Molecular insights into the biased signaling mechanism of the µ-opioid receptor.

GPCR functional selectivity opens new opportunities for the design of safer drugs. Ligands orchestrate GPCR signaling cascades by modulating the receptor conformational landscape. Our study provides insights into the dynamic mechanism enabling opioid ligands to preferentially activate the G protein over the ß-arrestin pathways through the µ-opioid receptor (µOR). We combine functional assays in living cells, solution NMR spectroscopy, and enhanced-sampling molecular dynamic simulations to identify the specific µOR conformations induced by G protein-biased agonists. In particular, we describe the dynamic and allosteric communications between the ligand-binding pocket and the receptor intracellular domains, through conserved motifs in class A GPCRs. Most strikingly, the biased agonists trigger µOR conformational changes in the intracellular loop 1 and helix 8 domains, which may impair ß-arrestin binding or signaling. The findings may apply to other GPCR families and provide key molecular information that could facilitate the design of biased ligands.

Author Correction: Structural insights into µ-opioid receptor activation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Structure of the µ-opioid receptor-Gi protein complex.

The µ-opioid receptor (µOR) is a G-protein-coupled receptor (GPCR) and the target of most clinically and recreationally used opioids. The induced positive effects of analgesia and euphoria are mediated by µOR signalling through the adenylyl cyclase-inhibiting heterotrimeric G protein Gi. Here we present the 3.5 Å resolution cryo-electron microscopy structure of the µOR bound to the agonist peptide DAMGO and nucleotide-free Gi. DAMGO occupies the morphinan ligand pocket, with its N terminus interacting with conserved receptor residues and its C terminus engaging regions important for opioid-ligand selectivity. Comparison of the µOR-Gi complex to previously determined structures of other GPCRs bound to the stimulatory G protein Gs reveals differences in the position of transmembrane receptor helix 6 and in the interactions between the G protein α-subunit and the receptor core. Together, these results shed light on the structural features that contribute to the Gi protein-coupling specificity of the µOR.

Molecular insights into mechanisms of GPCR hijacking by Staphylococcus aureus.

Atypical chemokine receptor 1 (ACKR1) is a G protein-coupled receptor (GPCR) targeted by Staphylococcus aureus bicomponent pore-forming leukotoxins to promote bacterial growth and immune evasion. Here, we have developed an integrative molecular pharmacology and structural biology approach in order to characterize the effect of leukotoxins HlgA and HlgB on ACKR1 structure and function. Interestingly, using cell-based assays and native mass spectrometry, we found that both components HlgA and HlgB compete with endogenous chemokines through a direct binding with the extracellular domain of ACKR1. Unexpectedly, hydrogen/deuterium exchange mass spectrometry analysis revealed that toxin binding allosterically modulates the intracellular G protein-binding domain of the receptor, resulting in dissociation and/or changes in the architecture of ACKR1-Gαi1 protein complexes observed in living cells. Altogether, our study brings important molecular insights into the initial steps of leukotoxins targeting a host GPCR.

Structural insights into adiponectin receptors suggest ceramidase activity.

Adiponectin receptors (ADIPORs) are integral membrane proteins that control glucose and lipid metabolism by mediating, at least in part, a cellular ceramidase activity that catalyses the hydrolysis of ceramide to produce sphingosine and a free fatty acid (FFA). The crystal structures of the two receptor subtypes, ADIPOR1 and ADIPOR2, show a similar overall seven-transmembrane-domain architecture with large unoccupied cavities and a zinc binding site within the seven transmembrane domain. However, the molecular mechanisms by which ADIPORs function are not known. Here we describe the crystal structure of ADIPOR2 bound to a FFA molecule and show that ADIPOR2 possesses intrinsic basal ceramidase activity that is enhanced by adiponectin. We also identify a ceramide binding pose and propose a possible mechanism for the hydrolytic activity of ADIPOR2 using computational approaches. In molecular dynamics simulations, the side chains of residues coordinating the zinc rearrange quickly to promote the nucleophilic attack of a zinc-bound hydroxide ion onto the ceramide amide carbonyl. Furthermore, we present a revised ADIPOR1 crystal structure exhibiting a seven-transmembrane-domain architecture that is clearly distinct from that of ADIPOR2. In this structure, no FFA is observed and the ceramide binding pocket and putative zinc catalytic site are exposed to the inner membrane leaflet. ADIPOR1 also possesses intrinsic ceramidase activity, so we suspect that the two distinct structures may represent key steps in the enzymatic activity of ADIPORs. The ceramidase activity is low, however, and further studies will be required to characterize fully the enzymatic parameters and substrate specificity of ADIPORs. These insights into ADIPOR function will enable the structure-based design of potent modulators of these clinically relevant enzymes.

Discovery and Mechanism of Action of Small Molecule Inhibitors of Ceramidases.

Sphingolipid metabolism is tightly controlled by enzymes to regulate essential processes in human physiology. The central metabolite is ceramide, a pro-apoptotic lipid catabolized by ceramidase enzymes to produce pro-proliferative sphingosine-1-phosphate. Alkaline ceramidases are transmembrane enzymes that recently attracted attention for drug development in fatty liver diseases. However, due to their hydrophobic nature, no specific small molecule inhibitors have been reported. We present the discovery and mechanism of action of the first drug-like inhibitors of alkaline ceramidase 3 (ACER3). In particular, we chemically engineered novel fluorescent ceramide substrates enabling screening of large compound libraries and characterized enzyme:inhibitor interactions using mass spectrometry and MD simulations. In addition to revealing a new paradigm for inhibition of lipid metabolising enzymes with non-lipidic small molecules, our data lay the ground for targeting ACER3 in drug discovery efforts.

Propagation of conformational changes during µ-opioid receptor activation.

µ-Opioid receptors (µORs) are G-protein-coupled receptors that are activated by a structurally diverse spectrum of natural and synthetic agonists including endogenous endorphin peptides, morphine and methadone. The recent structures of the µOR in inactive and agonist-induced active states (Huang et al., ref. 2) provide snapshots of the receptor at the beginning and end of a signalling event, but little is known about the dynamic sequence of events that span these two states. Here we use solution-state NMR to examine the process of µOR activation using a purified receptor (mouse sequence) preparation in an amphiphile membrane-like environment. We obtain spectra of the µOR in the absence of ligand, and in the presence of the high-affinity agonist BU72 alone, or with BU72 and a G protein mimetic nanobody. Our results show that conformational changes in transmembrane segments 5 and 6 (TM5 and TM6), which are required for the full engagement of a G protein, are almost completely dependent on the presence of both the agonist and the G protein mimetic nanobody, revealing a weak allosteric coupling between the agonist-binding pocket and the G-protein-coupling interface (TM5 and TM6), similar to that observed for the ß2-adrenergic receptor. Unexpectedly, in the presence of agonist alone, we find larger spectral changes involving intracellular loop 1 and helix 8 compared to changes in TM5 and TM6. These results suggest that one or both of these domains may play a role in the initial interaction with the G protein, and that TM5 and TM6 are only engaged later in the process of complex formation. The initial interactions between the G protein and intracellular loop 1 and/or helix 8 may be involved in G-protein coupling specificity, as has been suggested for other family A G-protein-coupled receptors.

Structural insights into µ-opioid receptor activation.

Activation of the µ-opioid receptor (µOR) is responsible for the efficacy of the most effective analgesics. To shed light on the structural basis for µOR activation, here we report a 2.1 Å X-ray crystal structure of the murine µOR bound to the morphinan agonist BU72 and a G protein mimetic camelid antibody fragment. The BU72-stabilized changes in the µOR binding pocket are subtle and differ from those observed for agonist-bound structures of the ß2-adrenergic receptor (ß2AR) and the M2 muscarinic receptor. Comparison with active ß2AR reveals a common rearrangement in the packing of three conserved amino acids in the core of the µOR, and molecular dynamics simulations illustrate how the ligand-binding pocket is conformationally linked to this conserved triad. Additionally, an extensive polar network between the ligand-binding pocket and the cytoplasmic domains appears to play a similar role in signal propagation for all three G-protein-coupled receptors.

Selective and Wash-Resistant Fluorescent Dihydrocodeinone Derivatives Allow Single-Molecule Imaging of µ-Opioid Receptor Dimerization.

µ-Opioid receptors (µ-ORs) play a critical role in the modulation of pain and mediate the effects of the most powerful analgesic drugs. Despite extensive efforts, it remains insufficiently understood how µ-ORs produce specific effects in living cells. We developed new fluorescent ligands based on the µ-OR antagonist E-p-nitrocinnamoylamino-dihydrocodeinone (CACO), that display high affinity, long residence time and pronounced selectivity. Using these ligands, we achieved single-molecule imaging of µ-ORs on the surface of living cells at physiological expression levels. Our results reveal a high heterogeneity in the diffusion of µ-ORs, with a relevant immobile fraction. Using a pair of fluorescent ligands of different color, we provide evidence that µ-ORs interact with each other to form short-lived homodimers on the plasma membrane. This approach provides a new strategy to investigate µ-OR pharmacology at single-molecule level.

Structure of the δ-opioid receptor bound to naltrindole.

The opioid receptor family comprises three members, the µ-, δ- and κ-opioid receptors, which respond to classical opioid alkaloids such as morphine and heroin as well as to endogenous peptide ligands like endorphins. They belong to the G-protein-coupled receptor (GPCR) superfamily, and are excellent therapeutic targets for pain control. The δ-opioid receptor (δ-OR) has a role in analgesia, as well as in other neurological functions that remain poorly understood. The structures of the µ-OR and κ-OR have recently been solved. Here we report the crystal structure of the mouse δ-OR, bound to the subtype-selective antagonist naltrindole. Together with the structures of the µ-OR and κ-OR, the δ-OR structure provides insights into conserved elements of opioid ligand recognition while also revealing structural features associated with ligand-subtype selectivity. The binding pocket of opioid receptors can be divided into two distinct regions. Whereas the lower part of this pocket is highly conserved among opioid receptors, the upper part contains divergent residues that confer subtype selectivity. This provides a structural explanation and validation for the 'message-address' model of opioid receptor pharmacology, in which distinct 'message' (efficacy) and 'address' (selectivity) determinants are contained within a single ligand. Comparison of the address region of the δ-OR with other GPCRs reveals that this structural organization may be a more general phenomenon, extending to other GPCR families as well.

Crystal structure of the µ-opioid receptor bound to a morphinan antagonist.

Opium is one of the world's oldest drugs, and its derivatives morphine and codeine are among the most used clinical drugs to relieve severe pain. These prototypical opioids produce analgesia as well as many undesirable side effects (sedation, apnoea and dependence) by binding to and activating the G-protein-coupled µ-opioid receptor (µ-OR) in the central nervous system. Here we describe the 2.8 Å crystal structure of the mouse µ-OR in complex with an irreversible morphinan antagonist. Compared to the buried binding pocket observed in most G-protein-coupled receptors published so far, the morphinan ligand binds deeply within a large solvent-exposed pocket. Of particular interest, the µ-OR crystallizes as a two-fold symmetrical dimer through a four-helix bundle motif formed by transmembrane segments 5 and 6. These high-resolution insights into opioid receptor structure will enable the application of structure-based approaches to develop better drugs for the management of pain and addiction.

Distinct roles of metabotropic glutamate receptor dimerization in agonist activation and G-protein coupling.

The eight metabotropic glutamate receptors (mGluRs) are key modulators of synaptic transmission and are considered promising targets for the treatment of various brain disorders. Whereas glutamate acts at a large extracellular domain, allosteric modulators have been identified that bind to the seven transmembrane domain (7TM) of these dimeric G-protein-coupled receptors (GPCRs). We show here that the dimeric organization of mGluRs is required for the modulation of active and inactive states of the 7TM by agonists, but is not necessary for G-protein activation. Monomeric mGlu2, either as an isolated 7TM or in full-length, purified and reconstituted into nanodiscs, couples to G proteins upon direct activation by a positive allosteric modulator. However, only a reconstituted full-length dimeric mGlu2 activates G protein upon glutamate binding, suggesting that dimerization is required for glutamate induced activation. These data show that, even for such well characterized GPCR dimers like mGluR2, a single 7TM is sufficient for G-protein coupling. Despite this observation, the necessity of dimeric architecture for signaling induced by the endogenous ligand glutamate confirms that the central core of signaling complex is dimeric.

Structural insights into biased G protein-coupled receptor signaling revealed by fluorescence spectroscopy.

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins that mediate most cellular responses to hormones and neurotransmitters, representing the largest group of therapeutic targets. Recent studies show that some GPCRs signal through both G protein and arrestin pathways in a ligand-specific manner. Ligands that direct signaling through a specific pathway are known as biased ligands. The arginine-vasopressin type 2 receptor (V2R), a prototypical peptide-activated GPCR, is an ideal model system to investigate the structural basis of biased signaling. Although the native hormone arginine-vasopressin leads to activation of both the stimulatory G protein (Gs) for the adenylyl cyclase and arrestin pathways, synthetic ligands exhibit highly biased signaling through either Gs alone or arrestin alone. We used purified V2R stabilized in neutral amphipols and developed fluorescence-based assays to investigate the structural basis of biased signaling for the V2R. Our studies demonstrate that the Gs-biased agonist stabilizes a conformation that is distinct from that stabilized by the arrestin-biased agonists. This study provides unique insights into the structural mechanisms of GPCR activation by biased ligands that may be relevant to the design of pathway-biased drugs.

Amphipols in G protein-coupled receptor pharmacology: what are they good for?

G protein-coupled receptors are at a central node of all cell communications. Investigating their molecular functioning is therefore crucial for both academic purposes and drug design. However, getting the receptors as isolated, stable and purified proteins for such studies still stumbles over their instability out of the membrane environment. Different membrane-mimicking environments have been developed so far to increase the stability of purified receptors. Among them are amphipols. These polymers not only preserve the native fold of receptors purified from membrane fractions but they also allow specific applications such as folding receptors purified from inclusion bodies back to their native state. Of importance, amphipol-trapped G protein-coupled receptors essentially maintain their pharmacological properties so that they are perfectly adapted to further investigate the molecular mechanisms underlying signaling processes. We review here how amphipols have been used to refold and stabilize detergent-solubilized purified receptors and what are the main subsequent molecular pharmacology analyses that were performed using this strategy.

A new era of GPCR structural and chemical biology.

G protein-coupled receptors (GPCRs) are versatile molecular machines that regulate the majority of physiological responses to chemically diverse hormones and neurotransmitters. Recent breakthroughs in structural studies have advanced our understanding of GPCR signaling, particularly the selectivity of ligand recognition and receptor activation of G proteins.

Structures of the arginine-vasopressin and oxytocin receptor signaling complexes.

Arginine-vasopressin (AVP) and oxytocin (OT) are neurohypophysial hormones which share a high sequence and structure homology. These are two cyclic C-terminally amidated nonapeptides with different residues at position 3 and 8. In mammals, AVP and OT exert their multiple biological functions through a specific G protein-coupled receptor family: four receptors are identified, the V1a, V1b, V2 receptors (V1aR, V1bR and V2R) and the OT receptor (OTR). The chemical structure of AVP and OT was elucidated in the early 1950s. Thanks to X-ray crystallography and cryo-electron microscopy, it took however 70 additional years to determine the three-dimensional structures of the OTR and the V2R in complex with their natural agonist ligands and with different signaling partners, G proteins and ß-arrestins. Today, the comparison of the different AVP/OT receptor structures gives structural insights into their orthosteric ligand binding pocket, their molecular mechanisms of activation, and their interfaces with canonical Gs, Gq and ß-arrestin proteins. It also paves the way to future rational drug design and therapeutic compound development. Indeed, agonist, antagonist, biased agonist, or pharmacological chaperone analogues of AVP and OT are promising candidates to regulate different physiological functions and treat several pathologies.

Bringing GPCR Structural Biology to Medical Applications: Insights from Both V2 Vasopressin and Mu-Opioid Receptors.

G-protein coupled receptors (GPCRs) are versatile signaling proteins that regulate key physiological processes in response to a wide variety of extracellular stimuli. The last decade has seen a revolution in the structural biology of clinically important GPCRs. Indeed, the improvement in molecular and biochemical methods to study GPCRs and their transducer complexes, together with advances in cryo-electron microscopy, NMR development, and progress in molecular dynamic simulations, have led to a better understanding of their regulation by ligands of different efficacy and bias. This has also renewed a great interest in GPCR drug discovery, such as finding biased ligands that can either promote or not promote specific regulations. In this review, we focus on two therapeutically relevant GPCR targets, the V2 vasopressin receptor (V2R) and the mu-opioid receptor (µOR), to shed light on the recent structural biology studies and show the impact of this integrative approach on the determination of new potential clinical effective compounds.

Structure, dynamics and transferability of the metal-dependent polyhistidine tetramerization motif TetrHis for single-chain Fv antibodies.

The polyhistidine (6XHis) motif is one of the most ubiquitous protein purification tags. The 6XHis motif enables the binding of tagged proteins to various metals, which can be advantageously used for purification with immobilized metal affinity chromatography. Despite its popularity, protein structures encompassing metal-bound 6XHis are rare. Here, we obtained a 2.5 Å resolution crystal structure of a single chain Fv antibody (scFv) bearing a C-terminal sortase motif, 6XHis and TwinStrep tags (LPETGHHHHHHWSHPQFEK[G3S]3WSHPQFEK). The structure, obtained in the presence of cobalt, reveals a unique tetramerization motif (TetrHis) stabilized by 8 Co2+ ions. The TetrHis motif contains four 6 residues-long ß-strands, and each metal center coordinates 3 to 5 residues, including all 6XHis histidines. By combining dynamic light scattering, small angle x-ray scattering and molecular dynamics simulations, We investigated the influence of Co2+ on the conformational dynamics of scFv 2A2, observing an open/close equilibrium of the monomer and the formation of cobalt-stabilized tetramers. By using a similar scFv design, we demonstrate the transferability of the tetramerization property. This novel metal-dependent tetramerization motif might be used as a fiducial marker for cryoelectron microscopy of scFv complexes, or even provide a starting point for designing metal-loaded biomaterials.

Signaling from beta1- and beta2-adrenergic receptors is defined by differential interactions with PDE4.

Beta1- and beta2-adrenergic receptors (betaARs) are highly homologous, yet they play clearly distinct roles in cardiac physiology and pathology. Myocyte contraction, for instance, is readily stimulated by beta1AR but not beta2AR signaling, and chronic stimulation of the two receptors has opposing effects on myocyte apoptosis and cell survival. Differences in the assembly of macromolecular signaling complexes may explain the distinct biological outcomes. Here, we demonstrate that beta1AR forms a signaling complex with a cAMP-specific phosphodiesterase (PDE) in a manner inherently different from a beta2AR/beta-arrestin/PDE complex reported previously. The beta1AR binds a PDE variant, PDE4D8, in a direct manner, and occupancy of the receptor by an agonist causes dissociation of this complex. Conversely, agonist binding to the beta2AR is a prerequisite for the recruitment of a complex consisting of beta-arrestin and the PDE4D variant, PDE4D5, to the receptor. We propose that the distinct modes of interaction with PDEs result in divergent cAMP signals in the vicinity of the two receptors, thus, providing an additional layer of complexity to enforce the specificity of beta1- and beta2-adrenoceptor signaling.