G protein-coupled receptors: a count of 1001 conformations

Allosteric modulation of the CXCR4:CXCL12 axis by targeting receptor nanoclustering via the TMV-TMVI domain

CXCR4 is a ubiquitously expressed chemokine receptor that regulates leukocyte trafficking and arrest in both homeostatic and pathological states. It also participates in organogenesis, HIV-1 infection, and tumor development. Despite the potential therapeutic benefit of CXCR4 antagonists, only one, plerixafor (AMD3100), which blocks the ligand-binding site, has reached the clinic. Recent advances in imaging and biophysical techniques have provided a richer understanding of the membrane organization and dynamics of this receptor. Activation of CXCR4 by CXCL12 reduces the number of CXCR4 monomers/dimers at the cell membrane and increases the formation of large nanoclusters, which are largely immobile and are required for correct cell orientation to chemoattractant gradients. Mechanistically, CXCR4 activation involves a structural motif defined by residues in TMV and TMVI. Using this structural motif as a template, we performed in silico molecular modeling followed by in vitro screening of a small compound library to identify negative allosteric modulators of CXCR4 that do not affect CXCL12 binding. We identified AGR1.137, a small molecule that abolishes CXCL12-mediated receptor nanoclustering and dynamics and blocks the ability of cells to sense CXCL12 gradients both in vitro and in vivo while preserving ligand binding and receptor internalization.

Synchronous and Asynchronous Response in Dynamically Perturbed Proteins

We present a dynamic perturbation-response model of proteins based on the Gaussian Network Model, where a residue is perturbed periodically, and the dynamic response of other residues is determined. The model shows that periodic perturbation causes a synchronous response in phase with the perturbation and an asynchronous response that is out of phase. The asynchronous component results from the viscous effects of the solvent and other dispersive factors in the system. The model is based on the solution of the Langevin equation in the presence of solvent, noise, and perturbation. We introduce several novel ideas: The concept of storage and loss compliance of the protein and their dependence on structure and frequency; the amount of work lost and the residues that contribute significantly to the lost work; new dynamic correlations that result from perturbation; causality, that is, the response of j when i is perturbed is not equal to the response of i when j is perturbed. As examples, we study two systems, namely, bovine rhodopsin and the class of nanobodies. The general results obtained are (i) synchronous and asynchronous correlations depend strongly on the frequency of perturbation, their magnitude decreases with increasing frequency, (ii) time-delayed mean-squared fluctuations of residues have only synchronous components. Asynchronicity is present only in cross correlations, that is, correlations between different residues, (iii) perturbation of loop residues leads to a large dissipation of work, (iv) correlations satisfy the hypothesis of pre-existing pathways according to which information transfer by perturbation rides on already existing equilibrium correlations in the system, (v) dynamic perturbation can introduce a selective response in the system, where the perturbation of each residue excites different sets of responding residues, and (vi) it is possible to identify nondissipative residues whose perturbation does not lead to dissipation in the protein. Despite its simplicity, the model explains several features of allosteric manipulation.

Theoretical identification of thermostabilizing amino acid mutations for G-protein-coupled receptors

Thermostabilization of a membrane proteins, especially G-protein-coupled receptors (GPCRs), is often necessary for biochemical applications and pharmaceutical studies involving structure-based drug design. Here we review our theoretical, physics-based method for identifying thermostabilizing amino acid mutations. Its novel aspects are the following: The entropic effect originating from the translational displacement of hydrocarbon groups within the lipid bilayer is treated as a pivotal factor; a reliable measure of thermostability is introduced and a mutation which enlarges the measure to a significant extent is chosen; and all the possible mutations can be examined with moderate computational effort. It was shown that mutating the residue at a position of N_BW = 3.39 (N_BW is the Ballesteros-Weinstein number) to Arg or Lys leads to the stabilization of significantly many different GPCRs of class A in the inactive state. Up to now, we have been successful in stabilizing several GPCRs and newly solving three-dimensional structures for the muscarinic acetylcholine receptor 2 (M2R), prostaglandin E receptor 4 (EP4), and serotonin 2A receptor (5-HT_2AR) using X-ray crystallography. The subjects to be pursued in future studies are also discussed.

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

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

Molecular basis for high-affinity agonist binding in GPCRs

G protein-coupled receptors (GPCRs) in the G protein-coupled active state have higher affinity for agonists as compared with when they are in the inactive state, but the molecular basis for this is unclear. We have determined four active-state structures of the β1-adrenoceptor (β1AR) bound to conformation-specific nanobodies in the presence of agonists of varying efficacy. Comparison with inactive-state structures of β1AR bound to the identical ligands showed a 24 to 42% reduction in the volume of the orthosteric binding site. Potential hydrogen bonds were also shorter, and there was up to a 30% increase in the number of atomic contacts between the receptor and ligand. This explains the increase in agonist affinity of GPCRs in the active state for a wide range of structurally distinct agonists.

Intramolecular and Intermolecular FRET Sensors for GPCRs - Monitoring Conformational Changes and Beyond

Within the past decade, a large increase in structural knowledge from crystallographic studies has significantly fostered our understanding of the structural biology of G protein-coupled receptors (GPCRs). However, information on dynamic events upon receptor activation or deactivation is not yet readily accessed by these structural approaches. GPCR-based fluorescence resonance energy transfer or bioluminescence resonance energy transfer sensors or sensors for interacting proteins (e.g., G proteins or arrestins) can in part cover this gap. The principal design of such sensors was reported 15 years ago. Since then, sensors for almost 20 different GPCRs have been designed. If used with necessary controls and cautious interpretation, such sensors can contribute significantly to our understanding of the basic mechanisms of GPCR function and beyond. In this review, we will discuss the recent developments in this area of GPCR dynamics.

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

CCR5 susceptibility to ligand-mediated down-modulation differs between human T lymphocytes and myeloid cells

CCR5 is a chemokine receptor expressed on leukocytes and a coreceptor used by HIV-1 to enter CD4(+) T lymphocytes and macrophages. Stimulation of CCR5 by chemokines triggers internalization of chemokine-bound CCR5 molecules in a process called down-modulation, which contributes to the anti-HIV activity of chemokines. Recent studies have shown that CCR5 conformational heterogeneity influences chemokine-CCR5 interactions and HIV-1 entry in transfected cells or activated CD4(+) T lymphocytes. However, the effect of CCR5 conformations on other cell types and on the process of down-modulation remains unclear. We used mAbs, some already shown to detect distinct CCR5 conformations, to compare the behavior of CCR5 on in vitro generated human T cell blasts, monocytes and MDMs and CHO-CCR5 transfectants. All human cells express distinct antigenic forms of CCR5 not detected on CHO-CCR5 cells. The recognizable populations of CCR5 receptors exhibit different patterns of down-modulation on T lymphocytes compared with myeloid cells. On T cell blasts, CCR5 is recognized by all antibodies and undergoes rapid chemokine-mediated internalization, whereas on monocytes and MDMs, a pool of CCR5 molecules is recognized by a subset of antibodies and is not removed from the cell surface. We demonstrate that this cell surface-retained form of CCR5 responds to prolonged treatment with more-potent chemokine analogs and acts as an HIV-1 coreceptor. Our findings indicate that the regulation of CCR5 is highly specific to cell type and provide a potential explanation for the observation that native chemokines are less-effective HIV-entry inhibitors on macrophages compared with T lymphocytes.

Structural libraries of protein models for multiple species to understand evolution of the renin-angiotensin system

The details of protein pathways at a structural level provides a bridge between genetics/molecular biology and physiology. The renin-angiotensin system is involved in many physiological pathways with informative structural details in multiple components. Few studies have been performed assessing structural knowledge across the system. This assessment allows use of bioinformatics tools to fill in missing structural voids. In this paper we detail known structures of the renin-angiotensin system and use computational approaches to estimate and model components that do not have their protein structures defined. With the subsequent large library of protein structures, we then created a species specific protein library for human, mouse, rat, bovine, zebrafish, and chicken for the system. The rat structural system allowed for rapid screening of genetic variants from 51 commonly used rat strains, identifying amino acid variants in angiotensinogen, ACE2, and AT1b that are in contact positions with other macromolecules. We believe the structural map will be of value for other researchers to understand their experimental data in the context of an environment for multiple proteins, providing pdb files of proteins for the renin-angiotensin system in six species. With detailed structural descriptions of each protein, it is easier to assess a species for use in translating human diseases with animal models. Additionally, as whole genome sequencing continues to decrease in cost, tools such as molecular modeling will gain use as an initial step in designing efficient hypothesis driven research, addressing potential functional outcomes of genetic variants with precompiled protein libraries aiding in rapid characterizations.

Analysis of functional selectivity through G protein-dependent and -independent signaling pathways at the adrenergic α(2C) receptor

Although G protein-coupled receptors (GPCRs) are traditionally categorized as Gs-, Gq-, or Gi/o-coupled, their signaling is regulated by multiple mechanisms. GPCRs can couple to several effector pathways, having the capacity to interact not only with more than one G protein subtype but also with alternative signaling or effector proteins such as arrestins. Moreover, GPCR ligands can have different efficacies for activating these signaling pathways, a characteristic referred to as biased agonism or functional selectivity. In this work our aim was to detect differences in the ability of various agonists acting at the α2C type of adrenergic receptors (α2C-ARs) to modulate cAMP accumulation, cytoplasmic Ca(2+) release, β-arrestin recruitment and receptor internalization. A detailed comparative pharmacological characterization of G protein-dependent and -independent signaling pathways was carried out using adrenergic agonists (norepinephrine, phenylephrine, brimonidine, BHT-920, oxymetazoline, clonidine, moxonidine, guanabenz) and antagonists (MK912, yohimbine). As initial analysis of agonist Emax and EC50 values suggested possible functional selectivity, ligand bias was quantified by applying the relative activity scale and was compared to that of the endogenous agonist norepinephrine. Values significantly different from 0 between pathways indicated an agonist that promoted different level of activation of diverse effector pathways most likely due to the stabilization of a subtly different receptor conformation from that induced by norepinephrine. Our results showed that a series of agonists acting at the α2C-AR displayed different degree of functional selectivity (bias factors ranging from 1.6 to 36.7) through four signaling pathways. As signaling via these pathways seems to have distinct functional and physiological outcomes, studying all these stages of receptor activation could have further implications for the development of more selective therapeutics with improved efficacy and/or fewer side effects.

Constitutively active chemokine CXC receptors

Chemokines are low-molecular-weight, secreted proteins that act as leukocyte-specific chemoattractants. The chemokine family has more than 40 members. Based on the position of two conserved cysteines in the N-terminal domain, chemokines can be divided into the CXC, C, CC, and CX3C subfamilies. The interaction of chemokines with their receptors mediates signaling pathways that play critical roles in cell migration, differentiation, and proliferation. The receptors for chemokines are G protein-coupled receptors (GPCRs), and thus far, seven CXC receptors have been cloned and are designated CXCR1-7. Constitutively active GPCRs are present in several human immune-mediated diseases and in tumors, and they have provided valuable information in understanding the molecular mechanism of GPCR activation. Several constitutively active CXC chemokine receptors include the V6.40A and V6.40N mutants of CXCR1; the D3.49V variant of CXCR2; the N3.35A, N3.35S, and T2.56P mutants of CXCR3; the N3.35 mutation of CXCR4; and the naturally occurring KSHV-GPCR. Here, we review the regulation of CXC chemokine receptor signaling, with a particular focus on the constitutive activation of these receptors and the implications in physiological conditions and in pathogenesis. Understanding the mechanisms behind the constitutive activation of CXC chemokine receptors may aid in pharmaceutical design and the screening of inverse agonists and allosteric modulators for the treatment of autoimmune diseases and cancers.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

Simulations of biased agonists in the β(2) adrenergic receptor with accelerated molecular dynamics

The biased agonism of the G protein-coupled receptors (GPCRs), where in addition to a traditional G protein-signaling pathway a GPCR promotes intracellular signals though β-arrestin, is a novel paradigm in pharmacology. Biochemical and biophysical studies have suggested that a GPCR forms a distinct ensemble of conformations signaling through the G protein and β-arrestin. Here we report on the dynamics of the β2 adrenergic receptor bound to the β-arrestin and G protein-biased agonists and the empty receptor to further characterize the receptor conformational changes caused by biased agonists. We use conventional and accelerated molecular dynamics (aMD) simulations to explore the conformational transitions of the GPCR from the active state to the inactive state. We found that aMD simulations enable monitoring of the transition within the nanosecond time scale while capturing the known microscopic characteristics of the inactive states, such as the ionic lock, the inward position of F6.44, and water clusters. Distinct conformational states are shown to be stabilized by each biased agonist. In particular, in simulations of the receptor with the β-arrestin-biased agonist N-cyclopentylbutanepherine, we observe a different pattern of motions in helix 7 when compared to simulations with the G protein-biased agonist salbutamol that involves perturbations of the network of interactions within the NPxxY motif. Understanding the network of interactions induced by biased ligands and the subsequent receptor conformational shifts will lead to development of more efficient drugs.

Differential mechanisms of activation of the Ang peptide receptors AT1, AT2, and MAS: using in silico techniques to differentiate the three receptors

The renin-angiotensin system is involved in multiple conditions ranging from cardiovascular disorders to cancer. Components of the pathway, including ACE, renin and angiotensin receptors are targets for disease treatment. This study addresses three receptors of the pathway: AT1, AT2, and MAS and how the receptors are similar and differ in activation by angiotensin peptides. Combining biochemical and amino acid variation data with multiple species sequence alignments, structural models, and docking site predictions allows for visualization of how angiotensin peptides may bind and activate the receptors; allowing identification of conserved and variant mechanisms in the receptors. MAS differs from AT1 favoring Ang-(1–7) and not Ang II binding, while AT2 recently has been suggested to preferentially bind Ang III. A new model of Ang peptide binding to AT1 and AT2 is proposed that correlates data from site directed mutagenesis and photolabled experiments that were previously considered conflicting. Ang II binds AT1 and AT2 through a conserved initial binding mode involving amino acids 111 (consensus 325) of AT1 (Asn) interacting with Tyr (4) of Ang II and 199 and 256 (consensus 512 and 621, a Lys and His respectively) interacting with Phe (8) of Ang II. In MAS these sites are not conserved, leading to differential binding and activation by Ang-(1–7). In both AT1 and AT2, the Ang II peptide may internalize through Phe (8) of Ang II propagating through the receptors’ conserved aromatic amino acids to the final photolabled positioning relative to either AT1 (amino acid 294, Asn, consensus 725) or AT2 (138, Leu, consensus 336). Understanding receptor activation provides valuable information for drug design and identification of other receptors that can potentially bind Ang peptides.

Leu128(3.43) (l128) and Val247(6.40) (V247) of CXCR1 are critical amino acid residues for g protein coupling and receptor activation

CXCR1, a classic GPCR that binds IL-8, plays a key role in neutrophil activation and migration by activating phospholipase C (PLC)β through Gα₁₅ and Gα_i which generates diacylglycerol and inositol phosphates (IPs). In this study, two conserved amino acid residues of CXCR1 on the transmembrane domain (TM) 3 and TM6, Leu128^3.43 (L128) and Val247^6.40 (V247), respectively, were selectively substituted with other amino acids to investigate the role of these conserved residues in CXCR1 activation. Although two selective mutants on Leu128, Leu128Ala (L128A) and Leu128Arg (L128R), demonstrated high binding affinity to IL-8, they were not capable of coupling to G proteins and consequently lost the functional response of the receptors. By contrast, among the four mutants at residue Val247 (TM6.40), replacing Val247 with Ala (V247A) and Asn (V247N) led to constitutive activation of mutant receptors when cotransfected with Gα₁₅. The V247N mutant also constitutively activated the Gα_i protein. These results indicate that L128 on TM3.43 is involved in G protein coupling and receptor activation but is unimportant for ligand binding. On the other hand, V247 on TM6.40 plays a critical role in maintaining the receptor in the inactive state, and the substitution of V247 impaired the receptor constraint and stabilized an active conformation. Functionally, there was an increase in chemotaxis in response to IL-8 in cells expressing V247A and V247N. Our findings indicate that Leu128^3.43 and Val247^6.40 are critical for G protein coupling and activation of signaling effectors, providing a valuable insight into the mechanism of CXCR1 activation.

Bihelix: Towards de novo structure prediction of an ensemble of G-protein coupled receptor conformations

G-Protein Coupled Receptors (GPCRs) play a critical role in cellular signal transduction pathways and are prominent therapeutic targets. Recently there has been major progress in obtaining experimental structures for a few GPCRs. Each GPCR, however, exhibits multiple conformations that play a role in their function and we have been developing methods aimed at predicting structures for all these conformations. Analysis of available structures shows that these conformations differ in relative helix tilts and rotations. The essential issue is, determining how to orient each of the seven helices about its axis since this determines how it interacts with the other six helices. Considering all possible helix rotations to ensure that no important packings are overlooked, and using rotation angle increments of 30° about the helical axis would still lead to 12(7) or 35 million possible conformations each with optimal residue positions. We show in this paper how to accomplish this. The fundamental idea is to optimize the interactions between each pair of contacting helices while ignoring the other 5 and then to estimate the energies of all 35 million combinations using these pair-wise interactions. This BiHelix approach dramatically reduces the effort to examine the complete set of conformations and correctly identifies the crystal packing for the experimental structures plus other near-native packings we believe may play an important role in activation. This approach also enables a detailed structural analysis of functionally distinct conformations using helix-helix interaction energy landscapes and should be useful for other helical transmembrane proteins as well.

Characterizing and predicting the functional and conformational diversity of seven-transmembrane proteins

The activation of seven-transmembrane receptors (7TMRs) allows cells to sense their environment and convert extracellular signals (like hormone binding) into intracellular signals (through G protein-coupled and/or β arrestin-coupled pathways). A single 7TMR is capable of transducing a wide spectrum of physiological responses inside a cell by coupling to these pathways. This intracellular pleiotropic action is enabled by multiple conformations exhibited by these receptors. Developments in membrane protein structure determination technologies have led to a rapid increase in crystal structures for many 7TMRs. Majority of these receptors have been crystallized in their inactive conformation and, for some, one of the many active conformations has also been crystallized. Given the topological constraints of a lipid bilayer that results in a single fold of seven almost parallel TM helices connected by mostly unstructured loops, these structures exhibit a diversity of conformations not only across the receptors but also across the different functional forms for receptors with structures for one of the functionally active conformations. Here we present a method to characterize this conformational diversity in terms of transmembrane helix topology (TMHTOP) parameters and how to use these helix orientation parameters to predict functionally-distinct multiple conformations for these receptors. The TMHTOP parameters enable a quantification of the structural changes that underlie 7TMR activation and also sheds a unique mechanistic light on the pleiotropic nature of these receptors. It provides a common language to describe the 7TMR activation mechanisms as well as differences across many receptors in terms of visually intuitive structural parameters. Protein structure prediction methods can use these parameters to describe 7TMR conformational ensembles, which coupled to experimental data can be used to develop testable hypotheses for the structural basis of 7TMR functions.

Mechanisms regulating chemokine receptor activity

Co-ordinated movement and controlled positioning of leucocytes is key to the development, maintenance and proper functioning of the immune system. Chemokines and their receptors play an essential role in these events by mediating directed cell migration, often referred to as chemotaxis. The chemotactic property of these molecules is also thought to contribute to an array of pathologies where inappropriate recruitment of specific chemokine receptor-expressing leucocytes is observed, including cancer and inflammatory diseases. As a result, chemokine receptors have become major targets for therapeutic intervention, and during the past 15 years much research has been devoted to understanding the regulation of their biological activity. From these studies, processes which govern the availability of functional chemokine receptors at the cell surface have emerged as playing a central role. In this review, we summarize and discuss current knowledge on the molecular mechanisms contributing to the regulation of chemokine receptor surface expression, from gene transcription and protein degradation to post-translational modifications, multimerization, intracellular transport and cross-talk.

Engineering an ultra-thermostable β(1)-adrenoceptor

Conformational thermostabilisation of G-protein-coupled receptors is a successful strategy for their structure determination. The thermostable mutants tolerate short-chain detergents, such as octylglucoside and nonylglucoside, which are ideal for crystallography, and in addition, the receptors are preferentially in a single conformational state. The first thermostabilised receptor to have its structure determined was the β(1)-adrenoceptor mutant β(1)AR-m23 bound to the antagonist cyanopindolol, and recently, additional structures have been determined with agonist bound. Here, we describe further stabilisation of β(1)AR-m23 by the addition of three thermostabilising mutations (I129V, D322K, and Y343L) to make a mutant receptor that is 31 °C more thermostable than the wild-type receptor in dodecylmaltoside and is 13 °C more thermostable than β(1)AR-m23 in nonylglucoside. Although a number of thermostabilisation methods were tried, including rational design of disulfide bonds and engineered zinc bridges, the two most successful strategies to improve the thermostability of β(1)AR-m23 were an engineered salt bridge and leucine scanning mutagenesis. The three additional thermostabilising mutations did not significantly affect the pharmacological properties of β(1)AR-m23, but the new mutant receptor was significantly more stable in short-chain detergents such as heptylthioglucoside and denaturing detergents such as SDS.

Agonist-bound adenosine A2A receptor structures reveal common features of GPCR activation

Adenosine receptors and β-adrenoceptors are G-protein-coupled receptors (GPCRs) that activate intracellular G proteins on binding the agonists adenosine or noradrenaline, respectively. GPCRs have similar structures consisting of seven transmembrane helices that contain well-conserved sequence motifs, indicating that they are probably activated by a common mechanism. Recent structures of β-adrenoceptors highlight residues in transmembrane region 5 that initially bind specifically to agonists rather than to antagonists, indicating that these residues have an important role in agonist-induced activation of receptors. Here we present two crystal structures of the thermostabilized human adenosine A(2A) receptor (A(2A)R-GL31) bound to its endogenous agonist adenosine and the synthetic agonist NECA. The structures represent an intermediate conformation between the inactive and active states, because they share all the features of GPCRs that are thought to be in a fully activated state, except that the cytoplasmic end of transmembrane helix 6 partially occludes the G-protein-binding site. The adenine substituent of the agonists binds in a similar fashion to the chemically related region of the inverse agonist ZM241385 (ref. 8). Both agonists contain a ribose group, not found in ZM241385, which extends deep into the ligand-binding pocket where it makes polar interactions with conserved residues in H7 (Ser 277(7.42) and His 278(7.43); superscripts refer to Ballesteros-Weinstein numbering) and non-polar interactions with residues in H3. In contrast, the inverse agonist ZM241385 does not interact with any of these residues and comparison with the agonist-bound structures indicates that ZM241385 sterically prevents the conformational change in H5 and therefore it acts as an inverse agonist. Comparison of the agonist-bound structures of A(2A)R with the agonist-bound structures of β-adrenoceptors indicates that the contraction of the ligand-binding pocket caused by the inward motion of helices 3, 5 and 7 may be a common feature in the activation of all GPCRs.

Understanding functional residues of the cannabinoid CB1

The brain cannabinoid (CB(1)) receptor that mediates numerous physiological processes in response to marijuana and other psychoactive compounds is a G protein coupled receptor (GPCR) and shares common structural features with many rhodopsin class GPCRs. For the rational development of therapeutic agents targeting the CB(1) receptor, understanding of the ligand-specific CB(1) receptor interactions responsible for unique G protein signals is crucial. For a more than a decade, a combination of mutagenesis and computational modeling approaches has been successfully employed to study the ligand-specific CB(1) receptor interactions. In this review, after a brief discussion about recent advances in understanding of some structural and functional features of GPCRs commonly applicable to the CB(1) receptor, the CB(1) receptor functional residues reported from mutational studies are divided into three different types, ligand binding (B), receptor stabilization (S) and receptor activation (A) residues, to delineate the nature of the binding pockets of anandamide, CP55940, WIN55212-2 and SR141716A and to describe the molecular events of the ligand-specific CB(1) receptor activation from ligand binding to G protein signaling. Taken these CB(1) receptor functional residues, some of which are unique to the CB(1) receptor, together with the biophysical knowledge accumulated for the GPCR active state, it is possible to propose the early stages of the CB(1) receptor activation process that not only provide some insights into understanding molecular mechanisms of receptor activation but also are applicable for identifying new therapeutic agents by applying the validated structure-based approaches, such as virtual high throughput screening (HTS) and fragment-based approach (FBA).

Chemogenomic analysis of G-protein coupled receptors and their ligands deciphers locks and keys governing diverse aspects of signalling

Understanding the molecular mechanism of signalling in the important super-family of G-protein-coupled receptors (GPCRs) is causally related to questions of how and where these receptors can be activated or inhibited. In this context, it is of great interest to unravel the common molecular features of GPCRs as well as those related to an active or inactive state or to subtype specific G-protein coupling. In our underlying chemogenomics study, we analyse for the first time the statistical link between the properties of G-protein-coupled receptors and GPCR ligands. The technique of mutual information (MI) is able to reveal statistical inter-dependence between variations in amino acid residues on the one hand and variations in ligand molecular descriptors on the other. Although this MI analysis uses novel information that differs from the results of known site-directed mutagenesis studies or published GPCR crystal structures, the method is capable of identifying the well-known common ligand binding region of GPCRs between the upper part of the seven transmembrane helices and the second extracellular loop. The analysis shows amino acid positions that are sensitive to either stimulating (agonistic) or inhibitory (antagonistic) ligand effects or both. It appears that amino acid positions for antagonistic and agonistic effects are both concentrated around the extracellular region, but selective agonistic effects are cumulated between transmembrane helices (TMHs) 2, 3, and ECL2, while selective residues for antagonistic effects are located at the top of helices 5 and 6. Above all, the MI analysis provides detailed indications about amino acids located in the transmembrane region of these receptors that determine G-protein signalling pathway preferences.

Selective modulation of follicle-stimulating hormone signaling pathways with enhancing equine chorionic gonadotropin/antibody immune complexes

The injection of equine chorionic gonadotropin (eCG) in dairy goats induces the production of anti-eCG antibodies (Abs) in some females. We have previously shown that Abs negatively modulate the LH and FSH-like bioactivities of eCG, in most cases, compromising fertility in treated females. Surprisingly, we found out that some anti-eCG Abs improved fertility and prolificity of the treated females, in vivo. These Abs, when complexed with eCG, enhanced LH and FSH ability to induce steroidogenesis on specific target cells, in vitro. In the present study, we analyzed the impact of three eCG/anti-eCG Ab-enhancing complexes on two transduction mechanisms triggered by the FSH receptor: guanine nucleotide-binding protein alphaS-subunit/cAMP/protein kinase A (PKA) and beta-arrestin-dependent pathways, respectively. In all cases, significant enhancing effects were observed on ERK phosphorylation compared with eCG alone. However, cAMP production and PKA activation induced by eCG could be differently modulated by Abs. By using a pharmacological inhibitor of PKA and small interfering RNA-mediated knock-down of endogenous beta-arrestin 1 and 2, we demonstrated that signaling bias was induced and was clearly dependent on the complexed Ab. Together, our data show that eCG/anti-eCG Ab-enhancing complexes can differentially modulate cAMP/PKA and beta-arrestin pathways as a function of the complexed Ab. We hypothesize that enhancing Abs may change the eCG conformation, the immune complex acquiring new "biased" pharmacological properties ultimately leading to the physiological effects observed in vivo. The modulation of ligand pharmacological properties by Abs opens promising research avenues towards the optimization of glycoprotein hormone biological activities and, more generally, the development of new therapeutics.

Tachykinins and Neurokinin Receptors in Bone Marrow Functions: Neural-Hematopoietic Link

After many decades of neuropeptide research, advances in the field of tachykinins have considerably increased and shown their implications in several physiological processes. In this review we focus on the role of the tachykinins in the regulation of hematopoietic functions. Evidence has shown that neural control of this process is emerging as a significant category in hematopoietic modulation. In the context of this regulation, we discuss the existence of a complex network involving the neurokinin receptors, tachykinins and cytokines. This network is tightly regulated by each of its components.

Antagonist-D2S-dopamine receptor interactions in intact recombinant Chinese hamster ovary cells [corrected]

D(2)-type dopamine receptors are major recognition sites for antipsychotic drugs. There are two splice variants: D(2S) and D(2L) with an additional 29 amino acid sequence in the third intracellular loop. Only little comparative information is hitherto available about their pharmacological properties and none of these studies dealt with intact cell systems. This prompted us to investigate the binding properties of [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone, to intact CHO cells expressing recombinant human D(2L)-receptors. Presently, we have repeated and extended this experimental approach to the human D(2S)-receptors in the same cell system. Except for a slower dissociation of [(3)H]-spiperone from D(2S), the binding properties of these and other antagonists were not significantly different for both isoforms (P > 0.05). The very slow dissociation of the atypical antipsychotic clozapine was surprising in light of its low affinity. Two experiments pointed out the existence of non-competitive interactions between raclopride and spiperone for D(2S) as well as D(2L) (A. Packeu, J. P. De Backer & G. Vauquelin, in preparation). Alongside the different physicochemical properties of these ligands, this finding fits with a model wherein the hydrophilic raclopride approaches the D(2L)-receptor from the aqueous phase, while the hydrophobic spiperone approaches the receptor by lateral diffusion between the membrane lipids. These different modes of approach could imply the existence of topologically distinct ligand binding sites at D(2)-receptors.

Non-competitive interaction between raclopride and spiperone on human D-receptors in intact Chinese hamster ovary cells

We recently investigated the binding properties of the antagonists [(3)H]-raclopride and [(3)H]-spiperone to intact Chinese hamster ovary cells expressing recombinant human D(2long)-dopamine receptors (CHO-D(2L) cells). Compared with saturation binding with [(3)H]-raclopride, raclopride reduced [(3)H]-spiperone binding with to low potency in competition binding experiments. The present findings illustrate the ability of spiperone to inhibit [(3)H]-raclopride binding non-competitively. While raclopride only decreases the apparent K(D) of [(3)H]-raclopride in saturation binding experiments, spiperone only decreases the number of sites to which [(3)H]-raclopride binds with high affinity. Also, while the IC(50) of raclopride depends on the concentration of [(3)H]-raclopride in competition experiments, this is not the case for spiperone. Kinetic studies reveal that the binding of raclopride at its high affinity sites does not affect the association of subsequently added [(3)H]-spiperone nor the rebinding of freshly dissociated [(3)H]-spiperone to the same or surrounding receptors. Yet, spiperone does not affect the dissociation rate of [(3)H]-raclopride and raclopride does not affect the (genuine) dissociation rate of [(3)H]-spiperone. The easiest way to interpret the present findings in molecular terms is to assume that D(2L)-receptors or their dimeric complexes possess two distinct binding sites: one with high affinity/accessibility for [(3)H]-raclopride and the other one with high affinity/accessibility for [(3)H]-spiperone. The ability of bound spiperone to inhibit high affinity raclopride binding while the reverse is not the case suggests for the occurrence of non-reciprocal allosteric interactions. These new findings could point at the occurrence of allosteric interactions between different classes of D(2)-receptor antagonists.

Coarse-grained modeling of allosteric regulation in protein receptors

Allosteric regulation provides highly specific ligand recognition and signaling by transmembrane protein receptors. Unlike functions of protein molecular machines that rely on large-scale conformational transitions, signal transduction in receptors appears to be mediated by more subtle structural motions that are difficult to identify. We describe a theoretical model for allosteric regulation in receptors that addresses a fundamental riddle of signaling: What are the structural origins of the receptor agonism (specific signaling response to ligand binding)? The model suggests that different signaling pathways in bovine rhodopsin or human beta(2)-adrenergic receptor can be mediated by specific structural motions in the receptors. We discuss implications for understanding the receptor agonism, particularly the recently observed "biased agonism" (selected activation of specific signaling pathways), and for developing rational structure-based drug-design strategies.

Sartan-AT1 receptor interactions: in vitro evidence for insurmountable antagonism and inverse agonism

Sartans are non-peptide AT(1) receptor antagonists used to treat hypertension and related pathologies. Their effects on the G protein-dependent responses of angiotensin II (Ang II) were the same in vascular tissues and in isolated cell systems. All are competitive but, when pre-incubated, they act surmountably (only rightward shift of the Ang II concentration-response curve) or insurmountably (also decreasing the maximal response). Insurmountable behaviour reflects the formation of tight sartan-receptor complexes; it is often partial due to the co-existence of tight and loose complexes. Their ratio positively correlates with the dissociation half-life of the tight complexes and depends on the sartan: i.e. candesartan>olmesartan>telmisartan approximately equal EXP3174>valsartan>irbesartan>>losartan. When AT(1) receptors display sufficient basal activity (in case of receptor over-expression, mutation and, especially, tissue stretching) sartans may also act as inverse agonists. This rather affects long-term, G protein-independent hypertrophic responses leading to cardiovascular remodelling.

Chapter 11. Subsecond analyses of G-protein coupled-receptor ternary complex dynamics by rapid mix flow cytometry

The binding of full and partial agonist ligands (L) to G-protein-coupled receptors (GPCRs) initiates the formation of ternary complexes with G-proteins (LRG complexes). We describe the assembly of detergent-solubilized LRG complexes on beads. Rapid mix flow cytometry is used to analyze the subsecond dynamics of guanine nucleotide-mediated ternary complex disassembly. Ternary complexes were assembled with three formyl peptide receptor constructs (wild type, FPR-Galpha(i2) fusion, and FPR-GFP fusion) and two isotypes of the alpha subunit (alpha(i2) and alpha(i3)) and betagamma dimer (beta(i)(1)gamma(2) and beta(4)gamma(2)). Experimental evidence suggests that thermodynamic stability of ternary complexes depends on subunit isotype. Comparison of assemblies derived from the three constructs of FPR and G-protein heterotrimers composed of the available subunit isotypes demonstrate that the fast step is associated with the separation of receptor and G-protein and that the dissociation of the ligand or of the alpha and betagamma subunits was slower. These results are compatible with a cell activation model involving G-protein conformational changes rather than disassembly of Galphabetagamma heterotrimer.

Antagonist-radioligand binding to D2L-receptors in intact cells

D(2)-dopamine receptors mediate most of the physiological actions of dopamine and are important recognition sites for antipsychotic drugs. Earlier binding studies were predominantly done with broken cell preparations with the tritiated D(2)-receptor antagonists [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone. Here we compared [(3)H]-raclopride and [(3)H]-spiperone binding properties in intact Chinese Hamster Ovary cells stably expressing recombinant human D(2L)-receptors. Specific binding of both radioligands occurred to a comparable number of sites. In contrast to the rapid dissociation of [(3)H]-raclopride in both medium only and in the presence of an excess of unlabelled ligand [(3)H]-spiperone dissociation was only observed in the latter condition, and it was still slower than in broken cell preparations. However, this could not explain the pronounced difference in the potency of some unlabelled ligands to compete with both radioligands. To integrate these new findings, a model is proposed in which raclopride approaches the receptor from the aqueous phase, while spiperone approaches the receptor by lateral diffusion within the membrane.

Conformational changes in G-protein-coupled receptors-the quest for functionally selective conformations is open

The G-protein-coupled receptors (GPCRs) represent one the largest families of drug targets. Upon agonist binding a receptor undergoes conformational rearrangements that lead to a novel protein conformation which in turn can interact with effector proteins. During the last decade significant progress has been made to prove that different conformational changes occur. Today it is mostly accepted that individual ligands can induce different receptor conformations. However, the nature or molecular identity of the different conformations is still ill-known. Knowledge of the potential functionally selective conformations will help to develop drugs that select specific conformations of a given GPCR which couple to specific signalling pathways and may, ultimately, lead to reduced side effects. In this review we will summarize recent progress in biophysical approaches that have led to the current understanding of conformational changes that occur during GPCR activation.

Identification of potent agonists acting at an endogenous atypical beta3-adrenoceptor state that modulate lipolysis in rodent fat cells

Small molecules interacting with aminergic G-protein coupled receptors represent a number of very successful drugs. G-protein coupled receptors continue to be a significant group of targets for pharmaceutical intervention, and modifying their activity through small molecules is a major focus of drug development. Previously, these small molecules could be easily fit in models, as agonists, partial agonists or antagonists. More recently, however, these lines have been blurred as it is increasingly recognized that ligands can interact with receptors in various ways. Analysis of beta-adrenoceptors has revealed that several sites or states exist for the individual receptors. The putative atypical beta(4)-adrenoceptor identified on heart and adipose tissue is now recognized as a unique beta(1)-adrenoceptor state. Similarly, a unique beta(3)-adrenoceptor state has been identified using the aryloxypropanolamine CGP-12,177 and cloned receptor systems. Here we expand upon these observations, by describing an atypical state of the beta(3)-adrenoceptor that exists endogenously in adipose tissue. Furthermore, we describe novel arylethanolamine ligands that interact with this atypical state of the beta(3)-adrenoceptor with high affinity and provide additional tools to investigate the atypical beta(3)-adrenoceptor state to determine whether it can be influenced for therapeutic purposes.

Rapid-mix flow cytometry measurements of subsecond regulation of G protein-coupled receptor ternary complex dynamics by guanine nucleotides

We have used rapid-mix flow cytometry to analyze the early subsecond dynamics of the disassembly of ternary complexes of G protein-coupled receptors (GPCRs) immobilized on beads to examine individual steps associated with guanine nucleotide activation. Our earlier studies suggested that the slow dissociation of Galpha and Gbetagamma subunits was unlikely to be an essential component of cell activation. However, these studies did not have adequate time resolution to define precisely the disassembly kinetics. Ternary complexes were assembled using three formyl peptide receptor constructs (wild type, formyl peptide receptor-Galpha(i2) fusion, and formyl peptide receptor-green fluorescent protein fusion) and two isotypes of the alpha subunit (alpha(i2) and alpha(i3)) and betagamma dimer (beta(1)gamma(2) and beta(4)gamma(2)). At saturating nucleotide levels, the disassembly of a significant fraction of ternary complexes occurred on a subsecond time frame for alpha(i2) complexes and tau(1/2)< or =4s for alpha(i3) complexes, time scales that are compatible with cell activation. beta(1)gamma(2) isotype complexes were generally more stable than beta(4)gamma(2)-associated complexes. The comparison of the three constructs, however, proved that the fast step was associated with the separation of receptor and G protein and that the dissociation of the ligand or of the alpha and betagamma subunits was slower. These results are compatible with a cell activation model involving G protein conformational changes rather than disassembly of Galphabetagamma heterotrimer.

Agonist selectivity in the oxytocin/vasopressin receptor family: new insights and challenges

The design and development of selective agonists acting at the OT (oxytocin)/AVP (vasopressin) receptors has been and continues to be a difficult task because of the great similarity among the different receptor subtypes as well as the high degree of chemical similarity between the active ligands. In recent decades, at least a thousand synthetic peptides have been synthesized and examined for their ability to bind to and activate the different OT/AVP receptors; an effort that has led to the identification of several receptor subtype-selective agonists in the rat. However, owing to species differences between rat and human AVP/OT receptors, these peptides do not exhibit the same selectivities in human receptor assays. Furthermore, the discovery of receptor promiscuity, which is the ability of a single receptor subtype to couple to several different G-proteins, has led to the definition of a completely new class of compounds, referred to here as coupling-selective ligands, which may activate, within a single receptor subtype, only a specific signalling pathway. Finally, the accumulating evidence that GPCRs (G-protein-coupled receptors) do not function as monomers, but as dimers/oligomers, opens up the design of another class of specific ligands, bivalent ligands, in which two agonist and/or antagonist moieties are joined by a spacer of the appropriate length to allow the simultaneous binding at the two subunits within the dimer. The pharmacological properties and selectivity profiles of these bivalent ligands, which remain to be investigated, could lead to highly novel research tools and potential therapeutic agents.

Lysophospholipids of Different Classes Mobilize Neutrophil Secretory Vesicles and Induce Redundant Signaling through G2A

Lysophosphatidylcholine has been shown to enhance neutrophil functions through a mechanism involving the G protein-coupled receptor G2A. Recent data support an indirect effect of lysophosphatidylcholine on G2A rather than direct ligand binding. These observations prompted the hypothesis that other lysophospholipids (lyso-PLs) may also signal for human neutrophil activation through G2A. To this end, 1-oleoyl-2-hydroxy-sn-glycero-3-[phospho-L-choline], but also C18:1/OH lyso-PLs bearing the phosphoserine and phosphoethanolamine head groups, presented on albumin, were shown to signal for calcium flux in a self- and cross-desensitizing manner, implicating a single receptor. Blocking Abs to G2A inhibited calcium signaling by all three lyso-PLs. Furthermore, inhibition by both pertussis toxin and U-73122 established signaling via the Galphai/phospholipase C pathway for calcium mobilization. Altered plasma membrane localization of G2A has been hypothesized to facilitate signaling. Accordingly, an increase in detectable G2A was demonstrated by 1 min after lyso-PL stimulation and was followed by visible patching of the receptor. Western blotting showed that G2A resides in the plasma membrane/secretory vesicle fraction and not in neutrophil primary, secondary, or tertiary granules. Enhanced detection of G2A induced by lyso-PLs was paralleled by enhanced detection of CD45, confirming mobilization of the labile secretory vesicle pool. Together, these data show that lyso-PLs bearing various head groups redundantly mobilize G2A latent within secretory vesicles and result in G2A receptor/Galphai/phospholipase C signaling for calcium flux in neutrophils.

Structural determinants for ligand-receptor conformational selection in a peptide G protein-coupled receptor

G protein coupled receptors (GPCRs) modulate the majority of physiological processes through specific intermolecular interactions with structurally diverse ligands and activation of differential intracellular signaling. A key issue yet to be resolved is how GPCRs developed selectivity and diversity of ligand binding and intracellular signaling during evolution. We have explored the structural basis of selectivity of naturally occurring gonadotropin-releasing hormones (GnRHs) from different species in the single functional human GnRH receptor. We found that the highly variable amino acids in position 8 of the naturally occurring isoforms of GnRH play a discriminating role in selecting receptor conformational states. The human GnRH receptor has a higher affinity for the cognate GnRH I but a lower affinity for GnRH II and GnRHs from other species possessing substitutions for Arg(8). The latter were partial agonists in the human GnRH receptor. Mutation of Asn(7.45) in transmembrane domain (TM) 7 had no effect on GnRH I affinity but specifically increased affinity for other GnRHs and converted them to full agonists. Using molecular modeling and site-directed mutagenesis, we demonstrated that the highly conserved Asn(7.45) makes intramolecular interactions with a highly conserved Cys(6.47) in TM 6, suggesting that disruption of this intramolecular interaction induces a receptor conformational change which allosterically alters ligand specific binding sites and changes ligand selectivity and signaling efficacy. These results reveal GnRH ligand and receptor structural elements for conformational selection, and support co-evolution of GnRH ligand and receptor conformations.

A novel, conformation-specific allosteric inhibitor of the tachykinin NK2 receptor (NK2R) with functionally selective properties

The orthosteric agonist neurokinin A (NKA) interacts with the tachykinin NK2 receptors (NK2Rs) via an apparent sequential binding process, which stabilizes the receptor in at least two different active conformations (A1L and A2L). The A1L conformation exhibits fast NKA dissociation kinetics and triggers intracellular calcium elevation; the A2L conformation exhibits slow NKA dissociation kinetics and triggers cAMP production. The new compound LPI805 is a partial and noncompetitive inhibitor of NKA binding to NK2Rs. Analysis of NKA dissociation in the presence of LPI805 suggests that LPI805 decreases the number of NKA-NK2R complexes in A2L conformation while increasing those in the A1L conformation. Analysis of signaling pathways of NK2Rs shows that LPI805 dramatically inhibits the NKA-induced cAMP response while slightly enhancing the NKA-induced calcium response. Analysis of NKA association kinetics reveals that LPI805 promotes strong and specific destabilization of the NKA-NK2R complexes in the A2L conformation whereas access of NKA to the A1L conformations is unchanged. Thus, to our knowledge, LPI805 is the first example of a conformation-specific allosteric antagonist of a G-protein-coupled receptor. This work establishes the use of allosteric modulators in order to promote functional selectivity on certain agonist-receptor interactions.

An acquired hypocalciuric hypercalcemia autoantibody induces allosteric transition among active human Ca-sensing receptor conformations

The seven-spanning calcium-sensing receptor (CaSR) activates multiple G proteins including Gq and Gi, and thereby activates a variety of second messengers and inhibits parathyroid hormone (PTH) secretion. However, the exact signaling mechanisms underlying the functional activity of CaSR are not yet fully understood. The heterozygous inactivation of CaSR or its inhibition by antibody blocking results in either familial hypocalciuric hypercalcemia or acquired hypocalciuric hypercalcemia (AHH), respectively. Here, we report the identification of a unique CaSR autoantibody in an AHH patient. Paradoxically, we find that this autoantibody potentiates the Ca(2+)/Gq-dependent accumulation of inositol phosphates by slightly shifting the dose dependence curve of the Ca(2+) mediated activation of phosphatidylinositol turnover to the left, whereas it inhibits the Ca(2+)/Gi-dependent phosphorylation of ERK1/2 in HEK293 cells stably expressing human CaSR. Treatment of these same cells with a calcimimetic, NPS-R-568, augments the CaSR response to Ca(2+), increasing phosphatidylinositol turnover and ERK1/2 phosphorylation, and overcoming the autoantibody effects. Our observations thus indicate that a calcium-stimulated CaSR primed by a specific autoantibody adopts a unique conformation that activates Gq but not Gi. Our findings also suggest that CaSR signaling may act via both Gq and Gi to inhibit PTH secretion. This is the first report of a disease-related autoantibody that functions as an allosteric modulator and maintains G protein-coupled receptors (GPCRs) in a unique active conformation with its agonist. We thus speculate that physiological modulators may exist that enable an agonist to specifically activate only one signaling pathway via a GPCR that activates multiple signaling pathways.

Protean agonism at the dopamine D2 receptor: (S)-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of Go1 but an antagonist/inverse agonist for Gi1,Gi2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D(2) receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins could be expressed in an inducible fashion. Varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [(35)S]GTPgammaS onto each of Galpha(i1),Galpha(i2),Galpha(i3), and Galpha(o1). By contrast, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when Galpha(o1) was the target G protein but an antagonist/inverse agonist at Galpha(i1),Galpha(i2), and Galpha(i3). In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D(2) receptor-G protein fusion proteins, and the high-affinity state was eliminated by guanine nucleotide. (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine bound to an apparent single state of the constructs in which the D(2) receptor was fused to Galpha(i1),Galpha(i2), or Galpha(i3). However, it bound to distinct high- and low-affinity states of the D(2) receptor-Galpha(o1) fusion, with the high-affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D(2) receptor when expression of pertussis toxin-resistant forms of each of Galpha(i1), Galpha(i2), Galpha(i3), and Galpha(o1) was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of Galpha(o1). p-Tyramine displayed a protean ligand profile similar to that of (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine but with lower potency. These results demonstrate (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine to be a protean agonist at the D(2) receptor and may explain in vivo actions of this ligand.

Thyrotropin-releasing hormone and its receptors — A hypothesis for binding and receptor activation

Thyrotropin-releasing hormone (TRH), a tripeptide, exerts its biological effects through stimulation of cell-surface receptors, TRH-R, belonging to the superfamily of G protein-coupled receptors (GPCR). Because of the intermediate size of TRH, it is smaller than polypeptide ligands that interact at GPCR ectodomains and larger than biogenic amines, which interact within GPCR transmembrane domains (TMD), the TRH/TRH-R complex probably shares properties of these 2 extremes, representing a unique system to study GPCR/ligand interactions. In this review, we summarize the current knowledge of the structure-activity relationships in the TRH/TRH-R system. Based on experimental data and the structural information acquired from computer simulations, we formulate a working hypothesis to describe the molecular events underlying the processes of TRH binding and TRH-R activation. This hypothesis represents a starting point for understanding the biology of the TRH/TRH-R system on a molecular level and provides a basis for potential design of new potent and selective modulators of TRH-R's activity.

Effects of N- and C-terminal addition of oligolysines or native loop residues on the biophysical properties of transmembrane domain peptides from a G-protein coupled receptor

Transmembrane domains (TMDs) of G-protein coupled receptors (GPCRs) have very low water solubility and often aggregate during purification and biophysical investigations. To circumvent this problem many laboratories add oligolysines to the N- and C-termini of peptides that correspond to a TMD. To systematically evaluate the effect of the oligolysines on the biophysical properties of a TMD we synthesized 21 peptides corresponding to either the second (TPIFIINQVSLFLIILHSALYFKY) or sixth (SFHILLIMSSQSLLVPSIIFILAYSLK) TMD of Ste2p, a GPCR from Saccharomyces cerevisiae. Added to the termini of these peptides were either Lys(n) (n = 1,2,3) or the corresponding native loop residues. The biophysical properties of the peptides were investigated by circular dichroism (CD) spectroscopy in trifluoroethanol-water mixtures, sodium dodecyl sulfate (SDS) micelles and dimyristoylphosphocholine (DMPC)-dimyristoylphosphoglycerol (DMPG) vesicles, and by attenuated total reflection Fourier transform infrared (ATR-FTIR) in DMPC/DMPG multilayers. The results show that the conformation assumed depends on the number of lysine residues and the sequence of the TMD. Identical peptides with native or an equal number of lysine residues exhibited different biophysical properties and structural tendencies.

The second extracellular loop of CCR5 contains the dominant epitopes for highly potent anti-human immunodeficiency virus monoclonal antibodies

Six mouse anti-human CCR5 monoclonal antibodies (mAbs) that showed potent antiviral activities were identified from over 26,000 mouse hybridomas. The epitopes for these mAbs were determined by using various CCR5 mutants, including CCR5/CCR2B chimeras. One mAb, ROAb13, was found to bind to a linear epitope in the N terminus of CCR5. Strikingly, the other five mAbs bind to epitopes derived from extracellular loop 2 (ECL2). The three most potent mAbs, ROAb12, ROAb14, and ROAb18, require residues from both the N-terminal (Lys171 and Glu172) and C-terminal (Trp190) halves of ECL2 for binding; two other mAbs, ROAb10 and ROAb51, which also showed potent antiviral activities, require Lys171 and Glu172 but not Trp190 for binding. Binding of the control mAb 2D7 completely relies on Lys171 and Glu172. Unlike 2D7, the novel mAbs ROAb12, ROAb14, and ROAb18 do not bind to the linear peptide 2D7-2SK. In addition, all three mAbs bind to monkey CCR5 (with Arg at position 171 instead of Lys); however, 2D7 does not. Since five of the six most potent CCR5 mAbs derived from the same pool of immunized mice require ECL2 as epitopes, we hypothesize that CCR5 ECL2 contains the dominant epitopes for mAbs with potent antiviral activities. These dominant epitopes were found in CCR5 from multiple species and were detected in large proportions of the total cell surface CCR5. mAbs recognizing these epitopes also showed high binding affinity. A homology model of CCR5 was generated to aid in the interpretation of these dominant epitopes in ECL2.

Some mechanistic insights into GPCR activation from detergent-solubilized ternary complexes on beads

The binding of full and partial agonist ligands (L) to G protein-coupled receptors (GPCRs) initiates the formation of ternary complexes with G proteins [ligand-receptor-G protein (LRG) complexes]. Cyclic ternary complex models are required to account for the thermodynamically plausible complexes. It has recently become possible to assemble solubilized formyl peptide receptor (FPR) and beta(2)-adrenergic receptor (beta(2)AR) ternary complexes for flow cytometric bead-based assays. In these systems, soluble ternary complex formation of the receptors with G proteins allows direct quantitative measurements which can be analyzed in terms of three-dimensional concentrations (molarity). In contrast to the difficulty of analyzing comparable measurements in two-dimensional membrane systems, the output of these flow cytometric experiments can be analyzed via ternary complex simulations in which all of the parameters can be estimated. An outcome from such analysis yielded lower affinity for soluble ternary complex assembly by partial agonists compared with full agonists for the beta(2)AR. In the four-sided ternary complex model, this behavior is consistent with distinct ligand-induced conformational states for full and partial agonists. Rapid mix flow cytometry is used to analyze the subsecond dynamics of guanine nucleotide-mediated ternary complex disassembly. The modular breakup of ternary complex components is highlighted by the finding that the fastest step involves the departure of the ligand-activated GPCR from the intact G protein heterotrimer. The data also show that, under these experimental conditions, G protein subunit dissociation does not occur within the time frame relevant to signaling. The data and concepts are discussed in the context of a review of current literature on signaling mechanism based on structural and spectroscopic (FRET) studies of ternary complex components.