single-cell RNA sequencing. Genetic association of all protein-altering G-protein coupled receptor single-nucleotide polymorphism was performed in an Ashkenazi Jewish UC case-control cohort. Variants of MRGPRX2 were transfected into Chinese hamster ovary (CHO) and human mast cell (HMC) 1.1 cells to detect genotype-dependent effects on β-arrestin recruitment, IP-1 accumulation, and phosphorylated extracellular signal-regulated kinase.

RESULTS

Mast cell-specific mediators and adrenomedullin (proteolytic precursor of PAMP-12, an MRGPRX2 agonist) are up-regulated in inflamed compared to uninflamed UC. MRGPRX2 stimulation induces carboxypeptidase secretion from inflamed UC. Of all protein-altering GPCR alleles, a unique variant of MRGPRX2, Asn62Ser, was most associated with and was bioinformatically predicted to alter arrestin recruitment. We validated that the UC protective serine allele enhances β-arrestin recruitment, decreases IP-1, and increases phosphorylated extracellular signal-regulated kinase with MRGPRX2 agonists. Single-cell RNA sequencing defines that adrenomedullin is expressed by activated fibroblasts and epithelial cells and that interferon gamma is a key upstream regulator of mast cell gene expression.

CONCLUSION

Inflamed UC regions are distinguished by MRGPRX2-mediated activation of mast cells, with decreased activation observed with a UC-protective genetic variant. These results define cell modules of UC activation and a new therapeutic target.

Prokineticin receptors interact unselectively with several G protein subtypes but bind selectively to β-arrestin 2

Prokineticin 1 (pk1) and prokineticin 2 (pk2) interact with two structurally related G-protein coupled receptors, prokineticin receptor 1 (PKR1) and prokineticin receptor 2 (PKR2). Cellular signalling studies show that the activated receptors can evoke Ca²⁺-mobilization, pertussis toxin-sensitive ERK phosphorylation, and intracellular cAMP accumulation, which suggests the partecipation of several G protein subtypes, such as G_q/11, G_i/o and G_s. However, direct interactions with these transduction proteins have not been studied yet. Here we measured by bioluminescence resonance energy transfer (BRET) the association of PKR1 and PKR2 with different heterotrimeric Gα proteins in response to pk1 and pk2 activation. Using host-cell lines carrying gene deletions of Gα_q/11 or Gα_s, and pertussis toxin treatment to abolish the receptor interactions with Gα_i/o, we determined that both receptors could couple with comparable efficiency to G_q/11 and G_i/o, but far less efficiently to G_s or other pertussis toxin-insensitive G proteins. We also used BRET methodology to assess the association of prokineticin receptors with β-arrestin isoforms. Fluorescent versions of the isoforms were transfected both in HEK293 cells and in double KO β-arrestin 1/2 mouse fibroblasts, to study receptor interaction with the reconstituted individual β-arrestins without background expression of the endogenous genes. Both receptors formed stable BRET-emitting complexes with β-arrestin 2 but not with β-arrestin 1, indicating strong selectivity for the former. In all the studied transducer interactions and in both receptors, pk2 was more potent than pk1 in promoting receptor binding to transduction proteins.

Mechanistic diversity involved in the desensitization of G protein-coupled receptors

The desensitization of G protein-coupled receptors (GPCRs), which involves rapid loss of responsiveness due to repeated or chronic exposure to agonists, can occur through various mechanisms at different levels of signaling pathways. In this review, the mechanisms of GPCR desensitization are classified according to their occurrence at the receptor level and downstream to the receptor. The desensitization at the receptor level occurs in a phosphorylation-dependent manner, wherein the activated receptors are phosphorylated by GPCR kinases (GRKs), thereby increasing their affinities for arrestins. Arrestins bind to receptors through the cavity on the cytoplasmic region of heptahelical domains and interfere with the binding and activation of G-protein. Diverse mechanisms are involved in the desensitization that occurs downstream of the receptor. Some of these include the sequestration of G proteins, such as G_q and G_i/o by GRK2/3 and deubiquitinated arrestins, respectively. Mechanistically, GRK2/3 attenuates GPCR signaling by sequestering the G_α subunits of the G_q family and G_βγ via regulators of G protein signaling and pleckstrin homology domains, respectively. Moreover, studies on G_i/o-coupled D2-like receptors have reported that arrestins are deubiquitinated under desensitization condition and form a stable complex with G_βγ, thereby preventing them from coupling with G_α and the receptor, eventually leading to receptor signaling inhibition. Notably, the desensitization mechanism that involves arrestin deubiquitination is interesting; however, this is a new mechanism and needs to be explored further.

The Open Question of How GPCRs Interact with GPCR Kinases (GRKs)

G protein-coupled receptors (GPCRs), which regulate a vast number of eukaryotic processes, are desensitized by various mechanisms but, most importantly, by the GPCR kinases (GRKs). Ever since GRKs were first identified, investigators have sought to determine which structural features of GRKs are used to select for the agonist-bound states of GPCRs and how this binding event in turn enhances GRK catalytic activity. Despite a wealth of molecular information from high-resolution crystal structures of GRKs, the mechanisms driving activation have remained elusive, in part because the GRK N-terminus and active site tether region, previously proposed to serve as a receptor docking site and to be key to kinase domain closure, are often disordered or adopt inconsistent conformations. However, two recent studies have implicated other regions of GRKs as being involved in direct interactions with active GPCRs. Atomic resolution structures of GPCR–GRK complexes would help refine these models but are, so far, lacking. Here, we assess three distinct models for how GRKs recognize activated GPCRs, discuss limitations in the approaches used to generate them, and then experimentally test a hypothetical GPCR interaction site in GRK2 suggested by the two newest models.

Targeting G protein-coupled receptor kinases (GRKs) to G protein-coupled receptors

G protein-coupled receptors (GPCRs) interact with three protein families following agonist binding: heterotrimeric G proteins, G protein-coupled receptor kinases (GRKs) and arrestins. GRK-mediated phosphorylation of GPCRs promotes arrestin binding to uncouple the receptor from G protein, a process called desensitization, and for many GPCRs, arrestin binding also promotes receptor endocytosis and intracellular signaling. Thus, GRKs play a central role in modulating GPCR signaling and localization. Here we review recent advances in this field which include additional insight into how GRKs target GPCRs and bias signaling, and the development of specific inhibitors to dissect GRK function in model systems.

Structural aspects of rod opsin and their implication in genetic diseases

Vision in dim-light conditions is triggered by photoactivation of rhodopsin, the visual pigment of rod photoreceptor cells. Rhodopsin is made of a protein, the G protein coupled receptor (GPCR) opsin, and the chromophore 11-cis-retinal. Vertebrate rod opsin is the GPCR best characterized at the atomic level of detail. Since the release of the first crystal structure 20 years ago, a huge number of structures have been released that, in combination with valuable spectroscopic determinations, unveiled most aspects of the photobleaching process. A number of spontaneous mutations of rod opsin have been found linked to vision-impairing diseases like autosomal dominant or autosomal recessive retinitis pigmentosa (adRP or arRP, respectively) and autosomal congenital stationary night blindness (adCSNB). While adCSNB is mainly caused by constitutive activation of rod opsin, RP shows more variegate determinants affecting different aspects of rod opsin function. The vast majority of missense rod opsin mutations affects folding and trafficking and is linked to adRP, an incurable disease that awaits light on its molecular structure determinants. This review article summarizes all major structural information available on vertebrate rod opsin conformational states and the insights gained so far into the structural determinants of adCSNB and adRP linked to rod opsin mutations. Strategies to design small chaperones with therapeutic potential for selected adRP rod opsin mutants will be discussed as well.

Differential Involvement of ACKR3 C-Tail in β-Arrestin Recruitment, Trafficking and Internalization

Background: The atypical chemokine receptor 3 (ACKR3) belongs to the superfamily of G protein-coupled receptors (GPCRs). Unlike classical GPCRs, this receptor does not activate G proteins in most cell types but recruits β-arrestins upon activation. ACKR3 plays an important role in cancer and vascular diseases. As recruitment of β-arrestins is triggered by phosphorylation of the C-terminal tail of GPCRs, we studied the role of different potential phosphorylation sites within the ACKR3 C-tail to further delineate the molecular mechanism of internalization and trafficking of this GPCR. Methods: We used various bioluminescence and fluorescence resonance energy transfer-based sensors and techniques in Human Embryonic Kidney (HEK) 293T cells expressing WT or phosphorylation site mutants of ACKR3 to measure CXCL12-induced recruitment of β-arrestins and G-protein-coupled receptor kinases (GRKs), receptor internalization and trafficking. Results: Upon CXCL12 stimulation, ACKR3 recruits both β-arrestin 1 and 2 with equivalent kinetic profiles. We identified interactions with GRK2, 3 and 5, with GRK2 and 3 being important for β-arrestin recruitment. Upon activation, ACKR3 internalizes and recycles back to the cell membrane. We demonstrate that β-arrestin recruitment to the receptor is mainly determined by a single cluster of phosphorylated residues on the C-tail of ACKR3, and that residue T³⁵² and in part S³⁵⁵ are important residues for β-arrestin1 recruitment. Phosphorylation of the C-tail appears essential for ligand-induced internalization and important for differential β-arrestin recruitment. GRK2 and 3 play a key role in receptor internalization. Moreover, ACKR3 can still internalize when β-arrestin recruitment is impaired or in the absence of β-arrestins, using alternative internalization pathways. Our data indicate that distinct residues within the C-tail of ACKR3 differentially regulate CXCL12-induced β-arrestin recruitment, ACKR3 trafficking and internalization.

Signaling at the endosome: cryo-EM structure of a GPCR-G protein-beta-arrestin megacomplex

G protein‐coupled receptors (GPCRs) are a large class of cell‐surface receptor involved in cellular signaling that are currently the target of over one third of all clinically approved therapeutics. Classically, an agonist‐bound, active GPCR couples to and activates G proteins through the receptor intracellular core. To attenuate G protein signaling, the GPCR is phosphorylated at its C‐terminal tail and/or relevant intracellular loops, allowing for the recruitment of β‐arrestins (βarrs). βarrs then couple to the receptor intracellular core in order to mediate receptor desensitization and internalization. However, our laboratory and others have observed that some GPCRs are capable of continuously signaling through G protein even after internalization. This mode of sustained signaling stands in contrast with our previous understanding of GPCR signaling, and its molecular mechanism is still not well understood. Recently, we have solved the structure of a GPCR–G protein–βarr megacomplex by cryo‐electron microscopy. This ‘megaplex’ structure illustrates the independent and simultaneous coupling of a G protein to the receptor intracellular core, and binding of a βarr to a phosphorylated receptor C‐terminal tail, with all three components maintaining their respective canonically active conformations. The structure provides evidence for the ability of a GPCR to activate G protein even while being bound to and internalized by βarr. It also reveals that the binding of G protein and βarr to the same GPCR is not mutually exclusive, and raises a number of future questions to be answered regarding the mechanism of sustained signaling.

An Eight Amino Acid Segment Controls Oligomerization and Preferred Conformation of the two Non-visual Arrestins

G protein coupled receptors signal through G proteins or arrestins. A long-standing mystery in the field is why vertebrates have two non-visual arrestins, arrestin-2 and arrestin-3. These isoforms are ~75% identical and 85% similar; each binds numerous receptors, and appear to have many redundant functions, as demonstrated by studies of knockout mice. We previously showed that arrestin-3 can be activated by inositol-hexakisphosphate (IP6). IP6 interacts with the receptor-binding surface of arrestin-3, induces arrestin-3 oligomerization, and this oligomer stabilizes the active conformation of arrestin-3. Here, we compared the impact of IP6 on oligomerization and conformational equilibrium of the highly homologous arrestin-2 and arrestin-3 and found that these two isoforms are regulated differently. In the presence of IP6, arrestin-2 forms "infinite" chains, where each promoter remains in the basal conformation. In contrast, full length and truncated arrestin-3 form trimers and higher-order oligomers in the presence of IP6; we showed previously that trimeric state induces arrestin-3 activation (Chen et al., 2017). Thus, in response to IP6, the two non-visual arrestins oligomerize in different ways in distinct conformations. We identified an insertion of eight residues that is conserved across arrestin-2 homologs, but absent in arrestin-3 that likely accounts for the differences in the IP6 effect. Because IP6 is ubiquitously present in cells, this suggests physiological consequences, including differences in arrestin-2/3 trafficking and JNK3 activation. The functional differences between two non-visual arrestins are in part determined by distinct modes of their oligomerization. The mode of oligomerization might regulate the function of other signaling proteins.

Supramolecular organization of rhodopsin in rod photoreceptor cell membranes

Rhodopsin is the light receptor in rod photoreceptor cells that initiates scotopic vision. Studies on the light receptor span well over a century, yet questions about the organization of rhodopsin within the photoreceptor cell membrane still persist and a consensus view on the topic is still elusive. Rhodopsin has been intensely studied for quite some time, and there is a wealth of information to draw from to formulate an organizational picture of the receptor in native membranes. Early experimental evidence in apparent support for a monomeric arrangement of rhodopsin in rod photoreceptor cell membranes is contrasted and reconciled with more recent visual evidence in support of a supramolecular organization of rhodopsin. What is known so far about the determinants of forming a supramolecular structure and possible functional roles for such an organization are also discussed. Many details are still missing on the structural and functional properties of the supramolecular organization of rhodopsin in rod photoreceptor cell membranes. The emerging picture presented here can serve as a springboard towards a more in-depth understanding of the topic.

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 α-arrestin family of ubiquitin ligase adaptors links metabolism with selective endocytosis

The regulation of nutrient uptake into cells is important, as it allows to either increase biomass for cell growth or to preserve homoeostasis. A key strategy to adjust cellular nutrient uptake is the reconfiguration of the nutrient transporter repertoire at the plasma membrane by the addition of nutrient transporters through the secretory pathway and by their endocytic removal. In this review, we focus on the mechanisms that regulate selective nutrient transporter endocytosis, which is mediated by the α-arrestin protein family. In the budding yeast Saccharomyces cerevisiae, 14 different α-arrestins (also named arrestin-related trafficking adaptors, ARTs) function as adaptors for the ubiquitin ligase Rsp5. They instruct Rsp5 to ubiquitinate subsets of nutrient transporters to orchestrate their endocytosis. The ART proteins are under multilevel control of the major nutrient sensing systems, including amino acid sensing by the general amino acid control and target of rapamycin pathways, and energy sensing by 5'-adenosine-monophosphate-dependent kinase. The function of the six human α-arrestins is comparably under-characterised. Here, we summarise the current knowledge about the function, regulation and substrates of yeast ARTs and human α-arrestins, and highlight emerging communalities and general principles.

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.

Dissecting the structural features of β-arrestins as multifunctional proteins

β-arrestins bind active G protein-coupled receptors (GPCRs) and play a crucial role in receptor desensitization and internalization. The classical paradigm of arrestin function has been expanded with the identification of many non-receptor-binding partners, which indicated the multifunctional role of β-arrestins in cellular functions. To elucidate the molecular mechanism of β-arrestin-mediated signaling, the structural features of β-arrestins were investigated using X-ray crystallography and cryogenic electron microscopy (cryo-EM). However, the intrinsic conformational flexibility of β-arrestins hampers the elucidation of structural interactions between β-arrestins and their binding partners using conventional structure determination tools. Therefore, structural information obtained using complementary structure analysis techniques would be necessary in combination with X-ray crystallography and cryo-EM data. In this review, we describe how β-arrestins interact with their binding partners from a structural point of view, as elucidated by both traditional methods (X-ray crystallography and cryo-EM) and complementary structure analysis techniques.

The G Protein-Coupled Receptor Kinases (GRKs) in Chemokine Receptor-Mediated Immune Cell Migration: From Molecular Cues to Physiopathology

Although G protein-coupled receptor kinases (GRKs) have long been known to regulate G protein-coupled receptor (GPCR) desensitization, their more recently characterized functions as scaffolds and signalling adapters underscore that this small family of proteins governs a larger array of physiological functions than originally suspected. This review explores how GRKs contribute to the complex signalling networks involved in the migration of immune cells along chemokine gradients sensed by cell surface GPCRs. We outline emerging evidence indicating that the coordinated docking of several GRKs on an active chemokine receptor determines a specific receptor phosphorylation barcode that will translate into distinct signalling and migration outcomes. The guidance cues for neutrophil migration are emphasized based on several alterations affecting GRKs or GPCRs reported to be involved in pathological conditions.

Potential of Nociceptin/Orphanin FQ Peptide Analogs for Drug Development

Nociceptin receptor (NOP) belongs to the family of opioid receptors but was discovered and characterized much later than the so called classical opioid receptors, μ, δ and κ (or MOP, DOP and KOP, resp.). Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand of this receptor and it controls numerous important functions in the central nervous system and in the periphery, so its analogs may be developed as innovative drugs for the treatment of a variety of conditions and pathological states. Availability of potent and selective ligands with high affinity to NOP receptor is essential to fully understand the role of NOP-N/OFQ system in the body, which in turn may lead to designing novel therapeutics. Here, we have focused on reviewing the structure of potent peptide-based agonists, antagonists, biased analogs and bivalent ligands that target NOP receptor.

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.

Pharmacological Characterization of µ-Opioid Receptor Agonists with Biased G Protein or β-Arrestin Signaling, and Computational Study of Conformational Changes during Receptor Activation

In recent years, G protein vs. β-arrestin biased agonism at opioid receptors has been proposed as an opportunity to produce antinociception with reduced adverse effects. However, at present this approach is highly debated, a reason why more information about biased ligands is required. While the practical relevance of bias in the case of µ-opioid receptors (MOP) still needs to be validated, it remains important to understand the basis of this bias of MOP (and other GPCRs). Recently, we reported two cyclopeptides with high affinity for MOP, the G protein biased Dmt-c[d-Lys-Phe-pCF3-Phe-Asp]NH2 (F-81), and the β-arrestin 2 biased Dmt-c[d-Lys-Phe-Asp]NH2 (C-33), as determined by calcium mobilization assay and bioluminescence resonance energy transfer-based assay. The biased character of F-81 and C-33 has been further analyzed in the [35S]GTPγS binding assay in human MOP-expressing cells, and the PathHunter enzyme complementation assay, used to measure β-arrestin 2 recruitment. To investigate the structural features of peptide-MOP complexes, we performed conformational analysis by NMR spectroscopy, molecular docking, and molecular dynamics simulation. These studies predicted that the two ligands form alternative complexes with MOP, engaging specific ligand-receptor contacts. This would induce different displays of the cytosolic side of the seven-helices bundle, in particular by stabilizing different angulations of helix 6, that could favor intracellular coupling to either G protein or β-arrestin.

The Conformational Equilibrium of the Neuropeptide Y2 Receptor in Bilayer Membranes

Dynamic structural transitions within the seven-transmembrane bundle represent the mechanism by which G-protein-coupled receptors convert an extracellular chemical signal into an intracellular biological function. Here, the conformational dynamics of the neuropeptide Y receptor type 2 (Y2R) during activation was investigated. The apo, full agonist-, and arrestin-bound states of Y2R were prepared by cell-free expression, functional refolding, and reconstitution into lipid membranes. To study conformational transitions between these states, all six tryptophans of Y2R were 13 C-labeled. NMR-signal assignment was achieved by dynamic-nuclear-polarization enhancement and the individual functional states of the receptor were characterized by monitoring 13 C NMR chemical shifts. Activation of Y2R is mediated by molecular switches involving the toggle switch residue Trp2816.48 of the highly conserved SWLP motif and Trp3277.55 adjacent to the NPxxY motif. Furthermore, a conformationally preserved "cysteine lock"-Trp11623.50 was identified.

Novel fluorescent GPCR biosensor detects retinal equilibrium binding to opsin and active G protein and arrestin signaling conformations

Rhodopsin is a canonical class A photosensitive G protein-coupled receptor (GPCR), yet relatively few pharmaceutical agents targeting this visual receptor have been identified, in part due to the unique characteristics of its light-sensitive, covalently bound retinal ligands. Rhodopsin becomes activated when light isomerizes 11-cis-retinal into an agonist, all-trans-retinal (ATR), which enables the receptor to activate its G protein. We have previously demonstrated that, despite being covalently bound, ATR can display properties of equilibrium binding, yet how this is accomplished is unknown. Here, we describe a new approach for both identifying compounds that can activate and attenuate rhodopsin and testing the hypothesis that opsin binds retinal in equilibrium. Our method uses opsin-based fluorescent sensors, which directly report the formation of active receptor conformations by detecting the binding of G protein or arrestin fragments that have been fused onto the receptor's C terminus. We show that these biosensors can be used to monitor equilibrium binding of the agonist, ATR, as well as the noncovalent binding of β-ionone, an antagonist for G protein activation. Finally, we use these novel biosensors to observe ATR release from an activated, unlabeled receptor and its subsequent transfer to the sensor in real time. Taken together, these data support the retinal equilibrium binding hypothesis. The approach we describe should prove directly translatable to other GPCRs, providing a new tool for ligand discovery and mutant characterization.

Direct evidence that the GPCR CysLTR2 mutant causative of uveal melanoma is constitutively active with highly biased signaling

Uveal melanoma is the most common eye cancer in adults and is clinically and genetically distinct from skin cutaneous melanoma. In a subset of cases, the oncogenic driver is an activating mutation in CYSLTR2, the gene encoding the G protein-coupled receptor cysteinyl-leukotriene receptor 2 (CysLTR2). The mutant CYSLTR2 encodes for the CysLTR2-L129Q receptor, with the substitution of Leu to Gln at position 129 (3.43). The ability of CysLTR2-L129Q to cause malignant transformation has been hypothesized to result from constitutive activity, but how the receptor could escape desensitization is unknown. Here, we characterize the functional properties of CysLTR2-L129Q. We show that CysLTR2-L129Q is a constitutively active mutant that strongly drives Gq/11 signaling pathways. However, CysLTR2-L129Q only poorly recruits β-arrestin. Using a modified Slack-Hall operational model, we quantified the constitutive activity for both pathways and conclude that CysLTR2-L129Q displays profound signaling bias for Gq/11 signaling pathways while escaping β-arrestin-mediated downregulation. CYSLTR2 is the first known example of a G protein-coupled receptor driver oncogene that encodes a highly biased constitutively active mutant receptor. These results provide new insights into the mechanism of CysLTR2-L129Q oncoprotein signaling and suggest CYSLTR2 as a promising potential therapeutic target in uveal melanoma.

How GPCR Phosphorylation Patterns Orchestrate Arrestin-Mediated Signaling

Binding of arrestin to phosphorylated G-protein-coupled receptors (GPCRs) controls many aspects of cell signaling. The number and arrangement of phosphates may vary substantially for a given GPCR, and different phosphorylation patterns trigger different arrestin-mediated effects. Here, we determine how GPCR phosphorylation influences arrestin behavior by using atomic-level simulations and site-directed spectroscopy to reveal the effects of phosphorylation patterns on arrestin binding and conformation. We find that patterns favoring binding differ from those favoring activation-associated conformational change. Both binding and conformation depend more on arrangement of phosphates than on their total number, with phosphorylation at different positions sometimes exerting opposite effects. Phosphorylation patterns selectively favor a wide variety of arrestin conformations, differently affecting arrestin sites implicated in scaffolding distinct signaling proteins. We also reveal molecular mechanisms of these phenomena. Our work reveals the structural basis for the long-standing "barcode" hypothesis and has important implications for design of functionally selective GPCR-targeted drugs.

The finger loop as an activation sensor in arrestin

The finger loop in the central crest of the receptor-binding site of arrestins engages the cavity between the transmembrane helices of activated G-protein-coupled receptors. Therefore, it was hypothesized to serve as the sensor that detects the activation state of the receptor. We performed comprehensive mutagenesis of the finger loop in bovine visual arrestin-1, generated mutant radiolabeled proteins by cell-free translation, and determined the effects of mutations on the in vitro binding of arrestin-1 to purified phosphorylated light-activated rhodopsin. This interaction is driven by two factors, rhodopsin activation and rhodopsin-attached phosphates. Therefore, the binding of arrestin-1 to light-activated unphosphorylated rhodopsin is low. To evaluate the role of the finger loop specifically in the recognition of the active receptor conformation, we tested the effects of these mutations in the context of truncated arrestin-1 that demonstrates much higher binding to unphosphorylated activated and phosphorylated inactive rhodopsin. The majority of finger loop residues proved important for arrestin-1 binding to light-activated rhodopsin, with six mutations affecting the binding exclusively to this form. Thus, the finger loop is the key element of arrestin-1 activation sensor. The data also suggest that arrestin-1 and its enhanced mutant bind various functional forms of rhodopsin differently.

Insights into the Activation Mechanism of the ALX/FPR2 Receptor

The formyl peptide receptor 2 (ALX/FPR2), a G-protein-coupled receptor (GPCR), plays an important role in host defense and inflammation. This receptor can be driven as pro- or anti-inflammatory depending on its agonist, such as N-formyl-Met-Leu-Phe-Lys (fMLFK) and resolvin D1 (RvD1) or its aspirin-triggered 17 (R)-epimer, AT-RvD1, respectively. However, the activation mechanism of ALX/FPR2 by pro- and anti-inflammatory agonists remains unclear. In this work, on the basis of molecular dynamics simulations, we evaluated a model of the ALX/FPR2 receptor activation process using two agonists, fMLFK and AT-RvD1, with opposite effects. The simulations by both fMLFK and AT-RvD1 induced the ALX/FPR2 activation through a set of receptor-core residues, in particular, R205, Q258, and W254. In addition, the activation was dependent on the disruption of electrostatic interactions in the cytoplasmic region of the receptor. We also found that in the AT-RvD1 simulations, the position of the H8 helix was similar to that of the same helix in other class-A GPCRs coupled to arrestin. Thus our results shed light on the mechanism of activation of the ALX/FPR2 receptor by pro-inflammatory and pro-resolution agonists.

Structural Insights into β-arrestin/CB1 Receptor Interaction: NMR and CD Studies on Model Peptides

Activation of the cannabinoid CB1 receptor induces different cellular signaling cascades through coupling to different effector proteins (G-proteins and β-arrestins), triggering numerous therapeutic effects. Conformational changes and rearrangements at the intracellular domain of this GPCR receptor that accompany ligand binding dictate the signaling pathways. The GPCR-binding interface for G proteins has been extensively studied, whereas β-arrestin/GPCR complexes are still poorly understood. To gain knowledge in this direction, we designed peptides that mimic the motifs involved in the putative interacting region: β-arrestin1 finger loop and the transmembrane helix 7-helix 8 (TMH7-H8) elbow located at the intracellular side of the CB1 receptor. According to circular dichroism and NMR data, these peptides form a native-like, helical conformation and interact with each other in aqueous solution, in the presence of trifluoroethanol, and using zwitterionic detergent micelles as membrane mimics. These results increase our understanding of the binding mode of β-arrestin and CB1 receptor and validate minimalist approaches to structurally comprehend complex protein systems.

Capturing Peptide-GPCR Interactions and Their Dynamics

Many biological functions of peptides are mediated through G protein-coupled receptors (GPCRs). Upon ligand binding, GPCRs undergo conformational changes that facilitate the binding and activation of multiple effectors. GPCRs regulate nearly all physiological processes and are a favorite pharmacological target. In particular, drugs are sought after that elicit the recruitment of selected effectors only (biased ligands). Understanding how ligands bind to GPCRs and which conformational changes they induce is a fundamental step toward the development of more efficient and specific drugs. Moreover, it is emerging that the dynamic of the ligand–receptor interaction contributes to the specificity of both ligand recognition and effector recruitment, an aspect that is missing in structural snapshots from crystallography. We describe here biochemical and biophysical techniques to address ligand–receptor interactions in their structural and dynamic aspects, which include mutagenesis, crosslinking, spectroscopic techniques, and mass-spectrometry profiling. With a main focus on peptide receptors, we present methods to unveil the ligand–receptor contact interface and methods that address conformational changes both in the ligand and the GPCR. The presented studies highlight a wide structural heterogeneity among peptide receptors, reveal distinct structural changes occurring during ligand binding and a surprisingly high dynamics of the ligand–GPCR complexes.

Position of rhodopsin photoisomerization on the disk surface confers variability to the rising phase of the single photon response in vertebrate rod photoreceptors

Retinal rods function as accurate photon counters to provide for vision under very dim light. To do so, rods must generate highly amplified, reproducible responses to single photons, yet outer segment architecture and randomness in the location of rhodopsin photoisomerization on the surface of an internal disk introduce variability to the rising phase of the photon response. Soon after a photoisomerization at a disk rim, depletion of cGMP near the plasma membrane closes ion channels and hyperpolarizes the rod. But with a photoisomerization in the center of a disk, local depletion of cGMP is distant from the channels in the plasma membrane. Thus, channel closure is delayed by the time required for the reduction of cGMP concentration to reach the plasma membrane. Moreover, the local fall in cGMP dissipates over a larger volume before affecting the channels, so response amplitude is reduced. This source of variability increases with disk radius. Using a fully space-resolved biophysical model of rod phototransduction, we quantified the variability attributable to randomness in the location of photoisomerization as a function of disk structure. In mouse rods that have small disks bearing a single incisure, this variability was negligible in the absence of the incisure. Variability was increased slightly by the incisure, but randomness in the shutoff of rhodopsin emerged as the main source of single photon response variability at all but the earliest times. Variability arising from randomness in the transverse location of photoisomerization increased in magnitude and persisted over a longer period in the photon response of large salamander rods. A symmetric arrangement of multiple incisures in the disks of salamander rods greatly reduced this variability during the rising phase, but the incisures had the opposite effect on variability arising from randomness in rhodopsin shutoff at later times.

Biological Role of Arrestin-1 Oligomerization

Members of the arrestin superfamily have great propensity of self-association, but the physiological significance of this phenomenon is unclear. To determine the biological role of visual arrestin-1 oligomerization in rod photoreceptors, we expressed mutant arrestin-1 with severely impaired self-association in mouse rods and analyzed mice of both sexes. We show that the oligomerization-deficient mutant is capable of quenching rhodopsin signaling normally, as judged by electroretinography and single-cell recording. Like wild type, mutant arrestin-1 is largely excluded from the outer segments in the dark, proving that the normal intracellular localization is not due the size exclusion of arrestin-1 oligomers. In contrast to wild type, supraphysiological expression of the mutant causes shortening of the outer segments and photoreceptor death. Thus, oligomerization reduces the cytotoxicity of arrestin-1 monomer, ensuring long-term photoreceptor survival.SIGNIFICANCE STATEMENT Visual arrestin-1 forms dimers and tetramers. The biological role of its oligomerization is unclear. To test the role of arrestin-1 self-association, we expressed oligomerization-deficient mutant in arrestin-1 knock-out mice. The mutant quenches light-induced rhodopsin signaling like wild type, demonstrating that in vivo monomeric arrestin-1 is necessary and sufficient for this function. In rods, arrestin-1 moves from the inner segments and cell bodies in the dark to the outer segments in the light. Nonoligomerizing mutant undergoes the same translocation, demonstrating that the size of the oligomers is not the reason for arrestin-1 exclusion from the outer segments in the dark. High expression of oligomerization-deficient arrestin-1 resulted in rod death. Thus, oligomerization reduces the cytotoxicity of high levels of arrestin-1 monomer.

Mu Opioids Induce Biased Signaling at the Full-Length Seven Transmembrane C-Terminal Splice Variants of the mu Opioid Receptor Gene, Oprm1

The biased signaling has been extensively studied in the original mu opioid receptor (MOR-1), particularly through G protein and β-arrestin2 signaling pathways. The concept that the G protein pathway is often linked to the therapeutic effect of the drug, while the β-arrestin pathway is associated to the side effects has been proposed to develop biased analgesic compounds with limited side-effects associated with traditional opiates. The mu opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, generating multiple splice variants or isoforms that are conserved from rodent to human. One type of the Oprm1 splice variants are the full-length 7 transmembrane (7TM) C-terminal splice variants, which have identical receptor structures including entire binding pocket, but contain a different intracellular C-terminal tail resulted from 3' alternative splicing. Increasing evidence suggest that these full-length 7TM C-terminal variants play important roles in mu opioid pharmacology, raising questions regarding biased signaling at these multiple C-terminal variants. In the present study, we investigated the effect of different C-terminal variants on mu agonist-induced G protein coupling, β-arrestin2 recruitment, and ultimately, signaling bias. We found that mu agonists produced marked differences in G protein activation and β-arrestin2 recruitment among various C-terminal variants, leading to biased signaling at various level. Particularly, MOR-1O, an exon 7-associated variant, showed greater β-arrestin2 bias for most mu agonists than MOR-1, an exon 4-associated variant. Biased signaling of G protein-coupled receptors has been defined by evidences that different agonists can produce divergent signaling transduction pathways through a single receptor. Our findings that a single mu agonist can induce differential signaling through multiple 7TM splice variants provide a new perspective on biased signaling at least for Oprm1, which perhaps is important for our understanding of the complex mu opioid actions in vivo where all the 7TM splice variants co-exist.

webPSN v2.0: a webserver to infer fingerprints of structural communication in biomacromolecules

A mixed Protein Structure Network (PSN) and Elastic Network Model-Normal Mode Analysis (ENM-NMA)-based strategy (i.e. PSN-ENM) was developed to investigate structural communication in bio-macromolecules. Protein Structure Graphs (PSGs) are computed on a single structure, whereas information on system dynamics is supplied by ENM-NMA. The approach was implemented in a webserver (webPSN), which was significantly updated herein. The webserver now handles both proteins and nucleic acids and relies on an internal upgradable database of network parameters for ions and small molecules in all PDB structures. Apart from the radical restyle of the server and some changes in the calculation setup, other major novelties concern the possibility to: a) compute the differences in nodes, links, and communication pathways between two structures (i.e. network difference) and b) infer links, hubs, communities, and metapaths from consensus networks computed on a number of structures. These new features are useful to identify commonalties and differences between two different functional states of the same system or structural-communication signatures in homologous or analogous systems. The output analysis relies on 3D-representations, interactive tables and graphs, also available for download. Speed and accuracy make this server suitable to comparatively investigate structural communication in large sets of bio-macromolecular systems. URL: http://webpsn.hpc.unimore.it.

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

DeSiphering receptor core-induced and ligand-dependent conformational changes in arrestin via genetic encoded trimethylsilyl 1H-NMR probe

Characterization of the dynamic conformational changes in membrane protein signaling complexes by nuclear magnetic resonance (NMR) spectroscopy remains challenging. Here we report the site-specific incorporation of 4-trimethylsilyl phenylalanine (TMSiPhe) into proteins, through genetic code expansion. Crystallographic analysis revealed structural changes that reshaped the TMSiPhe-specific amino-acyl tRNA synthetase active site to selectively accommodate the trimethylsilyl (TMSi) group. The unique up-field 1H-NMR chemical shift and the highly efficient incorporation of TMSiPhe enabled the characterization of multiple conformational states of a phospho-β2 adrenergic receptor/β-arrestin-1(β-arr1) membrane protein signaling complex, using only 5 μM protein and 20 min of spectrum accumulation time. We further showed that extracellular ligands induced conformational changes located in the polar core or ERK interaction site of β-arr1 via direct receptor transmembrane core interactions. These observations provided direct delineation and key mechanism insights that multiple receptor ligands were able to induce distinct functionally relevant conformational changes of arrestin.

Experimental and computational analysis of biased agonism on full-length and a C-terminally truncated adenosine A2A receptor

Biased agonism, the ability of agonists to differentially activate downstream signaling pathways by stabilizing specific receptor conformations, is a key issue for G protein-coupled receptor (GPCR) signaling. The C-terminal domain might influence this functional selectivity of GPCRs as it engages G proteins, GPCR kinases, β-arrestins, and several other proteins. Thus, the aim of this paper is to compare the agonist-dependent selectivity for intracellular pathways in a heterologous system expressing the full-length (A_2AR) and a C-tail truncated (A_2A^Δ40R lacking the last 40 amino acids) adenosine A_2A receptor, a GPCR that is already targeted in Parkinson’s disease using a first-in-class drug. Experimental data such as ligand binding, cAMP production, β-arrestin recruitment, ERK1/2 phosphorylation and dynamic mass redistribution assays, which correspond to different aspects of signal transduction, were measured upon the action of structurally diverse compounds (the agonists adenosine, NECA, CGS-21680, PSB-0777 and LUF-5834 and the SCH-58261 antagonist) in cells expressing A_2AR and A_2A^Δ40R. The results show that taking cAMP levels and the endogenous adenosine agonist as references, the main difference in bias was obtained with PSB-0777 and LUF-5834. The C-terminus is dispensable for both G-protein and β-arrestin recruitment and also for MAPK activation. Unrestrained molecular dynamics simulations, at the μs timescale, were used to understand the structural arrangements of the binding cavity, triggered by these chemically different agonists, facilitating G protein binding with different efficacy.

Biased Opioid Ligands

Achieving effective pain management is one of the major challenges associated with modern day medicine. Opioids, such as morphine, have been the reference treatment for moderate to severe acute pain not excluding chronic pain modalities. Opioids act through the opioid receptors, the family of G-protein coupled receptors (GPCRs) that mediate pain relief through both the central and peripheral nervous systems. Four types of opioid receptors have been described, including the μ-opioid receptor (MOR), κ-opioid receptor (KOR), δ-opioid receptor (DOR), and the nociceptin opioid peptide receptor (NOP receptor). Despite the proven success of opioids in treating pain, there are still some inherent limitations. All clinically approved MOR analgesics are associated with adverse effects, which include tolerance, dependence, addiction, constipation, and respiratory depression. On the other hand, KOR selective analgesics have found limited clinical utility because they cause sedation, anxiety, dysphoria, and hallucinations. DOR agonists have also been investigated but they have a tendency to cause convulsions. Ligands targeting NOP receptor have been reported in the preclinical literature to be useful as spinal analgesics and as entities against substance abuse disorders while mixed MOR/NOP receptor agonists are useful as analgesics. Ultimately, the goal of opioid-related drug development has always been to design and synthesize derivatives that are equally or more potent than morphine but most importantly are devoid of the dangerous residual side effects and abuse potential. One proposed strategy is to take advantage of biased agonism, in which distinct downstream pathways can be activated by different molecules working through the exact same receptor. It has been proposed that ligands not recruiting β-arrestin 2 or showing a preference for activating a specific G-protein mediated signal transduction pathway will function as safer analgesic across all opioid subtypes. This review will focus on the design and the pharmacological outcomes of biased ligands at the opioid receptors, aiming at achieving functional selectivity.

Exploring GPCR-arrestin interfaces with genetically encoded crosslinkers

β‐arrestins (βarr1 and βarr2) are ubiquitous regulators of G protein‐coupled receptor (GPCR) signaling. Available data suggest that β‐arrestins dock to different receptors in different ways. However, the structural characterization of GPCR‐arrestin complexes is challenging and alternative approaches to study GPCR‐arrestin complexes are needed. Here, starting from the finger loop as a major site for the interaction of arrestins with GPCRs, we genetically incorporate non‐canonical amino acids for photo‐ and chemical crosslinking into βarr1 and βarr2 and explore binding topologies to GPCRs forming either stable or transient complexes with arrestins: the vasopressin receptor 2 (rhodopsin‐like), the corticotropin‐releasing factor receptor 1, and the parathyroid hormone receptor 1 (both secretin‐like). We show that each receptor leaves a unique footprint on arrestins, whereas the two β‐arrestins yield quite similar crosslinking patterns. Furthermore, we show that the method allows defining the orientation of arrestin with respect to the GPCR. Finally, we provide direct evidence for the formation of arrestin oligomers in the cell.

Distinct phosphorylation sites in a prototypical GPCR differently orchestrate β-arrestin interaction, trafficking, and signaling

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a key determinant for their interaction with β-arrestins (βarrs) and subsequent functional responses. Therefore, it is important to decipher the contribution and interplay of different receptor phosphorylation sites in governing βarr interaction and functional outcomes. Here, we find that several phosphorylation sites in the human vasopressin receptor (V₂R), positioned either individually or in clusters, differentially contribute to βarr recruitment, trafficking, and ERK1/2 activation. Even a single phosphorylation site in V₂R, suitably positioned to cross-talk with a key residue in βarrs, has a decisive contribution in βarr recruitment, and its mutation results in strong G-protein bias. Molecular dynamics simulation provides mechanistic insights into the pivotal role of this key phosphorylation site in governing the stability of βarr interaction and regulating the interdomain rotation in βarrs. Our findings uncover important structural aspects to better understand the framework of GPCR-βarr interaction and biased signaling.

Keys to the Kingdom: GPCR phosphorylation patterns direct β-arrestin

The β-arrestin proteins are key regulators of G protein-coupled receptors, serving at least three distinct functions: inhibiting receptor signaling through G proteins, directing receptor trafficking from the cell surface after activation, and transmitting receptor-initiated signals directly. How the two β-arrestin proteins perform these many functions for hundreds of receptor types throughout the body, and specifically how β-arrestin-mediated signaling can be tuned to cellular conditions, remains an open question. Function-based evidence and recent structure-based evidence have suggested that patterns of receptor phosphorylation ("barcodes") may be a critical determinant of β-arrestin action. In this issue of EMBO Reports, Baidya and colleagues (Baidya et al, 2020a) report that specific receptor phosphorylation site clusters ("codes") determine whether β-arrestin 1 acts to promote or inhibit receptor activation of Erk mitogen-activated protein kinases. They provide direct evidence for a functional barcode system by transferring inhibitory and stimulatory codes between receptors, suggesting future work to understand just how code site location in a receptor and its phosphorylation status can lead to very different functions of bound β-arrestin proteins.

The Effect of Cell Surface Expression and Linker Sequence on the Recruitment of Arrestin to the GIP Receptor

The glucose-dependent insulinotropic polypeptide (GIP) and the glucagon-like peptide-1 (GLP-1) receptor are important targets in the treatment of both type 2 diabetes mellitus (T2DM) and obesity. Originally identified for their role in desensitization, internalization and recycling of G protein-coupled receptors (GPCRs), arrestins have since been shown to act as scaffolding proteins that allow GPCRs to signal in a G protein-independent manner. While GLP-1R has been reported to interact with arrestins, this aspect of cell signaling remains controversial for GIPR. Using a (FRET)-based assay we have previously shown that yellow fluorescent protein (YFP)-labeled GIPR does not recruit arrestin. This GIPR-YFP construct contained a 10 amino acid linker between the receptor and a XbaI restriction site upstream of the YFP. This linker was not present in the modified GIPR-SYFP2 used in subsequent FRET and bioluminescence resonance energy transfer (BRET) assays. However, its removal results in the introduction of a serine residue adjacent to the end of GIPR’s C-terminal tail which could potentially be a phosphorylation site. The resulting receptor was indeed able to recruit arrestin. To find out whether the serine/arginine (SR) coded by the XbaI site was indeed the source of the problem, it was substituted with glycine/glycine (GG) by site-directed mutagenesis. This substitution abolished arrestin recruitment in the BRET assay but only significantly reduced it in the FRET assay. In addition, we show that the presence of a N-terminal FLAG epitope and influenza hemagglutinin signal peptide were also required to detect arrestin recruitment to the GIPR, most likely by increasing receptor cell surface expression. These results demonstrate how arrestin recruitment assay configuration can dramatically alter the result. This becomes relevant when drug discovery programs aim to identify ligands with “biased agonist” properties.

A non-GPCR-binding partner interacts with a novel surface on β-arrestin1 to mediate GPCR signaling

The multifaceted adaptor protein β-arr1 (β-arrestin1) promotes activation of focal adhesion kinase (FAK) by the chemokine receptor CXCR4, facilitating chemotaxis. This function of β-arr1 requires the assistance of the adaptor protein STAM1 (signal-transducing adaptor molecule 1) because disruption of the interaction between STAM1 and β-arr1 reduces CXCR4-mediated activation of FAK and chemotaxis. To begin to understand the mechanism by which β-arr1 together with STAM1 activates FAK, we used site-directed spin-labeling EPR spectroscopy-based studies coupled with bioluminescence resonance energy transfer-based cellular studies to show that STAM1 is recruited to activated β-arr1 by binding to a novel surface on β-arr1 at the base of the finger loop, at a site that is distinct from the receptor-binding site. Expression of a STAM1-deficient binding β-arr1 mutant that is still able to bind to CXCR4 significantly reduced CXCL12-induced activation of FAK but had no impact on ERK-1/2 activation. We provide evidence of a novel surface at the base of the finger loop that dictates non-GPCR interactions specifying β-arrestin-dependent signaling by a GPCR. This surface might represent a previously unidentified switch region that engages with effector molecules to drive β-arrestin signaling.

Key phosphorylation sites in GPCRs orchestrate the contribution of β-Arrestin 1 in ERK1/2 activation

β-arrestins (βarrs) are key regulators of G protein-coupled receptor (GPCR) signaling and trafficking, and their knockdown typically leads to a decrease in agonist-induced ERK1/2 MAP kinase activation. Interestingly, for some GPCRs, knockdown of βarr1 augments agonist-induced ERK1/2 phosphorylation although a mechanistic basis for this intriguing phenomenon is unclear. Here, we use selected GPCRs to explore a possible correlation between the spatial positioning of receptor phosphorylation sites and the contribution of βarr1 in ERK1/2 activation. We discover that engineering a spatially positioned double-phosphorylation-site cluster in the bradykinin receptor (B₂ R), analogous to that present in the vasopressin receptor (V₂ R), reverses the contribution of βarr1 in ERK1/2 activation from inhibitory to promotive. An intrabody sensor suggests a conformational mechanism for this role reversal of βarr1, and molecular dynamics simulation reveals a bifurcated salt bridge between this double-phosphorylation site cluster and Lys²⁹⁴ in the lariat loop of βarr1, which directs the orientation of the lariat loop. Our findings provide novel insights into the opposite roles of βarr1 in ERK1/2 activation for different GPCRs with a direct relevance to biased agonism and novel therapeutics.

Lysine in the lariat loop of arrestins does not serve as phosphate sensor

Arrestins demonstrate strong preference for phosphorylated over unphosphorylated receptors, but how arrestins "sense" receptor phosphorylation is unclear. A conserved lysine in the lariat loop of arrestins directly binds the phosphate in crystal structures of activated arrestin-1, -2, and -3. The lariat loop supplies two negative charges to the central polar core, which must be disrupted for arrestin activation and high-affinity receptor binding. Therefore, we hypothesized that receptor-attached phosphates pull the lariat loop via this lysine, thus removing the negative charges and destabilizing the polar core. We tested the role of this lysine by introducing charge elimination (Lys->Ala) and reversal (Lys->Glu) mutations in arrestin-1, -2, and -3. These mutations in arrestin-1 only moderately reduced phospho-rhodopsin binding and had no detectable effect on arrestin-2 and -3 binding to cognate non-visual receptors in cells. The mutations of Lys300 in bovine and homologous Lys301 in mouse arrestin-1 on the background of pre-activated mutants had variable effects on the binding to light-activated phosphorylated rhodopsin, while affecting the binding to unphosphorylated rhodopsin to a greater extent. Thus, conserved lysine in the lariat loop participates in receptor binding, but does not play a critical role in phosphate-induced arrestin activation.

Phosphorylated peptide of G protein-coupled receptor induces dimerization in activated arrestin

Termination of the G-protein-coupled receptor signaling involves phosphorylation of its C-terminus and subsequent binding of the regulatory protein arrestin. In the visual system, arrestin-1 preferentially binds to photoactivated and phosphorylated rhodopsin and inactivates phototransduction. Here, we have investigated binding of a synthetic phosphopeptide of bovine rhodopsin (residues 323-348) to the active variants of visual arrestin-1: splice variant p44, and the mutant R175E. Unlike the wild type arrestin-1, both these arrestins are monomeric in solution. Solution structure analysis using small angle X-ray scattering supported by size exclusion chromatography results reveal dimerization in both the arrestins in the presence of phosphopeptide. Our results are the first report, to our knowledge, on receptor-induced oligomerization in arrestin, suggesting possible roles for the cellular function of arrestin oligomers. Given high structural homology and the similarities in their activation mechanism, these results are expected to have implications for all arrestin isoforms.

Conformational Sensors and Domain Swapping Reveal Structural and Functional Differences between β-Arrestin Isoforms

Desensitization, signaling, and trafficking of G-protein-coupled receptors (GPCRs) are critically regulated by multifunctional adaptor proteins, β-arrestins (βarrs). The two isoforms of βarrs (βarr1 and 2) share a high degree of sequence and structural similarity; still, however, they often mediate distinct functional outcomes in the context of GPCR signaling and regulation. A mechanistic basis for such a functional divergence of βarr isoforms is still lacking. By using a set of complementary approaches, including antibody-fragment-based conformational sensors, we discover structural differences between βarr1 and 2 upon their interaction with activated and phosphorylated receptors. Interestingly, domain-swapped chimeras of βarrs display robust complementation in functional assays, thereby linking the structural differences between receptor-bound βarr1 and 2 with their divergent functional outcomes. Our findings reveal important insights into the ability of βarr isoforms to drive distinct functional outcomes and underscore the importance of integrating this aspect in the current framework of biased agonism.

Mechanism of β-arrestin recruitment by the μ-opioid G protein-coupled receptor

Agonists to the μ-opioid G protein-coupled receptor (μOR) can alleviate pain through activation of G protein signaling, but they can also induce β-arrestin activation, leading to such side effects as respiratory depression. Biased ligands to μOR that induce G protein signaling without inducing β-arrestin signaling can alleviate pain while reducing side effects. However, the mechanism for stimulating β-arrestin signaling is not known, making it difficult to design optimum biased ligands. We use extensive molecular dynamics simulations to determine three-dimensional (3D) structures of activated β-arrestin2 stabilized by phosphorylated μOR bound to the morphine and D-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) nonbiased agonists and to the TRV130 biased agonist. For nonbiased agonists, we find that the β-arrestin2 couples to the phosphorylated μOR by forming strong polar interactions with intracellular loop 2 (ICL2) and either the ICL3 or cytoplasmic region of transmembrane (TM6). Strikingly, Gi protein makes identical strong bonds with these same ICLs. Thus, the Gi protein and β-arrestin2 compete for the same binding site even though their recruitment leads to much different outcomes. On the other hand, we find that TRV130 has a greater tendency to bind the extracellular portion of TM2 and TM3, which repositions TM6 in the cytoplasmic region of μOR, hindering β-arrestin2 from making polar anchors to the ICL3 or to the cytosolic end of TM6. This dramatically reduces the affinity between μOR and β-arrestin2.

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.