A mutation-induced activated state of the beta 2-adrenergic receptor. Extending the ternary complex model

G Protein-Coupled Receptor Systems and Their Role in Cellular Senescence

Aging is a complex biological process that is inevitable for nearly all organisms. Aging is the strongest risk factor for development of multiple neurodegenerative disorders, cancer and cardiovascular disorders. Age-related disease conditions are mainly caused by the progressive degradation of the integrity of communication systems within and between organs. This is in part mediated by, i) decreased efficiency of receptor signaling systems and ii) an increasing inability to cope with stress leading to apoptosis and cellular senescence. Cellular senescence is a natural process during embryonic development, more recently it has been shown to be also involved in the development of aging disorders and is now considered one of the major hallmarks of aging. G-protein-coupled receptors (GPCRs) comprise a superfamily of integral membrane receptors that are responsible for cell signaling events involved in nearly every physiological process. Recent advances in the molecular understanding of GPCR signaling complexity have expanded their therapeutic capacity tremendously. Emerging data now suggests the involvement of GPCRs and their associated proteins in the development of cellular senescence. With the proven efficacy of therapeutic GPCR targeting, it is reasonable to now consider GPCRs as potential platforms to control cellular senescence and the consequently, age-related disorders.

Molecular pharmacology of metabotropic receptors targeted by neuropsychiatric drugs

Metabotropic receptors are responsible for so-called 'slow synaptic transmission' and mediate the effects of hundreds of peptide and non-peptide neurotransmitters and neuromodulators. Over the past decade or so, a revolution in membrane-protein structural determination has clarified the molecular determinants responsible for the actions of these receptors. This Review focuses on the G protein-coupled receptors (GPCRs) that are targets of neuropsychiatric drugs and shows how insights into the structure and function of these important synaptic proteins are accelerating understanding of their actions. Notably, elucidating the structure and function of GPCRs should enhance the structure-guided discovery of novel chemical tools with which to manipulate and understand these synaptic proteins.

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.

Prescient Indices of Activity: The Application of Functional System Sensitivity to Measurement of Drug Effect

Through pharmacological procedures, indices of drug activity can be obtained that transcend the systems in which they are measured. If (i) affinity, (ii) efficacies, (iii) orthosteric versus allosteric interaction, and (iv) rate of receptor offset can be determined, activity can be predicted in all systems. This can yield more detailed profiles (fingerprints) of efficacy to better define the required activities of follow-up molecules should the original candidates fail in the clinic. The use of functional assays of varying sensitivity is a major tool in the lead optimization process and the observation of candidate molecule profiles in multiple functional assays can reveal all properties of candidate molecules. In this review, the different indices for agonists, antagonists, and allosteric modulators are defined while highlighting the application of functional assays in deriving these indices.

Differentiating between Inactive and Active States of Rhodopsin by Atomic Force Microscopy in Native Membranes

Membrane proteins, including G protein-coupled receptors (GPCRs), present a challenge in studying their structural properties under physiological conditions. Moreover, to better understand the activity of proteins requires examination of single molecule behaviors rather than ensemble averaged behaviors. Force-distance curve-based AFM (FD-AFM) was utilized to directly probe and localize the conformational states of a GPCR within the membrane at nanoscale resolution based on the mechanical properties of the receptor. FD-AFM was applied to rhodopsin, the light receptor and a prototypical GPCR, embedded in native rod outer segment disc membranes from photoreceptor cells of the retina in mice. Both FD-AFM and computational studies on coarse-grained models of rhodopsin revealed that the active state of the receptor has a higher Young's modulus compared to the inactive state of the receptor. Thus, the inactive and active states of rhodopsin could be differentiated based on the stiffness of the receptor. Differentiating the states based on the Young's modulus allowed for the mapping of the different states within the membrane. Quantifying the active states present in the membrane containing the constitutively active G90D rhodopsin mutant or apoprotein opsin revealed that most receptors adopt an active state. Traditionally, constitutive activity of GPCRs has been described in terms of two-state models where the receptor can achieve only a single active state. FD-AFM data are inconsistent with a two-state model but instead require models that incorporate multiple active states.

The role of NMR spectroscopy in mapping the conformational landscape of GPCRs

Over recent years, nuclear magnetic resonance (NMR) spectroscopy has developed into a powerful mechanistic tool for the investigation of G protein-coupled receptors (GPCRs). NMR provides insights which underpin the dynamic nature of these important receptors and reveals experimental evidence for a complex conformational energy landscape that is explored during receptor activation resulting in signalling. NMR studies have highlighted both the dynamic properties of different receptor states as well as the exchange pathways and intermediates formed during activation, extending the static view of GPCRs obtained from other techniques. NMR studies can be undertaken in realistic membrane-like phospholipid environments and an ever-increasing choice of labelling strategies provides comprehensive, receptor-wide information. Combined with other structural methods, NMR is contributing to our understanding of allosteric signal propagation and the interaction of GPCRs with intracellular binding partners (IBP), crucial to explaining cellular signalling.

The structural basis of the arrestin binding to GPCRs

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins targeted by more clinically used drugs than any other protein family. GPCR signaling via G proteins is quenched (desensitized) by the phosphorylation of the active receptor by specific GPCR kinases (GRKs) followed by tight binding of arrestins to active phosphorylated receptors. Thus, arrestins engage two types of receptor elements: those that contain GRK-added phosphates and those that change conformation upon activation. GRKs attach phosphates to serines and threonines in the GPCR C-terminus or any one of the cytoplasmic loops. In addition to these phosphates, arrestins engage the cavity that appears between trans-membrane helices upon receptor activation and several other non-phosphorylated elements. The residues that bind GPCRs are localized on the concave side of both arrestin domains. Arrestins undergo a global conformational change upon receptor binding (become activated). Arrestins serve as important hubs of cellular signaling, emanating from activated GPCRs and receptor-independent.

Microswitches for the Activation of the Nociceptin Receptor Induced by Cebranopadol: Hints from Microsecond Molecular Dynamics

Cebranopadol (CBP) is a novel analgesic acting as agonist at the nociceptin (NOP) and μ-opioid (MOP) receptors, exhibiting high potency and efficacy as an antinociceptive and antihypersensitive drug. The binding conformation and the dynamical interactions of CBP with the NOP receptor have been investigated by molecular docking, molecular dynamics (MD) in the microsecond time scale, and hybrid quantum mechanics/molecular mechanics (QM/MM). CBP binds to the NOP receptor as a bidentate ligand of the aspartate D130^3,32 by means of both its fluoroindole and dimethyl nitrogens. Starting from the known crystal structure of the inactive state of the receptor, in complex with the antagonist compound-24 (NOP-C24) the comparative analysis of 1 μs MD trajectories of the NOP-C24 complex itself and the NOP_free and NOP-CBP complexes provides new insights on the already known microswitches related to receptor activation, in the frame of the extended ternary complex model. The agonist acts by destabilizing the inactive conformation of the NOP receptor, by inducing a conformational change of M134^3,36, which allows W276^6,48 to flip around its χ₂ dihedral, going in close proximity to the receptor hydrophobic core (T138^3,40, P227^5,50, F272^6,44), which is known to be fundamental for the activation of the opioid receptors. A complete rational picture is also provided for the role of N133^3,35 and W276^6,48 undergoing critical conformational changes related to an anticooperativity effect, i.e. the well-known role of sodium as negative modulator of agonist binding. Finally, the movement of residue Y319^7,53 belonging to the NPxxY motif is also induced by the binding of the agonist in the inactive state, opening a gate for a water channel just as upon receptor activation.

Emergent Concepts of Receptor Pharmacology

Pharmacology, the chemical control of physiology, emerged as an offshoot of physiology when the physiologists using chemicals to probe physiological systems became more interested in the probes than the systems. Pharmacologists were always, and in many ways still are, bound to study drugs in systems they do not fully understand. Under these circumstances, null methods were the main ways in which conclusions about biologically active molecules were made. However, as understanding of the basic mechanisms of cellular function and biochemical systems were elucidated, so too did the understanding of how drugs affected these systems. Over the past 20 years, new ideas have emerged in the field that have completely changed and revitalized it; these are described herein. It will be seen how null methods in isolated tissues gave way to, first biochemical radioligand binding studies, and then to a wide array of functional assay technologies that can measure the effects of molecules on drug targets. In addition, the introduction of molecular dynamics, the appreciation of the allosteric nature of receptors, protein X-ray crystal structures, genetic manipulations in the form of knock-out and knock-in systems and Designer Receptors Exclusively Activated by Designer Drugs have revolutionized pharmacology.

Hot spots for GPCR signaling: lessons from single-molecule microscopy

G protein-coupled receptors (GPCRs) are among the best-studied membrane receptors, mainly due to their central role in human physiology, involvement in disease and relevance as drug targets. Although biochemical and pharmacological studies have characterized the main steps in GPCR signaling, how GPCRs produce highly specific responses in our cells remains insufficiently understood. New developments in single-molecule microscopy have made it possible to study the protein-protein interactions at the basis of GPCR signaling in previously inconceivable detail. Using this approach, it was recently possible to follow individual receptors and G proteins as they diffuse, interact and signal on the surface of living cells. This has revealed hot spots on the plasma membrane, where receptors and G proteins undergo transient interactions to produce rapid and local signals. Overall, these recent findings reveal a high degree of dynamicity and complexity in signaling by GPCRs, which provides a new basis to understand how these important receptors produce specific effects and might pave the way to innovative pharmacological approaches.

G Protein-Coupled Receptor Systems as Crucial Regulators of DNA Damage Response Processes

G protein-coupled receptors (GPCRs) and their associated proteins represent one of the most diverse cellular signaling systems involved in both physiological and pathophysiological processes. Aging represents perhaps the most complex biological process in humans and involves a progressive degradation of systemic integrity and physiological resilience. This is in part mediated by age-related aberrations in energy metabolism, mitochondrial function, protein folding and sorting, inflammatory activity and genomic stability. Indeed, an increased rate of unrepaired DNA damage is considered to be one of the 'hallmarks' of aging. Over the last two decades our appreciation of the complexity of GPCR signaling systems has expanded their functional signaling repertoire. One such example of this is the incipient role of GPCRs and GPCR-interacting proteins in DNA damage and repair mechanisms. Emerging data now suggest that GPCRs could function as stress sensors for intracellular damage, e.g., oxidative stress. Given this role of GPCRs in the DNA damage response process, coupled to the effective history of drug targeting of these receptors, this suggests that one important future activity of GPCR therapeutics is the rational control of DNA damage repair systems.

The Role of CXCR3 and Its Chemokine Ligands in Skin Disease and Cancer

Chemokines and their receptors play an important role in the recruitment, activation and differentiation of immune cells. The chemokine receptor, CXCR3, and its ligands, CXCL9, CXCL10, and CXCL11 are key immune chemoattractants during interferon-induced inflammatory responses. Inflammation of the skin resulting from infections or autoimmune disease drives expression of CXCL9/10/11 and the subsequent recruitment of effector, CXCR3⁺ T cells from the circulation. The relative contributions of the different CXCR3 chemokines and the three variant isoforms of CXCR3 (CXCR3A, CXCR3B, CXCR3alt) to the inflammatory process in human skin requires further investigation. In skin cancers, the CXCR3 receptor can play a dual role whereby expression on tumor cells can lead to cancer metastasis to systemic sites while receptor expression on immune cells can frequently promote anti-tumor immune responses. This review will discuss the biology of CXCR3 and its associated ligands with particular emphasis on the skin during inflammation and carcinogenesis.

G protein-coupled receptor kinases (GRKs) orchestrate biased agonism at the β2-adrenergic receptor

Biased agonists of G protein-coupled receptors (GPCRs), which selectively activate either G protein- or β-arrestin-mediated signaling pathways, are of major therapeutic interest because they have the potential to show improved efficacy and specificity as drugs. Efforts to understand the mechanistic basis of this phenomenon have focused on the hypothesis that G proteins and β-arrestins preferentially couple to distinct GPCR conformations. However, because GPCR kinase (GRK)-dependent receptor phosphorylation is a critical prerequisite for the recruitment of β-arrestins to most GPCRs, GRKs themselves may play an important role in establishing biased signaling. We showed that an alanine mutant of the highly conserved residue tyrosine 219 (Y219A) in transmembrane domain five of the β₂-adrenergic receptor (β₂AR) was incapable of β-arrestin recruitment, receptor internalization, and β-arrestin-mediated activation of extracellular signal-regulated kinase (ERK), whereas it retained the ability to signal through G protein. We found that the impaired β-arrestin recruitment in cells was due to reduced GRK-mediated phosphorylation of the β₂AR Y219A C terminus, which was recapitulated in vitro with purified components. Furthermore, in vitro ligation of a synthetically phosphorylated peptide onto the C terminus of β₂AR Y219A rescued both the initial recruitment of β-arrestin and its engagement with the intracellular core of the receptor. These data suggest that the Y219A mutation generates a G protein-biased state primarily by conformational selection against GRK coupling, rather than against β-arrestin. Together, these findings highlight the importance of GRKs in modulating the biased agonism of GPCRs.

PAM-Antagonists: A Better Way to Block Pathological Receptor Signaling?

Seven transmembrane receptor (7TMR) responses are modulated by orthosteric and allosteric ligands to great therapeutic advantage. Here we introduce a unique class of negative allosteric modulator (NAM) - the positive allosteric modulator (PAM)-antagonist - that increases the affinity of the receptor for the agonist but concomitantly decreases agonist efficacy when cobound. Notably, the reciprocation of allosteric energy causes the orthosteric agonist to increase the affinity of the receptor for the PAM-antagonist; thus, this modulator seeks out and destroys agonist-bound receptor complexes. When contrasted with standard orthosteric and allosteric antagonists it is clear that PAM-antagonists are uniquely well suited to reversing ongoing persistent agonism and provide favorable target coverage in vivo. Specifically, the therapeutic application of PAM-antagonists to reverse pathological overactivation (e.g., endothelin vasoconstriction) is emphasized.

GPCRs and Signal Transducers: Interaction Stoichiometry

Until the late 1990s, class A G protein-coupled receptors (GPCRs) were believed to function as monomers. Indirect evidence that they might internalize or even signal as dimers has emerged, along with proof that class C GPCRs are obligatory dimers. Crystal structures of GPCRs and their much larger binding partners were consistent with the idea that two receptors might engage a single G protein, GRK, or arrestin. However, recent biophysical, biochemical, and structural evidence invariably suggests that a single GPCR binds G proteins, GRKs, and arrestins. Here we review existing evidence of the stoichiometry of GPCR interactions with signal transducers and discuss potential biological roles of class A GPCR oligomers, including proposed homo- and heterodimers.

Amphetamine-Induced Striatal Dopamine Release Measured With an Agonist Radiotracer in Schizophrenia

BACKGROUND

Receptor imaging studies have reported increased amphetamine-induced dopamine release in subjects with schizophrenia (SCH) relative to healthy control subjects (HCs). A limitation of these studies, performed with D2/3 antagonist radiotracers, is the failure to provide information about D2/3 receptors configured in a state of high affinity for the agonists (i.e., D2/3 receptors coupled to G proteins [D2/3 HIGH]). The endogenous agonist dopamine binds with preference to D2/3 HIGH receptors relative to D2/3 LOW receptors, making it critical to understand the status of D2/3 HIGH receptors in SCH.

METHODS

D2/3 agonist positron emission tomography radiotracer [11C]N-propyl-norapomorphine ([11C]NPA) binding potential (BPND) was measured in 14 off-medication subjects with SCH and 14 matched HCs at baseline and after the administration of 0.5 mg kg-1 oral D-amphetamine. The amphetamine-induced change in BPND (ΔBPND) was calculated as the difference between BPND in the postamphetamine condition and BPND in the baseline condition and was expressed as a percentage of BPND at baseline.

RESULTS

A trend-level increase was observed in comparing baseline [11C]NPA BPND (repeated-measures analysis of variance, F1,26 = 3.34, p = .08) between the SCH and HC groups. Amphetamine administration significantly decreased BPND in all striatal regions across all subjects in both groups. No differences were observed in [11C]NPA ΔBPND (repeated-measures analysis of variance, F1,26 = 1.9, p = .18) between HCs and subjects with SCH. Amphetamine significantly increased positive symptoms in subjects with SCH (19.5 ± 5.3 vs. 23.7 ± 4.1, paired t test, p < .0001); however, no correlations were noted with [11C]NPA BPND or ΔBPND.

CONCLUSIONS

This study provides in vivo indication of a role for postsynaptic factors in amphetamine-induced psychosis in SCH.

Thyrotropin receptor, still much to be learned from the patients

In the absence of crystal available for the full-length thyrotropin receptor, knowledge of its structure and functioning has benefitted from the identification and characterization of mutations in patients with various thyroid dysfunctions. The characterization of activating mutations has contributed to the elaboration of a model involving the extracellular domain of the receptor as an inverse tethered agonist which, upon binding of the ligand, relieves the transmembrane domain from an inhibiting interaction and activates it. The models derived from comparisons with other receptors, enriched with the information provided by the study of mutations, have proven useful for the design of small-molecule agonists and antagonists that may be used in the future to treat thyroid dysfunctions. In this review, extrathyroidal expression of the thyrotropin receptor is described, the role of which is still poorly defined.

The N54-αs Mutant Has Decreased Affinity for βγ and Suggests a Mechanism for Coupling Heterotrimeric G Protein Nucleotide Exchange with Subunit Dissociation

Ser54 of G_sα binds guanine nucleotide and Mg²⁺ as part of a conserved sequence motif in GTP binding proteins. Mutating the homologous residue in small and heterotrimeric G proteins generates dominant-negative proteins, but by protein-specific mechanisms. For α_i/o, this results from persistent binding of α to βγ, whereas for small GTP binding proteins and α_s this results from persistent binding to guanine nucleotide exchange factor or receptor. This work examined the role of βγ interactions in mediating the properties of the Ser54-like mutants of Gα subunits. Unexpectedly, WT-α_s or N54-α_s coexpressed with α_1B-adrenergic receptor in human embryonic kidney 293 cells decreased receptor stimulation of IP3 production by a cAMP-independent mechanism, but WT-α_s was more effective than the mutant. One explanation for this result would be that α_s, like Ser47 α_i/o, blocks receptor activation by sequestering βγ; implying that N54-α_S has reduced affinity for βγ since it was less effective at blocking IP3 production. This possibility was more directly supported by the observation that WT-α_s was more effective than the mutant in inhibiting βγ activation of phospholipase Cβ2. Further, in vitro synthesized N54-α_s bound biotinylated-βγ with lower apparent affinity than did WT-α_s The Cys54 mutation also decreased βγ binding but less effectively than N54-α_s Substitution of the conserved Ser in α_o with Cys or Asn increased βγ binding, with the Cys mutant being more effective. This suggests that Ser54 of α_s is involved in coupling changes in nucleotide binding with altered subunit interactions, and has important implications for how receptors activate G proteins.

Kinetic operational models of agonism for G-protein-coupled receptors

The application of kinetics to research and therapeutic development of G-protein-coupled receptors has become increasingly valuable. Pharmacological models provide the foundation of pharmacology, providing concepts and measurable parameters such as efficacy and potency that have underlain decades of successful drug discovery. Currently there are few pharmacological models that incorporate kinetic activity in such a way as to yield experimentally-accessible drug parameters. In this study, a kinetic model of pharmacological response was developed that provides a kinetic descriptor of efficacy (the transduction rate constant, k_τ) and allows measurement of receptor-ligand binding kinetics from functional data. The model assumes: (1) receptor interacts with a precursor of the response ("Transduction potential") and converts it to the response. (2) The response can decay. Familiar response vs time plots emerge, depending on whether transduction potential is depleted and/or response decays. These are the straight line, the "association" exponential curve, and the rise-and-fall curve. Convenient, familiar methods are described for measuring the model parameters and files are provided for the curve-fitting program Prism (GraphPad Software) that can be used as a guide. The efficacy parameter k_τ is straightforward to measure and accounts for receptor reserve; all that is required is measurement of response over time at a maximally-stimulating concentration of agonist. The modular nature of the model framework allows it to be extended. Here this is done to incorporate antagonist-receptor binding kinetics and slow agonist-receptor equilibration. In principle, the modular framework can incorporate other cellular processes, such as receptor desensitization. The kinetic response model described here can be applied to measure kinetic pharmacological parameters than can be used to advance the understanding of GPCR pharmacology and optimize new and improved therapeutics.

Calcium receptor signaling and citrate transport

The calcium sensing receptor (CaSR) in the distal nephron decreases the propensity for calcium stones. Here we investigate if the apical CaSR in the proximal tubule also prevents stone formation acting via regulation of apical dicarboxylate and citrate transport. Urinary citrate, partially reabsorbed as a dicarboxylate in the proximal tubule lumen, inhibits stone formation by complexing calcium. We previously demonstrated a novel apical calcium-sensitive dicarboxylate transport system in OK proximal tubule cells. This calcium-sensitive process has the potential to modulate the amount of citrate available to complex increased urinary calcium. Using isotope labeled succinate uptake in OK cells along with various pharmacologic tools we examined whether the CaSR alters apical dicarboxylate transport and through which signal transduction pathways this occurs. Our results indicate that in the proximal tubule CaSR adjusts apical dicarboxylate transport, and does so via a CaSR → G_q → PKC signaling pathway. Thus, the CaSR may decrease the propensity for stone formation via actions in both proximal and distal nephron segments.

Structure of the Nanobody-Stabilized Active State of the Kappa Opioid Receptor

The κ-opioid receptor (KOP) mediates the actions of opioids with hallucinogenic, dysphoric, and analgesic activities. The design of KOP analgesics devoid of hallucinatory and dysphoric effects has been hindered by an incomplete structural and mechanistic understanding of KOP agonist actions. Here, we provide a crystal structure of human KOP in complex with the potent epoxymorphinan opioid agonist MP1104 and an active-state-stabilizing nanobody. Comparisons between inactive- and active-state opioid receptor structures reveal substantial conformational changes in the binding pocket and intracellular and extracellular regions. Extensive structural analysis and experimental validation illuminate key residues that propagate larger-scale structural rearrangements and transducer binding that, collectively, elucidate the structural determinants of KOP pharmacology, function, and biased signaling. These molecular insights promise to accelerate the structure-guided design of safer and more effective κ-opioid receptor therapeutics.

Gγ identity dictates efficacy of Gβγ signaling and macrophage migration

G protein βγ subunit (Gβγ) is a major signal transducer and controls processes ranging from cell migration to gene transcription. Despite having significant subtype heterogeneity and exhibiting diverse cell- and tissue-specific expression levels, Gβγ is often considered a unified signaling entity with a defined functionality. However, the molecular and mechanistic basis of Gβγ's signaling specificity is unknown. Here, we demonstrate that Gγ subunits, bearing the sole plasma membrane (PM)-anchoring motif, control the PM affinity of Gβγ and thereby differentially modulate Gβγ effector signaling in a Gγ-specific manner. Both Gβγ signaling activity and the migration rate of macrophages are strongly dependent on the PM affinity of Gγ. We also found that the type of C-terminal prenylation and five to six pre-CaaX motif residues at the PM-interacting region of Gγ control the PM affinity of Gβγ. We further show that the overall PM affinity of the Gβγ pool of a cell type is a strong predictor of its Gβγ signaling-activation efficacy. A kinetic model encompassing multiple Gγ types and parameterized for empirical Gβγ behaviors not only recapitulated experimentally observed signaling of Gβγ, but also suggested a Gγ-dependent, active-inactive conformational switch for the PM-bound Gβγ, regulating effector signaling. Overall, our results unveil crucial aspects of signaling and cell migration regulation by Gγ type-specific PM affinities of Gβγ.

A Serendipitous Scientist

Growing up in a middle-class Jewish home in the Bronx, I had only one professional goal: to become a physician. However, as with most of my Vietnam-era MD colleagues, I found my residency training interrupted by the Doctor Draft in 1968. Some of us who were academically inclined fulfilled this obligation by serving in the US Public Health Service as commissioned officers stationed at the National Institutes of Health. This experience would eventually change the entire trajectory of my career. Here I describe how, over a period of years, I transitioned from the life of a physician to that of a physician-scientist; my 50 years of work on cellular receptors; and some miscellaneous thoughts on subjects as varied as Nobel prizes, scientific lineages, mentoring, publishing, and funding.

Biased signalling: from simple switches to allosteric microprocessors

G protein-coupled receptors (GPCRs) are the largest class of receptors in the human genome and some of the most common drug targets. It is now well established that GPCRs can signal through multiple transducers, including heterotrimeric G proteins, GPCR kinases and β-arrestins. While these signalling pathways can be activated or blocked by 'balanced' agonists or antagonists, they can also be selectively activated in a 'biased' response. Biased responses can be induced by biased ligands, biased receptors or system bias, any of which can result in preferential signalling through G proteins or β-arrestins. At many GPCRs, signalling events mediated by G proteins and β-arrestins have been shown to have distinct biochemical and physiological actions from one another, and an accurate evaluation of biased signalling from pharmacology through physiology is crucial for preclinical drug development. Recent structural studies have provided snapshots of GPCR-transducer complexes, which should aid in the structure-based design of novel biased therapies. Our understanding of GPCRs has evolved from that of two-state, on-and-off switches to that of multistate allosteric microprocessors, in which biased ligands transmit distinct structural information that is processed into distinct biological outputs. The development of biased ligands as therapeutics heralds an era of increased drug efficacy with reduced drug side effects.

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

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

Molecular Mechanisms of GPCR Signaling: A Structural Perspective

G protein-coupled receptors (GPCRs) are cell surface receptors that respond to a wide variety of stimuli, from light, odorants, hormones, and neurotransmitters to proteins and extracellular calcium. GPCRs represent the largest family of signaling proteins targeted by many clinically used drugs. Recent studies shed light on the conformational changes that accompany GPCR activation and the structural state of the receptor necessary for the interactions with the three classes of proteins that preferentially bind active GPCRs, G proteins, G protein-coupled receptor kinases (GRKs), and arrestins. Importantly, structural and biophysical studies also revealed activation-related conformational changes in these three types of signal transducers. Here, we summarize what is already known and point out questions that still need to be answered. Clear understanding of the structural basis of signaling by GPCRs and their interaction partners would pave the way to designing signaling-biased proteins with scientific and therapeutic potential.

GPR62 constitutively activates cAMP signaling but is dispensable for male fertility in mice

G-protein-coupled receptors (GPCRs) participate in diverse physiological functions and are promising targets for drug discovery. However, there are still over 140 orphan GPCRs whose functions remain to be elucidated. Gpr62 is one of the orphan GPCRs that is expressed in the rat and human brain. In this study, we found that Gpr62 is also expressed in male germ cells in mice, and its expression increases along with sperm differentiation. GPR62 lacks the BBXXB and DRY motifs, which are conserved across many GPCRs, and it was able to induce cAMP signaling in the absence of a ligand. These structural and functional features are conserved among mammals, and the mutant analysis of GPR62 has revealed that lacking of these motifs is involved in the constitutive activity. We also found that GPR62 can homooligomerize, but it is not sufficient for its constitutive activity. We further investigated its physiological function by using Gpr62 knockout (Gpr62^-/-) mice. Gpr62^-/- mice were born normally and did not show any abnormality in growth and behavior. In addition, both male and female Gp62^-/- mice were fertile, and the differentiation and motility of spermatozoa were normal. We also found that Gpr61, the gene most similar to Gpr62 in the GPCR family shows a constitutive activity and an expression pattern similar to those of Gpr62 Our results suggest that GPR62 constitutively activates the cAMP pathway in male germ cells but is dispensable for male fertility, which is probably due to its functional redundancy with GPR61.

How Ligands Illuminate GPCR Molecular Pharmacology

G protein-coupled receptors (GPCRs), which are modulated by a variety of endogenous and synthetic ligands, represent the largest family of druggable targets in the human genome. Recent structural and molecular studies have both transformed and expanded classical concepts of receptor pharmacology and have begun to illuminate the distinct mechanisms by which structurally, chemically, and functionally diverse ligands modulate GPCR function. These molecular insights into ligand engagement and action have enabled new computational methods and accelerated the discovery of novel ligands and tool compounds, especially for understudied and orphan GPCRs. These advances promise to streamline the development of GPCR-targeted medications.

Comparison of the Human A2A Adenosine Receptor Recognition by Adenosine and Inosine: New Insight from Supervised Molecular Dynamics Simulations

Adenosine deaminase converts adenosine into inosine. In contrast to adenosine, relatively little attention has been paid to the physiological roles of inosine. Nevertheless, recent studies have demonstrated that inosine has neuroprotective, cardioprotective, immunomodulatory, and antidepressive effects. Inosine was recently shown to be a less potent agonist than adenosine at the A_2A adenosine receptor. To better depict the differences in the mechanisms of receptor recognition between adenosine and inosine, we carried out supervised molecular dynamics (SuMD) simulations, and the results are analyzed herein.

Translating in vitro ligand bias into in vivo efficacy

It is increasingly apparent that ligand structure influences both the efficiency with which G protein-coupled receptors (GPCRs) engage their downstream effectors and the manner in which they are activated. Thus, 'biased' agonists, synthetic ligands whose intrinsic efficacy differs from the native ligand, afford a strategy for manipulating GPCR signaling in ways that promote beneficial signals while blocking potentially deleterious ones. Still, there are significant challenges in relating in vitro ligand efficacy, which is typically measured in heterologous expression systems, to the biological response in vivo, where the ligand is acting on natively expressed receptors and in the presence of the endogenous ligand. This is particularly true of arrestin pathway-selective 'biased' agonists. The type 1 parathyroid hormone receptor (PTH₁R) is a case in point. Parathyroid hormone (PTH) is the principal physiological regulator of calcium homeostasis, and PTH₁R expressed on cells of the osteoblast lineage are an established therapeutic target in osteoporosis. In vitro, PTH₁R signaling is highly sensitive to ligand structure, and PTH analogs that affect the selectivity/kinetics of G protein coupling or that engage arrestin-dependent signaling mechanisms without activating heterotrimeric G proteins have been identified. In vivo, intermittent administration of conventional PTH analogs accelerates the rate of osteoblastic bone formation, largely through known cAMP-dependent mechanisms. Paradoxically, both intermittent and continuous administration of an arrestin pathway-selective PTH analog, which in vivo would be expected to antagonize endogenous PTH₁R-cAMP signaling, also increases bone mass. Transcriptomic analysis of tissue from treated animals suggests that conventional and arrestin pathway-selective PTH1R ligands act in largely different ways, with the latter principally affecting pathways involved in the regulation of cell cycle, survival, and migration/cytoskeletal dynamics. Such multi-dimensional in vitro and in vivo analyses of ligand bias may provide insights into the physiological roles of non-canonical arrestin-mediated signaling pathways in vivo, and provide a conceptual framework for translating arrestin pathway-selective ligands into viable therapeutics.

Biased signaling at neural melanocortin receptors in regulation of energy homeostasis

The global prevalence of obesity highlights the importance of understanding on regulation of energy homeostasis. The central melanocortin system is an important intersection connecting the neural pathways controlling satiety and energy expenditure to regulate energy homeostasis by sensing and integrating the signals of external stimuli. In this system, neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play crucial roles in the regulation of energy homeostasis. Recently, multiple intracellular signaling pathways and biased signaling at neural MCRs have been discovered, providing new insights into neural MCR signaling. This review attempts to summarize biased signaling including biased receptor mutants (both naturally occurring and lab-generated) and biased ligands at neural MCRs, and to provide a better understanding of obesity pathogenesis and new therapeutic implications for obesity treatment.

The DRY motif and the four corners of the cubic ternary complex model

Recent structural data on GPCRs using a variety of spectroscopic approaches suggest that GPCRs adopt a dynamic conformational landscape, with ligands stabilizing subsets of these states to activate one or more downstream signaling effectors. A key outstanding question posed by this emerging dynamic structural model of GPCRs is what states, active, inactive, or intermediate are captured by the numerous crystal structures of GPCRs complexed with a variety of agonists, partial agonists, and antagonists. In the early nineties the discovery of inverse agonists and constitutive activity led to the idea that the active receptor state (R^⁎) is an intrinsic property of the receptor itself rather than of the RG complex, eventually leading to the formulation of the cubic ternary complex model (CTC). Here, by a careful analysis of a series of data obtained with a number of mutants of the highly conserved E/DRY motif, we show evidences for the existence of all the receptor states theorized by the CTC, four 'uncoupled (R, R^⁎ and HR and HR^⁎), and, consequently four 'coupled' (RG, R^⁎G, HRG and HR^⁎G). The E/DRY motif located at the cytosolic end of transmembrane helix III of Class A GPCRs has been widely studied and analyzed because it forms a network of interactions believed to lock receptors in the inactive conformation (R), and, thus, to play a key role in receptor activation. Our conclusions are supported by recent crystal and NMR spectra, as well as by results obtained with two prototypical GPCRs using a new FRET technology that de-couples G protein binding to the receptor from signal transduction. Thus, despite its complexity and limitations, we propose that the CTC is a useful framework to reconcile pharmacological, biochemical and structural data.

Priming GPCR signaling through the synergistic effect of two G proteins

Although individual G-protein-coupled receptors (GPCRs) are known to activate one or more G proteins, the GPCR-G-protein interaction is viewed as a bimolecular event involving the formation of a ternary ligand-GPCR-G-protein complex. Here, we present evidence that individual GPCR-G-protein interactions can reinforce each other to enhance signaling through canonical downstream second messengers, a phenomenon we term "GPCR priming." Specifically, we find that the presence of noncognate Gq protein enhances cAMP stimulated by two Gs-coupled receptors, β2-adrenergic receptor (β2-AR) and D₁ dopamine receptor (D₁-R). Reciprocally, Gs enhances IP₁ through vasopressin receptor (V_1A-R) but not α1 adrenergic receptor (α1-AR), suggesting that GPCR priming is a receptor-specific phenomenon. The C terminus of either the Gαs or Gαq subunit is sufficient to enhance Gα subunit activation and cAMP levels. Interaction of Gαs or Gαq C termini with the GPCR increases signaling potency, suggesting an altered GPCR conformation as the underlying basis for GPCR priming. We propose three parallel mechanisms involving (i) sequential G-protein interactions at the cognate site, (ii) G-protein interactions at distinct allosteric and cognate sites on the GPCR, and (iii) asymmetric GPCR dimers. GPCR priming suggests another layer of regulation in the classic GPCR ternary-complex model, with broad implications for the multiplicity inherent in signaling networks.

Gonadotropin-Releasing Hormone (GnRH) Receptor Structure and GnRH Binding

Gonadotropin-releasing hormone (GnRH) regulates reproduction. The human GnRH receptor lacks a cytoplasmic carboxy-terminal tail but has amino acid sequence motifs characteristic of rhodopsin-like, class A, G protein-coupled receptors (GPCRs). This review will consider how recent descriptions of X-ray crystallographic structures of GPCRs in inactive and active conformations may contribute to understanding GnRH receptor structure, mechanism of activation and ligand binding. The structures confirmed that ligands bind to variable extracellular surfaces, whereas the seven membrane-spanning α-helices convey the activation signal to the cytoplasmic receptor surface, which binds and activates heterotrimeric G proteins. Forty non-covalent interactions that bridge topologically equivalent residues in different transmembrane (TM) helices are conserved in class A GPCR structures, regardless of activation state. Conformation-independent interhelical contacts account for a conserved receptor protein structure and their importance in the GnRH receptor structure is supported by decreased expression of receptors with mutations of residues in the network. Many of the GnRH receptor mutations associated with congenital hypogonadotropic hypogonadism, including the Glu^2.53(90) Lys mutation, involve amino acids that constitute the conserved network. Half of the ~250 intramolecular interactions in GPCRs differ between inactive and active structures. Conformation-specific interhelical contacts depend on amino acids changing partners during activation. Conserved inactive conformation-specific contacts prevent receptor activation by stabilizing proximity of TM helices 3 and 6 and a closed G protein-binding site. Mutations of GnRH receptor residues involved in these interactions, such as Arg^3.50(139) of the DRY/S motif or Tyr^7.53(323) of the N/DPxxY motif, increase or decrease receptor expression and efficiency of receptor coupling to G protein signaling, consistent with the native residues stabilizing the inactive GnRH receptor structure. Active conformation-specific interhelical contacts stabilize an open G protein-binding site. Progress in defining the GnRH-binding site has recently slowed, with evidence that Tyr^6.58(290) contacts Tyr⁵ of GnRH, whereas other residues affect recognition of Trp³ and Gly¹⁰NH₂. The surprisingly consistent observations that GnRH receptor mutations that disrupt GnRH binding have less effect on "conformationally constrained" GnRH peptides may now be explained by crystal structures of agonist-bound peptide receptors. Analysis of GPCR structures provides insight into GnRH receptor function.

Yeast GPCR signaling reflects the fraction of occupied receptors, not the number

According to receptor theory, the effect of a ligand depends on the amount of agonist-receptor complex. Therefore, changes in receptor abundance should have quantitative effects. However, the response to pheromone in Saccharomyces cerevisiae is robust (unaltered) to increases or reductions in the abundance of the G-protein-coupled receptor (GPCR), Ste2, responding instead to the fraction of occupied receptor. We found experimentally that this robustness originates during G-protein activation. We developed a complete mathematical model of this step, which suggested the ability to compute fractional occupancy depends on the physical interaction between the inhibitory regulator of G-protein signaling (RGS), Sst2, and the receptor. Accordingly, replacing Sst2 by the heterologous hsRGS4, incapable of interacting with the receptor, abolished robustness. Conversely, forcing hsRGS4:Ste2 interaction restored robustness. Taken together with other results of our work, we conclude that this GPCR pathway computes fractional occupancy because ligand-bound GPCR-RGS complexes stimulate signaling while unoccupied complexes actively inhibit it. In eukaryotes, many RGSs bind to specific GPCRs, suggesting these complexes with opposing activities also detect fraction occupancy by a ratiometric measurement. Such complexes operate as push-pull devices, which we have recently described.

Mechanistic insights into GPCR-G protein interactions

G protein-coupled receptors (GPCRs) respond to extracellular stimuli and interact with several intracellular binding partners to elicit cellular responses, including heterotrimeric G proteins. Recent structural and biophysical studies have highlighted the dynamic nature of GPCRs and G proteins and have identified specific conformational changes important for receptor-mediated nucleotide exchange on Gα. While domain separation within Gα is necessary for GDP release, opening the inter-domain interface is insufficient to stimulate nucleotide exchange. Rather, an activated receptor promotes GDP release by allosterically disrupting the nucleotide-binding site via interactions with the Gα N-termini and C-termini. Highlighting the allosteric nature of GPCRs, recent studies suggest that agonist binding alone poorly stabilizes an active conformation of several receptors. Rather, full stabilization of the receptor in an active state requires formation of the agonist-receptor-G protein ternary complex. In turn, nucleotide-free Gα is able to stabilize conformational changes around the receptor's agonist-binding site to enhance agonist affinity.

GPCR Signaling Rides a Wave of Conformational Changes

The ability of structurally distinct ligands to "bias" G protein-coupled receptor signaling affords the opportunity to tailor efficacy to suit specific therapeutic needs. Furness et al. demonstrate that ligand structure controls not only which effectors are activated, but also the way they are activated and the kinetics of downstream signaling.

Allosteric nanobodies reveal the dynamic range and diverse mechanisms of G-protein-coupled receptor activation

G-protein coupled receptors (GPCRs) modulate many physiological processes by transducing a variety of extracellular cues into intracellular responses. Ligand binding to an extracellular orthosteric pocket propagates conformational change to the receptor cytosolic region to promote binding and activation of downstream signaling effectors such as G proteins and β-arrestins. It is widely appreciated that different agonists can share the same binding pocket but evoke unique receptor conformations leading to a wide range of downstream responses (i.e., ‘efficacy’)¹. Furthermore, mounting biophysical evidence, primarily using the β-adrenergic receptor (β₂AR) as a model system, supports the existence of multiple active and inactive conformational states^2–5. However, how agonists with varying efficacy modulate these receptor states to initiate cellular responses is not well understood. Here we report stabilization of two distinct β₂AR conformations using single domain camelid antibodies (nanobodies): a previously described positive allosteric nanobody (Nb80) and a newly identified negative allosteric nanobody (Nb60)^6,7. We show that Nb60 stabilizes a previously unappreciated low affinity receptor state which corresponds to one of two inactive receptor conformations as delineated by X-ray crystallography and NMR spectroscopy. We find that the agonist isoproterenol has a 15,000-fold higher affinity for the β₂AR in the presence of Nb80 compared to Nb60, highlighting the full allosteric range of a GPCR. Assessing the binding of 17 ligands of varying efficacy to the β₂AR in the absence and presence of Nb60 or Nb80 reveals large ligand-specific effects that can only be explained using an allosteric model which assumes equilibrium amongst at least three receptor states. Agonists generally exert efficacy by stabilizing the active Nb80-stabilized receptor state (R₈₀). In contrast, for a number of partial agonists, both stabilization of R₈₀ and destabilization of the inactive, Nb60-bound state (R₆₀) contribute to their ability to modulate receptor activation. These data demonstrate that ligands can initiate a wide range of cellular responses by differentially stabilizing multiple receptor states.

Allosteric coupling from G protein to the agonist-binding pocket in GPCRs

G protein-coupled receptors (GPCRs) remain the primary conduit by which cells detect environmental stimuli and communicate with each other¹. Upon activation by extracellular agonists, these seven transmembrane domain (7TM)-containing receptors interact with heterotrimeric G proteins to regulate downstream second messenger and/or protein kinase cascades¹. Crystallographic evidence from a prototypic GPCR, the β₂-adrenergic receptor (β₂AR), in complex with its cognate G protein, Gs, has provided a model for how agonist binding promotes conformational changes that propagate through the GPCR and into the nucleotide binding pocket of the G protein α-subunit to catalyze GDP release, the key step required for GTP binding and activation of G proteins². The structure also offers hints on how G protein binding may, in turn, allosterically influence ligand binding. Here we provide functional evidence that G protein coupling to β₂AR stabilizes a ‘closed’ receptor conformation characterized by restricted access to and egress from the hormone binding site. Surprisingly, the effects of G protein on the hormone binding site can be observed in the absence of a bound agonist, where G protein coupling driven by basal receptor activity impedes the association of agonists, partial agonists, antagonists and inverse agonists. The ability of bound ligands to dissociate from the receptor is also hindered, providing a structural explanation for the G protein-mediated enhancement of agonist affinity, which has been observed for many GPCR-G protein pairs. Our studies also suggest that in contrast to agonist binding alone, coupling of a G protein in the absence of an agonist stabilizes large structural changes in a GPCR. The effects of nucleotide-free G protein on ligand binding kinetics are shared by other members of the superfamily of GPCRs, suggesting that a common mechanism may underlie G protein-mediated enhancement of agonist affinity.

DREADDs for Neuroscientists

To understand brain function, it is essential that we discover how cellular signaling specifies normal and pathological brain function. In this regard, chemogenetic technologies represent valuable platforms for manipulating neuronal and non-neuronal signal transduction in a cell-type-specific fashion in freely moving animals. Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-based chemogenetic tools are now commonly used by neuroscientists to identify the circuitry and cellular signals that specify behavior, perceptions, emotions, innate drives, and motor functions in species ranging from flies to nonhuman primates. Here I provide a primer on DREADDs highlighting key technical and conceptual considerations and identify challenges for chemogenetics going forward.

The multi-facet aspects of cell sentience and their relevance for the integrative brain actions: role of membrane protein energy landscape

Several ion channels can be randomly and spontaneously in an open state, allowing the exchange of ion fluxes between extracellular and intracellular environments. We propose that the random changes in the state of ion channels could be also due to proteins exploring their energy landscapes. Indeed, proteins can modify their steric conformation under the effects of the physicochemical parameters of the environments with which they are in contact, namely, the extracellular, intramembrane and intracellular environments. In particular, it is proposed that the random walk of proteins in their energy landscape is towards attractors that can favor the open or close condition of the ion channels and/or intrinsic activity of G-protein-coupled receptors. The main aspect of the present proposal is that some relevant physicochemical parameters of the environments (e.g. molecular composition, temperature, electrical fields) with which some signaling-involved plasma membrane proteins are in contact alter their conformations. In turn, these changes can modify their information handling via a modulatory action on their random walk towards suitable attractors of their energy landscape. Thus, spontaneous and/or signal-triggered electrical activities of neurons occur that can have emergent properties capable of influencing the integrative actions of brain networks. Against this background, Cook’s hypothesis on ‘cell sentience’ is developed by proposing that physicochemical parameters of the environments with which the plasma-membrane proteins of complex cellular networks are in contact fulfill a fundamental role in their spontaneous and/or signal-triggered activity. Furthermore, it is proposed that a specialized organelle, the primary cilium, which is present in most cells (also neurons and astrocytes), could be of peculiar importance to pick up chemical signals such as ions and transmitters and to detect physical signals such as pressure waves, thermal gradients, and local field potentials.

G protein-coupled receptor kinases as regulators of dopamine receptor functions

Actions of the neurotransmitter dopamine in the brain are mediated by dopamine receptors that belong to the superfamily of G protein-coupled receptors (GPCRs). Mammals have five dopamine receptor subtypes, D1 through D5. D1 and D5 couple to Gs/olf and activate adenylyl cyclase, whereas D2, D3, and D4 couple to Gi/o and inhibit it. Most GPCRs upon activation by an agonist are phosphorylated by GPCR kinases (GRKs). The GRK phosphorylation makes receptors high-affinity binding partners for arrestin proteins. Arrestin binding to active phosphorylated receptors stops further G protein activation and promotes receptor internalization, recycling or degradation, thereby regulating their signaling and trafficking. Four non- visual GRKs are expressed in striatal neurons. Here we describe known effects of individual GRKs on dopamine receptors in cell culture and in the two in vivo models of dopamine-mediated signaling: behavioral response to psychostimulants and L-DOPA- induced dyskinesia. Dyskinesia, associated with dopamine super-sensitivity of striatal neurons, is a debilitating side effect of L-DOPA therapy in Parkinson's disease. In vivo, GRK subtypes show greater receptor specificity than in vitro or in cultured cells. Overexpression, knockdown, and knockout of individual GRKs, particularly GRK2 and GRK6, have differential effects on signaling of dopamine receptor subtypes in the brain. Furthermore, deletion of GRK isoforms in select striatal neuronal types differentially affects psychostimulant-induced behaviors. In addition, anti-dyskinetic effect of GRK3 does not require its kinase activity: it is mediated by the binding of its RGS-like domain to Gαq/11, which suppresses Gq/11 signaling. The data demonstrate that the dopamine signaling in defined neuronal types in vivo is regulated by specific and finely orchestrated actions of GRK isoforms.

GPCR-radioligand binding assays

Radioligand binding assays provide sensitive and quantitative information about guanine nucleotide protein G protein-coupled receptor (GPCR) expression and affinity for a wide variety of ligands, making them essential for drug structure-activity studies and basic GPCR research. Three basic radioligand binding protocols, saturation, indirect (competition, displacement, or modulation), and kinetic binding assays, are used to assess GPCR expression (Bmax), equilibrium dissociation constants for radioligands (Kd) and nonradioactive ligands (Ki), association and dissociation rates, and to distinguish competitive and allosteric mechanisms of GPCR-ligand interactions. Nonspecific radioligand binding may be mitigated by appropriate choices of reaction conditions. Radioligand depletion (bound radioactivity >10% of total radioligand), which compromises accuracy of Kd and Ki measurements, can be limited by adjusting receptor concentration and appropriate radioligand choice. Accurate Kd and Ki values in saturation and indirect binding assays depend on binding equilibrium. Equilibration time for high-affinity ligands, with slow dissociation rates, may require much extended incubation times or increased incubation temperature.

Theoretical Aspects of GPCR-Ligand Complex Pharmacology

Over the past 50 years in pharmacology, an understanding of seven transmembrane (7TMR) function has been gained from the comparison of experimental data to receptor models. These models have been constructed from building blocks composed of systems consisting of series and parallel mass action binding reactions. Basic functions such as the the isomerization of receptors upon ligand binding, the sequential binding of receptors to membrane coupling proteins, and the selection of multiple receptor conformations have been combined in various ways to build receptor systems such as the ternary complex, extended ternary complex, and cubic ternary complex models for 7TMR function. Separately, the Black/Leff operational model has furnished an extremely valuable method of quantifying drug agonism. In the past few years, incorporation of the basic allosteric nature of 7TMRs has led to additional useful models of functional receptor allosteric mechanisms; these models yield valuable methods for quantifying allosteric effects. Finally, molecular dynamics has provided yet another new set of models describing the probability of formation of multiple receptor states; these radically new models are extremely useful in the prediction of functionally selective drug effects.