Multiple residues in the second extracellular loop are critical for M3 muscarinic acetylcholine receptor activation

Pharmacological Characterization of the Zebrafish (Danio Rerio) Histamine H1 Receptor Reveals the Involvement of the Second Extracellular Loop in the Binding of Histamine

The zebrafish (Danio rerio) histamine H1 receptor gene (zfH1R) was cloned in 2007 and reported to be involved in fish locomotion. Yet, no detailed characterization of its pharmacology and signaling properties have so far been reported. In this study, we pharmacologically characterized the zfH1R expressed in HEK-293T cells by means of [3H]-mepyramine binding and G protein-signaling assays. The zfH1R [dissociation constant (KD), 0.7 nM] displayed similar affinity for the antagonist [3H]-mepyramine as the human histamine H1 receptor (hH1R) (KD, 1.5 nM), whereas the affinity for histamine is 100-fold higher than for the human H1R. The zfH1R couples to Gαq/11 proteins and activates several reporter genes, i.e., NFAT, NFϰB, CRE, VEGF, COX-2, SRE, and AP-1, and zfH1R-mediated signaling is prevented by the Gαq/11 inhibitor YM-254890 and the antagonist mepyramine. Molecular modeling of the zfH1R and human H1R shows that the binding pockets are identical, implying that variations along the ligand binding pathway could underly the differences in histamine affinity instead. Targeting differentially charged residues in extracellular loop 2 (ECL2) using site-directed mutagenesis revealed that Arg21045x55 is most likely involved in the binding process of histamine in zfH1R. This study aids the understanding of the pharmacological differences between H1R orthologs and the role of ECL2 in histamine binding and provides fundamental information for the understanding of the histaminergic system in the zebrafish. SIGNIFICANCE STATEMENT: The use of the zebrafish as in vivo models in neuroscience is growing exponentially, which asks for detailed characterization of the aminergic neurotransmitter systems in this model. This study is the first to pharmacologically characterize the zebrafish histamine H1 receptor after expression in HEK-293T cells. The results show a high pharmacological and functional resemblance with the human ortholog but also reveal interesting structural differences and unveils an important role of the second extracellular loop in histamine binding.

Autoimmune autonomic nervous system imbalance and conditions: Chronic fatigue syndrome, fibromyalgia, silicone breast implants, COVID and post-COVID syndrome, sick building syndrome, post-orthostatic tachycardia syndrome, autoimmune diseases and autoimmune/inflammatory syndrome induced by adjuvants

Chronic fatigue syndrome (CFS), fibromyalgia, silicone breast implants syndrome (SBIs), COVID and post-COVID syndrome (PCS), sick building syndrome (SBS), post-orthostatic tachycardia syndrome (POTS), autoimmune diseases and autoimmune/inflammatory syndrome induced by adjuvants (ASIA) are frequently accompanied by clinical symptoms characteristic for dysautonomia: severe fatigue, dizziness, fogginess, memory loss, dry mouth and eyes, hearing dysfunction, tachycardia etc. The recent discovery of an imbalance of autoantibodies against G protein-coupled receptors (GPCR) in some autoimmune diseases, post-COVID syndrome, SBIs allowed researchers to assume the novel mechanism in these conditions - autoimmune autonomic nervous system imbalance. In this review, all data published on an imbalance of autoantibodies against GPCR, clinical symptoms and pathogenic mechanisms in CFS, Fibromyalgia, SBIs, COVID and PCS, SBS, POTS, and some autoimmune diseases were analyzed. Possible criteria to diagnose the autoimmune autonomic nervous system imbalance were created.

WITHDRAWN: Autoimmune autonomic nervous system imbalance and conditions: Chronic fatigue syndrome, fibromyalgia, silicone breast implants, COVID and post-COVID syndrome, sick building syndrome, post-orthostatic tachycardia syndrome, autoimmune diseases and autoimmune/inflammatory syndrome induced by adjuvants

The Publisher regrets that this article is an accidental duplication of an article that has already been published, https://doi.org/10.1016/j.autrev.2022.103230. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

Engineering G protein-coupled receptor signalling in yeast for biotechnological and medical purposes

G protein-coupled receptors (GPCRs) comprise the largest class of membrane proteins in the human genome, with a common denominator of seven-transmembrane domains largely conserved among eukaryotes. Yeast is naturally armoured with three different GPCRs for pheromone and sugar sensing, with the pheromone pathway being extensively hijacked for characterising heterologous GPCR signalling in a model eukaryote. This review focusses on functional GPCR studies performed in yeast and on the elucidated hotspots for engineering, and discusses both endogenous and heterologous GPCR signalling. Key emphasis will be devoted to studies describing important engineering parameters to consider for successful coupling of GPCRs to the yeast mating pathway. We also review the various means of applying yeast for studying GPCRs, including the use of yeast armed with heterologous GPCRs as a platform for (i) deorphanisation of orphan receptors, (ii) metabolic engineering of yeast for production of bioactive products and (iii) medical applications related to pathogen detection and drug discovery. Finally, this review summarises the current challenges related to expression of functional membrane-bound GPCRs in yeast and discusses the opportunities to continue capitalising on yeast as a model chassis for functional GPCR signalling studies.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

Comparative analyses of site-directed mutagenesis of human melatonin MTNR1A and MTNR1B receptors using a yeast fluorescent biosensor

Melatonin is an indoleamine neurohormone made by the pineal gland. Its receptors, MTNR1A and MTNR1B, are members of the G-protein-coupled receptor (GPCR) family and are involved in sleep, circadian rhythm, and mood disorders, and in the inhibition of cancer growth. These receptors, therefore, represent significant molecular targets for insomnia, circadian sleep disorders, and cancer. The yeast Saccharomyces cerevisiae is an attractive host for assaying agonistic activity for human GPCR. We previously constructed a GPCR-based biosensor employing a high-sensitivity yeast strain that incorporated both a chimeric yeast-human Gα protein and a bright fluorescent reporter gene (ZsGreen). Similar approaches have been used for simple and convenient measurements of various GPCR activities. In the current study, we constructed a fluorescence-based yeast biosensor for monitoring the signaling activation of human melatonin receptors. We used this system to analyze point mutations, including previously unreported mutations of the consensus sequences of MTNR1A and MTNR1B melatonin receptors and compared their effects. Most mutations in the consensus sequences significantly affected the signaling capacities of both receptors, but several mutations showed differences between these subtype receptors. Thus, this yeast biosensor holds promise for revealing the functions of melatonin receptors.

Molecular dynamics of the histamine H3 membrane receptor reveals different mechanisms of GPCR signal transduction

In this work, we studied the mechanisms of classical activation and inactivation of signal transduction by the histamine H3 receptor, a 7-helix transmembrane bundle G-Protein Coupled Receptor through long-time-scale atomistic molecular dynamics simulations of the receptor embedded in a hydrated double layer of dipalmitoyl phosphatidyl choline, a zwitterionic polysaturated ordered lipid. Three systems were prepared: the apo receptor, representing the constitutively active receptor; and two holo-receptors-the receptor coupled to the antagonist/inverse agonist ciproxifan, representing the inactive state of the receptor, and the receptor coupled to the endogenous agonist histamine and representing the active state of the receptor. An extensive analysis of the simulation showed that the three states of H3R present significant structural and dynamical differences as well as a complex behavior given that the measured properties interact in multiple and interdependent ways. In addition, the simulations described an unexpected escape of histamine from the orthosteric binding site, in agreement with the experimental modest affinities and rapid off-rates of agonists.

M3 muscarinic acetylcholine receptor-reactive Th17 cells in primary Sjögren's syndrome

M3 muscarinic acetylcholine receptor (M3R) is one of the autoantigens associated with Sjögren's syndrome (SS) and is localized in exocrine glands where disease-specific inflammation occurs. The inflammatory lesion is characterized by infiltration of CD4+ T cells, including clonally expanded Th17 cells. We undertook this study to identify circulating M3R-specific Th17 cells and to determine functional properties of those cells. Using the enzyme-linked immunospot assay (ELISpot) method, we identified M3R-reactive Th17 cells in the peripheral blood of patients with primary SS (pSS). Among 10 examined pSS patients, 10 healthy subjects (HS), and 5 IgG4-related disease (IgG4-RD) patients, M3R-reactive IL-17 secreting cells were significantly increased in 5 pSS patients specifically. The most common T cell epitope, which was analyzed and confirmed by coculture of isolated CD4+ T cells with antigen presenting cells plus M3R peptides in vitro, was peptide 83-95 of M3R. Peptide recognition was partly in an HLA-DR-restricted manner, confirmed by blocking assay. M3R-reactive Th17 cells positivity correlated with higher titers of anti-M3R antibodies, whose systemic disease activity score tended to be higher. Our studies highlight the role of tissue-specific autoantigen-derived circulating Th17 cells in pSS, for which further work might lead to antigen-specific targeted therapy.

Probing the Druggablility on the Interface of the Protein-Protein Interaction and Its Allosteric Regulation Mechanism on the Drug Screening for the CXCR4 Homodimer

Modulating protein–protein interactions (PPIs) with small drug-like molecules targeting it exhibits great promise in modern drug discovery. G protein-coupled receptors (GPCRs) are the largest family of targeted proteins and could form dimers in living biological cells through PPIs. However, compared to drug development of the orthosteric site, there has been lack of investigations on the druggability of the PPI interface for GPCRs and its functional implication on experiments. Thus, in order to address these issues, we constructed a novel computational strategy, which involved in molecular dynamics simulation, virtual screening and protein structure network (PSN), to study one representative GPCR homodimer (CXCR4). One druggable pocket was identified in the PPI interface and one small molecule targeting it was screened, which could strengthen PPI mainly through hydrophobic interaction between the benzene rings of the PPI molecule and TM4 of the receptor. The PSN results further reveals that the PPI molecule could increase the number of the allosteric regulation pathways between the druggable pocket of the dimer interface to the orthostatic site for the subunit A but only play minor role for the other subunit B, leading to the asymmetric change in the volume of the binding pockets for the two subunits (increase for the subunit A and minor change for the subunit B). Consequently, the screening performance of the subunit A to the antagonists is enhanced while the subunit B is unchanged nearly, implying that the PPI molecule may be beneficial to enhance the drug efficacies of the antagonists. In addition, one main regulation pathway with the highest frequency was identified for the subunit A, which consists of Trp195^5.34–Tyr190^ECL2–Val196^5.35–Gln200^5.39–Asp262^6.58–Cys28^N-term, revealing their importance in the allosteric regulation from the PPI molecule. The observations from the work could provide valuable information for the development of the PPI drug-like molecule for GPCRs.

Aminergic GPCR-Ligand Interactions: A Chemical and Structural Map of Receptor Mutation Data

The aminergic family of G protein-coupled receptors (GPCRs) plays an important role in various diseases and represents a major drug discovery target class. Structure determination of all major aminergic subfamilies has enabled structure-based ligand design for these receptors. Site-directed mutagenesis data provides an invaluable complementary source of information for elucidating the structural determinants of binding of different ligand chemotypes. The current study provides a comparative analysis of 6692 mutation data points on 34 aminergic GPCR subtypes, covering the chemical space of 540 unique ligands from mutagenesis experiments and information from experimentally determined structures of 52 distinct aminergic receptor-ligand complexes. The integrated analysis enables detailed investigation of structural receptor-ligand interactions and assessment of the transferability of combined binding mode and mutation data across ligand chemotypes and receptor subtypes. An overview is provided of the possibilities and limitations of using mutation data to guide the design of novel aminergic receptor ligands.

Exploring the mechanism of F282L mutation-caused constitutive activity of GPCR by a computational study

G-protein-coupled receptors (GPCRs) are important drug targets and generally activated by ligands. However, some experiments found that GPCRs also give rise to constitutive activity through some mutations (viz., CAM), which are usually associated with different kinds of diseases. However, the mechanisms of CAMs and their roles in interactions with drug-ligands are unclear in experiments. Herein, we used microsecond molecular dynamics simulations to study the effect of one important F282L mutation on β2AR in order to address the questions above. With the aid of principle component and correlation analysis, our results revealed that the F282L mutation could increase the instability of the overall structure, increase the dramatic fluctuations of NPxxY and extracellular loops, and decrease restraint of the helices through weakening interhelical H-bonding and correlations between residues, which could partly contribute to the constitutive activity reported by the experiments. The observations from the protein structure network (PSN) analysis indicate that the mutant exhibits less information flow than the wild β2AR and weakens the role of TM5 and TM6 in the signal transmission, but it enhances the impact of TM3 on the orthosteric pathway and TM4 on the allosteric one. In addition, the results from the virtual screening reveal that the mutant prefers to select agonists rather than antagonists, similar to the active state but opposite of the inactive state, further confirming that the F282L mutation advances the activation of β2AR. Our observations provide valuable information for understanding the mechanism of the mutation-caused constitutive activity of GPCR and related drug-design.

Probing the cooperative mechanism of the μ–δ opioid receptor heterodimer by multiscale simulation

The activation-cooperativity of the μ–δ opioid receptor heterodimer was probed by multiscale simulation coupled with a protein structure network.

Structural Features and Ligand Selectivity for 10 Intermediates in the Activation Process of β2-Adrenergic Receptor

It has already been suggested by researchers that there should be multiple intermediate states in the activation process for G-protein-coupled receptors (GPCRs). However, the intermediate states are very short-lived and hardly captured by the experiments, leading to very limited understanding of their structural features and drug efficacies. In this work, a novel joint strategy of targeted molecular dynamics simulation, conventional molecular dynamics simulation, and virtual screening is developed to address the problems. The results from 10 intermediate conformations obtained from the work reveal that the ligand pocket is very unstable and fluctuates between the inactive state and the active one in the case of ligand-free, in particular for ECL2 as a gate-keeper of the ligand-binding. The ligand-binding site could be stable in the active state with a small volume and a completely closed ECL2, only when the G-protein-binding region is fully activated. In addition, the activations of the ligand-binding pocket and G-protein-binding site are relatively independent and exhibit a loose allosteric coupling, which contributes to the existence of multiple intermediate conformations. Interestingly, the screening performance of the agonists does not increase on increasing the overall activity of the intermediate state, but is dependent on the activated extent of the ligand pocket. The receptor is prone to bind the agonist when closing ECL2 and reducing the ligand-binding pocket volume, whereas it is more favorable for binding the antagonist when opening ECL2 and increasing the pocket volume. These observations added to previous studies could help us better understand the activation mechanism of GPCRs and provide valuable information for drug design.

Hybrid approach to structure modeling of the histamine H3 receptor: Multi-level assessment as a tool for model verification

The crucial role of G-protein coupled receptors and the significant achievements associated with a better understanding of the spatial structure of known receptors in this family encouraged us to undertake a study on the histamine H3 receptor, whose crystal structure is still unresolved. The latest literature data and availability of different software enabled us to build homology models of higher accuracy than previously published ones. The new models are expected to be closer to crystal structures; and therefore, they are much more helpful in the design of potential ligands. In this article, we describe the generation of homology models with the use of diverse tools and a hybrid assessment. Our study incorporates a hybrid assessment connecting knowledge-based scoring algorithms with a two-step ligand-based docking procedure. Knowledge-based scoring employs probability theory for global energy minimum determination based on information about native amino acid conformation from a dataset of experimentally determined protein structures. For a two-step docking procedure two programs were applied: GOLD was used in the first step and Glide in the second. Hybrid approaches offer advantages by combining various theoretical methods in one modeling algorithm. The biggest advantage of hybrid methods is their intrinsic ability to self-update and self-refine when additional structural data are acquired. Moreover, the diversity of computational methods and structural data used in hybrid approaches for structure prediction limit inaccuracies resulting from theoretical approximations or fuzziness of experimental data. The results of docking to the new H3 receptor model allowed us to analyze ligand-receptor interactions for reference compounds.

Flavonoid allosteric modulation of mutated visual rhodopsin associated with retinitis pigmentosa

Dietary flavonoids exhibit many biologically-relevant functions and can potentially have beneficial effects in the treatment of pathological conditions. In spite of its well known antioxidant properties, scarce structural information is available on the interaction of flavonoids with membrane receptors. Advances in the structural biology of a specific class of membrane receptors, the G protein-coupled receptors, have significantly increased our understanding of drug action and paved the way for developing improved therapeutic approaches. We have analyzed the effect of the flavonoid quercetin on the conformation, stability and function of the G protein-coupled receptor rhodopsin, and the G90V mutant associated with the retinal degenerative disease retinitis pigmentosa. By using a combination of experimental and computational methods, we suggest that quercetin can act as an allosteric modulator of opsin regenerated with 9-cis-retinal and more importantly, that this binding has a positive effect on the stability and conformational properties of the G90V mutant associated with retinitis pigmentosa. These results open new possibilities to use quercetin and other flavonoids, in combination with specific retinoids like 9-cis-retinal, for the treatment of retinal degeneration associated with retinitis pigmentosa. Moreover, the use of flavonoids as allosteric modulators may also be applicable to other members of the G protein-coupled receptors superfamily.

Serotonin 2A receptor disulfide bridge integrity is crucial for ligand binding to different signalling states but not for its homodimerization

The serotonin 2_A (5-HT_2A) receptor is a G-protein coupled receptor (GPCR) with a conserved disulfide bridge formed by Cys¹⁴⁸ (transmembrane helix 3, TM3) and Cys²²⁷ (extracellular loop 2, ECL-2). We hypothesized that disulfide bridges may determine serotonin 5-HT_2A receptor functions such as receptor activation, functional selectivity and ligand recognition. We used the reducing agent dithiothreitol (DTT) to determine how the reduction of disulfide bridges affects radioligand binding, second messenger mobilization and receptor dimerization. A DTT-induced decrease in the number of binding sites (1190 ± 63.55 fmol/mg protein for control cells compared with 921.2 ± 60.84 fmol/mg protein for DTT-treated cells) as well as in the efficacy of both signalling pathways characterized was observed, although the affinity and potency were unchanged. Bioluminiscence resonance energy transfer (BRET) assays revealed the DTT treatment did not modify the homodimeric nature of serotonin 5-HT_2A receptors. In molecular dynamic simulations, the ECL-2 of the receptor with a broken cysteine bond adopts a wider variety of conformations, some of which protrude deeper into the receptor orthosteric binding pocket leading to collapse of the pocket. A shrunken binding pocket would be incapable of accommodating lysergic acid diethylamide (LSD). Our findings suggest that the decrease of efficacy may be due to disruption of disulfide bridge between TM3 and ECL-2. This reveals the integrity of the ECL-2 epitope, which should be explored in the development of novel ligands acting as allosteric modulators of serotonin 5-HT_2A receptors.

Functional autoantibodies targeting G protein-coupled receptors in rheumatic diseases

G protein-coupled receptors (GPCRs) comprise the largest and most diverse family of integral membrane proteins that participate in different physiological processes such as the regulation of the nervous and immune systems. Besides the endogenous ligands of GPCRs, functional autoantibodies are also able to bind GPCRs to trigger or block intracellular signalling pathways, resulting in agonistic or antagonistic effects, respectively. In this Review, the effects of functional GPCR-targeting autoantibodies on the pathogenesis of autoimmune diseases, including rheumatic diseases, are discussed. Autoantibodies targeting β1 and β2 adrenergic receptors, which are expressed by cardiac and airway smooth muscle cells, respectively, have an important role in the development of asthma and cardiovascular diseases. In addition, high levels of autoantibodies against the muscarinic acetylcholine receptor M3 as well as those targeting endothelin receptor type A and type 1 angiotensin II receptor have several implications in the pathogenesis of rheumatic diseases such as Sjögren syndrome and systemic sclerosis. Expanding the knowledge of the pathophysiological roles of autoantibodies against GPCRs will shed light on the biology of these receptors and open avenues for new therapeutic approaches.

Binding of N-methylscopolamine to the extracellular domain of muscarinic acetylcholine receptors

Interaction of orthosteric ligands with extracellular domain was described at several aminergic G protein-coupled receptors, including muscarinic acetylcholine receptors. The orthosteric antagonists quinuclidinyl benzilate (QNB) and N-methylscopolamine (NMS) bind to the binding pocket of the muscarinic acetylcholine receptor formed by transmembrane α-helices. We show that high concentrations of either QNB or NMS slow down dissociation of their radiolabeled species from all five subtypes of muscarinic acetylcholine receptors, suggesting allosteric binding. The affinity of NMS at the allosteric site is in the micromolar range for all receptor subtypes. Using molecular modelling of the M₂ receptor we found that E172 and E175 in the second extracellular loop and N419 in the third extracellular loop are involved in allosteric binding of NMS. Mutation of these amino acids to alanine decreased affinity of NMS for the allosteric binding site confirming results of molecular modelling. The allosteric binding site of NMS overlaps with the binding site of some allosteric, ectopic and bitopic ligands. Understanding of interactions of NMS at the allosteric binding site is essential for correct analysis of binding and action of these ligands.

Pharmacological Characterization of Human Histamine Receptors and Histamine Receptor Mutants in the Sf9 Cell Expression System

A large problem of histamine receptor research is data heterogeneity. Various experimental approaches, the complex signaling pathways of mammalian cells, and the use of different species orthologues render it difficult to compare and interpret the published results. Thus, the four human histamine receptor subtypes were analyzed side-by-side in the Sf9 insect cell expression system, using radioligand binding assays as well as functional readouts proximal to the receptor activation event (steady-state GTPase assays and [35S]GTPγS assays). The human H1R was co-expressed with the regulators of G protein signaling RGS4 or GAIP, which unmasked a productive interaction between hH1R and insect cell Gαq. By contrast, functional expression of the hH2R required the generation of an hH2R-Gsα fusion protein to ensure close proximity of G protein and receptor. Fusion of hH2R to the long (GsαL) or short (GsαS) splice variant of Gαs resulted in comparable constitutive hH2R activity, although both G protein variants show different GDP affinities. Medicinal chemistry studies revealed profound species differences between hH1R/hH2R and their guinea pig orthologues gpH1R/gpH2R. The causes for these differences were analyzed by molecular modeling in combination with mutational studies. Co-expression of the hH3R with Gαi1, Gαi2, Gαi3, and Gαi/o in Sf9 cells revealed high constitutive activity and comparable interaction efficiency with all G protein isoforms. A comparison of various cations (Li+, Na+, K+) and anions (Cl-, Br-, I-) revealed that anions with large radii most efficiently stabilize the inactive hH3R state. Potential sodium binding sites in the hH3R protein were analyzed by expressing specific hH3R mutants in Sf9 cells. In contrast to the hH3R, the hH4R preferentially couples to co-expressed Gαi2 in Sf9 cells. Its high constitutive activity is resistant to NaCl or GTPγS. The hH4R shows structural instability and adopts a G protein-independent high-affinity state. A detailed characterization of affinity and activity of a series of hH4R antagonists/inverse agonists allowed first conclusions about structure/activity relationships for inverse agonists at hH4R. In summary, the Sf9 cell system permitted a successful side-by-side comparison of all four human histamine receptor subtypes. This chapter summarizes the results of pharmacological as well as medicinal chemistry/molecular modeling approaches and demonstrates that these data are not only important for a deeper understanding of HxR pharmacology, but also have significant implications for the molecular pharmacology of GPCRs in general.

Extracellular Loop 2 of the Adenosine A1 Receptor Has a Key Role in Orthosteric Ligand Affinity and Agonist Efficacy

The adenosine A₁ G protein-coupled receptor (A₁AR) is an important therapeutic target implicated in a wide range of cardiovascular and neuronal disorders. Although it is well established that the A₁AR orthosteric site is located within the receptor's transmembrane (TM) bundle, prior studies have implicated extracellular loop 2 (ECL2) as having a significant role in contributing to orthosteric ligand affinity and signaling for various G protein-coupled receptors (GPCRs). We thus performed extensive alanine scanning mutagenesis of A₁AR-ECL2 to explore the role of this domain on A₁AR orthosteric ligand pharmacology. Using quantitative analytical approaches and molecular modeling, we identified ECL2 residues that interact either directly or indirectly with orthosteric agonists and antagonists. Discrete mutations proximal to a conserved ECL2-TM3 disulfide bond selectively affected orthosteric ligand affinity, whereas a cluster of five residues near the TM4-ECL2 juncture influenced orthosteric agonist efficacy. A combination of ligand docking, molecular dynamics simulations, and mutagenesis results suggested that the orthosteric agonist 5'-N-ethylcarboxamidoadenosine binds transiently to an extracellular vestibule formed by ECL2 and the top of TM5 and TM7, prior to entry into the canonical TM bundle orthosteric site. Collectively, this study highlights a key role for ECL2 in A₁AR orthosteric ligand binding and receptor activation.

Muscarinic type 3 receptor autoantibodies are associated with anti-SSA/Ro autoantibodies in Sjögren's syndrome

Anti-muscarinic type 3 receptor autoantibodies (anti-M3R) are reported as potential inhibitors of saliva secretion in Sjögren's syndrome (SjS). However, despite extensive efforts to establish an anti-M3R detection method, there is no clinical test available for these autoantibodies. The purpose of this study was to propose inclusion of anti-M3R testing for SjS diagnosis through investigation of their prevalence using a modified In-Cell Western (ICW) assay. A stable cell line expressing human M3R tagged with GFP (M3R-GFP) was established to screen unadsorbed and adsorbed plasma from primary SjS (n=24), rheumatoid arthritis (RA, n=18), systemic lupus erythematosus (SLE, n=18), and healthy controls (HC, n=23). Anti-M3R abundance was determined by screening for the intensity of human IgG interacting with M3R-GFP cells by ICW assay, as detected by an anti-human IgG IRDye800-conjugated secondary antibody and normalized to GFP. Method comparisons and receiver-operating-characteristic (ROC)-curve analyses were performed to evaluate the diagnostic value of our current approaches. Furthermore, clinical parameters of SjS were also analyzed in association with anti-M3R. Anti-M3R was significantly elevated in SjS plasma in comparison with HC, SLE, or RA (P<0.01). SjS anti-M3R intensities were greater than two-standard deviations above the HC mean for both unadsorbed (16/24, 66.67%) and adsorbed (18/24, 75%) plasma samples. Furthermore, anti-M3R was associated with anti-SjS-related-antigen A/Ro positivity (P=0.0353). Linear associations for anti-M3R intensity indicated positive associations with focus score (R(2)=0.7186, P<0.01) and negative associations with saliva flow rate (R(2)=0.3052, P<0.05). Our study strongly supports our rationale to propose inclusion of anti-M3R for further testing as a non-invasive serological marker for SjS diagnosis.

Crystal structures of the M1 and M4 muscarinic acetylcholine receptors

Muscarinic M1-M5 acetylcholine receptors are G-protein-coupled receptors that regulate many vital functions of the central and peripheral nervous systems. In particular, the M1 and M4 receptor subtypes have emerged as attractive drug targets for treatments of neurological disorders, such as Alzheimer's disease and schizophrenia, but the high conservation of the acetylcholine-binding pocket has spurred current research into targeting allosteric sites on these receptors. Here we report the crystal structures of the M1 and M4 muscarinic receptors bound to the inverse agonist, tiotropium. Comparison of these structures with each other, as well as with the previously reported M2 and M3 receptor structures, reveals differences in the orthosteric and allosteric binding sites that contribute to a role in drug selectivity at this important receptor family. We also report identification of a cluster of residues that form a network linking the orthosteric and allosteric sites of the M4 receptor, which provides new insight into how allosteric modulation may be transmitted between the two spatially distinct domains.

Autoantibodies against the Second Extracellular Loop of M3R Do neither Induce nor Indicate Primary Sjögren's Syndrome

Objectives

Anti-muscarinic acetylcholine type-3 receptor (anti-M3R) autoantibodies have been suggested to be pathogenic for primary Sjögren’s syndrome (pSS), and the second extracellular loop of M3R is suspected to carry a disease-promoting epitope. In this study, we aimed to evaluate the pathogenicity of autoantibodies against peptides derived from the second extracellular loop of M3R in mice and to determine whether those autoantibodies could be used as biomarker for pSS.

Methods

BALB/c mice were immunized with modified linear or cyclic peptides of the second extracellular loop of M3R. The function of exocrine glands was evaluated by measuring the secretion of saliva and tears. The histological evaluations were performed by using H&E staining or direct immunofluorescence staining. Autoantibodies against linear or cyclic peptides of the second extracellular loop of M3R in human and mice were determined using ELISA.

Results

Immunization induced mice to produce autoantibodies against the linear or cyclic peptides of the second extracellular loop of M3R, and those autoantibodies could bind onto salivary glands. However, those mice showed neither impairment in the secretion of tears or saliva nor histological abnormality in the exocrine glands. Furthermore, passive transfer of the IgG isolated from the immunized mice into healthy mice did not induced the dysfunction of the exocrine glands. The prevalence of autoantibodies against the peptides of the second extracellular loop of M3R was low in pSS patients, and it did not differ significantly from that in healthy controls.

Conclusions

Our results suggest that the autoantibodies against peptides of the second extracellular loop of M3R are not pathogenic in vivo and they are not suitable as biomarkers for pSS diagnosis.

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

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

Structure-Function Basis of Attenuated Inverse Agonism of Angiotensin II Type 1 Receptor Blockers for Active-State Angiotensin II Type 1 Receptor

Ligand-independent signaling by the angiotensin II type 1 receptor (AT1R) can be activated in clinical settings by mechanical stretch and autoantibodies as well as receptor mutations. Transition of the AT1R to the activated state is known to lower inverse agonistic efficacy of clinically used AT1R blockers (ARBs). The structure-function basis for reduced efficacy of inverse agonists is a fundamental aspect that has been understudied not only in relation to the AT1R but also regarding other homologous receptors. Here, we demonstrate that the active-state transition in the AT1R indeed attenuates an inverse agonistic effect of four biphenyl-tetrazole ARBs through changes in specific ligand-receptor interactions. In the ground state, tight interactions of four ARBs with a set of residues (Ser109(TM3), Phe182(ECL2), Gln257(TM6), Tyr292(TM7), and Asn295(TM7)) results in potent inverse agonism. In the activated state, the ARB-AT1R interactions shift to a different set of residues (Val108(TM3), Ser109(TM3), Ala163(TM4), Phe182(ECL2), Lys199(TM5), Tyr292(TM7), and Asn295(TM7)), resulting in attenuated inverse agonism. Interestingly, V108I, A163T, N295A, and F182A mutations in the activated state of the AT1R shift the functional response to the ARB binding toward agonism, but in the ground state the same mutations cause inverse agonism. Our data show that the second extracellular loop is an important regulator of the functional states of the AT1R. Our findings suggest that the quest for discovering novel ARBs, and improving current ARBs, fundamentally depends on the knowledge of the unique sets of residues that mediate inverse agonistic potency in the two states of the AT1R.

Agonist binding by the β2-adrenergic receptor: an effect of receptor conformation on ligand association-dissociation characteristics

The β₂-adrenergic receptor (β₂-AR), a G protein-coupled receptor (GPCR), is a physiologically important transmembrane protein that is a target for drugs used for treatment of asthma and cardiovascular diseases. Study of the first steps of ligand recognition and the molecular basis of ligand binding to the orthosteric site is essential for understanding the pharmacological properties of the receptor. In this work we investigated the characteristic features of the agonist association–dissociation process to and from the different conformational forms of β₂-AR by use of advanced molecular modeling techniques. The investigation was focused on estimating the free energy profiles (FEPs) corresponding to the process of a full agonist ((R,R)-fenoterol) and an inverse agonist (carazolol) binding and unbinding to and from β₂-AR. The two different conformational forms of β₂-AR, i.e. active β₂-AR–PDB: 3P0G and inactive β₂-AR–PDB: 2RH1 were included in this stage of the study. We revealed several significant qualitative differences between FEPs characteristic of both conformational forms. Both FEPs suggest the existence of three transient binding sites in the extracellular domain of β₂-AR. Comparison of the residues surrounding these transient binding sites in both β₂-AR states revealed the importance of the aromatic residues F194, H93^2.64, H296^6.58, and H178 (extracellular part of β₂-AR) in the early stages of the binding process. In addition, slightly different exit and entry paths are preferred by the ligand molecule in the extracellular part of β₂-AR, depending on the conformation of the receptor.

The extracellular loop 2 (ECL2) of the human histamine H4 receptor substantially contributes to ligand binding and constitutive activity

In contrast to the corresponding mouse and rat orthologs, the human histamine H4 receptor (hH4R) shows extraordinarily high constitutive activity. In the extracellular loop (ECL), replacement of F169 by V as in the mouse H4R significantly reduced constitutive activity. Stabilization of the inactive state was even more pronounced for a double mutant, in which, in addition to F169V, S179 in the ligand binding site was replaced by M. To study the role of the FF motif in ECL2, we generated the hH4R-F168A mutant. The receptor was co-expressed in Sf9 insect cells with the G-protein subunits Gαi2 and Gβ1γ2, and the membranes were studied in [3H]histamine binding and functional [35S]GTPγS assays. The potency of various ligands at the hH4R-F168A mutant decreased compared to the wild-type hH4R, for example by 30- and more than 100-fold in case of the H4R agonist UR-PI376 and histamine, respectively. The high constitutive activity of the hH4R was completely lost in the hH4R-F168A mutant, as reflected by neutral antagonism of thioperamide, a full inverse agonist at the wild-type hH4R. By analogy, JNJ7777120 was a partial inverse agonist at the hH4R, but a partial agonist at the hH4R-F168A mutant, again demonstrating the decrease in constitutive activity due to F168A mutation. Thus, F168 was proven to play a key role not only in ligand binding and potency, but also in the high constitutive activity of the hH4R.

Understanding the conformation transition in the activation pathway of β2 adrenergic receptor via a targeted molecular dynamics simulation

The conformation transition in the activation pathway of β2 adrenergic receptor was explored mainly using a target molecular dynamics simulation.

Identification, tissue distribution and functional characterization of the ghrelin receptor in West African lungfish, Protopterus annectens

We identified two ghrelin receptor isoforms, the ghrelin receptor type-1a (GHS-R1a) and its alternative splice form (GHS-R1b) for West African lungfish, Protopterus annectens. Lungfish GHS-R1a and 1b comprised 361 and 281 amino acids, respectively. Lungfish GHS-R1a showed the highest identity to coelacanth GHS-R1a (80.4%). The highest expression of GHS-R1a mRNAs was seen in the brain, liver, ovary, heart, intestine, and gills. GHS-R1b mRNAs were also detected in the same tissues with GHS-R1a, but their expression level was 1/20 that of GHS-R1a. In human embryonic kidney 293 cells transiently expressing lungfish GHS-R1a, rat and bullfrog ghrelin, and two GHS-R1a agonists, GHRP-6 and hexarelin, increased intracellular Ca(2+) concentrations. The intensity of the Ca(2+) increases induced by GHS-R1a agonists was twice when compared to that induced by ghrelin, although the median effective doses (ED50) were similar, suggesting a long-lasting effect of GHS-R1a agonists with similar affinity. We also examined changes in the GHS-R gene expression during an eight-week estivation. Body weight was slightly lowered, but plasma sodium and glucose concentrations decreased; plasma urea concentration increased significantly 4weeks after the start of estivation. Overall, expression of GHS-R1a mRNA decreased, but changes in GHS-R1b mRNA expression were inconsistent with those of GHS-R1a during estivation, suggesting an involvement of GHS-R in energy homeostasis, as seen in mammals. Our results suggest that the ghrelin-GHS-R1a system is present in this lungfish although ghrelin has not yet been found. The structure of GHS-R1a is closer to that of tetrapods than Actinopterygian fish, indicating a process of evolution that follows the Crossopterygii such as coelacanth.

Molecular mechanisms of bitopic ligand engagement with the M1 muscarinic acetylcholine receptor

TBPB and 77-LH-28-1 are selective agonists of the M1 muscarinic acetylcholine receptor (mAChR) that may gain their selectivity through a bitopic mechanism, interacting concomitantly with the orthosteric site and part of an allosteric site. The current study combined site-directed mutagenesis, analytical pharmacology,and molecular modeling to gain further insights into the structural basis underlying binding and signaling by these agonists. Mutations within the orthosteric binding site caused similar reductions in affinity and signaling efficacy for both selective and prototypical orthosteric ligands. In contrast, the mutation of residues within transmembrane helix (TM) 2 and the second extracellular loop (ECL2) discriminated between the different classes of ligand. In particular, ECL2 appears to be involved in the selective binding of bitopic ligands and in coordinating biased agonism between intracellular calcium mobilization and ERK1/2 phosphorylation. Molecular modeling of the interaction between TBPB and the M1 mAChR revealed a binding pose predicted to extend from the orthosteric site up toward a putative allosteric site bordered by TM2, TM3, and TM7, thus consistent with a bitopic mode of binding. Overall, these findings provide valuable structural and mechanistic insights into bitopic ligand actions and receptor activation and support a role for ECL2 in dictating the active states that can be adopted by a G protein-coupled receptor. This may enable greater selective ligand design and development for mAChRs and facilitate improved identification of bitopic ligands.

Characterization of the simultaneous decay kinetics of metarhodopsin states II and III in rhodopsin by solution-state NMR spectroscopy

The mammalian visual dim-light photoreceptor rhodopsin is considered a prototype G protein-coupled receptor. Here, we characterize the kinetics of its light-activation process. Milligram quantities of α,ε-(15)N-labeled tryptophan rhodopsin were produced in stably transfected HEK293 cells. Assignment of the chemical shifts of the indole signals was achieved by generating the single-point-tryptophan to phenylalanine mutants, and the kinetics of each of the five tryptophan residues were recorded. We find kinetic partitioning in rhodopsin decay, including three half-lives, that reveal two parallel processes subsequent to rhodopsin activation that are related to the photocycle. The meta II and meta III states emerge in parallel with a relative ratio of about 3:1. Transient formation of the meta III state was confirmed by flash photolysis experiments. From analysis of the site-resolved kinetic data we propose the involvement of the E2 -loop in the formation of the meta III state.

The role of M3 muscarinic acetylcholine receptor reactive T cells in Sjögren's syndrome: a critical review

CD4+ T cells constitute the majority of infiltrating cells in salivary glands and lachrymal glands of patients with Sjögren's syndrome (SS). The pathophysiology of SS involves T cell recognition of antigens through the T cell antigen receptor, which triggers cytokine production and chronic inflammation. The M3 muscarinic acetylcholine receptor (M3R) molecule is expressed in exocrine glands, such as salivary glands and lachrymal glands, and plays an important role in exocrine secretion. Previous studies indicated the presence of M3R reactive T cells in peripheral blood of 40% of patients with SS and autoantibodies against M3R in sera of 9-100% of the same patients. Thus, M3R is considered a candidate receptor for autoantigen recognition by T and B cells. The relationship between B cell epitopes and the function of anti-M3R antibodies has been reported, suggesting the pathogenic role of anti-M3R antibodies in xerostomia commonly seen in SS patients. We generated new experimental mouse model, M3R-induced sialadenitis (MIS), using Rag1(-/-) mice inoculated with splenocytes from M3R(-/-) mice immunized with M3R synthetic peptides. Mice with MIS developed severe SS-like sialadenitis. Cell transfer experiments using M3R(-/-)xIFNγ(-/-) mice and M3R(-/-)xIL-17(-/-) mice showed that IFNγ and IL-17 are key cytokines in the pathogenesis of sialadenitis. These findings indicate the crucial roles of M3R-reactive Th1 and Th17 cells in autoimmune sialadenitis, and suggest that these cells, in addition to anti-M3R antibodies, are potential targets in new treatments for SS.

Allosteric and biased g protein-coupled receptor signaling regulation: potentials for new therapeutics

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins that participate in many aspects of the endocrine function and are important targets for drug development. They transduce signals mainly, but not exclusively, via hetero-trimeric G proteins, leading to a diversity of intracellular signaling cascades. Ligands binding at the hormone orthosteric sites of receptors have been classified as agonists, antagonists, and/or inverse agonists based on their ability to mainly modulate G protein signaling. Accumulating evidence also indicates that such ligands, alone or in combination with other ones such as those acting outside the orthosteric hormone binding sites (e.g., allosteric modulators), have the ability to selectively engage subsets of signaling responses as compared to the natural endogenous ligands. Such modes of functioning have been variously referred to as "functional selectivity" or "ligand-biased signaling." In this review, we provide an overview of the current knowledge regarding GPCR-biased signaling and their functional regulation with a focus on the evolving concept that receptor domains can also be targeted to allosterically bias signaling, and discuss the usefulness of such modes of regulation for the design of more efficient therapeutics.

Global fold of human cannabinoid type 2 receptor probed by solid-state 13C-, 15N-MAS NMR and molecular dynamics simulations

The global fold of human cannabinoid type 2 (CB2 ) receptor in the agonist-bound active state in lipid bilayers was investigated by solid-state (13)C- and (15)N magic-angle spinning (MAS) NMR, in combination with chemical-shift prediction from a structural model of the receptor obtained by microsecond-long molecular dynamics (MD) simulations. Uniformly (13)C- and (15)N-labeled CB2 receptor was expressed in milligram quantities by bacterial fermentation, purified, and functionally reconstituted into liposomes. (13)C MAS NMR spectra were recorded without sensitivity enhancement for direct comparison of Cα, Cβ, and C=O bands of superimposed resonances with predictions from protein structures generated by MD. The experimental NMR spectra matched the calculated spectra reasonably well indicating agreement of the global fold of the protein between experiment and simulations. In particular, the (13) C chemical shift distribution of Cα resonances was shown to be very sensitive to both the primary amino acid sequence and the secondary structure of CB2. Thus the shape of the Cα band can be used as an indicator of CB2 global fold. The prediction from MD simulations indicated that upon receptor activation a rather limited number of amino acid residues, mainly located in the extracellular Loop 2 and the second half of intracellular Loop 3, change their chemical shifts significantly (≥ 1.5 ppm for carbons and ≥ 5.0 ppm for nitrogens). Simulated two-dimensional (13) Cα(i)-(13)C=O(i) and (13)C=O(i)-(15)NH(i + 1) dipolar-interaction correlation spectra provide guidance for selective amino acid labeling and signal assignment schemes to study the molecular mechanism of activation of CB2 by solid-state MAS NMR.

Novel insights into CB1 cannabinoid receptor signaling: a key interaction identified between the extracellular-3 loop and transmembrane helix 2

Activation of the cannabinoid CB1 receptor (CB1) is modulated by aspartate residue D2.63(176) in transmembrane helix (TMH) 2. Interestingly, D2.63 does not affect the affinity for ligand binding at the CB1 receptor. Studies in class A G protein-coupled receptors have suggested an ionic interaction between residues of TMH2 and 7. In this report, modeling studies identified residue K373 in the extracellular-3 (EC-3) loop in charged interactions with D2.63. We investigated this possibility by performing reciprocal mutations and biochemical studies. D2.63(176)A, K373A, D2.63(176)A-K373A, and the reciprocal mutant with the interacting residues juxtaposed D2.63(176)K-K373D were characterized using radioligand binding and guanosine 5'-3-O-(thio)triphosphate functional assays. None of the mutations resulted in a significant change in the binding affinity of N-(piperidiny-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A) or (-)-3cis -[2-hydroxyl-4-(1,1-dimethyl-heptyl)phenyl]-trans-4-[3-hydroxyl-propyl] cyclohexan-1-ol (CP55,940). Modeling studies indicated that binding-site interactions and energies of interaction for CP55,940 were similar between wild-type and mutant receptors. However, the signaling of CP55,940, and (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)-methanone mesylate (WIN55,212-2) was impaired at the D2.63(176)A-K373A and the single-alanine mutants. In contrast, the reciprocal D2.63(176)K-K373D mutant regained function for both CP55,940 and WIN55,212-2. Computational results indicate that the D2.63(176)-K373 ionic interaction strongly influences the conformation(s) of the EC-3 loop, providing a structure-based rationale for the importance of the EC-3 loop to signal transduction in CB1. The putative ionic interaction results in the EC-3 loop pulling over the top (extracellular side) of the receptor; this EC-3 loop conformation may serve protective and mechanistic roles. These results suggest that the ionic interaction between D2.63(176) and K373 is important for CB1 signal transduction.

Anti-M3 muscarinic acetylcholine receptor antibodies in patients with Sjögren's syndrome

Sjögren's syndrome (SS) is an autoimmune disease that affects exocrine glands including salivary and lacrimal glands. Recently, autoantibodies against muscarinic acetylcholine receptor M3 (M3R) have been detected in serum from 9 to 100 % of patients with SS in addition to anti-SS-A and anti-SS-B antibodies. These observations suggest the possibility that anti-M3R antibodies could serve as a new diagnostic test in patients with SS. Some anti-M3R antibodies are directly responsible for salivary underproduction in patients with SS. Thus, strategies designed to eliminate such pathogenic antibodies could help cure SS sufferers. In this review, we summarize the current state of knowledge of anti-M3R autoantibodies in patients with SS and the correlation between B cell epitopes and the function of anti-M3R antibodies.

Effects of scleroderma antibodies and pooled human immunoglobulin on anal sphincter and colonic smooth muscle function

BACKGROUND & AIMS

Patients with systemic sclerosis (SSc) have impairments in gastrointestinal smooth muscle function. The disorder has been associated with circulating antibodies to cholinergic muscarinic the type-3 receptor (M(3)-R). We investigated whether it is possible to neutralize these antibodies with pooled human IgGs (pooledhIgG).

METHODS

We studied the effects of IgGs purified from patients with SSc (SScIgGs) on cholinergic nerve stimulation in rat colon tissues. We also examined the effects of SScIgGs on M(3)-R activation by bethanechol (BeCh), M(3)-R occupancy, and receptor binding using immunofluorescence, immunoblot, and enzyme-linked immunosorbent analyses of human internal anal sphincter (IAS) smooth muscle cells, before and after administration of pooledhIgG. Functional displacement of M(3)-R occupancy by the SScIgGs was compared with that of other IgGs during the sustained phase of BeCh-induced contraction of intact smooth muscles from rats.

RESULTS

SScIgG significantly attenuated neurally mediated contraction and acetylcholine release in rat colon as well as BeCh-induced sustained contraction of the IAS smooth muscle. In immunofluorescence analysis, SScIgG co-localized with M(3)-R. In immunoblot and enzyme-linked immunosorbent analyses, M(3)-R loop-2 peptide and human IAS SMC membrane lysates bound significant amounts of SScIgG, compared with IgGs from healthy individuals and pooledhIgG. Binding was attenuated significantly by application of pooledhIgG, which by itself had no significant effect. Incubation of samples with pooledhIgG, or mixing pooledhIgG with SScIgG before administration to tissues, significantly reduced binding of SScIgG, indicating that pooledhIgG prevents SScIgG blockade of M(3)-R.

CONCLUSIONS

In studies of rat and human tissues, pooled human IgG prevent and reverses the cholinergic dysfunction associated with the progressive gastrointestinal manifestations of SSc by neutralizing functional M(3)-R antibodies present in the circulation of patients with SSc.

Extracellular loop II modulates GTP sensitivity of the prostaglandin EP3 receptor

Unlike the majority of G protein-coupled receptors, the prostaglandin E(2) (PGE(2)) E-prostanoid 3 (EP3) receptor binds agonist with high affinity that is insensitive to the presence of guanosine 5[prime]-O-(3-thio)triphosphate (GTPγS). We report the identification of mutations that confer GTPγS sensitivity to agonist binding. Seven point mutations were introduced into the conserved motif in the second extracellular loop (ECII) of EP3, resulting in acquisition of GTP-sensitive agonist binding. One receptor mutation W203A was studied in detail. Loss of agonist binding was observed on intact human embryonic kidney 293 cells expressing the W203A receptor, conditions where high GTP levels are present; however, high affinity binding [(3)H]PGE(2) was observed in broken cell preparations washed free of GTP. The [(3)H]PGE(2) binding of W203A in broken cell membrane fractions was inhibited by addition of GTPγS (IC(50) 21 ± 1.8 nM). Taken together, these results suggest that the wild-type EP3 receptor displays unusual characteristics of the complex coupled equilibria between agonist-receptor and receptor-G protein interaction. Moreover, mutation of ECII can alter this coupled equilibrium from GTP-insensitive agonist binding to more conventional GTP-sensitive binding. This suggests that for the mutant receptors, ECII plays a critical role in linking the agonist bound receptor conformation to the G protein nucleotide bound state.

Lifting the lid on GPCRs: the role of extracellular loops

GPCRs exhibit a common architecture of seven transmembrane helices (TMs) linked by intracellular loops and extracellular loops (ECLs). Given their peripheral location to the site of G-protein interaction, it might be assumed that ECL segments merely link the important TMs within the helical bundle of the receptor. However, compelling evidence has emerged in recent years revealing a critical role for ECLs in many fundamental aspects of GPCR function, which supported by recent GPCR crystal structures has provided mechanistic insights. This review will present current understanding of the key roles of ECLs in ligand binding, activation and regulation of both family A and family B GPCRs.

Spinophilin as a novel regulator of M3 muscarinic receptor-mediated insulin release in vitro and in vivo

Spinophilin (SPL), a multidomain scaffolding protein known to modulate the activity of different G-protein-coupled receptors, regulates various central nervous system (CNS) functions. However, little is known about the role of SPL expressed in peripheral cell types including pancreatic β cells. In this study, we examined the ability of SPL to modulate the activity of β-cell M(3) muscarinic acetylcholine receptors (M3Rs), which play an important role in facilitating insulin release and maintaining normal blood glucose levels. We demonstrated, by using both in vitro and in vivo approaches (mouse insulinoma cells and SPL-deficient mice), that SPL is a potent negative regulator of M3R-mediated signaling and insulin release. Additional biochemical and biophysical studies, including the use of bioluminescence resonance energy transfer technology, suggested that SPL is able to recruit regulator of G-protein signaling 4 (RGS4) to the M3R signaling complex in an agonist-dependent fashion. Since RGS4 is a member of the RGS family of proteins that act to reduce the lifetime of activated G proteins, these findings support the concept that the inhibitory effects of SPL on M3R activity are mediated by RGS4. These data suggest that SPL or other G-protein-coupled receptor-associated proteins may serve as novel targets for drug therapy aimed at improving β-cell function for the treatment of type 2 diabetes.

Homology modeling and molecular dynamics simulations of the active state of the nociceptin receptor reveal new insights into agonist binding and activation

The opioid receptor-like receptor, also known as the nociceptin receptor (NOP), is a class A G protein-coupled receptor (GPCR) in the opioid receptor family. Although NOP shares a significant homology with the other opioid receptors, it does not bind known opioid ligands and has been shown to have a distinct mechanism of activation compared to the closely related opioid receptors mu, delta, and kappa. Previously reported homology models of the NOP receptor, based on the inactive-state GPCR crystal structures, give limited information on the activation and selectivity features of this fourth member of the opioid receptor family. We report here the first active-state homology model of the NOP receptor based on the opsin GPCR crystal structure. An inactive-state homology model of NOP was also built using a multiple template approach. Molecular dynamics simulation of the active-state NOP model and comparison to the inactive-state model suggest that NOP activation involves movements of transmembrane (TM)3 and TM6 and several activation microswitches, consistent with GPCR activation. Docking of the selective nonpeptidic NOP agonist ligand Ro 64-6198 into the active-state model reveals active-site residues in NOP that play a role in the high selectivity of this ligand for NOP over the other opioid receptors. Docking the shortest active fragment of endogenous agonist nociceptin/orphaninFQ (residues 1-13) shows that the NOP extracellular loop 2 (EL2) loop interacts with the positively charged residues (8-13) of N/OFQ. Both agonists show extensive polar interactions with residues at the extracellular end of the TM domain and EL2 loop, suggesting agonist-induced reorganization of polar networks, during receptor activation.