Distinct profiles of functional discrimination among G proteins determine the actions of G protein-coupled receptors

GzESTY as an optimized cell-based assay for initial steps in GPCR deorphanization

G protein-coupled receptors (GPCRs) are key pharmacological targets, yet many remain underutilized due to unknown activation mechanisms and ligands. Orphan GPCRs, lacking identified natural ligands, are a high priority for research, as identifying their ligands will aid in understanding their functions and potential as drug targets. Most GPCRs, including orphans, couple to Gi/o/z family members, however current assays to detect their activation are limited, hindering ligand identification efforts. We introduce GzESTY, a sensitive, cell-based assay developed in an easily deliverable format designed to study the pharmacology of Gi/o/z-coupled GPCRs and assist in deorphanization. We optimized assay conditions and developed an all-in-one vector employing cloning methods to ensure the correct expression ratio of GzESTY components. GzESTY successfully assessed activation of a library of ligand-activated GPCRs, detecting both full and partial agonism, and responses from endogenous GPCRs. Notably, with GzESTY we established the presence of endogenous ligands for GPR176 and GPR37 in brain extracts, validating its use in deorphanization efforts. This assay enhances the ability to find ligands for orphan GPCRs, expanding the toolkit for GPCR pharmacologists.

G Protein Selectivity in Dopamine Receptors is Determined before GDP Release

Dopaminergic signaling in neurophysiological processes utilizes multiple G proteins. The dopamine receptor subtypes D1R/D5R selectively couple to Gs/olf proteins, while D2R/D3R/D4R is selective for Gi/o proteins. The molecular mechanisms underlying this selectivity are not clear, so structural models of D1R and D2R were built in complex with their cognate and noncognate G proteins, in either GDP-bound or nucleotide-free states. These eight complexes were relaxed in a membrane environment through 2 μs-long molecular dynamics (MD) simulations. A thermodynamic analysis of these complexes provided free energies of G protein binding to the receptors that was consistent with D1R's preference for Gs protein and D2R's preference for Gi protein, but only for the GDP-bound states of the G proteins, suggesting that Gs vs Gi selectivity happens before GDP release. Biophysical measurements of receptor preassociation with G proteins in cells were also consistent with these preferences. The role of the Gα protein's α5-helix in G protein selectivity was probed by switching the last 18 residues of Gα between Gαs and Gαi to create chimeric Gi18s and Gs18i proteins. Thermodynamic analysis of MD-relaxed chimeric complexes revealed a complete switch in G protein binding selectivity for both D1R and D2R receptors, but again only for the GDP-bound G proteins. Biophysical measurements of receptor preassociation with G proteins in cells also overall supported this selectivity alteration. These studies have shown that G protein selectivity for dopamine receptors is conferred before GDP release; however, additional molecular events may be needed for a productive coupling to enable a successful GDP/GTP exchange.

Cyclic peptide inhibitors function as molecular glues to stabilize Gq/11 heterotrimers

Heterotrimeric Gα:Gβγ G proteins function as molecular switches downstream of G protein-coupled receptors (GPCRs). They alternate between a heterotrimeric GDP-bound OFF-state and a GTP-bound ON-state in which GαGTP is separated from the Gβγ dimer. Consequently, pharmacological tools to securely prevent the OFF-ON transition are of utmost importance to investigate their molecular switch function, specific contribution to GPCR signal transduction, and potential as drug targets. FR900359 (FR) and YM-254890 (YM), two natural cyclic peptides and highly specific inhibitors of Gq/11 heterotrimers, are exactly such tools. To date, their efficient and long-lasting inhibition of Gq/11 signaling has been attributed solely to a wedge-like binding to Gα, thereby preventing separation of the GTPase and α-helical domains and thus GDP release. Here, we use X-ray crystallography, biochemical and signaling assays, and BRET-based biosensors to show that FR and YM also function as stabilizers of the Gα:Gβγ subunit interface. Our high-resolution structures reveal a network of residues in Gα and two highly conserved amino acids in Gβ that are targeted by FR and YM to glue the Gβγ complex to the inactive GαGDP subunit. Unlike all previously developed nucleotide-state specific inhibitors that sequester Gα in its OFF-state but compete with Gβγ, FR and YM actively promote the inhibitory occlusion of GαGDP by Gβγ. In doing so, they securely lock the entire heterotrimer, not just Gα, in its inactive state. Our results identify FR and YM as molecular glues for Gα and Gβγ that combine simultaneous binding to both subunits with inhibition of G protein signaling.

Loss-of-function Thr347Ala Variant in the G Protein Subunit-Α11 Causes Familial Hypocalciuric Hypercalcemia 2

CONTEXT AND OBJECTIVES

To date, only 4 loss-of-function variants in the GNA11 gene encoding the G protein subunit α11 (Gα11) leading to familial hypocalciuric hypercalcemia (FHH) 2 have been characterized. Gα11 is involved in calcium-sensing receptor (CaSR) signaling, and loss-of-function variants in GNA11 lead to reduced agonist potency at CaSR and an FHH phenotype.

DESIGN AND PARTICIPANTS

We have identified a family with a heterozygous GNA11 Thr347Ala variant and characterized its impact on calcium homeostasis in FHH2 patients and the signaling properties of CaSR through the Gα11-Thr347Ala variant in vitro.

MAIN OUTCOME MEASURES

The index patient and her family had clinical, biochemical, and genetic analyses performed. The expression levels of Gα11 and the cell-surface expression levels of CaSR in human embryonic kidney 293A Gq/11 knockout cells (ΔGq/11-HEK293A) cotransfected with CaSR and Gα11 [wild type (WT) or Thr347Ala] were determined, and the functional properties exhibited by calcium at CaSR were characterized in an inositol monophosphate (IP1) accumulation assay.

RESULTS

Heterozygous carriers of the GNA11 Thr347Ala variant had mild asymptomatic hypercalcemia, hypocalciuria, and inappropriately high normal PTH levels considering their elevated serum calcium levels. Whereas the variant did not impact Gα11 expression or CaSR cell surface expression levels, calcium displayed a moderately but significantly lower agonist potency at CaSR/Gα11-Thr347Ala-transfected cells compared with CaSR/Gα11-WT-transfected cells in the IP1 accumulation assay (EC50 values of 5.67 mM and 4.38 mM, respectively).

CONCLUSION

This identification of a novel GNA11 variant causing FHH2 substantiates the important role of Gα11 for CaSR signaling and Ca2+ homeostasis.

Biased signaling in drug discovery and precision medicine

Receptors are crucial for converting chemical and environmental signals into cellular responses, making them prime targets in drug discovery, with about 70% of drugs targeting these receptors. Biased signaling, or functional selectivity, has revolutionized drug development by enabling precise modulation of receptor signaling pathways. This concept is more firmly established in G protein-coupled receptor and has now been applied to other receptor types, including ion channels, receptor tyrosine kinases, and nuclear receptors. Advances in structural biology have further refined our understanding of biased signaling. This targeted approach enhances therapeutic efficacy and potentially reduces side effects. Numerous biased drugs have been developed and approved as therapeutics to treat various diseases, demonstrating their significant therapeutic potential. This review provides a comprehensive overview of biased signaling in drug discovery and disease treatment, highlighting recent advancements and exploring the therapeutic potential of these innovative modulators across various diseases.

A sensitive biosensor of endogenous Gαi activity enables the accurate characterization of endogenous GPCR agonist responses

The activation of heterotrimeric G proteins (Gαβγ) by G protein-coupled receptors (GPCRs) is a mechanism broadly used by eukaryotes to transduce signals across the plasma membrane and a target for many clinical drugs. Many optical biosensors commonly used for measuring GPCR-stimulated G protein activity rely on exogenously expressed GPCRs and/or G proteins, which compromise readout fidelity. Biosensors that measure endogenous signaling may interfere with the signaling process under investigation or have a limited dynamic range of detection, hindering applicability. Here, we developed an optical BRET-based biosensor, Gαi bONE-GO, that detects endogenous GTP-bound (active) Gαi upon stimulation of endogenous GPCRs more robustly than existing sensors of endogenous activity. Its design leverages the Gαi-binding protein GINIP as a high-affinity and specific detector of Gαi-GTP. We optimized this design to prevent interference with downstream Gi-dependent signaling and to enable implementation in different experimental systems having endogenous GPCRs, including adenosine receptors in primary astroglial cells and opioid receptors in cell lines. In a neuronal cell line, Gαi bONE-GO revealed activation profiles indicating that several natural opioid neuropeptides acted as partial agonists, in contrast with their characterization as full agonists using biosensors that depend on exogenously expressed receptors and G proteins. The Gαi bONE-GO biosensor is a direct and sensitive detector of endogenous activation of Gαi proteins by GPCRs in different experimental settings but does not interfere with the subsequent propagation of signaling.

Structure and dynamics determine G protein coupling specificity at a class A GPCR

G protein-coupled receptors (GPCRs) exhibit varying degrees of selectivity for different G protein isoforms. Despite the abundant structures of GPCR-G protein complexes, little is known about the mechanism of G protein coupling specificity. The β2-adrenergic receptor is an example of GPCR with high selectivity for Gαs, the stimulatory G protein for adenylyl cyclase, and much weaker for the Gαi family of G proteins inhibiting adenylyl cyclase. By developing a Gαi-biased agonist (LM189), we provide structural and biophysical evidence supporting that distinct conformations at ICL2 and TM6 are required for coupling of the different G protein subtypes Gαs and Gαi. These results deepen our understanding of G protein specificity and bias and can accelerate the design of ligands that select for preferred signaling pathways.

The structure and function of the ghrelin receptor coding for drug actions

Drugs targeting the ghrelin receptor hold therapeutic potential in anorexia, obesity and diabetes. However, developing effective drugs is challenging. To tackle this common issue across a broad drug target, this study aims to understand how anamorelin, the only approved drug targeting the ghrelin receptor, operates compared to other synthetic drugs. Our research elucidated the receptor's structure with anamorelin and miniGq, unveiling anamorelin's superagonistic activity. We demonstrated that ligands with distinct chemical structures uniquely bind to the receptor, resulting in diverse conformations and biasing signal transduction. Moreover, our study showcased the utility of structural information in effectively identifying natural genetic variations altering drug action and causing severe functional deficiencies, offering a basis for selecting the right medication on the basis of the individual's genomic sequence. Thus, by building on structural analysis, this study enhances the foundational framework for selecting therapeutic agents targeting the ghrelin receptor, by effectively leveraging signaling bias and genetic variations.

Genetic variants of accessory proteins and G proteins in human genetic disease

We present a series of three articles on the genetics and pharmacogenetics of G protein- coupled receptors (GPCR). In the first article, we discuss genetic variants of the G protein subunits and accessory proteins that are associated with human phenotypes; in the second article, we build upon this to discuss "G protein-coupled receptor (GPCR) gene variants and human genetic disease" and in the third article, we survey "G protein-coupled receptor pharmacogenomics". In the present article, we review the processes of ligand binding, GPCR activation, inactivation, and receptor trafficking to the membrane in the context of human genetic disease resulting from pathogenic variants of accessory proteins and G proteins. Pathogenic variants of the genes encoding G protein α and β subunits are examined in diverse phenotypes. Variants in the genes encoding accessory proteins that modify or organize G protein coupling have been associated with disease; these include the contribution of variants of the regulator of G protein signaling (RGS) to hypertension; the role of variants of activator of G protein signaling type III in phenotypes such as hypoxia; the contribution of variation at the RGS10 gene to short stature and immunological compromise; and the involvement of variants of G protein-coupled receptor kinases (GRKs), such as GRK4, in hypertension. Variation in genes that encode proteins involved in GPCR signaling are outlined in the context of the changes in structure and function that may be associated with human phenotypes.

A Fluorescent Probe Enables the Discovery of Improved Antagonists Targeting the Intracellular Allosteric Site of the Chemokine Receptor CCR7

Intracellular ligands of G protein-coupled receptors (GPCRs) are gaining significant interest in drug discovery. Here, we report the development of the fluorescent ligand Mz437 (4) targeting the CC chemokine receptor CCR7 at an intracellular allosteric site. We demonstrate its experimental power by applying 4 to identify two improved intracellular CCR7 antagonists, SLW131 (10) and SLW132 (21m), developed by converting two weakly active antagonists into single- or double-digit nanomolar ligands with minimal modifications. The thiadiazoledioxide 10 was derived from the CCR7 antagonist Cmp2105 by removing a methyl group from the benzamide moiety, while the squaramide 21m was obtained from the CXCR1/CXCR2 antagonist and clinical candidate navarixin by replacing the ethyl substituent by a tert-butyl group to engage a lipophilic subpocket. We show that 10 and 21m qualify to probe CCR7 biology in recombinant and primary immune cells and expect our novel probes to facilitate the design of next-generation intracellular CCR7 ligands.

Buprenorphine Pharmacodynamics: A Bridge to Understanding Buprenorphine Clinical Benefits

Buprenorphine is an agonist at the mu opioid receptor (MOR) and antagonist at the kappa (KOR) and delta (DOR) receptors and a nociceptin receptor (NOR) ligand. Buprenorphine has a relatively low intrinsic efficacy for G-proteins and a long brain and MOR dwell time. Buprenorphine ceiling on respiratory depression has theoretically been related multiple factors such as low intrinsic efficacy at MOR, binding to six-transmembrane MOR and interactions in MOR/NOR heterodimers. Buprenorphine reduces analgesic tolerance by acting as a delta opioid receptor (DOR) antagonist. As a kappa opioid receptor (KOR) antagonist, buprenorphine reduces craving associated with addiction. Buprenorphine is a model opioid for the ordinal bifunctional analogs BU10038, BU08028 which have been shown to be potent analgesics in non-human primates without reinforcing effects and little to no respiratory depression.

Structure and dynamics determine G protein coupling specificity at a class A GPCR

G protein coupled receptors (GPCRs) exhibit varying degrees of selectivity for different G protein isoforms. Despite the abundant structures of GPCR-G protein complexes, little is known about the mechanism of G protein coupling specificity. The β2-adrenergic receptor is an example of GPCR with high selectivity for Gαs, the stimulatory G protein for adenylyl cyclase, and much weaker for the Gαi family of G proteins inhibiting adenylyl cyclase. By developing a new Gαi-biased agonist (LM189), we provide structural and biophysical evidence supporting that distinct conformations at ICL2 and TM6 are required for coupling of the different G protein subtypes Gαs and Gαi. These results deepen our understanding of G protein specificity and bias and can accelerate the design of ligands that select for preferred signaling pathways.

Targeting the activated allosteric conformation of the endothelin receptor B in melanoma with an antibody-drug conjugate: mechanisms and therapeutic efficacy

BACKGROUND

Endothelin 1 receptors are one of the drivers of tumor progression in many cancers. Inhibition of their signaling pathways with antagonist drugs has been the subject of numerous clinical trials, but the results have not met expectations probably due to the high endothelin concentrations in the tumor microenvironment and their unusually high affinity for their receptors.

METHODS

We previously reported the rendomab B49 antibody (RB49) exhibiting a preferential affinity for the activated conformation of human endothelin B receptor (ETB), not displaced by high endothelin levels, and without any pharmacological properties that could inhibit the division of melanoma cells. In this context, we have developed xiRB49-MMAE, a chimeric antibody-drug conjugated (ADC) to monomethyl auristatin E. We have characterized its physicochemical properties, studied its binding mechanisms, and evaluated its therapeutic potential in a preclinical model. Immunohistochemical analysis of metastatic melanoma lymph nodes evaluated RB49 as a diagnostic tool for patient stratification.

RESULTS

xiRB49-MMAE showed high efficacy against melanoma cells and ETB+ xenograft tumor models. IHC studies indicated that 100% of melanoma patient lymph node biopsies were RB49-positive.

CONCLUSIONS

xiRB49-MMAE is a promising drug candidate for clinical trials in ETB+ tumors. RB49 could be used as a diagnostic tool for patient stratification.

Exploring Bias in GPCR Signaling and its Implication in Drug Development: A One-Sided Affair

G protein-coupled receptors (GPCRs) play a pivotal role in regulating numerous physiological processes through their interactions with two key effectors: G proteins and β-arrestins (βarrs). This makes them crucial targets for therapeutic drug development. Interestingly, the evolving concept of biased signaling where ligands selectively activate either the G proteins or the βarrs has not only refined our understanding of segregation of physiological responses downstream of GPCRs but has also revolutionized drug discovery, offering the potential for treatments with enhanced efficacy and minimal side effects. This Review explores the mechanisms behind biased agonism, exploring it through various lenses, including ligand, receptor, cellular systems, location, and tissue-specific biases. It also offers structural insights into both orthosteric and allosteric ligand-binding pockets, structural rearrangements associated with the loops, and how ligand-engineering can contribute to biased signaling. Moreover, we also discuss the unique conformational signature in an intrinsically biased GPCR, which currently remains relatively less explored and adds a new dimension in biased signaling. Lastly, we address the translational challenges and practical considerations in characterizing bias, emphasizing its therapeutic potential and the latest advancements in drug development. By designing ligands that target specific signaling pathways, biased signaling presents a transformative approach to creating safer and more effective therapies. This Review focuses on our current understanding of GPCR-biased signaling, discussing potential mechanisms that lead to bias, the effect of bias on GPCR structures at a molecular level, recent advancements, and its profound potential to drive innovation in drug discovery.

Astrocytic G Protein-Coupled Receptors in Drug Addiction

Understanding the cellular mechanisms of drug addiction remains a key task in current brain research. While neuron-based mechanisms have been extensively explored over the past three decades, recent evidence indicates a critical involvement of astrocytes, the main type of non-neuronal cells in the brain. In response to extracellular stimuli, astrocytes modulate the activity of neurons, synaptic transmission, and neural network properties, collectively influencing brain function. G protein-coupled receptors (GPCRs) expressed on astrocyte surfaces respond to neuron- and environment-derived ligands by activating or inhibiting astrocytic signaling, which in turn regulates adjacent neurons and their circuitry. In this review, we focus on the dopamine D1 receptors (D1R) and metabotropic glutamate receptor 5 (mGLUR5 or GRM5)-two GPCRs that have been critically implicated in the acquisition and maintenance of addiction-related behaviors. Positioned as an introductory-level review, this article briefly discusses astrocyte biology, outlines earlier discoveries about the role of astrocytes in substance-use disorders (SUDs), and provides detailed discussion about astrocytic D1Rs and mGLUR5s in regulating synapse and network functions in the nucleus accumbens (NAc)-a brain region that mediates addiction-related emotional and motivational responses. This review serves as a stepping stone for readers of Engineering to explore links between astrocytic GPCRs and drug addiction and other psychiatric disorders.

Detailed functional characterization of four nanobodies as positive allosteric modulators of the human calcium-sensing receptor

The calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis, and small-molecule and peptide positive allosteric modulators (PAMs) of CaSR, so-called calcimimetics, are used in the treatment of hyperparathyroidism and hypocalcemic disorders. In this study, four monovalent nanobodies - representing four distinct nanobody families with CaSR PAM activity - were subjected to elaborate pharmacological profiling at the receptor. While Nb5 displayed negligible PAM activity at CaSR in all assays, Nb4, Nb10 and Nb45 all potently potentiated Ca2+-evoked signalling through a myc epitope-tagged CaSR expressed in HEK293 or HEK293T cells in Gαq and Gαi1 protein activation assays and in a Ca2+/Fluo-4 assay. Nb4 and Nb10 also displayed comparable PAM properties at a stable CaSR-HEK293 cell line in a Ca2+/Fura-2 imaging assay, but surprisingly Nb45 was completely inactive at this cell line in both the Ca2+/Fura-2 and Ca2+/Fluo-4 assays. Investigations into this binary difference in Nb45 activity revealed that the nanobody only possesses modulatory activity at CaSRs tagged N-terminally with various epitopes (myc, HA, Flag-SNAP), whereas it is inactive at the untagged wild-type receptor. In conclusion, overall each of the four nanobodies exhibit similar CaSR PAM properties in a range of assays, and thus none of them display pathway bias as modulators. However, of the four nanobodies Nb4 and Nb10 would be applicable as pharmacological tools for the wild-type CaSR, whereas the complete inactivity of Nb45 at the untagged CaSR serves as an reminder that epitope-tagging of a receptor, even if deemed functionally silent, can have profound implications for ligand discovery efforts.

Distinct autoregulatory roles of ELFN1 intracellular and extracellular domains on membrane trafficking, synaptic localization, and dimerization

Synaptic adhesion molecules are essential components of the synapse, yet the diversity of these molecules and their associated functions remain to be fully characterized. Extracellular leucine rich repeat and fibronectin type III domain containing 1 (ELFN1) is a postsynaptic adhesion molecule in the brain that has been increasingly implicated in human neurological disease. ELFN1 is best known for trans-synaptically modulating group III metabotropic glutamate receptors (mGluRs). However, little is known about ELFN1 organization and regulation, which likely govern and precede its ultimate trans-synaptic engagement with group III mGluRs. Herein, we report that the intracellular ELFN1 domain controls membrane trafficking and post-synaptic localization of ELFN1. We pinpoint a ∼30 amino acid juxtamembranous region required for membrane-targeting and discover that ELFN1 exists as an obligate homodimer prior to its trafficking to the membrane. We determine that ELFN1 homodimerization is not appreciably affected by the intracellular region and instead utilizes the extracellular leucine rich repeats (LRR) domain. We find that a single membrane-targeting motif located in one protomer is sufficient for effective trafficking of the ELFN1 homodimer. We further demonstrate that the closest ELFN1 homolog, synaptic adhesion molecule ELFN2, exhibits similar properties and participates in heterodimerization with ELFN1. This establishes distinct autoregulatory roles of ELFN1 intracellular and extracellular domains on membrane trafficking, post-synaptic localization, and dimerization while indicating conservation of the mechanisms across the ELFN subfamily of cell adhesion molecules.

Disease-Associated Dopamine Receptor D2 Variants Exhibit Functional Consequences Depending on Different Heterotrimeric G-Protein Subunit Combinations

Background: Dopamine receptors (DRs) are G-protein-coupled receptors (GPCRs) found in the central nervous system (CNS). DRs are essential for mediating various downstream signaling cascades and play a critical role in regulating the dopaminergic nigrostriatal pathway, which is involved in motor control. Recently, mutations in DRD2 (WT), p.Ile212Phe (I212F), and p.Met345Arg (M345R) have been associated with hyperkinetic movement disorders and shown to alter heterotrimeric G-protein complex signaling and β-arrestin recruitment. Methods: To conduct a detailed investigation of the I212F and M345R functional phenotypes, we used the TRansdUcer PATHway (TRUPATH) assay to study heterotrimeric G-protein recruitment and the Parallel Receptorome Expression and Screening via Transcriptional Output (PRESTO-Tango) assay to evaluate transcriptional activation following arrestin translocation for β-arrestin recruitment. Results: In our study, we could confirm the reported mutant's loss-of-function phenotype in β-arrestin 2 recruitment (reduced agonist potency and decreased maximal signaling efficacy in comparison to the WT). However, a detailed analysis of basal/constitutive activity also revealed a gain-of-function phenotype for mutant M345R. For a more comprehensive investigation of heterotrimeric G-protein complex signaling, we investigated the impact of WT mutants in combination with (i) a specifically suggested assay, and (ii) the most abundantly expressed heterotrimeric G-protein complex combinations in WT receptor-enriched regions. We were able to confirm the reported gain-of-function phenotype by Rodriguez-Contreras et al. and extend it by the use of the most abundant heterotrimeric G-protein subunits, GαoA and Gαi1, β1 and β2, and γ3 and γ7, in mouse and human basal ganglia. Conclusions: Although our results indicate that the interaction of the two variants with the most highly expressed heterotrimeric G-protein complex subunit combinations also results in a gain-of-function phenotype, they also clearly demonstrate that the phenotype can be significantly altered, dependent on heterotrimeric G-protein complex expression.

Essential strategies for the detection of constitutive and ligand-dependent Gi-directed activity of 7TM receptors using bioluminescence resonance energy transfer

The constitutive (ligand-independent) signaling of G protein-coupled receptors (GPCRs) is being increasingly appreciated as an integral aspect of their function; however, it can be technically hard to detect for poorly characterized, e.g. orphan, receptors of the cAMP-inhibitory Gi-coupled (GiPCR) family. In this study, we delineate the optimal strategies for the detection of such activity across several GiPCRs in two cell lines. As our study examples, we chose two canonical GiPCRs - the constitutively active Smoothened and the ligand-activated CXCR4, - and one atypical GPCRs, the chemokine receptor ACKR3. We verified the applicability of three Bioluminescence Resonance Energy Transfer (BRET)-based assays - one measuring changes in intracellular cAMP, another in Gβγ/GRK3ct association and third in Gαi-Gβγ dissociation, - for assessing both constitutive and ligand-modulated activity of these receptors. We also revealed the possible caveats and sources of false positives, and proposed optimization strategies. All three types of assays confirmed the ligand-dependent activity of CXCR4, the controversial G protein incompetence of ACKR3, the constitutive Gi-directed activity of SMO, and its modulation by PTCH1. We also demonstrated that PTCH1 promotes SMO localization to the cell surface, thus enhancing its responsiveness not only to agonists but also to antagonists, which is a novel mechanism of regulation of a Class F GiPCR Smoothened.

Receptor-dependent influence of R7 RGS proteins on neuronal GIRK channel signaling dynamics

Most neurons are influenced by multiple neuromodulatory inputs that converge on common effectors. Mechanisms that route these signals are key to selective neuromodulation but are poorly understood. G protein-gated inwardly rectifying K+ (GIRK or Kir3) channels mediate postsynaptic inhibition evoked by G protein-coupled receptors (GPCRs) that signal via inhibitory G proteins. GIRK-dependent signaling is modulated by Regulator of G protein Signaling proteins RGS6 and RGS7, but their selectivity for distinct GPCR-GIRK signaling pathways in defined neurons is unclear. We compared how RGS6 and RGS7 impact GIRK channel regulation by the GABAB receptor (GABABR), 5HT1A receptor (5HT1AR), and A1 adenosine receptor (A1R) in hippocampal neurons. Our data show that RGS6 and RGS7 make non-redundant contributions to GABABR- and 5HT1AR-GIRK signaling and compartmentalization and suggest that GPCR-G protein preferences and the substrate bias of RGS proteins, as well as receptor-dependent differences in Gαo engagement and effector access, shape GPCR-GIRK signaling dynamics in hippocampal neurons.

Design of allosteric modulators that change GPCR G protein subtype selectivity

G protein-coupled receptors (GPCRs), the largest family of drug targets, can signal through 16 subtypes of Gα proteins. Biased compounds that selectively activate therapy-relevant pathways promise to be safer, more effective medications. The determinants of bias are poorly understood, however, and rationally-designed, G protein-subtype-selective compounds are lacking. Here, using the prototypical class A GPCR neurotensin receptor 1 (NTSR1), we find that small molecules binding the intracellular GPCR-transducer interface change G protein coupling by subtype-specific and predictable mechanisms, enabling rational drug design. We demonstrate that the compound SBI-553 switches NTSR1 G protein preference by acting both as a molecular bumper and a molecular glue. Structurally, SBI-553 occludes G protein binding determinants on NTSR1, promoting association with select G protein subtypes for which an alternative, shallow-binding conformation is energetically favorable. Minor modifications to the SBI-553 scaffold produce allosteric modulators with distinct G protein subtype selectivity profiles. Selectivity profiles are probe-independent, conserved across species, and translate to differences in in vivo activity. These studies demonstrate that G protein selectivity can be tailored with small changes to a single chemical scaffold targeting the receptor-transducer interface and, as this pocket is broadly conserved, present a strategy for pathway-selective drug discovery applicable to the diverse GPCR superfamily.

Unique pharmacology of mGlu homo- and heterodimers

Background and Purpose

Metabotropic glutamate receptors (mGlus) are obligate dimer G protein coupled receptors that can all homodimerize and heterodimerize in select combinations. Responses of mGlu heterodimers to selective ligands, including orthosteric agonists and allosteric modulators, are largely unknown.

Experimental Approach

The pharmacological properties of each group II and III mGlu homodimer (except mGlu6) and several heterodimers were examined when stochastically assembled in HEK293T cells, or specifically measured using an improved G protein mediated BRET assay employing complimented fragments of NanoLuciferase.

Results

Stochastically assembled receptors adopted unique signaling characteristics. Some favored the potency, efficacy or signaling kinetics of a dominant subunit, while others exhibited blended profiles reflective of a combination of homo- and heterodimers at various ratios of expressed receptor. Finally, group II and III mGlu dimers were examined for responses to selective agonists and allosteric modulators. Effects of glutamate and selective group II and III orthosteric agonists were found to result in unique concentration response profiles when examining each combination of group II and II mGlu. Effects of select allosteric modulators were examined for each mGlu2 containing dimer as well as several group III dimer pairs. Likewise, allosteric modulator effects were often unique across dimers containing the targeted subunit of the ligand being tested.

Conclusions

Results demonstrate that mGlu dimers respond uniquely to selective ligands, and show that the mGlu family is not governed by generalizable rules dictating consequences of dimeric subunit interactions leading to signaling consequences.

Receptor Determinants for β-Arrestin Functional Specificity at C-X-C Chemokine Receptor 5

β-arrestins are multifaceted adaptor proteins that mediate G protein-coupled receptor (GPCR) desensitization, internalization, and signaling. It is emerging that receptor-specific determinants specify these divergent functions at GPCRs, yet this remains poorly understood. Here, we set out to identify the receptor determinants responsible for β-arrestin-mediated regulation of the chemokine receptor C-X-C motif chemokine receptor 5 (CXCR5). Using bioluminescence resonance energy transfer, we show that β-arrestin1 and β-arrestin2 are dose-dependently recruited to CXCR5 by its cognate ligand C-X-C motif chemokine ligand 13 (CXCL13). The carboxy-terminal tail of CXCR5 contains several serine/threonine residues that can be divided into three discrete phospho-site clusters based on their position relative to transmembrane domain 7. Mutagenesis experiments revealed that the distal and medial phospho-site clusters, but not the proximal, are required for agonist-stimulated β-arrestin1 or β-arrestin2 recruitment to CXCR5. Consistent with this, we provide evidence that the distal and medial, but not proximal, phospho-site clusters are required for receptor desensitization. Surprisingly, the individual phospho-site clusters are not required for agonist-stimulated internalization of CXCR5. Further, we show that CXCL13-stimulated CXCR5 internalization and ERK1/2 phosphorylation, but not desensitization, remain intact in human embryonic kidney 293 cells lacking β-arrestin1 and β-arrestin2. Our study provides evidence that β-arrestins are recruited to CXCR5 and are required for desensitization but are dispensable for internalization or signaling, suggesting that discrete receptor determinants specify the divergent functions of β-arrestins. SIGNIFICANCE STATEMENT: C-X-C motif ligand 13 (CXCL13) and C-X-C motif chemokine receptor 5 (CXCR5) are important in the immune system and are linked to diseases, yet regulation of CXCR5 signaling remains poorly understood. We provide evidence that a phospho-site cluster located at the extreme distal carboxyl-terminal tail of the receptor is responsible for β-arrestin recruitment and receptor desensitization. β-arrestins are not required for CXCL13-stimulated internalization or signaling, indicating that β-arrestins perform only one of their functions at CXCR5 and that discrete receptor determinants specify the divergent functions of β-arrestins.

Negative allosteric modulator of Group Ⅰ mGluRs: Recent advances and therapeutic perspective for neuropathic pain

Neuropathic pain (NP) is a widespread public health problem that existing therapeutic treatments cannot manage adequately; therefore, novel treatment strategies are urgently required. G-protein-coupled receptors are important for intracellular signal transduction, and widely participate in physiological and pathological processes, including pain perception. Group I metabotropic glutamate receptors (mGluRs), including mGluR1 and mGluR5, are predominantly implicated in central sensitization, which can lead to hyperalgesia and allodynia. Many orthosteric site antagonists targeting Group I mGluRs have been found to alleviate NP, but their poor efficacy, low selectivity, and numerous side effects limit their development in NP treatment. Here we reviewed the advantages of Group I mGluRs negative allosteric modulators (NAMs) over orthosteric site antagonists based on allosteric modulation mechanism, and the challenges and opportunities of Group I mGluRs NAMs in NP treatment. This article aims to elucidate the advantages and future development potential of Group I mGluRs NAMs in the treatment of NP.

Mechanistic insights into G-protein coupling with an agonist-bound G-protein-coupled receptor

G-protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by promoting guanine nucleotide exchange. Here, we investigate the coupling of G proteins with GPCRs and describe the events that ultimately lead to the ejection of GDP from its binding pocket in the Gα subunit, the rate-limiting step during G-protein activation. Using molecular dynamics simulations, we investigate the temporal progression of structural rearrangements of GDP-bound Gs protein (Gs·GDP; hereafter GsGDP) upon coupling to the β2-adrenergic receptor (β2AR) in atomic detail. The binding of GsGDP to the β2AR is followed by long-range allosteric effects that significantly reduce the energy needed for GDP release: the opening of α1-αF helices, the displacement of the αG helix and the opening of the α-helical domain. Signal propagation to the Gs occurs through an extended receptor interface, including a lysine-rich motif at the intracellular end of a kinked transmembrane helix 6, which was confirmed by site-directed mutagenesis and functional assays. From this β2AR-GsGDP intermediate, Gs undergoes an in-plane rotation along the receptor axis to approach the β2AR-Gsempty state. The simulations shed light on how the structural elements at the receptor-G-protein interface may interact to transmit the signal over 30 Å to the nucleotide-binding site. Our analysis extends the current limited view of nucleotide-free snapshots to include additional states and structural features responsible for signaling and G-protein coupling specificity.

A molecular mechanism to diversify Ca2+ signaling downstream of Gs protein-coupled receptors

A long-held tenet in inositol-lipid signaling is that cleavage of membrane phosphoinositides by phospholipase Cβ (PLCβ) isozymes to increase cytosolic Ca2+ in living cells is exclusive to Gq- and Gi-sensitive G protein-coupled receptors (GPCRs). Here we extend this central tenet and show that Gs-GPCRs also partake in inositol-lipid signaling and thereby increase cytosolic Ca2+. By combining CRISPR/Cas9 genome editing to delete Gαs, the adenylyl cyclase isoforms 3 and 6, or the PLCβ1-4 isozymes, with pharmacological and genetic inhibition of Gq and G11, we pin down Gs-derived Gβγ as driver of a PLCβ2/3-mediated cytosolic Ca2+ release module. This module does not require but crosstalks with Gαs-dependent cAMP, demands Gαq to release PLCβ3 autoinhibition, but becomes Gq-independent with mutational disruption of the PLCβ3 autoinhibited state. Our findings uncover the key steps of a previously unappreciated mechanism utilized by mammalian cells to finetune their calcium signaling regulation through Gs-GPCRs.

Direct detection of endogenous Gαi activity in cells with a sensitive conformational biosensor

Activation of heterotrimeric G-proteins (Gαβγ) by G-protein-coupled receptors (GPCRs) is not only a mechanism broadly used by eukaryotes to transduce signals across the plasma membrane, but also the target for a large fraction of clinical drugs. However, approaches typically used to assess this signaling mechanism by directly measuring G-protein activity, like optical biosensors, suffer from limitations. On one hand, many of these biosensors require expression of exogenous GPCRs and/or G-proteins, compromising readout fidelity. On the other hand, biosensors that measure endogenous signaling may still interfere with the signaling process under investigation or suffer from having a small dynamic range of detection, hindering broad applicability. Here, we developed an optical biosensor that detects the endogenous G-protein active species Gαi-GTP upon stimulation of endogenous GPCRs more robustly than current state-of-the-art sensors for the same purpose. Its design is based on the principle of bystander Bioluminescence Resonance Energy Transfer (BRET) and leverages the Gαi-binding protein named GINIP as a high affinity and specific detector module of the GTP-bound conformation of Gαi. We optimized this design to prevent interference with Gi-dependent signaling (cAMP inhibition) and to enable implementation in different experimental systems with endogenous GPCRs, including neurotransmitter receptors in primary astroglial cells or opioid receptors in cell lines, which revealed opioid neuropeptide-mediated activation profiles different from those observed with other biosensors involving exogenous GPCRs and G-proteins. Overall, we introduce a biosensor that directly and sensitively detects endogenous activation of G-proteins by GPCRs across different experimental settings without interfering with the subsequent propagation of signaling.

Get Ready to Sharpen Your Tools: A Short Guide to Heterotrimeric G Protein Activity Biosensors

G protein-coupled receptors (GPCRs) are the largest class of transmembrane receptors encoded in the human genome, and they initiate cellular responses triggered by a plethora of extracellular stimuli ranging from neurotransmitters and hormones to photons. Upon stimulation, GPCRs activate heterotrimeric G proteins (Gαβγ) in the cytoplasm, which then convey signals to their effectors to elicit cellular responses. Given the broad biological and biomedical relevance of GPCRs and G proteins in physiology and disease, there is great interest in developing and optimizing approaches to measure their signaling activity with high accuracy and across experimental systems pertinent to their functions in cellular communication. This review provides a historical perspective on approaches to measure GPCR-G protein signaling, from quantification of second messengers and other indirect readouts of activity to biosensors that directly detect the activity of G proteins. The latter is the focus of a more detailed overview of the evolution of design principles for various optical biosensors of G protein activity with different experimental capabilities. We will highlight advantages and limitations of biosensors that detect different G protein activation hallmarks, like dissociation of Gα and Gβγ or nucleotide exchange on Gα, as well as their suitability to detect signaling mediated by endogenous versus exogenous signaling components or in physiologically relevant systems like primary cells. Overall, this review intends to provide an assessment of the state-of-the-art for biosensors that directly measure G protein activity to allow readers to make informed decisions on the selection and implementation of currently available tools. SIGNIFICANCE STATEMENT: G protein activity biosensors have become essential and widespread tools to assess GPCR signaling and pharmacology. Yet, investigators face the challenge of choosing from a growing list of G protein activity biosensors. This review provides an overview of the features and capabilities of different optical biosensor designs for the direct detection of G protein activity in cells, with the aim of facilitating the rational selection of systems that align with the specific scientific questions and needs of investigators.

A neurodevelopmental disorder mutation locks G proteins in the transitory pre-activated state

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.

Gz Enhanced Signal Transduction assaY (GZESTY) for GPCR deorphanization

G protein-coupled receptors (GPCRs) are key pharmacological targets, yet many remain underutilized due to unknown activation mechanisms and ligands. Orphan GPCRs, lacking identified natural ligands, are a high priority for research, as identifying their ligands will aid in understanding their functions and potential as drug targets. Most GPCRs, including orphans, couple to Gi/o/z family members, however current assays to detect their activation are limited, hindering ligand identification efforts. We introduce GZESTY, a highly sensitive, cell-based assay developed in an easily deliverable format designed to study the pharmacology of Gi/o/z-coupled GPCRs and assist in deorphanization. We optimized assay conditions and developed an all-in-one vector employing novel cloning methods to ensure the correct expression ratio of GZESTY components. GZESTY successfully assessed activation of a library of ligand-activated GPCRs, detecting both full and partial agonism, as well as responses from endogenous GPCRs. Notably, with GZESTY we established the presence of endogenous ligands for GPR176 and GPR37 in brain extracts, validating its use in deorphanization efforts. This assay enhances the ability to find ligands for orphan GPCRs, expanding the toolkit for GPCR pharmacologists.

Generation of Comprehensive GPCR-Transducer-Deficient Cell Lines to Dissect the Complexity of GPCR Signaling

G-protein-coupled receptors (GPCRs) compose the largest family of transmembrane receptors and are targets of approximately one-third of Food and Drug Administration-approved drugs owing to their involvement in almost all physiologic processes. GPCR signaling occurs through the activation of heterotrimeric G-protein complexes and β-arrestins, both of which serve as transducers, resulting in distinct cellular responses. Despite seeming simple at first glance, accumulating evidence indicates that activation of either transducer is not a straightforward process as a stimulation of a single molecule has the potential to activate multiple signaling branches. The complexity of GPCR signaling arises from the aspects of G-protein-coupling selectivity, biased signaling, interpathway crosstalk, and variable molecular modifications generating these diverse signaling patterns. Numerous questions relative to these aspects of signaling remained unanswered until the recent development of CRISPR genome-editing technology. Such genome editing technology presents opportunities to chronically eliminate the expression of G-protein subunits, β-arrestins, G-protein-coupled receptor kinases (GRKs), and many other signaling nodes in the GPCR pathways at one's convenience. Here, we review the practicality of using CRISPR-derived knockout (KO) cells in the experimental contexts of unraveling the molecular details of GPCR signaling mechanisms. To mention a few, KO cells have revealed the contribution of β-arrestins in ERK activation, Gα protein selectivity, GRK-based regulation of GPCRs, and many more, hence validating its broad applicability in GPCR studies. SIGNIFICANCE STATEMENT: This review emphasizes the practical application of G-protein-coupled receptor (GPCR) transducer knockout (KO) cells in dissecting the intricate regulatory mechanisms of the GPCR signaling network. Currently available cell lines, along with accumulating KO cell lines in diverse cell types, offer valuable resources for systematically elucidating GPCR signaling regulation. Given the association of GPCR signaling with numerous diseases, uncovering the system-based signaling map is crucial for advancing the development of novel drugs targeting specific diseases.

Growth factor-dependent phosphorylation of Gαi shapes canonical signaling by G protein-coupled receptors

A long-standing question in the field of signal transduction is how distinct signaling pathways interact with each other to control cell behavior. Growth factor receptors and G protein-coupled receptors (GPCRs) are the two major signaling hubs in eukaryotes. Given that the mechanisms by which they signal independently have been extensively characterized, we investigated how they may cross-talk with each other. Using linear ion trap mass spectrometry and cell-based biophysical, biochemical, and phenotypic assays, we found at least three distinct ways in which epidermal growth factor affected canonical G protein signaling by the Gi-coupled GPCR CXCR4 through the phosphorylation of Gαi. Phosphomimicking mutations in two residues in the αE helix of Gαi (tyrosine-154/tyrosine-155) suppressed agonist-induced Gαi activation while promoting constitutive Gβγ signaling. Phosphomimicking mutations in the P loop (serine-44, serine-47, and threonine-48) suppressed Gi activation entirely, thus completely segregating growth factor and GPCR pathways. As expected, most of the phosphorylation events appeared to affect intrinsic properties of Gαi proteins, including conformational stability, nucleotide binding, and the ability to associate with and to release Gβγ. However, one phosphomimicking mutation, targeting the carboxyl-terminal residue tyrosine-320, promoted mislocalization of Gαi from the plasma membrane, a previously uncharacterized mechanism of suppressing GPCR signaling through G protein subcellular compartmentalization. Together, these findings elucidate not only how growth factor and chemokine signals cross-talk through the phosphorylation-dependent modulation of Gαi but also how such cross-talk may generate signal diversity.

Bayesian network models identify cooperative GPCR:G protein interactions that contribute to G protein coupling

Cooperative interactions in protein-protein interfaces demonstrate the interdependency or the linked network-like behavior and their effect on the coupling of proteins. Cooperative interactions also could cause ripple or allosteric effects at a distance in protein-protein interfaces. Although they are critically important in protein-protein interfaces, it is challenging to determine which amino acid pair interactions are cooperative. In this work, we have used Bayesian network modeling, an interpretable machine learning method, combined with molecular dynamics trajectories to identify the residue pairs that show high cooperativity and their allosteric effect in the interface of G protein-coupled receptor (GPCR) complexes with Gα subunits. Our results reveal six GPCR:Gα contacts that are common to the different Gα subtypes and show strong cooperativity in the formation of interface. Both the C terminus helix5 and the core of the G protein are codependent entities and play an important role in GPCR coupling. We show that a promiscuous GPCR coupling to different Gα subtypes, makes all the GPCR:Gα contacts that are specific to each Gα subtype (Gαs, Gαi, and Gαq). This work underscores the potential of data-driven Bayesian network modeling in elucidating the intricate dependencies and selectivity determinants in GPCR:G protein complexes, offering valuable insights into the dynamic nature of these essential cellular signaling components.

Agonist-selective activation of individual G-proteins by muscarinic receptors

Selective activation of individual subtypes of muscarinic receptors is a promising way to safely alleviate a wide range of pathological conditions in the central nervous system and the periphery as well. The flexible G-protein interface of muscarinic receptors allows them to interact with several G-proteins with various efficacy, potency, and kinetics. Agonists biased to the particular G-protein mediated pathway may result in selectivity among muscarinic subtypes and, due to the non-uniform expression of individual G-protein alpha subunits, possibly achieve tissue specificity. Here, we demonstrate that novel tetrahydropyridine-based agonists exert specific signalling profiles in coupling with individual G-protein α subunits. These signalling profiles profoundly differ from the reference agonist carbachol. Moreover, coupling with individual Gα induced by these novel agonists varies among subtypes of muscarinic receptors which may lead to subtype selectivity. Thus, the novel tetrahydropyridine-based agonist can contribute to the elucidation of the mechanism of pathway-specific activation of muscarinic receptors and serve as a starting point for the development of desired selective muscarinic agonists.

GPCR signaling bias: an emerging framework for opioid drug development

Biased signaling, also known as functional selectivity, has emerged as an important concept in drug development targeting G-protein-coupled receptors (GPCRs). Drugs that provoke biased signaling are expected to offer an opportunity for enhanced therapeutic effectiveness with minimized side effects. Opioid analgesics, whilst exerting potent pain-relieving effects, have become a social problem owing to their serious side effects. For the development of safer pain medications, there has been extensive exploration of agonists with a distinct balance of G-protein and β-arrestin (βarr) signaling. Recently, several approaches based on protein-protein interactions have been developed to precisely evaluate individual signal pathways, paving the way for the comprehensive analysis of biased signals. In this review, we describe an overview of bias signaling in opioid receptors, especially the μ-opioid receptor (MOR), and how to evaluate signaling bias in the GPCR field. We also discuss future directions for rational drug development through the integration of diverse signal datasets.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins encoded in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Activation of Multiple G Protein Pathways to Characterize the Five Dopamine Receptor Subtypes Using Bioluminescence Technology

G protein-coupled receptors show preference for G protein subtypes but can recruit multiple G proteins with various downstream signaling cascades. This functional selection can guide drug design. Dopamine receptors are both stimulatory (D1-like) and inhibitory (D2-like) with diffuse expression across the central nervous system. Functional selectivity of G protein subunits may help with dopamine receptor targeting and their downstream effects. Three bioluminescence-based assays were used to characterize G protein coupling and function with the five dopamine receptors. Most proximal to ligand binding was the miniG protein assay with split luciferase technology used to measure recruitment. For endogenous and selective ligands, the G-CASE bioluminescence resonance energy transfer (BRET) assay measured G protein activation and receptor selectivity. Downstream, the BRET-based CAMYEN assay quantified cyclic adenosine monophosphate (cAMP) changes. Several dopamine receptor agonists and antagonists were characterized for their G protein recruitment and cAMP effects. G protein selectivity with dopamine revealed potential Gq coupling at all five receptors, as well as the ability to activate subtypes with the "opposite" effects to canonical signaling. D1-like receptor agonist (+)-SKF-81297 and D2-like receptor agonist pramipexole showed selectivity at all receptors toward Gs or Gi/o/z activation, respectively. The five dopamine receptors show a wide range of potentials for G protein coupling and activation, reflected in their downstream cAMP signaling. Targeting these interactions can be achieved through drug design. This opens the door to pharmacological treatment with more selectivity options for inducing the correct physiological events.

Specific pharmacological and Gi/o protein responses of some native GPCRs in neurons

G protein-coupled receptors (GPCRs) constitute the largest family of membrane proteins and are important drug targets. The discovery of drugs targeting these receptors and their G protein signaling properties are based on assays mainly performed with modified receptors expressed in heterologous cells. However, GPCR responses may differ in their native environment. Here, by using highly sensitive Gi/o sensors, we reveal specific properties of Gi/o protein-mediated responses triggered by GABAB, α2 adrenergic and cannabinoid CB1 receptors in primary neurons, different from those in heterologous cells. These include different profiles in the Gi/o protein subtypes-mediated responses, and differences in the potencies of some ligands even at similar receptor expression levels. Altogether, our results show the importance of using biosensors compatible with primary cells for evaluating the activities of endogenous GPCRs in their native environment.

Current strategic trends in drug discovery: the present as prologue

INTRODUCTION

Escalating costs and inherent uncertainties associated with drug discovery invite initiatives to improve its efficiency and de-risk campaigns for inventing better therapeutics. One such initiative involves recognizing and exploiting current approaches in therapeutics invention with molecular mechanisms of action that hold promise for designing and targeting new chemical entities as drugs.

AREAS COVERED

This perspective considers the current contextual framework around three drug-discovery approaches and evaluates their potential to help identify new targets/modalities in small-molecule molecular pharmacology: diversifying ligand-directed phenotypes for G protein-coupled receptor (GPCR) pharmacotherapeutic signaling; developing therapeutic-protein degraders and stabilizers for proximity-inducing pharmacology; and mining organelle biology for druggable therapeutic targets.

EXPERT OPINION

The contemporary drug-discovery approaches examined appear generalizable and versatile to have applications in therapeutics invention beyond those case studies discussed herein. Accordingly, they may be considered strategic trends worthy of note in advancing the field toward novel ways of addressing pharmacotherapeutically unmet medical needs.

Allosteric modulation and G-protein selectivity of the Ca2+-sensing receptor

The calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor1 (GPCR) that has a central role in regulating systemic calcium homeostasis2,3. Here we use cryo-electron microscopy and functional assays to investigate the activation of human CaSR embedded in lipid nanodiscs and its coupling to functional Gi versus Gq proteins in the presence and absence of the calcimimetic drug cinacalcet. High-resolution structures show that both Gi and Gq drive additional conformational changes in the activated CaSR dimer to stabilize a more extensive asymmetric interface of the seven-transmembrane domain (7TM) that involves key protein-lipid interactions. Selective Gi and Gq coupling by the receptor is achieved through substantial rearrangements of intracellular loop 2 and the C terminus, which contribute differentially towards the binding of the two G-protein subtypes, resulting in distinct CaSR-G-protein interfaces. The structures also reveal that natural polyamines target multiple sites on CaSR to enhance receptor activation by zipping negatively charged regions between two protomers. Furthermore, we find that the amino acid L-tryptophan, a well-known ligand of CaSR extracellular domains, occupies the 7TM bundle of the G-protein-coupled protomer at the same location as cinacalcet and other allosteric modulators. Together, these results provide a framework for G-protein activation and selectivity by CaSR, as well as its allosteric modulation by endogenous and exogenous ligands.

Signaling Specificity and Kinetics of the Human Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluRs) are obligate dimer G protein coupled receptors that can all function as homodimers. Here, each mGluR homodimer was examined for its G protein coupling profile using a bioluminescence resonance energy transfer-based assay that detects the interaction between a split YFP-tagged Gβ 1γ2 and a Nanoluciferase tagged free Gβγ sensor, MAS-GRK3-ct- nanoluciferase with 14 specific Gα proteins heterologously expressed, representing each family. Canonically, the group II and III mGluRs (2 and 3 and 4, 6, 7, and 8, respectively) are thought to couple to Gi/o exclusively. In addition, the group I mGluRs (1 and 5) are known to couple to the Gq/11 family and generally thought to also couple to the pertussis toxin-sensitive Gi/o family some reports have suggested Gs coupling is possible as cAMP elevations have been noted. In this study, coupling was observed with all eight mGluRs through the Gi/o proteins and only mGluR1 and mGluR5 through Gq/11, and, perhaps surprisingly, not G14 None activated any Gs protein. Interestingly, coupling was seen with the group I and II but not the group III mGluRs to G16 Slow but significant coupling to Gz was also seen with the group II receptors. SIGNIFICANCE STATEMENT: Metabotropic glutamate receptor (mGluR)-G protein coupling has not been thoroughly examined, and some controversy remains about whether some mGluRs can activate Gαs family members. Here we examine the ability of each mGluR to activate representative members of every Gα protein family. While all mGluRs can activate Gαi/o proteins, only the group I mGluRs couple to Gαq/11, and no members of the family can activate Gαs family members, including the group I receptors alone or with positive allosteric modulators.

GproteinDb in 2024: new G protein-GPCR couplings, AlphaFold2-multimer models and interface interactions

G proteins are the major signal proteins of ∼800 receptors for medicines, hormones, neurotransmitters, tastants and odorants. GproteinDb offers integrated genomic, structural, and pharmacological data and tools for analysis, visualization and experiment design. Here, we present the first major update of GproteinDb greatly expanding its coupling data and structural templates, adding AlphaFold2 structure models of GPCR-G protein complexes and advancing the interactive analysis tools for their interfaces underlying coupling selectivity. We present insights on coupling agreement across datasets and parameters, including constitutive activity, agonist-induced activity and kinetics. GproteinDb is accessible at https://gproteindb.org.

Dissecting the molecular basis for the modulation of neurotransmitter GPCR signaling by GINIP

It is well established that G-protein-coupled receptors (GPCRs) stimulated by neurotransmitters are critical for neuromodulation. Much less is known about how heterotrimeric G-protein (Gαβγ) regulation after receptor-mediated activation contributes to neuromodulation. Recent evidence indicates that the neuronal protein GINIP shapes GPCR inhibitory neuromodulation via a unique mechanism of G-protein regulation that controls pain and seizure susceptibility. However, the molecular basis of this mechanism remains ill-defined because the structural determinants of GINIP responsible for binding and regulating G proteins are not known. Here, we combined hydrogen-deuterium exchange mass spectrometry, computational structure predictions, biochemistry, and cell-based biophysical assays to demonstrate an effector-like binding mode of GINIP to Gαi. Specific amino acids of GINIP's PHD domain first loop are essential for G-protein binding and subsequent regulation of Gαi-GTP and Gβγ signaling upon neurotransmitter GPCR stimulation. In summary, these findings shed light onto the molecular basis for a post-receptor mechanism of G-protein regulation that fine-tunes inhibitory neuromodulation.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically-relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed new insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally-occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Eyeless razor clam Sinonovacula constricta discriminates light spectra through opsins to guide Ca2+ and cAMP signaling pathways

Phototransduction is based on opsins that drive distinct types of Gα cascades. Although nonvisual photosensitivity has long been known in marine bivalves, the underlying molecular basis and phototransduction mechanism are poorly understood. Here, we introduced the eyeless razor clam Sinonovacula constricta as a model to clarify this issue. First, we showed that S. constricta was highly diverse in opsin family members, with a significant expansion in xenopsins. Second, the expression of putative S. constricta opsins was highly temporal-spatio specific, indicating their potential roles in S. constricta development and its peripheral photosensitivity. Third, by cloning four S. constricta opsins with relatively higher expression (Sc_opsin1, 5, 7, and 12), we found that they exhibited different expression levels in response to different light environments. Moreover, we demonstrated that these opsins (excluding Sc_opsin7) couple with Gαq and Gαi cascades to mediate the light-dependent Ca2+ (Sc_opsin1 and 5) and cAMP (Sc_opsin12) signaling pathways. The results indicated that Sc_opsin1 and 5 belonged to Gq-opsins, Sc_opsin12 belonged to Gi-opsins, while Sc_opsin7 might act as a photo-isomerase. Furthermore, we found that the phototransduction function of S. constricta Gq-opsins was dependent on the lysine at the seventh transmembrane domain, and greatly influenced by the external light spectra in a complementary way. Thus, a synergistic photosensitive system mediated by opsins might exist in S. constricta to rapidly respond to the transient or subtle changes of the external light environment. Collectively, our findings provide valuable insights into the evolution of opsins in marine bivalves and their potential functions in nonvisual photosensitivity.

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Modulating signalling lifetime to optimise a prototypical animal opsin for optogenetic applications

Animal opsins are light activated G-protein-coupled receptors, capable of optogenetic control of G-protein signalling for research or therapeutic applications. Animal opsins offer excellent photosensitivity, but their temporal resolution can be limited by long photoresponse duration when expressed outside their native cellular environment. Here, we explore methods for addressing this limitation for a prototypical animal opsin (human rod opsin) in HEK293T cells. We find that the application of the canonical rhodopsin kinase (GRK1)/visual arrestin signal termination mechanism to this problem is complicated by a generalised suppressive effect of GRK1 expression. This attenuation can be overcome using phosphorylation-independent mutants of arrestin, especially when these are tethered to the opsin protein. We further show that point mutations targeting the Schiff base stability of the opsin can also reduce signalling lifetime. Finally, we apply one such mutation (E122Q) to improve the temporal fidelity of restored visual responses following ectopic opsin expression in the inner retina of a mouse model of retinal degeneration (rd1). Our results reveal that these two strategies (targeting either arrestin binding or Schiff-base hydrolysis) can produce more time-delimited opsin signalling under heterologous expression and establish the potential of this approach to improve optogenetic performance.

Gα Protein Signaling Bias at Serotonin 1A Receptor

Serotonin 1A receptor (5-HT1AR) is a clinically relevant target because of its involvement in several central and peripheral functions, including sleep, temperature homeostasis, processing of emotions, and response to stress. As a G protein coupled receptor (GPCR) activating numerous Gα i/o/z family members, 5-HT1AR can potentially modulate multiple intracellular signaling pathways in response to different therapeutics. Here, we applied a cell-based bioluminescence resonance energy transfer assay to quantify how ten structurally diverse 5-HT1AR agonists exert biased signaling by differentially stimulating Gα i/o/z family members. Our concentration-response analysis of the activation of each Gα i/o/z protein revealed unique potency and efficacy profiles of selected agonists when compared with the reference 5-hydroxytryptamine, serotonin. Overall, our analysis of signaling bias identified groups of ligands sharing comparable G protein activation selectivity and also drugs with unique selectivity profiles. We observed, for example, a strong bias of F-15599 toward the activation of Gα i3 that was unique among the agonists tested: we found a biased factor of +2.19 when comparing the activation of Gα i3 versus Gα i2 by F-15599, while it was -0.29 for 8-hydroxy-2-(di-n-propylamino) tetralin. Similarly, vortioxetine showed a biased factor of +1.06 for Gα z versus Gα oA, while it was -1.38 for vilazodone. Considering that alternative signaling pathways are regulated downstream of each Gα protein, our data suggest that the unique pharmacological properties of the tested agonists could result in multiple unrelated cellular outcomes. Further investigation is needed to reveal how this type of ligand bias could affect cellular responses and to illuminate the molecular mechanisms underlying therapeutic profile and side effects of each drug. SIGNIFICANCE STATEMENT: Serotonin 1a receptor (5-HT1AR) activates several members of the Gi/o/z protein family. Here, we examined ten structurally diverse and clinically relevant agonists acting on 5-HT1AR and identified distinctive bias patterns among G proteins. Considering the diversity of their intracellular effectors and signaling properties, this data reveal novel mechanisms underlying both therapeutic and undesirable effects.

Severity of GNAO1-Related Disorder Correlates with Changes in G-Protein Function

OBJECTIVE

GNAO1-related disorders (OMIM #615473 and #617493), caused by variants in the GNAO1 gene, are characterized by developmental delay or intellectual disability, hypotonia, movement disorders, and epilepsy. Neither a genotype-phenotype correlation nor a clear severity score have been established for this disorder. The objective of this prospective and retrospective observational study was to develop a severity score for GNAO1-related disorders, and to delineate the correlation between the underlying molecular mechanisms and clinical severity.

METHODS

A total of 16 individuals with GNAO1-related disorders harboring 12 distinct missense variants, including four novel variants (p.K46R, p.T48I, p.R209P, and p.L235P), were examined with repeated clinical assessments, video-electroencephalogram monitoring, and brain magnetic resonance imaging. The molecular pathology of each variant was delineated using a molecular deconvoluting platform.

RESULTS

The patients displayed a wide variability in the severity of their symptoms. This heterogeneity was well represented in the GNAO1-related disorders severity score, with a broad range of results. Patients with the same variant had comparable severity scores, indicating that differences in disease profiles are not due to interpatient variability, but rather, to unique disease mechanisms. Moreover, we found a significant correlation between clinical severity scores and molecular mechanisms.

INTERPRETATION

The clinical score proposed here provides further insight into the correlation between pathophysiology and phenotypic severity in GNAO1-related disorders. We found that each variant has a unique profile of clinical phenotypes and pathological molecular mechanisms. These findings will contribute to better understanding GNAO1-related disorders. Additionally, the severity score will facilitate standardization of patients categorization and assessment of response to therapies in development. ANN NEUROL 2023;94:987-1004.

Rules and mechanisms governing G protein coupling selectivity of GPCRs

G protein-coupled receptors (GPCRs) convert extracellular stimuli into intracellular signaling by coupling to heterotrimeric G proteins of four classes: Gi/o, Gq, Gs, and G12/13. However, our understanding of the G protein selectivity of GPCRs is incomplete. Here, we quantitatively measure the enzymatic activity of GPCRs in living cells and reveal the G protein selectivity of 124 GPCRs with the exact rank order of their G protein preference. Using this information, we establish a classification of GPCRs by functional selectivity, discover the existence of a G12/13-coupled receptor, G15-coupled receptors, and a variety of subclasses for Gi/o-, Gq-, and Gs-coupled receptors, culminating in development of the predictive algorithm of G protein selectivity. We further identify the structural determinants of G protein selectivity, allowing us to synthesize non-existent GPCRs with de novo G protein selectivity and efficiently identify putative pathogenic variants.