Plasma membrane preassociation drives ß-arrestin coupling to receptors and activation.

ß-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-ß-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 ß-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that ß-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 ß-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 ß-arrestin function at the plasma membrane, revealing a critical role for ß-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

Intrinsic bias at non-canonical, ß-arrestin-coupled seven transmembrane receptors.

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors (7TMRs), typically interact with two distinct signal-transducers, i.e., G proteins and ß-arrestins (ßarrs). Interestingly, there are some non-canonical 7TMRs that lack G protein coupling but interact with ßarrs, although an understanding of their transducer coupling preference, downstream signaling, and structural mechanism remains elusive. Here, we characterize two such non-canonical 7TMRs, namely, the decoy D6 receptor (D6R) and the complement C5a receptor subtype 2 (C5aR2), in parallel with their canonical GPCR counterparts. We discover that D6R and C5aR2 efficiently couple to ßarrs, exhibit distinct engagement of GPCR kinases (GRKs), and activate non-canonical downstream signaling pathways. We also observe that ßarrs adopt distinct conformations for D6R and C5aR2, compared to their canonical GPCR counterparts, in response to common natural agonists. Our study establishes D6R and C5aR2 as ßarr-coupled 7TMRs and provides key insights into their regulation and signaling with direct implication for biased agonism.

Entering the Pocket: Crystal Structure of a Prostaglandin D2 Receptor.

In this issue of Molecular Cell, crystal structures of a prostaglandin D2 receptor determined by Wang et al. (2018) reveal novel insights into differential ligand recognition among the members of lipid-binding GPCRs, and provide a structural framework for the identification of novel therapeutics in inflammatory disorders.

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

ß-arrestins (ßarrs) 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 ßarr1 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 ßarr1 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 ßarr1 in ERK1/2 activation from inhibitory to promotive. An intrabody sensor suggests a conformational mechanism for this role reversal of ßarr1, and molecular dynamics simulation reveals a bifurcated salt bridge between this double-phosphorylation site cluster and Lys294 in the lariat loop of ßarr1, which directs the orientation of the lariat loop. Our findings provide novel insights into the opposite roles of ßarr1 in ERK1/2 activation for different GPCRs with a direct relevance to biased agonism and novel therapeutics.

Genetically encoded intrabody sensors report the interaction and trafficking of ß-arrestin 1 upon activation of G-protein-coupled receptors.

Agonist stimulation of G-protein-coupled receptors (GPCRs) typically leads to phosphorylation of GPCRs and binding to multifunctional proteins called ß-arrestins (ßarrs). The GPCR-ßarr interaction critically contributes to GPCR desensitization, endocytosis, and downstream signaling, and GPCR-ßarr complex formation can be used as a generic readout of GPCR and ßarr activation. Although several methods are currently available to monitor GPCR-ßarr interactions, additional sensors to visualize them may expand the toolbox and complement existing methods. We have previously described antibody fragments (FABs) that recognize activated ßarr1 upon its interaction with the vasopressin V2 receptor C-terminal phosphopeptide (V2Rpp). Here, we demonstrate that these FABs efficiently report the formation of a GPCR-ßarr1 complex for a broad set of chimeric GPCRs harboring the V2R C terminus. We adapted these FABs to an intrabody format by converting them to single-chain variable fragments and used them to monitor the localization and trafficking of ßarr1 in live cells. We observed that upon agonist simulation of cells expressing chimeric GPCRs, these intrabodies first translocate to the cell surface, followed by trafficking into intracellular vesicles. The translocation pattern of intrabodies mirrored that of ßarr1, and the intrabodies co-localized with ßarr1 at the cell surface and in intracellular vesicles. Interestingly, we discovered that intrabody sensors can also report ßarr1 recruitment and trafficking for several unmodified GPCRs. Our characterization of intrabody sensors for ßarr1 recruitment and trafficking expands currently available approaches to visualize GPCR-ßarr1 binding, which may help decipher additional aspects of GPCR signaling and regulation.

Partial ligand-receptor engagement yields functional bias at the human complement receptor, C5aR1.

The human complement component, C5a, binds two different seven-transmembrane receptors termed C5aR1 and C5aR2. C5aR1 is a prototypical G-protein-coupled receptor that couples to the Gαi subfamily of heterotrimeric G-proteins and ß-arrestins (ßarrs) following C5a stimulation. Peptide fragments derived from the C terminus of C5a can still interact with the receptor, albeit with lower affinity, and can act as agonists or antagonists. However, whether such fragments might display ligand bias at C5aR1 remains unexplored. Here, we compare C5a and a modified C-terminal fragment of C5a, C5apep, in terms of G-protein coupling, ßarr recruitment, endocytosis, and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase activation at the human C5aR1. We discover that C5apep acts as a full agonist for Gαi coupling as measured by cAMP response and extracellular signal-regulated kinase 1/2 phosphorylation, but it displays partial agonism for ßarr recruitment and receptor endocytosis. Interestingly, C5apep exhibits full-agonist efficacy with respect to inhibiting lipopolysaccharide-induced interleukin-6 secretion in human macrophages, but its ability to induce human neutrophil migration is substantially lower compared with C5a, although both these responses are sensitive to pertussis toxin treatment. Taken together, our data reveal that compared with C5a, C5apep exerts partial efficacy for ßarr recruitment, receptor trafficking, and neutrophil migration. Our findings therefore uncover functional bias at C5aR1 and also provide a framework that can potentially be extended to chemokine receptors, which also typically interact with chemokines through a biphasic mechanism.

Purification of native CCL7 and its functional interaction with selected chemokine receptors.

Chemokine receptors form a major sub-family of G protein-coupled receptors (GPCRs) and they are involved in a number of cellular and physiological processes related to our immune response and regulation. A better structural understanding of ligand-binding, activation, signaling and regulation of chemokine receptors is very important to design potentially therapeutic interventions for human disorders arising from aberrant chemokine signaling. One of the key limitations in probing the structural details of chemokine receptors is the availability of large amounts of purified, homogenous and fully functional chemokine ligands, and the commercially available products, are not affordable for in-depth structural studies. Moreover, production of uniformly isotope-labeled chemokines, for example, suitable for NMR-based structural investigation, also remains challenging. Here, we have designed a streamlined approach to express and purify the human chemokine CCL7 as well as its 15N-, 15N/13C-, 2H/15N/13C- isotope-labeled derivatives, at milligram levels using E. coli expression system. Purified CCL7 not only maintains a well-folded three-dimensional structure as analyzed using circular dichroism and 1H/15N NMR but it also induces coupling of heterotrimeric G-proteins and ß-arrestins for selected chemokine receptors in cellular system. We compared cAMP response induced by histidine tagged CCL7 and native CCL7 and found that modification of the N-terminus of CCL7 compromises its functionality. Our strategy presented here may be applicable to other chemokines and therefore, provide a potentially generic and cost-effective approach to produce chemokines in large amounts for functional and structural studies.

Designing a thiol specific fluorescent probe for possible use as a reagent for intracellular detection and estimation in blood serum: kinetic analysis to probe the role of intramolecular hydrogen bonding.

A new and simple chemodosimetric probe L1 is utilized for the selective detection of biothiols in the presence of other relevant amino acids under physiological conditions (pH = 7.4). This eventually led to a turn-off luminescence response due to an effective photoinduced electron transfer based signaling mechanism. A comparison of the results of the fluorescence kinetic analysis and (1)H NMR studies of the reaction between thiol and L1 or the analogous compound L2 revealed the role of intramolecular hydrogen bonding in activating the imine functionality towards nucleophilic addition. Such an example is not common in contemporary literature. Conventional MTT assay studies revealed that this probe (L1) has low cytotoxicity. Results of the cell imaging studies revealed that this probe was cell membrane permeable and could detect the intracellular distribution of biothiols within living HeLa cells. Furthermore, our studies with human blood plasma demonstrated the possibility of using this reagent for the quantitative optical detection of total biothiols in biological fluid. Such an example for the detection of biothiols in real biological samples is rare in the contemporary literature. These results clearly demonstrate the possibility of using this reagent in medicinal biology and diagnostic applications.

New chemodosimetric reagents as ratiometric probes for cysteine and homocysteine and possible detection in living cells and in blood plasma.

In this work, we have rationally designed and synthesized two new reagents (L(1) and L(2)), each bearing a pendant aldehyde functionality. This aldehyde group can take part in cyclization reactions with ß- or γ-amino thiols to yield the corresponding thiazolidine and thiazinane derivatives, respectively. The intramolecular charge-transfer (ICT) bands of these thiazolidine and thiazinane derivatives are distinctly different from those of the molecular probes (L(1) and L(2)). Such changes could serve as a potential platform for using L(1) and L(2) as new colorimetric/fluorogenic as well as ratiometric sensors for cysteine (Cys) and homocysteine (Hcy) under physiological conditions. Both reagents proved to be specific towards Cys and Hcy even in the presence of various amino acids, glucose, and DNA. Importantly, these two chemodosimetric reagents could be used for the quantitative detection of Cys present in blood plasma by using a pre-column HPLC technique. Such examples are not common in contemporary literature. MTT assay studies have revealed that these probes have low cytotoxicity. Confocal laser scanning micrographs of cells demonstrated that these probes could penetrate cell membranes and could be used to detect intracellular Cys/Hcy present within living cells. Thus, the results presented in this article not only demonstrate the efficiency and specificity of two ratiometric chemodosimeter molecules for the quantitative detection of Cys and Hcy, but also provide a strategy for developing reagents for analysis of these vital amino acids in biological samples.

Synthesis, photophysical, photochemical, DNA cleavage/binding and cytotoxic properties of pyrene oxime ester conjugates.

A new series of (E)-pyrene oxime ester conjugates of carboxylic acids including amino acids were synthesized by coupling with an environment sensitive fluorophore 1-acetylpyrene. (E)-Pyrene oxime esters exhibited strong fluorescence properties and interestingly their fluorescence properties were found to be highly sensitive to the surrounding environment. Direct irradiation of the (E)-pyrene oxime esters by UV light (≥350 nm) resulted in both the photo-Beckmann rearrangement product and products resulting from N-O bond homolysis. Photoproduct formation and their distribution were found to be solvent dependent. Further, we also showed (E)-pyrene oxime esters intercalated into DNA efficiently and photo-cleaved DNA. Finally we also showed these oxime esters can permeate cells efficiently and may cause cytotoxicity upon irradiation of light.

Recognition of Hg2+ and Cr3+ in physiological conditions by a rhodamine derivative and its application as a reagent for cell-imaging studies.

A new rhodamine-based receptor, derivatized with an additional fluorophore (quinoline), was synthesized for selective recognition of Hg(2+) and Cr(3+) in an acetonitrile/HEPES buffer medium of pH 7.3. This reagent could be used as a dual probe and allowed detection of these two ions by monitoring changes in absorption and the fluorescence spectral pattern. In both instances, the extent of the changes was significant enough to allow visual detection. More importantly, the receptor molecule could be used as an imaging reagent for detection of Hg(2+) and Cr(3+) uptake in live human cancer cells (MCF7) using laser confocal microscopic studies. Unlike Hg(ClO(4))(2) or Hg(NO(3))(2) salts, HgCl(2) or HgI(2) failed to induce any visually detectable change in color or fluorescence upon interaction with L(1) under identical experimental conditions. Presumably, the higher covalent nature of Hg(II) in HgCl(2) or HgI(2) accounts for its lower acidity and its inability to open up the spirolactam ring of the reagent L(1). The issue has been addressed on the basis of the single-crystal X-ray structures of L(1)·HgX(2) (X(-) = Cl(-) or I(-)) and results from other spectral studies.

Allosteric modulation of GPCR-induced ß-arrestin trafficking and signaling by a synthetic intrabody.

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a primary determinant of ß-arrestin (ßarr) recruitment and trafficking. For several GPCRs such as the vasopressin receptor subtype 2 (V2R), agonist-stimulation first drives the translocation of ßarrs to the plasma membrane, followed by endosomal trafficking, which is generally considered to be orchestrated by multiple phosphorylation sites. We have previously shown that mutation of a single phosphorylation site in the V2R (i.e., V2RT360A) results in near-complete loss of ßarr translocation to endosomes despite robust recruitment to the plasma membrane, and compromised ERK1/2 activation. Here, we discover that a synthetic intrabody (Ib30), which selectively recognizes activated ßarr1, efficiently rescues the endosomal trafficking of ßarr1 and ERK1/2 activation for V2RT360A. Molecular dynamics simulations reveal that Ib30 enriches active-like ßarr1 conformation with respect to the inter-domain rotation, and cellular assays demonstrate that it also enhances ßarr1-ß2-adaptin interaction. Our data provide an experimental framework to positively modulate the receptor-transducer-effector axis for GPCRs using intrabodies, which can be potentially integrated in the paradigm of GPCR-targeted drug discovery.

Structural insights into ligand recognition and activation of angiotensin receptors.

G protein-coupled angiotensin II receptors, AT1R and AT2R, are integral components of the renin-angiotensin system (RAS) that regulates blood pressure and fluid balance in humans. While AT1R is a well-established target of angiotensin receptor blockers (ARBs) for managing hypertension and a prime system for studying biased signaling, AT2R has been recognized as a promising target against neuropathic pain and lung fibrosis. In this review, we discuss how recent structural advances illuminate ligand-binding modes and subtype selectivity, shared and distinct features of the receptors, their transducer-coupling patterns, and downstream signaling responses. We also underscore the key ATR aspects that require further studies to fully appreciate the mechanistic framework that fine-tunes their cellular and physiological functions, providing untapped potential for drug discovery.

Site-directed labeling of ß-arrestin with monobromobimane for measuring their interaction with G protein-coupled receptors.

ß-arrestins (ßarrs) are multifunctional proteins that interact with activated and phosphorylated G protein-coupled receptors (GPCRs) to regulate their signaling and trafficking. Understanding the intricate details of GPCR-ßarr interaction continues to be a key research area in the field of GPCR biology. Bimane fluorescence spectroscopy has been one of the key approaches among a broad range of methods employed to study GPCR-ßarr interaction using purified and reconstituted system. Here, we present a step-by-step protocol for labeling ßarrs with monobromobimane (mBBr) in a site-directed fashion for measuring their interaction with GPCRs and the resulting conformational changes. This simple protocol can be directly applied to other protein-protein interaction modules as well for measuring interactions and conformational changes in reconstituted systems in vitro.

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

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a key determinant for their interaction with ß-arrestins (ßarrs) and subsequent functional responses. Therefore, it is important to decipher the contribution and interplay of different receptor phosphorylation sites in governing ßarr 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 ßarr recruitment, trafficking, and ERK1/2 activation. Even a single phosphorylation site in V2R, suitably positioned to cross-talk with a key residue in ßarrs, has a decisive contribution in ßarr 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 ßarr interaction and regulating the interdomain rotation in ßarrs. Our findings uncover important structural aspects to better understand the framework of GPCR-ßarr interaction and biased signaling.

Crystal Structure of ß-Arrestin 2 in Complex with CXCR7 Phosphopeptide.

ß-Arrestins (ßarrs) critically regulate G-protein-coupled receptor (GPCR) signaling and trafficking. ßarrs have two isoforms, ßarr1 and ßarr2. Receptor phosphorylation is a key determinant for the binding of ßarrs, and understanding the intricate details of receptor-ßarr interaction is the next frontier in GPCR structural biology. The high-resolution structure of active ßarr1 in complex with a phosphopeptide derived from GPCR has been revealed, but that of ßarr2 remains elusive. Here, we present a 2.3-Å crystal structure of ßarr2 in complex with a phosphopeptide (C7pp) derived from the carboxyl terminus of CXCR7. The structural analysis of C7pp-bound ßarr2 reveals key differences from the previously determined active conformation of ßarr1. One of the key differences is that C7pp-bound ßarr2 shows a relatively small inter-domain rotation. Antibody-fragment-based conformational sensor and hydrogen/deuterium exchange experiments further corroborated the structural features of ßarr2 and suggested that ßarr2 adopts a range of inter-domain rotations.

Measuring agonist-induced ERK MAP kinase phosphorylation for G-protein-coupled receptors.

Agonist stimulation of G-protein-coupled receptors (GPCRs) typically results in phosphorylation and activation of ERK (Extracellular-signal Regulated Kinase) which is a member of MAP kinase (Mitogen-Activated Protein kinase) family. Detection of phosphorylated ERK1/2 MAP kinase has been widely used as readout of GPCR signaling in heterologous cells, primary cells, tissues and even in animal studies. ERK1/2 phosphorylation downstream of GPCRs is now well established to arise from the activation of both, the heterotrimeric G-proteins and ß-arrestins (ßarrs) with distinct spatio-temporal components. Here, we present a step-by-step protocol for measuring agonist-induced ERK1/2 MAP kinase activation downstream of GPCRs using standard Western blotting assay. Note: ERK1/2 is also referred to as p44/42 MAP kinase. ERK1 and ERK2 are same as Mitogen-Activated Protein Kinase 3 (MAP3) and Mitogen-Activated Protein Kinase 1 (MAP1), respectively.

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

Desensitization, signaling, and trafficking of G-protein-coupled receptors (GPCRs) are critically regulated by multifunctional adaptor proteins, ß-arrestins (ßarrs). The two isoforms of ßarrs (ßarr1 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 ßarr isoforms is still lacking. By using a set of complementary approaches, including antibody-fragment-based conformational sensors, we discover structural differences between ßarr1 and 2 upon their interaction with activated and phosphorylated receptors. Interestingly, domain-swapped chimeras of ßarrs display robust complementation in functional assays, thereby linking the structural differences between receptor-bound ßarr1 and 2 with their divergent functional outcomes. Our findings reveal important insights into the ability of ßarr isoforms to drive distinct functional outcomes and underscore the importance of integrating this aspect in the current framework of biased agonism.

GPCR Signaling: The Interplay of Gαi and ß-arrestin.

Biased agonism at G-protein-coupled receptors is generally conceptualized as the ability of certain stimuli to trigger downstream signaling exclusively through one of two effectors. Recent studies reveal that signaling downstream of the ß1 adrenergic receptor and the angiotensin II type 1 receptor induced by biased stimuli actually involves both effectors.