FRET-Based Sensors Unravel Activation and Allosteric Modulation of the GABAB Receptor

Metabotropic GABAB Receptor Activation Induced by G Protein Coupling

G protein-coupled receptors (GPCRs) play central roles in regulating cellular responses through heterotrimeric G proteins (GP). Extensive studies have elucidated the complex cellular signaling mediated by GPCRs that accompany dynamic conformational changes upon activation. However, there has been less focus on the role of the GP on the activation process, particularly for class C GPCRs that function as obligate dimers. Herein, we report the pivotal role of GP coupling on the dynamic activation process for the metabotropic γ-aminobutyric acid receptor (GABABR) based on extensive atomistic simulations. We find that GP coupling triggers drastic conformational changes in the GABABR transmembrane domain (TMD), while an agonist alone is insufficient to shift the equilibrium state from the inactive to the active states. These conformational changes induced by GP coupling destabilize the inactive TM5/TM5 interface, shifting the equilibrium toward the activated TM6/TM6 interface. This active role of the GP in activation provides fresh insights into the activation mechanism of GABABR and perhaps other class C GPCRs. These insights should aid in the development of more potent and selective drugs.

Adaptive Workflows of Machine Learning Illuminate the Sequential Operation Mechanism of the TAK1's Allosteric Network

Allostery is a fundamental mechanism driving biomolecular processes that holds significant therapeutic concern. Our study rigorously investigates how two distinct machine-learning algorithms uniquely classify two already close-to-active DFG-in states of TAK1, differing just by the presence or absence of its allosteric activator TAB1, from an ensemble mixture of conformations (obtained from 2.4 μs molecular dynamics (MD) simulations). The novelty, however, lies in understanding the deeper algorithmic potentials to systematically derive a diverse set of differential residue connectivity features that reconstruct the essential mechanistic architecture for TAK1-TAB1 allostery in such a close-to-active biochemical scenario. While the recursive, random forest-based workflow displays the potential of conducting discretized, hierarchical derivation of allosteric features, a multilayer perceptron-based approach gains considerable efficacy in revealing fluid connected patterns of features when hybridized with mutual information scoring. Interestingly, both pipelines benchmark similar directions of functional conformational changes for TAK1's activation. The findings significantly advance the depth of mechanistic understanding by highlighting crucial activation signatures along a directed C-lobe → activation loop → ATP pocket channel of information flow, including (1) the αF-αE biterminal alignments and (2) the "catalytic" drift of the activation loop toward kinase active site. Besides, some novel allosteric hotspots (K253, Y206, N189, etc.) are further recognized as TAB1 sensors, transducers, and responders, including a benchmark E70 mutation site, precisely mapping the important structural segments for sequential allosteric execution. Hence, our work demonstrates how to navigate through greater structural depths and dimensions of dynamic allosteric machineries just by leveraging standard ML methods in suitable streamlined workflows adaptive to the specific system and objectives.

Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges

G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.

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.

Asymmetric activation of dimeric GABAB and metabotropic glutamate receptors

G protein-coupled receptors (GPCRs) represent the largest family of membrane proteins and are important drug targets. GPCRs are allosteric machines that transduce an extracellular signal to the cell by activating heterotrimeric G proteins. Herein, we summarize the recent advancements in the molecular activation mechanism of the γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors, the most important class C GPCRs that modulate synaptic transmission in the brain. Both are mandatory dimers, this quaternary structure being needed for their function The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of mGlu heterodimers, where the eight mGlu subunits can form specific and functional heterodimers. Finally, the development of allosteric modulators has revealed new possibilities for regulating the function of these receptors by targeting the transmembrane dimer interface. This family of receptors never ceases to astonish and serve as models to better understand the diversity and asymmetric functioning of GPCRs.NEW & NOTEWORTHY γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors form constitutive dimers, which are required for their function. They serve as models to better understand the diversity and activation of G protein-coupled receptors (GPCRs). The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of specific and functional mGlu heterodimers. Allosteric modulators can be developed to target the transmembrane interface and modulate the asymmetry.

Soluble amyloid-β precursor peptide does not regulate GABAB receptor activity

Amyloid-β precursor protein (APP) regulates neuronal activity through the release of secreted APP (sAPP) acting at cell surface receptors. APP and sAPP were reported to bind to the extracellular sushi domain 1 (SD1) of GABA_B receptors (GBRs). A 17 amino acid peptide (APP17) derived from APP was sufficient for SD1 binding and shown to mimic the inhibitory effect of sAPP on neurotransmitter release and neuronal activity. The functional effects of APP17 and sAPP were similar to those of the GBR agonist baclofen and blocked by a GBR antagonist. These experiments led to the proposal that sAPP activates GBRs to exert its neuronal effects. However, whether APP17 and sAPP influence classical GBR signaling pathways in heterologous cells was not analyzed. Here, we confirm that APP17 binds to GBRs with nanomolar affinity. However, biochemical and electrophysiological experiments indicate that APP17 does not influence GBR activity in heterologous cells. Moreover, APP17 did not regulate synaptic GBR localization, GBR-activated K⁺ currents, neurotransmitter release, or neuronal activity in vitro or in vivo. Our results show that APP17 is not a functional GBR ligand and indicate that sAPP exerts its neuronal effects through receptors other than GBRs.

GABAB Receptors: Anxiety and Mood Disorders

Gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, acts at the ionotropic GABA_A and GABA_C receptors, and the metabotropic GABA_B receptor. This chapter summarizes the studies that have investigated the role of the GABA_B receptor in stress-related psychiatric disorders including anxiety and mood disorders. Overall, clinical and preclinical evidences strongly suggest that the GABA_B receptor is a therapeutic candidate for depression and anxiety disorders. However, the clinical development of GABA_B receptor-based drugs to treat these disorders has been hampered by their potential side-effects, particularly those of agonists. Nevertheless, the discovery of novel GABA_B receptor allosteric modulators, and increasing understanding of the influence of specific intracellular GABA_B receptor-associated proteins on GABA_B receptor activity, may now pave the way towards GABA_B receptor therapeutics in the treatment of mood and anxiety disorders.

Allosteric ligands control the activation of a class C GPCR heterodimer by acting at the transmembrane interface

G protein-coupled receptors (GPCRs) are among the most promising drug targets. They often form homo- and heterodimers with allosteric cross-talk between receptor entities, which contributes to fine-tuning of transmembrane signaling. Specifically controlling the activity of GPCR dimers with ligands is a good approach to clarify their physiological roles and validate them as drug targets. Here, we examined the mode of action of positive allosteric modulators (PAMs) that bind at the interface of the transmembrane domains of the heterodimeric GABA_B receptor. Our site-directed mutagenesis results show that mutations of this interface impact the function of the three PAMs tested. The data support the inference that they act at the active interface between both transmembrane domains, the binding site involving residues of the TM6s of the GABA_B1 and the GABA_B2 subunit. Importantly, the agonist activity of these PAMs involves a key region in the central core of the GABA_B2 transmembrane domain, which also controls the constitutive activity of the GABA_B receptor. This region corresponds to the sodium ion binding site in class A GPCRs that controls the basal state of the receptors. Overall, these data reveal the possibility of developing allosteric compounds able to specifically modulate the activity of GPCR homo- and heterodimers by acting at their transmembrane interface.

The GABAB receptor mediates neuroprotection by coupling to G13

[Figure: see text].

Identification of thyroid hormone/thyroid hormone receptor interaction based on aptamer-assisted protein-induced fluorescence enhancement

We herein describe a novel method to identify thyroid hormone (TH)/thyroid hormone receptor (TR) interaction, termed aptamer-assisted protein-induced fluorescence enhancement (AptPIFE). In this method, a detection probe consisting of an RNA strand incorporating TH-specific aptamer and a Cy3-labeled DNA strand holds TH in close proximity to Cy3. The corresponding TR then binds to the TH near Cy3, consequently stimulating Cy3 to emit a significantly enhanced fluorescence through PIFE phenomenon. Based on this simple yet efficient design principle, we successfully identified the interaction of TH with TR within 10 min, down to 0.37 pM with excellent specificity. The practical and robust applicability of this method was also successfully validated by properly screening TR antagonists and reliably quantifying TH present in real clinical serum samples from patients with hyperthyroidism and healthy volunteers.

Multiple GPCR Functional Assays Based on Resonance Energy Transfer Sensors

G protein-coupled receptors (GPCRs) represent one of the largest membrane protein families that participate in various physiological and pathological activities. Accumulating structural evidences have revealed how GPCR activation induces conformational changes to accommodate the downstream G protein or β-arrestin. Multiple GPCR functional assays have been developed based on Förster resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) sensors to monitor the conformational changes in GPCRs, GPCR/G proteins, or GPCR/β-arrestin, especially over the past two decades. Here, we will summarize how these sensors have been optimized to increase the sensitivity and compatibility for application in different GPCR classes using various labeling strategies, meanwhile provide multiple solutions in functional assays for high-throughput drug screening.

Molecular mechanisms of metabotropic GABAB receptor function

Metabotropic γ-aminobutyric acid G protein-coupled receptors (GABA_B) represent one of the two main types of inhibitory neurotransmitter receptors in the brain. These receptors act both pre- and postsynaptically by modulating the transmission of neuronal signals and are involved in a range of neurological diseases, from alcohol addiction to epilepsy. A series of recent cryo-EM studies revealed critical details of the activation mechanism of GABA_B Structures are now available for the receptor bound to ligands with different modes of action, including antagonists, agonists, and positive allosteric modulators, and captured in different conformational states from the inactive apo to the fully active state bound to a G protein. These discoveries provide comprehensive insights into the activation of the GABA_B receptor, which not only broaden our understanding of its structure, pharmacology, and physiological effects but also will ultimately facilitate the discovery of new therapeutic drugs and neuromodulators.

COR758, a negative allosteric modulator of GABAB receptors

Allosteric modulators of G protein coupled receptors (GPCRs), including GABA_BRs (GABA_BRs), are promising therapeutic candidates. While several positive allosteric modulators (PAM) of GABA_BRs have been characterized, only recently the first negative allosteric modulator (NAM) has been described. In the present study, we report the characterization of COR758, which acts as GABA_BR NAM in rat cortical membranes and CHO cells stably expressing GABA_BRs (CHO-GABA_B). COR758 failed to displace the antagonist [³H]CGP54626 from the orthosteric binding site of GABA_BRs showing that it acts through an allosteric binding site. Docking studies revealed a possible new allosteric binding site for COR758 in the intrahelical pocket of the GABA_B1 monomer. COR758 inhibited basal and GABA_BR-stimulated O-(3-[³⁵Sthio)-triphosphate ([³⁵S]GTPγS) binding in brain membranes and blocked the enhancement of GABA_BR-stimulated [³⁵S]GTPγS binding by the PAM GS39783. Bioluminescent resonance energy transfer (BRET) measurements in CHO-GABA_B cells showed that COR758 inhibited G protein activation by GABA and altered GABA_BR subunit rearrangements. Additionally, the compound altered GABA_BR-mediated signaling such as baclofen-induced inhibition of cAMP production in transfected HEK293 cells, agonist-induced Ca²⁺ mobilization as well as baclofen and the ago-PAM CGP7930 induced phosphorylation of extracellular signal-regulated kinases (ERK1/2) in CHO-GABA_B cells. COR758 also prevented baclofen-induced outward currents recorded from rat dopamine neurons, substantiating its property as a NAM for GABA_BRs. Altogether, these data indicate that COR758 inhibits G protein signaling by GABA_BRs, likely by interacting with an allosteric binding-site. Therefore, COR758 might serve as a scaffold to develop additional NAMs for therapeutic intervention.

Structural Characterization of Receptor-Receptor Interactions in the Allosteric Modulation of G Protein-Coupled Receptor (GPCR) Dimers

G protein-coupled receptor (GPCR) oligomerization, while contentious, continues to attract the attention of researchers. Numerous experimental investigations have validated the presence of GPCR dimers, and the relevance of dimerization in the effectuation of physiological functions intensifies the attractiveness of this concept as a potential therapeutic target. GPCRs, as a single entity, have been the main source of scrutiny for drug design objectives for multiple diseases such as cancer, inflammation, cardiac, and respiratory diseases. The existence of dimers broadens the research scope of GPCR functions, revealing new signaling pathways that can be targeted for disease pathogenesis that have not previously been reported when GPCRs were only viewed in their monomeric form. This review will highlight several aspects of GPCR dimerization, which include a summary of the structural elucidation of the allosteric modulation of class C GPCR activation offered through recent solutions to the three-dimensional, full-length structures of metabotropic glutamate receptor and γ-aminobutyric acid B receptor as well as the role of dimerization in the modification of GPCR function and allostery. With the growing influence of computational methods in the study of GPCRs, we will also be reviewing recent computational tools that have been utilized to map protein-protein interactions (PPI).

Optogenetic Techniques for Manipulating and Sensing G Protein-Coupled Receptor Signaling

G protein-coupled receptors (GPCRs) form the largest class of membrane receptors in the mammalian genome with nearly 800 human genes encoding for unique subtypes. Accordingly, GPCR signaling is implicated in nearly all physiological processes. However, GPCRs have been difficult to study due in part to the complexity of their function which can lead to a plethora of converging or diverging downstream effects over different time and length scales. Classic techniques such as pharmacological control, genetic knockout and biochemical assays often lack the precision required to probe the functions of specific GPCR subtypes. Here we describe the rapidly growing set of optogenetic tools, ranging from methods for optical control of the receptor itself to optical sensing and manipulation of downstream effectors. These tools permit the quantitative measurements of GPCRs and their downstream signaling with high specificity and spatiotemporal precision.

Structural and Biophysical Mechanisms of Class C G Protein-Coupled Receptor Function

Groundbreaking structural and spectroscopic studies of class A G protein-coupled receptors (GPCRs), such as rhodopsin and the β2 adrenergic receptor, have provided a picture of how structural rearrangements between transmembrane helices control ligand binding, receptor activation, and effector coupling. However, the activation mechanism of other GPCR classes remains more elusive, in large part due to complexity in their domain assembly and quaternary structure. In this review, we focus on the class C GPCRs, which include metabotropic glutamate receptors (mGluRs) and gamma-aminobutyric acid B (GABAB) receptors (GABABRs) most prominently. We discuss the unique biophysical questions raised by the presence of large extracellular ligand-binding domains (LBDs) and constitutive homo/heterodimerization. Furthermore, we discuss how recent studies have begun to unravel how these fundamental class C GPCR features impact the processes of ligand binding, receptor activation, signal transduction, regulation by accessory proteins, and crosstalk with other GPCRs.

Illuminating the allosteric modulation of the calcium-sensing receptor

Many membrane receptors are regulated by nutrients. However, how these nutrients control a single receptor remains unknown, even in the case of the well-studied calcium-sensing receptor CaSR, which is regulated by multiple factors, including ions and amino acids. Here, we developed an innovative cell-free Förster resonance energy transfer (FRET)-based conformational CaSR biosensor to clarify the main conformational changes associated with activation. By allowing a perfect control of ambient nutrients, this assay revealed that Ca²⁺ alone fully stabilizes the active conformation, while amino acids behave as pure positive allosteric modulators. Based on the identification of Ca²⁺ activation sites, we propose a molecular basis for how these different ligands cooperate to control CaSR activation. Our results provide important information on CaSR function and improve our understanding of the effects of genetic mutations responsible for human diseases. They also provide insights into how a receptor can integrate signals from various nutrients to better adapt to the cell response.

The GABAB Receptor-Structure, Ligand Binding and Drug Development

The γ-aminobutyric acid (GABA) type B receptor (GABA_B-R) belongs to class C of the G-protein coupled receptors (GPCRs). Together with the GABA_A receptor, the receptor mediates the neurotransmission of GABA, the main inhibitory neurotransmitter in the central nervous system (CNS). In recent decades, the receptor has been extensively studied with the intention being to understand pathophysiological roles, structural mechanisms and develop drugs. The dysfunction of the receptor is linked to a broad variety of disorders, including anxiety, depression, alcohol addiction, memory and cancer. Despite extensive efforts, few compounds are known to target the receptor, and only the agonist baclofen is approved for clinical use. The receptor is a mandatory heterodimer of the GABA_B1 and GABA_B2 subunits, and each subunit is composed of an extracellular Venus Flytrap domain (VFT) and a transmembrane domain of seven α-helices (7TM domain). In this review, we briefly present the existing knowledge about the receptor structure, activation and compounds targeting the receptor, emphasizing the role of the receptor in previous and future drug design and discovery efforts.

Fluorescent Biosensors for Neurotransmission and Neuromodulation: Engineering and Applications

Understanding how neuronal activity patterns in the brain correlate with complex behavior is one of the primary goals of modern neuroscience. Chemical transmission is the major way of communication between neurons, however, traditional methods of detection of neurotransmitter and neuromodulator transients in mammalian brain lack spatiotemporal precision. Modern fluorescent biosensors for neurotransmitters and neuromodulators allow monitoring chemical transmission in vivo with millisecond precision and single cell resolution. Changes in the fluorescent biosensor brightness occur upon neurotransmitter binding and can be detected using fiber photometry, stationary microscopy and miniaturized head-mounted microscopes. Biosensors can be expressed in the animal brain using adeno-associated viral vectors, and their cell-specific expression can be achieved with Cre-recombinase expressing animals. Although initially fluorescent biosensors for chemical transmission were represented by glutamate biosensors, nowadays biosensors for GABA, acetylcholine, glycine, norepinephrine, and dopamine are available as well. In this review, we overview functioning principles of existing intensiometric and ratiometric biosensors and provide brief insight into the variety of neurotransmitter-binding proteins from bacteria, plants, and eukaryotes including G-protein coupled receptors, which may serve as neurotransmitter-binding scaffolds. We next describe a workflow for development of neurotransmitter and neuromodulator biosensors. We then discuss advanced setups for functional imaging of neurotransmitter transients in the brain of awake freely moving animals. We conclude by providing application examples of biosensors for the studies of complex behavior with the single-neuron precision.

Rearrangement of the transmembrane domain interfaces associated with the activation of a GPCR hetero-oligomer

G protein-coupled receptors (GPCRs) can integrate extracellular signals via allosteric interactions within dimers and higher-order oligomers. However, the structural bases of these interactions remain unclear. Here, we use the GABA_B receptor heterodimer as a model as it forms large complexes in the brain. It is subjected to genetic mutations mainly affecting transmembrane 6 (TM6) and involved in human diseases. By cross-linking, we identify the transmembrane interfaces involved in GABA_B1-GABA_B2, as well as GABA_B1-GABA_B1 interactions. Our data are consistent with an oligomer made of a row of GABA_B1. We bring evidence that agonist activation induces a concerted rearrangement of the various interfaces. While the GB1-GB2 interface is proposed to involve TM5 in the inactive state, cross-linking of TM6s lead to constitutive activity. These data bring insight for our understanding of the allosteric interaction between GPCRs within oligomers.

G protein-coupled receptors (GPCRs), such as GABA_B, can integrate extracellular signals via allosteric interactions within dimers and oligomers. Here authors use crosslinking and identify two transmembrane interfaces in GABA_B which undergo a concerted rearrangement upon agonist activation.

Quantitative Single-Residue Bioorthogonal Labeling of G Protein-Coupled Receptors in Live Cells

High-end microscopy studies of G protein-coupled receptors (GPCRs) require installing onto the receptors bright and photostable dyes. Labeling must occur in quantitative yields, to allow stoichiometric data analysis, and in a minimally invasive fashion, to avoid perturbing GPCR function. We demonstrate here that the genetic incorporation of trans-cyclooct-2-ene lysine (TCO*) allows achieving quantitative single-residue labeling of the extracellular loops of the β₂-adrenergic and the muscarinic M₂ class A GPCRs, as well as of the corticotropin releasing factor class B GPCR. Labeling occurs within a few minutes by reaction with dye-tetrazine conjugates on the surface of live cells and preserves the functionality of the receptors. To precisely quantify the labeling yields, we devise a method based on fluorescence fluctuation microscopy that extracts the number of labeling sites at the single-cell level. Further, we show that single-residue labeling is better suited for studies of GPCR diffusion than fluorescent-protein tags, since the latter can affect the mobility of the receptor. Finally, by performing dual-color competitive labeling on a single TCO* site, we devise a method to estimate the oligomerization state of a GPCR without the need for a biological monomeric reference, which facilitates the application of fluorescence methods to oligomerization studies. As TCO* and the dye-tetrazines used in this study are commercially available and the described microscopy techniques can be performed on a commercial microscope, we expect our approach to be widely applicable to fluorescence microscopy studies of membrane proteins in general.

Profiling of orthosteric and allosteric group-III metabotropic glutamate receptor ligands on various G protein-coupled receptors with Tag-lite® assays

Group-III metabotropic glutamate (mGlu) receptors are important synaptic regulators and are potential druggable targets for Parkinson disease, autism and pain. Potential drugs include orthosteric agonists in the glutamate binding extracellular domain and positive allosteric modulators interacting with seven-pass transmembrane domains. Orthosteric agonists are rarely completely specific for an individual group-III mGlu subtype. Furthermore they often fail to pass the blood-brain barrier and they constitutively activate their target receptor. These properties limit the potential therapeutic use of orthosteric agonists. Allosteric modulators are more specific and maintain the biological activity of the targeted receptor. However, they bind in a hydrophobic pocket and this limits their bio-availability and increases possible off-target action. It is therefore important to characterize the action of potential drug targets with a multifaceted and deeply informative assay. Here we aimed at multifaceted deep profiling of the effect of seven different agonists, and seven positive allosteric modulators on 34 different G protein-coupled receptors by a Tag-lite^® assay. Our results did not reveal off-target activity of mGlu orthosteric agonists. However, five allosteric modulators had either positive or negative effects on non-cognate G protein-coupled receptors. In conclusion, we demonstrate the power of the Tag-lite^® assay for potential drug ligand profiling on G protein-coupled receptors and its potential to identify positive allosteric compounds.

Oligomerization of a G protein-coupled receptor in neurons controlled by its structural dynamics

G protein coupled receptors (GPCRs) play essential roles in intercellular communication. Although reported two decades ago, the assembly of GPCRs into dimer and larger oligomers in their native environment is still a matter of intense debate. Here, using number and brightness analysis of fluorescently labeled receptors in cultured hippocampal neurons, we confirm that the metabotropic glutamate receptor type 2 (mGlu₂) is a homodimer at expression levels in the physiological range, while heterodimeric GABA_B receptors form larger complexes. Surprisingly, we observed the formation of larger mGlu₂ oligomers upon both activation and inhibition of the receptor. Stabilizing the receptor in its inactive conformation using biochemical constraints also led to the observation of oligomers. Following our recent observation that mGlu receptors are in constant and rapid equilibrium between several states under basal conditions, we propose that this structural heterogeneity limits receptor oligomerization. Such assemblies are expected to stabilize either the active or the inactive state of the receptor.

Structural biology of GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor (GPCR) that mediates slow and prolonged inhibitory neurotransmission in the brain. It functions as a constitutive heterodimer composed of the GABAB1 and GABAB2 subunits. Each subunit contains three domains; the extracellular Venus flytrap module, seven-helix transmembrane region and cytoplasmic tail. In recent years, the three-dimensional structures of GABAB receptor extracellular and intracellular domains have been elucidated. These structures reveal the molecular basis of ligand recognition, receptor heterodimerization and receptor activation. Here we provide a brief review of the GABAB receptor structures, with an emphasis on describing the different ligand-bound states of the receptor. We will also compare these with the known structures of related GPCRs to shed light on the molecular mechanisms of activation and regulation in the GABAB system, as well as GPCR dimers in general. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Protein-induced fluorescence enhancement for a simple and universal detection of protein/small molecule interactions

We herein describe a novel and efficient method for the detection of protein/small molecule (SM) interactions, which relies on the protein-induced fluorescence enhancement (PIFE). In this method, a duplex probe is designed to position Cy3 and SM at the optimal distance to maximize the effect of PIFE, which is utilized as the key component. In the presence of target proteins that bind to SM, the Cy3 is guided close to the target proteins, which significantly enhances the fluorescence signal through a process of PIFE. With this approach, we successfully analyzed a model target protein, streptavidin (STV) that interacts with biotin (BTN) in less than 10 min without any washing steps. In addition, the practical applicability of this method was demonstrated by reliably determining STV in human serum. Finally, the universal applicability of this method was demonstrated by monitoring the interaction between folate and folate receptors.

We herein describe a novel and efficient method for the detection of protein/small molecule (SM) interactions, which relies on the protein-induced fluorescence enhancement (PIFE).

Potent Prearranged Positive Allosteric Modulators of the Glucagon-like Peptide-1 Receptor

Drugs that allosterically modulate G protein-coupled receptor (GPCR) activity display higher specificity and may improve disease treatment. However, the rational design of compounds that target the allosteric site is difficult, as conformations required for receptor activation are poorly understood. Guided by photopharmacology, a set of prearranged positive allosteric modulators (PAMs) with restricted degrees of freedom was designed and tested against the glucagon-like peptide-1 receptor (GLP-1R), a GPCR involved in glucose homeostasis. Compounds incorporating a trans-stilbene comprehensively outperformed those with a cis-stilbene, as well as the benchmark BETP, as GLP-1R PAMs. We also identified major effects of ligand conformation on GLP-1R binding kinetics and signal bias. Thus, we describe a photopharmacology-directed approach for rational drug design, and introduce a new class of stilbene-containing PAM for the specific regulation of GPCR activity.

Elucidation of the neural circuits activated by a GABAB receptor positive modulator: Relevance to anxiety

Although there is much evidence for a role of GABA_B receptors in the pathophysiology of anxiety, the underlying neuronal mechanisms are largely unclear. The GABA_B receptor allosteric positive modulator, GS39783, exerts anxiolytic effects without interfering with GABA_B-mediated modulation of body temperature, cognitive performance and locomotor activity thus offering advantages over GABA_B receptor agonists. However, the precise neural circuits underlying the anxiolytic effects of GS39783 are unknown. The aim of the present study was to identify brain structures and associated neuronal circuits that are modulated by GS39783 under either basal or mild stress conditions. To this end, the expression pattern of c-Fos, a marker of neuronal activation, was examined in mice acutely treated with GS39783 under basal conditions or following a mild anxiogenic challenge induced by exposure to the Open Arm (OA) of an Elevated Plus Maze. OA exposure enhanced c-Fos expression in vehicle-treated animals in several brain regions, including the medial prefrontal cortex, lateral septum, amygdala, hippocampus, paraventricular nucleus of the hypothalamus and the periaqueductal gray (PAG). Under basal conditions, GS39783 increased c-Fos in a restricted panel of areas notably amygdala nuclei, cortical areas and PAG subregions, while it inhibited c-Fos expression in the dorsal raphe nucleus (DRN). Under stress conditions, GS39783 reversed OA-induced c-Fos expression in the granular cell layer of the dentate gyrus, no longer increased c-Fos expression in the amygdala nor reduced c-Fos expression in the DRN. These specific patterns of neural activation by GS39783 might explain the neurobiological correlates implicated in GABA_B-mediated anti-anxiety effects. This article is part of the "Special Issue Dedicated to Norman G. Bowery".