Xylazine is an α2-adrenergic agonist typically used in as a sedative and analgesic in veterinary medicine. For some years, xylazine has been reported as an additive to fentanyl on the illicit drug market and has been associated with severe side-effects including severe ulcerations and potential amputations at the sites of injection along with an increased risk of respiratory depression and death. We recently reported that xylazine has modest κ opioid agonist activity in vitro and in vivo and asked if other α2-adrenergic agonists had similar off-target activities. To test this hypothesis, we profiled US Food and Drug Administration-approved α2-adrenergic agonists at 320 G protein coupled receptors to identify potentially deleterious and/or beneficial off-targets. Although all other tested α2-adrenergic agonists were devoid of κ opioid agonist activity, each had a distinct pattern of activity at various G protein coupled receptors and differential patterns of signaling bias at α2-receptor subtypes. These findings suggest potential molecular targets for both side-effects and therapeutic activities among known α2-adrenergic agonists. Significance Statement Xylazine was recently shown to be a κ-opioid receptor agonist and we examined other US Food and Drug Administration-approved α2-agonists for κ-opioid activity. Xylazine is unique in having κ-agonist activity.

G-protein-coupled receptors (GPCRs) have key roles in physiology and are central targets for drug discovery and development1,2, but the design of protein agonists and antagonists has been challenging as GPCRs are integral membrane proteins and conformationally dynamic3-6. Here we describe de novo design methods and a high-throughput receptor-diversion microscopy-based screen for generating GPCR-binding miniproteins with high affinity, potency and selectivity. We design miniprotein agonists that activate receptors involved in itch and pain, as well as antagonists that inhibit receptors implicated in cancer, metabolic disorders such as diabetes and obesity, and migraines. The cryo-electron microscopy (cryo-EM) structures of five receptor-bound designs are close to the computational design models. A designed chemokine receptor antagonist mobilizes haematopoietic stem and progenitor cells in vivo at a level comparable to a clinically used drug, with fewer adverse effects.

Xylazine is a α2-adrenergic agonist typically used in as a sedative and analgesic in veterinary medicine. For some years, xylazine has been reported as an additive to fentanyl on the illicit drug market and has been associated with severe side-effects including severe ulcerations and potential amputations at the sites of injection along with an increased risk of respiratory depression and death. We recently reported that xylazine has modest κ opioid agonist activity in vitro and in vivo and asked if other α2-adrenergic agonists had similar off-target activities. To test this hypothesis, we profiled US FDA-approved α2-adrenergic agonists at 320 G protein coupled receptors (GPCRs) to identify potentially deleterious and/or beneficial off-targets. Although all other tested α2-adrenergic agonists were devoid of κ opioid agonist activity, each had a distinct pattern of activity at various GPCRs and differential patterns of signaling bias at α2-receptor subtypes. These findings suggest potential molecular targets for both side-effects and therapeutic activities among known α2-adrenergic agonists.

G-protein-coupled receptors (GPCRs) have proven to be the most successful target class for drug discovery but their complicated signal transduction pathways cause difficulties for drug development. Recently, ligands have been identified that engage an intracellular binding site which promotes pathway biased signal in cooperation with orthosteric ligands. Here, we explore the topic of biased signaling and intracellular modulators to understand their application for precision pharmacology of Class A or Rhodopsin-Like GPCRs.

G protein-coupled receptors (GPCRs) play key roles in physiology and are central targets for drug discovery and development, yet the design of protein agonists and antagonists has been challenging as GPCRs are integral membrane proteins and conformationally dynamic. Here we describe computational de novo design methods and a high throughput "receptor diversion" microscopy-based screen for generating GPCR binding miniproteins with high affinity, potency and selectivity, and the use of these methods to generate MRGPRX1 agonists and CXCR4, GLP1R, GIPR, GCGR and CGRPR antagonists. Cryo-electron microscopy data reveals atomic-level agreement between designed and experimentally determined structures for CGRPR-bound antagonists and MRGPRX1-bound agonists, confirming precise conformational control of receptor function. Our de novo design and screening approach opens new frontiers in GPCR drug discovery and development.

There is currently a resurgence in exploring the utility of classical psychedelics to treat depression, addiction, anxiety disorders, cluster headaches, and many other neuropsychiatric disorders. A biological target of these compounds, and a hypothesized target for their therapeutic actions, is the 5-HT2A serotonin receptor. Here, we present 7 cryo-EM structures covering all major compound classes of psychedelic and non-psychedelic agonists, including a β-arrestin-biased compound RS130-180. Identifying the molecular interactions between various psychedelics and the 5-HT2A receptor reveals both common and distinct motifs among the examined psychedelic chemotypes. These findings lead to a broader mechanistic understanding of 5-HT2A activation, which can catalyze the development of novel chemotypes with potential therapeutic utility and fewer side effects. The authors present 7 cryo-EM structures of hallucinogenic and non-hallucinogenic compounds across multiple chemotypes bound to the 5-HT2A receptor, shedding light onto ligand specificity and signaling bias.

Sodium ions are classically conceptualized as negative allosteric modulators for G protein-coupled receptors, although there have been reports of either positive allosteric modulation or no effect of sodium on GPCR function. Here, we identified opposing actions of sodium on μ and κ opioid receptors. We utilized a variety of methods including radioligand binding, real-time conformational monitoring of transitions using bioluminescence resonance energy transfer, and signaling assays using the TRUPATH resource. At the μ receptors, sodium behaved as a negative allosteric modulator of binding, conformational transitions, and signaling. Intriguingly, bitopic μ agonists displayed transducer-specific effects on conformational transitions and signaling sodium concentrations. By contrast, at the κ opioid receptor, sodium negatively modulated agonist binding and positively modulated conformational transitions and signaling. Taken together, these findings support the notion that the differential sensitivities to sodium concentrations will result in opposing effects on the cell surface and intracellular signaling.

Biased allosteric modulators provide great therapeutic poten-tial by selectively directing signal bias in the presence of endogenous ligand under(patho)physiological conditions.In a recent Cell Research paper,Sun et al.revealed the structural mechanisms underlying the biased allosteric modulation exerted by SBI-533 directly at the neurotensin receptor 1-β-arrestin1 interface.

Many neurotransmitter receptors activate G proteins through exchange of GDP for GTP. The intermediate nucleotide-free state has eluded characterization, due largely to its inherent instability. Here we characterize a G protein variant associated with a rare neurological disorder in humans. GαoK46E has a charge reversal that clashes with the phosphate groups of GDP and GTP. As anticipated, the purified protein binds poorly to guanine nucleotides yet retains wild-type affinity for G protein βγ subunits. In cells with physiological concentrations of nucleotide, GαoK46E forms a stable complex with receptors and Gβγ, impeding effector activation. Further, we demonstrate that the mutant can be easily purified in complex with dopamine-bound D2 receptors, and use cryo-electron microscopy to determine the structure, including both domains of Gαo, without nucleotide or stabilizing nanobodies. These findings reveal the molecular basis for the first committed step of G protein activation, establish a mechanistic basis for a neurological disorder, provide a simplified strategy to determine receptor-G protein structures, and a method to detect high affinity agonist binding in cells. Many neurotransmitters act on receptors coupled to GTP-binding G proteins. Here authors report the structure and activity of a mutant that locks the nucleotide-free and receptor-bound state of the G protein, leading to a rare neurological disorder.

Abstract ID 95565Poster Board 354G protein heterotrimers (Galpha, Gbeta, and Ggamma) are important signal transducers upon activation of G protein coupled receptors (GPCRs). Opioid receptors belong to the class A GPCRs and play major roles in pain, mood, reward, and motivation. Opioids are used for pain management despite the side effects that contribute to the opioid crisis. The pursuit of non-addictive opioid analgesics remains unattained due to the unresolved intricacies of opioid actions, receptor signaling cascades, and neuronal plasticity. Opioid receptor activation couples to up to seven Ga subtypes (Gai1, Gai2, Gai3, GaoA, GaoB, Gaz, and/or Gustducin). The molecular basis for the G protein subtype selectivity are largely unclear; the functional consequence of individual G protein signaling remains unknown. We thus determined the active-state structures of kappa opioid receptor (KOR) in a complex with multiple G-protein heterotrimers using cryo-electron microscopy. Comparisons of these structures reveal molecular determinants critical for KOR–G-protein interactions as well as key elements governing Gi/o-family subtype selectivity. Furthermore, the G-protein subtypes display an intrinsically different allosteric activity on agonist activity at KOR. These results provide insights into the actions of opioids and G-protein-coupling specificity at KOR and establish a foundation to examine the therapeutic potential of pathway-selective agonists of KOR.The work is supported by NIH grants R35GM143061 (to T.C.)

Xylazine is in the unregulated drug supply at increasing rates, usually combined with fentanyl, necessitating understanding of its pharmacology. Despite commentary from politicians, and public health officials, it is unknown how xylazine impacts naloxone efficacy, and. few studies have examined it alone. Here, we examine the impact of xylazine alone and in combination with fentanyl on several behaviors in mice. Surprisingly, naloxone precipitates withdrawal from xylazine and fentanyl/xylazine coadministration, with enhanced sensitivity in females. Further, xylazine is a full agonist at kappa opioid receptors, a potential mechanism for its naloxone sensitivity. Finally, we demonstrate surprising effects of xylazine to kappa opioid antagonism, which are relevant for public health considerations. These data address an ongoing health crisis and will help inform critical policy and healthcare decisions.One-Sentence SummaryWe present surprising new insights into xylazine and fentanyl pharmacology with immediate implications for clinical practice and frontline public health.

"Molecular Glues" are defined as small molecules that can either be endogenous or synthetic which promote interactions between proteins at their interface. Allosteric modulators, specifically GPCR allosteric modulators, can promote both the association and the dissociation of a given receptor's transducer but accomplishes this "at a distance" from the interface. However, recent structures of GPCR G protein complexes in the presence of allosteric modulators indicate that some GPCR allosteric modulators can act as "molecular glues" interacting with both the receptor and the transducer at the interface biasing transducer signaling in both a positive and negative manner depending on the transducer. Given these phenomena we discuss the implications for this class of allosteric modulators to be used as molecular tools and for future drug development.

The human MRGPRD protein is a member of the Mas-related G protein-coupled receptors (MRGPRs) that is involved in the sensing of pain, itch, and other inflammatory stimuli. As with other MRGPRs, MRGPRD is a relatively understudied receptor with few known agonists. The most potent small-molecule agonist of MRGPRD reported so far is β-alanine, with an affinity in the micromole range, which largely restricts its functional study. Here, we report two MRGPRD agonists, EP-2825 and EP-3945, that are approximately 100-fold more potent than β-alanine and determine the structures of MRGPRD-Gq in complex with EP-2825 and EP-3945, respectively. The structures reveal distinct agonist binding modes of MRGPRD and large conformational plasticity of the orthosteric pocket. Collectively, the discovery of high-affinity MRGPRD agonists and their distinct binding modes will facilitate the functional study and the structure-based design of ligands targeting this understudied receptor.

Designer receptors exclusively activated by designer drugs (DREADDs) are chemogenetic tools for remotely controlling cellular signaling, neural activity, behavior, and physiology. Using a structure-guided approach, we provide a peripherally restricted Gi-DREADD, hydroxycarboxylic acid receptor DREADD (HCAD), whose native receptor is minimally expressed in the brain, and a chemical actuator that does not cross the blood-brain barrier (BBB). This was accomplished by combined mutagenesis, analoging via an ultra-large make-on-demand library, structural determination of the designed DREADD receptor via cryoelectron microscopy (cryo-EM), and validation of HCAD function. Expression and activation of HCAD in dorsal root ganglion (DRG) neurons inhibit action potential (AP) firing and reduce both acute and tissue-injury-induced inflammatory pain. The HCAD chemogenetic system expands the possibilities for studying numerous peripheral systems with little adverse effects on the central nervous system (CNS). The structure-guided approach used to generate HCAD also has the potential to accelerate the development of emerging chemogenetic tools for basic and translational sciences.

Bitter taste sensing is mediated by type 2 taste receptors (TAS2Rs (also known as T2Rs)), which represent a distinct class of G-protein-coupled receptors1. Among the 26 members of the TAS2Rs, TAS2R14 is highly expressed in extraoral tissues and mediates the responses to more than 100 structurally diverse tastants2-6, although the molecular mechanisms for recognizing diverse chemicals and initiating cellular signalling are still poorly understood. Here we report two cryo-electron microscopy structures for TAS2R14 complexed with Ggust (also known as gustducin) and Gi1. Both structures have an orthosteric binding pocket occupied by endogenous cholesterol as well as an intracellular allosteric site bound by the bitter tastant cmpd28.1, including a direct interaction with the α5 helix of Ggust and Gi1. Computational and biochemical studies validate both ligand interactions. Our functional analysis identified cholesterol as an orthosteric agonist and the bitter tastant cmpd28.1 as a positive allosteric modulator with direct agonist activity at TAS2R14. Moreover, the orthosteric pocket is connected to the allosteric site via an elongated cavity, which has a hydrophobic core rich in aromatic residues. Our findings provide insights into the ligand recognition of bitter taste receptors and suggest activities of TAS2R14 beyond bitter taste perception via intracellular allosteric tastants.

The NTSR1 neurotensin receptor (NTSR1) is a G protein-coupled receptor (GPCR) found in the brain and peripheral tissues with neurotensin (NTS) being its endogenous peptide ligand. In the brain, NTS modulates dopamine neuronal activity, induces opioid-independent analgesia, and regulates food intake. Recent studies indicate that biasing NTSR1 toward β-arrestin signaling can attenuate the actions of psychostimulants and other drugs of abuse. Here, we provide the cryoEM structures of NTSR1 ternary complexes with heterotrimeric Gq and GoA with and without the brain-penetrant small-molecule SBI-553. In functional studies, we discovered that SBI-553 displays complex allosteric actions exemplified by negative allosteric modulation for G proteins that are Gα subunit selective and positive allosteric modulation and agonism for β-arrestin translocation at NTSR1. Detailed structural analysis of the allosteric binding site illuminated the structural determinants for biased allosteric modulation of SBI-553 on NTSR1.

The dopaminergic system, including five dopamine receptors (D1R to D5R), plays essential roles in the central nervous system (CNS); and ligands that activate dopamine receptors have been used to treat many neuropsychiatric disorders, including Parkinson's Disease (PD) and schizophrenia. Here, we report cryo-EM structures of all five subtypes of human dopamine receptors in complex with G protein and bound to the pan-agonist, rotigotine, which is used to treat PD and restless legs syndrome. The structures reveal the basis of rotigotine recognition in different dopamine receptors. Structural analysis together with functional assays illuminate determinants of ligand polypharmacology and selectivity. The structures also uncover the mechanisms of dopamine receptor activation, unique structural features among the five receptor subtypes, and the basis of G protein coupling specificity. Our work provides a comprehensive set of structural templates for the rational design of specific ligands to treat CNS diseases targeting the dopaminergic system.

The κ-opioid receptor (KOR) represents a highly desirable therapeutic target for treating not only pain but also addiction and affective disorders 1 . However, the development of KOR analgesics has been hindered by the associated hallucinogenic side effects 2 . The initiation of KOR signalling requires the G i/o -family proteins including the conventional (G i1 , G i2 , G i3 , G oA and G oB ) and nonconventional (G z and G g ) subtypes. How hallucinogens exert their actions through KOR and how KOR determines G-protein subtype selectivity are not well understood. Here we determined the active-state structures of KOR in a complex with multiple G-protein heterotrimers—G i1 , G oA , G z and G g —using cryo-electron microscopy. The KOR–G-protein complexes are bound to hallucinogenic salvinorins or highly selective KOR agonists. Comparisons of these structures reveal molecular determinants critical for KOR–G-protein interactions as well as key elements governing G i/o -family subtype selectivity and KOR ligand selectivity. Furthermore, the four G-protein subtypes display an intrinsically different binding affinity and allosteric activity on agonist binding at KOR. These results provide insights into the actions of opioids and G-protein-coupling specificity at KOR and establish a foundation to examine the therapeutic potential of pathway-selective agonists of KOR.