Structural flexibility of the G alpha s alpha-helical domain in the beta2-adrenoceptor Gs complex

Beneath the surface: endosomal GPCR signaling

G protein-coupled receptors (GPCRs) located at the cell surface bind extracellular ligands and convey intracellular signals via activation of heterotrimeric G proteins. Traditionally, G protein signaling was viewed to occur exclusively at this subcellular region followed by rapid desensitization facilitated by β-arrestin (βarr)-mediated G protein uncoupling and receptor internalization. However, emerging evidence over the past 15 years suggests that these βarr-mediated events do not necessarily terminate receptor signaling and that some GPCRs continue to activate G proteins after having been internalized into endosomes. Here, we review the recently elucidated mechanistic basis underlying endosomal GPCR signaling and discuss physiological implications and pharmacological targeting of this newly appreciated signaling mode.

Mechanistic insights into G-protein activation via phosphorylation mediated non-canonical pathway

Activation of heterotrimeric G-proteins (Gαβγ) downstream to receptor tyrosine kinases (RTKs) is a well-established crosstalk between the signaling pathways mediated by G-protein coupled receptors (GPCRs) and RTKs. While GPCR serves as a guanine exchange factor (GEF) in the canonical activation of Gα that facilitates the exchange of GDP for GTP, the mechanism through which RTK phosphorylations induce Gα activation remains unclear. Recent experimental studies revealed that the epidermal growth factor receptor (EGFR), a well-known RTK, phosphorylates the helical domain tyrosine residues Y154 and Y155 and accelerates the GDP release from the Gαi3, a subtype of Gα-protein. Using well-tempered metadynamics and extensive unbiased molecular dynamics simulations, we captured the GDP release event and identified the intermediates between bound and unbound states through Markov state models. In addition to weakened salt bridges at the domain interface, phosphorylations induced the unfolding of helix αF, which contributed to increased flexibility near the hinge region, facilitating a greater distance between domains in the phosphorylated Gαi3. Although the larger domain separation in the phosphorylated system provided an unobstructed path for the nucleotide, the accelerated release of GDP was attributed to increased fluctuations in several conserved regions like P-loop, switch 1, and switch 2. Overall, this study provides atomistic insights into the activation of G-proteins induced by RTK phosphorylations and identifies the specific structural motifs involved in the process. The knowledge gained from the study could establish a foundation for targeting non-canonical signaling pathways and developing therapeutic strategies against the ailments associated with dysregulated G-protein signaling.

In-depth analysis of Gαs protein activity by probing different fluorescently labeled guanine nucleotides

G proteins are interacting partners of G protein-coupled receptors (GPCRs) in eukaryotic cells. Upon G protein activation, the ability of the Gα subunit to exchange GDP for GTP determines the intracellular signal transduction. Although various studies have successfully shown that both Gαs and Gαi have an opposite effect on the intracellular cAMP production, with the latter being commonly described as "more active", the functional analysis of Gαs is a comparably more complicated matter. Additionally, the thorough investigation of the ubiquitously expressed variants of Gαs, Gαs(short) and Gαs(long), is still pending. Since the previous experimental evaluation of the activity and function of the Gαs isoforms is not consistent, the focus was laid on structural investigations to understand the GTPase activity. Herein, we examined recombinant human Gαs by applying an established methodological setup developed for Gαi characterization. The ability for GTP binding was evaluated with fluorescence and fluorescence anisotropy assays, whereas the intrinsic hydrolytic activity of the isoforms was determined by a GTPase assay. Among different nucleotide probes, BODIPY FL GTPγS exhibited the highest binding affinity towards the Gαs subunit. This work provides a deeper understanding of the Gαs subunit and provides novel information concerning the differences between the two protein variants.

Time-resolved cryo-EM of G-protein activation by a GPCR

G-protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by stimulating guanine nucleotide exchange in the Gα subunit1. To visualize this mechanism, we developed a time-resolved cryo-EM approach that examines the progression of ensembles of pre-steady-state intermediates of a GPCR-G-protein complex. By monitoring the transitions of the stimulatory Gs protein in complex with the β2-adrenergic receptor at short sequential time points after GTP addition, we identified the conformational trajectory underlying G-protein activation and functional dissociation from the receptor. Twenty structures generated from sequential overlapping particle subsets along this trajectory, compared to control structures, provide a high-resolution description of the order of main events driving G-protein activation in response to GTP binding. Structural changes propagate from the nucleotide-binding pocket and extend through the GTPase domain, enacting alterations to Gα switch regions and the α5 helix that weaken the G-protein-receptor interface. Molecular dynamics simulations with late structures in the cryo-EM trajectory support that enhanced ordering of GTP on closure of the α-helical domain against the nucleotide-bound Ras-homology domain correlates with α5 helix destabilization and eventual dissociation of the G protein from the GPCR. These findings also highlight the potential of time-resolved cryo-EM as a tool for mechanistic dissection of GPCR signalling events.

Structure and function of an intermediate GPCR-Gαβγ complex

Unraveling the signaling roles of intermediate complexes is pivotal for G protein-coupled receptor (GPCR) drug development. Despite hundreds of GPCR-Gαβγ structures, these snapshots primarily capture the fully activated end-state complex. Consequently, a comprehensive understanding of the conformational transitions during GPCR activation and the roles of intermediate GPCR-G protein complexes in signaling remain elusive. Guided by a conformational landscape profiled by 19 F quantitative NMR ( 19 F-qNMR) and Molecular Dynamics (MD) simulations, we resolved the structure of an unliganded GPCR-G protein intermediate complex by blocking its transition to the fully activated end-state complex. More importantly, we presented direct evidence that the intermediate GPCR-Gαsβγ complex initiates a rate-limited nucleotide exchange without progressing to the fully activated end-state complex, thereby bridging a significant gap in our understanding the complexity of GPCR signaling. Understanding the roles of individual conformational states and their complexes in signaling efficacy and bias will help us to design drugs that discriminately target a disease-related conformation.

Molecular mechanism of muscarinic acetylcholine receptor M3 interaction with Gq

Muscarinic acetylcholine receptor M3 (M3) and its downstream effector Gq/11 are critical drug development targets due to their involvement in physiopathological processes. Although the structure of the M3-miniGq complex was recently published, the lack of information on the intracellular loop 3 (ICL3) of M3 and extensive modification of Gαq impedes the elucidation of the molecular mechanism of M3-Gq coupling under more physiological condition. Here, we describe the molecular mechanism underlying the dynamic interactions between full-length wild-type M3 and Gq using hydrogen-deuterium exchange mass spectrometry and NanoLuc Binary Technology-based cell systems. We propose a detailed analysis of M3-Gq coupling through examination of previously well-defined binding interfaces and neglected regions. Our findings suggest potential binding interfaces between M3 and Gq in pre-assembled and functionally active complexes. Furthermore, M3 ICL3 negatively affected M3-Gq coupling, and the Gαq AHD underwent unique conformational changes during M3-Gq coupling.

β-Arrestin-independent endosomal cAMP signaling by a polypeptide hormone GPCR

Many G protein-coupled receptors (GPCRs) initiate a second phase of stimulatory heterotrimeric G protein (Gs)-coupled cAMP signaling after endocytosis. The prevailing current view is that the endosomal signal is inherently β-arrestin-dependent because β-arrestin is necessary for receptor internalization and, for some GPCRs, to prolong the endosomal signal. Here we revise this view by showing that the vasoactive intestinal peptide receptor 1 (VIPR1), a secretin-family polypeptide hormone receptor, does not require β-arrestin to internalize or to generate an endosomal signal. β-Arrestin instead resolves the plasma membrane and endosomal signaling phases into sequential cAMP peaks by desensitizing the plasma membrane phase without affecting the endosomal phase. This appears to occur through the formation of functionally distinct VIPR1-β-arrestin complexes at each location that differ in their phosphorylation dependence. We conclude that endosomal GPCR signaling can occur in the absence of β-arrestin and that β-arrestin sculpts the spatiotemporal profile of cellular GPCR-G protein signaling through location-specific remodeling of GPCR-β-arrestin complexes.

Sample Preparation for Electron Cryo-Microscopy of Macromolecular Machines

High-resolution structure determination by electron cryo-microscopy underwent a step change in recent years. This now allows study of challenging samples which previously were inaccessible for structure determination, including membrane proteins. These developments shift the focus in the field to the next bottlenecks which are high-quality sample preparations. While the amounts of sample required for cryo-EM are relatively small, sample quality is the key challenge. Sample quality is influenced by the stability of complexes which depends on buffer composition, inherent flexibility of the sample, and the method of solubilization from the membrane for membrane proteins. It further depends on the choice of sample support, grid pre-treatment and cryo-grid freezing protocol. Here, we discuss various widely applicable approaches to improve sample quality for structural analysis by cryo-EM.

Single-molecule visualization of human A2A adenosine receptor activation by a G protein and constitutively activating mutations

Mutations that constitutively activate G protein-coupled receptors (GPCRs), known as constitutively activating mutations (CAMs), modify cell signaling and interfere with drugs, resulting in diseases with limited treatment options. We utilize fluorescence imaging at the single-molecule level to visualize the dynamic process of CAM-mediated activation of the human A2A adenosine receptor (A2AAR) in real time. We observe an active-state population for all CAMs without agonist stimulation. Importantly, activating mutations significantly increase the population of an intermediate state crucial for receptor activation, notably distinct from the addition of a partner G protein. Activation kinetics show that while CAMs increase the frequency of transitions to the intermediate state, mutations altering sodium sensitivity increase transitions away from it. These findings indicate changes in GPCR function caused by mutations may be predicted based on whether they favor or disfavor formation of an intermediate state, providing a framework for designing receptors with altered functions or therapies that target intermediate states.

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins, composed of Gα, Gβ, and Gγ subunits, are a class of signal transduction complexes with broad roles in human health and agriculturally relevant plant physiological and developmental traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. We sought to develop improved methods for heterologous expression and rapid purification of Gα subunits. Using GPA1, the sole canonical Gα subunit of the model plant species, Arabidopsis thaliana, we observed that, compared to conventional purification methods, rapid StrepII-tag mediated purification facilitates isolation of protein with increased GTP binding and hydrolysis activities. Human GNAI1 purified using our approach also displayed the expected binding and hydrolysis activities, indicating our protocol is applicable to mammalian Gα subunits, potentially including those for which purification of enzymatically active protein has been historically problematic. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate that the inherent instability of GPA1 is a function of the interaction between the Ras and helical domains. Additionally, we found that GPA1-GNAO1 domain swaps partially uncouple the instability from the rapid nucleotide binding kinetics displayed by GPA1.

Gαs slow conformational transition upon GTP binding and a novel Gαs regulator

G proteins are major signaling partners for G protein-coupled receptors (GPCRs). Although stepwise structural changes during GPCR-G protein complex formation and guanosine diphosphate (GDP) release have been reported, no information is available with regard to guanosine triphosphate (GTP) binding. Here, we used a novel Bayesian integrative modeling framework that combines data from hydrogen-deuterium exchange mass spectrometry, tryptophan-induced fluorescence quenching, and metadynamics simulations to derive a kinetic model and atomic-level characterization of stepwise conformational changes incurred by the β2-adrenergic receptor (β2AR)-Gs complex after GDP release and GTP binding. Our data suggest rapid GTP binding and GTP-induced dissociation of Gαs from β2AR and Gβγ, as opposed to a slow closing of the Gαs α-helical domain (AHD). Yeast-two-hybrid screening using Gαs AHD as bait identified melanoma-associated antigen D2 (MAGE D2) as a novel AHD-binding protein, which was also shown to accelerate the GTP-induced closing of the Gαs AHD.

Mapping the conformational landscape of the stimulatory heterotrimeric G protein

Heterotrimeric G proteins serve as membrane-associated signaling hubs, in concert with their cognate G-protein-coupled receptors. Fluorine nuclear magnetic resonance spectroscopy was employed to monitor the conformational equilibria of the human stimulatory G-protein α subunit (G_sα) alone, in the intact G_sαβ₁γ₂ heterotrimer or in complex with membrane-embedded human adenosine A_2A receptor (A_2AR). The results reveal a concerted equilibrium that is strongly affected by nucleotide and interactions with the βγ subunit, the lipid bilayer and A_2AR. The α1 helix of G_sα exhibits significant intermediate timescale dynamics. The α4β6 loop and α5 helix undergo membrane/receptor interactions and order-disorder transitions respectively, associated with G-protein activation. The αN helix adopts a key functional state that serves as an allosteric conduit between the βγ subunit and receptor, while a significant fraction of the ensemble remains tethered to the membrane and receptor upon activation.

Time-resolved cryo-EM of G protein activation by a GPCR

G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by stimulating the exchange of guanine nucleotide in the Gα subunit. To visualize this mechanism, we developed a time-resolved cryo-EM approach that examines the progression of ensembles of pre-steady-state intermediates of a GPCR-G protein complex. Using variability analysis to monitor the transitions of the stimulatory Gs protein in complex with the β 2 -adrenergic receptor (β 2 AR) at short sequential time points after GTP addition, we identified the conformational trajectory underlying G protein activation and functional dissociation from the receptor. Twenty transition structures generated from sequential overlapping particle subsets along this trajectory, compared to control structures, provide a high-resolution description of the order of events driving G protein activation upon GTP binding. Structural changes propagate from the nucleotide-binding pocket and extend through the GTPase domain, enacting alterations to Gα Switch regions and the α5 helix that weaken the G protein-receptor interface. Molecular dynamics (MD) simulations with late structures in the cryo-EM trajectory support that enhanced ordering of GTP upon closure of the alpha-helical domain (AHD) against the nucleotide-bound Ras-homology domain (RHD) correlates with irreversible α5 helix destabilization and eventual dissociation of the G protein from the GPCR. These findings also highlight the potential of time-resolved cryo-EM as a tool for mechanistic dissection of GPCR signaling events.

Intracellular VHHs to monitor and modulate GPCR signaling

Single-domain antibody fragments, also known as VHHs or nanobodies, have opened promising avenues in therapeutics and in exploration of intracellular processes. Because of their unique structural properties, they can reach cryptic regions in their cognate antigen. Intracellular VHHs/antibodies primarily directed against cytosolic proteins or transcription factors have been described. In contrast, few of them target membrane proteins and even less recognize G protein-coupled receptors. These receptors are major therapeutic targets, which reflects their involvement in a plethora of physiological responses. Hence, they elicit a tremendous interest in the scientific community and in the industry. Comprehension of their pharmacology has been obscured by their conformational complexity, that has precluded deciphering their structural properties until the early 2010's. To that respect, intracellular VHHs have been instrumental in stabilizing G protein-coupled receptors in active conformations in order to solve their structure, possibly bound to their primary transducers, G proteins or β-arrestins. In contrast, the modulatory properties of VHHs recognizing the intracellular regions of G protein-coupled receptors on the induced signaling network have been poorly studied. In this review, we will present the advances that the intracellular VHHs have permitted in the field of GPCR signaling and trafficking. We will also discuss the methodological hurdles that linger the discovery of modulatory intracellular VHHs directed against GPCRs, as well as the opportunities they open in drug discovery.

Nanobodies as Probes and Modulators of Cardiovascular G Protein-Coupled Receptors

ABSTRACT

Understanding the activation of G protein-coupled receptors (GPCRs) is of paramount importance to the field of cardiovascular medicine due to the critical physiological roles of these receptors and their prominence as drug targets. Although many cardiovascular GPCRs have been extensively studied as model receptors for decades, new complexities in their regulation continue to emerge. As a result, there is an ongoing need to develop novel approaches to monitor and to modulate GPCR activation. In less than a decade, nanobodies, or recombinant single-domain antibody fragments from camelids, have become indispensable tools for interrogating GPCRs both in purified systems and in living cells. Nanobodies have gained traction rapidly due to their biochemical tractability and their ability to recognize defined states of native proteins. Here, we review how nanobodies have been adopted to elucidate the structure, pharmacology, and signaling of cardiovascular GPCRs, resolving long-standing mysteries and revealing unexpected mechanisms. We also discuss how advancing technologies to discover nanobodies with tailored specificities may expand the impact of these tools for both basic science and therapeutic applications.

G protein-coupled receptor-effector macromolecular membrane assemblies (GEMMAs)

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in cellular signaling across the plasma membrane and a major class of drug targets. The canonical model for GPCR signaling involves three components - the GPCR, a heterotrimeric G protein and a proximal plasma membrane effector - that have been generally thought to be freely mobile molecules able to interact by 'collision coupling'. Here, we synthesize evidence that supports the existence of GPCR-effector macromolecular membrane assemblies (GEMMAs) comprised of specific GPCRs, G proteins, plasma membrane effector molecules and other associated transmembrane proteins that are pre-assembled prior to receptor activation by agonists, which then leads to subsequent rearrangement of the GEMMA components. The GEMMA concept offers an alternative and complementary model to the canonical collision-coupling model, allowing more efficient interactions between specific signaling components, as well as the integration of the concept of GPCR oligomerization as well as GPCR interactions with orphan receptors, truncated GPCRs and other membrane-localized GPCR-associated proteins. Collision-coupling and pre-assembled mechanisms are not exclusive and likely both operate in the cell, providing a spectrum of signaling modalities which explains the differential properties of a multitude of GPCRs in their different cellular environments. Here, we explore the unique pharmacological characteristics of individual GEMMAs, which could provide new opportunities to therapeutically modulate GPCR signaling.

Endomembrane-Based Signaling by GPCRs and G-Proteins

G-protein-coupled receptors (GPCRs) and G-proteins have a range of roles in many physiological and pathological processes and are among the most studied signaling proteins. A plethora of extracellular stimuli can activate the GPCR and can elicit distinct intracellular responses through the activation of specific transduction pathways. For many years, biologists thought that GPCR signaling occurred entirely on the plasma membrane. However, in recent decades, many lines of evidence have proved that the GPCRs and G-proteins may reside on endomembranes and can start or propagate signaling pathways through the organelles that form the secretory route. How these alternative intracellular signaling pathways of the GPCR and G-proteins influence the physiological and pathological function of the endomembranes is still under investigation. Here, we review the general role and classification of GPCRs and G-proteins with a focus on their signaling pathways in the membrane transport apparatus.

The Conformational Dynamics of Heterotrimeric G Proteins During GPCR-Mediated Activation

Heterotrimeric G proteins (G proteins) are essential cellular signaling proteins that mediate extracellular signals to achieve various cellular functions. G-protein-coupled receptors (GPCRs) are the major guanine nucleotide exchange factors (GEFs) that induce G proteins to release guanosine diphosphate and rapidly bind to guanosine triphosphate, resulting in G protein activation. G proteins undergo dynamic conformational changes during the activation/inactivation process, and the precise structural mechanism of GPCR-mediated G protein activation is of great interest. Over the last decade, a number of GPCR-G protein complex structures have been identified, yet an understanding of the mechanisms underlying allosteric conformational changes during receptor-mediated G protein activation and GPCR-G protein coupling selectivity is only now emerging. This review discusses recent studies on the dynamic conformational changes of G proteins and provides insight into the structural mechanism of GPCR-mediated G protein activation.

Optical tools to study the subcellular organization of GPCR neuromodulation

Modulation of neuronal circuit activity is key to information processing in the brain. G protein-coupled receptors (GPCRs), the targets of most neuromodulatory ligands, show extremely diverse expression patterns in neurons and receptors can be localized in various sub-neuronal membrane compartments. Upon activation, GPCRs promote signaling cascades that alter the level of second messengers, drive phosphorylation changes, modulate ion channel function, and influence gene expression, all of which critically impact neuron physiology. Because of its high degree of complexity, this form of interneuronal communication has remained challenging to integrate into our conceptual understanding of brain function. Recent technological advances in fluorescence microscopy and the development of optical biosensors now allow investigating neuromodulation with unprecedented resolution on the level of individual cells. In this review, we will highlight recent imaging techniques that enable determining the precise localization of GPCRs in neurons, with specific focus on the subcellular and nanoscale level. Downstream of receptors, we describe novel conformation-specific biosensors that allow for real-time monitoring of GPCR activation and of distinct signal transduction events in neurons. Applying these new tools has the potential to provide critical insights into the function and organization of GPCRs in neuronal cells and may help decipher the molecular and cellular mechanisms that underlie neuromodulation.

Integrative RNA-omics discovers GNAS alternative splicing as a phenotypic driver of splicing factor-mutant neoplasms

Mutations in splicing factors (SFs) are the predominant class of mutations in myelodysplastic syndrome (MDS), but convergent downstream disease drivers remain elusive. To identify common direct targets of mis-splicing by mutant U2AF1 and SRSF2, we performed RNA-Seq and eCLIP in human hematopoietic stem/progenitor cells (HSPCs) derived from isogenic induced pluripotent stem cell (iPSC) models. Integrative analyses of alternative splicing and differential binding converged on a long isoform of GNAS (GNAS-L), promoted by both mutant factors. MDS population genetics, functional and biochemical analyses support that GNAS-L is a driver of MDS and encodes a hyperactive long form of the stimulatory G protein alpha subunit, Gas-L, that activates ERK/MAPK signaling. SF-mutant MDS cells have activated ERK signaling and consequently are sensitive to MEK inhibitors. Our findings highlight an unexpected and unifying mechanism by which SRSF2 and U2AF1 mutations drive oncogenesis with potential therapeutic implications for MDS and other SF-mutant neoplasms.

HDX-MS-optimized approach to characterize nanobodies as tools for biochemical and structural studies of class IB phosphoinositide 3-kinases

There is considerable interest in developing antibodies as modulators of signaling pathways. One of the most important signaling pathways in higher eukaryotes is the phosphoinositide 3-kinase (PI3K) pathway, which plays fundamental roles in growth, metabolism, and immunity. The class IB PI3K, PI3Kγ, is a heterodimeric complex composed of a catalytic p110γ subunit bound to a p101 or p84 regulatory subunit. PI3Kγ is a critical component in multiple immune signaling processes and is dependent on activation by Ras and G protein-coupled receptors (GPCRs) to mediate its cellular roles. Here we describe the rapid and efficient characterization of multiple PI3Kγ binding single-chain camelid nanobodies using hydrogen-deuterium exchange (HDX) mass spectrometry (MS) for structural and biochemical studies. We identify nanobodies that stimulated lipid kinase activity, block Ras activation, and specifically inhibited p101-mediated GPCR activation. Overall, our work reveals insight into PI3Kγ regulation and identifies sites that may be exploited for therapeutic development.

Crystal structure of dopamine D1 receptor in complex with G protein and a non-catechol agonist

Dopamine D1 receptor (D1R) is an important drug target implicated in many psychiatric and neurological disorders. Selective agonism of D1R are sought to be the therapeutic strategy for these disorders. Most selective D1R agonists share a dopamine-like catechol moiety in their molecular structure, and their therapeutic potential is therefore limited by poor pharmacological properties in vivo. Recently, a class of non-catechol D1R selective agonists with a distinct scaffold and pharmacological properties were reported. Here, we report the crystal structure of D1R in complex with stimulatory G protein (Gs) and a non-catechol agonist Compound 1 at 3.8 Å resolution. The structure reveals the ligand bound to D1R in an extended conformation, spanning from the orthosteric site to extracellular loop 2 (ECL2). Structural analysis reveals that the unique features of D1R ligand binding pocket explains the remarkable selectivity of this scaffold for D1R over other aminergic receptors, and sheds light on the mechanism for D1R activation by the non-catechol agonist.

Evolving cryo-EM structural approaches for GPCR drug discovery

G protein-coupled receptors (GPCRs) are the largest class of cell surface drug targets. Advances in stabilization of GPCR:transducer complexes, together with improvements in cryoelectron microscopy (cryo-EM) have recently been applied to structure-assisted drug design for GPCR agonists. Nonetheless, limitations in the commercial application of these approaches, including the use of nanobody 35 (Nb35) to aid complex stabilization and the high cost of 300 kV imaging, have restricted broad application of cryo-EM in drug discovery. Here, using the PF 06882961-bound GLP-1R as exemplar, we validated the formation of stable complexes with a modified Gs protein in the absence of Nb35. In parallel, we compare 200 versus 300 kV image acquisition using a Falcon 4 or K3 direct electron detector. Moreover, the 200 kV Glacios-Falcon 4 yielded a 3.2 Å map with clear density for bound drug and multiple structurally ordered waters. Our work paves the way for broader commercial application of cryo-EM for GPCR drug discovery.

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.

A universal allosteric mechanism for G protein activation

G proteins play a central role in signal transduction and pharmacology. Signaling is initiated by cell-surface receptors, which promote guanosine triphosphate (GTP) binding and dissociation of Gα from the Gβγ subunits. Structural studies have revealed the molecular basis of subunit association with receptors, RGS proteins, and downstream effectors. In contrast, the mechanism of subunit dissociation is poorly understood. We use cell signaling assays, molecular dynamics (MD) simulations, and biochemistry and structural analyses to identify a conserved network of amino acids that dictates subunit release. In the presence of the terminal phosphate of GTP, a glycine forms a polar network with an arginine and glutamate, putting torsional strain on the subunit binding interface. This "G-R-E motif" secures GTP and, through an allosteric link, discharges the Gβγ dimer. Replacement of network residues prevents subunit dissociation regardless of agonist or GTP binding. These findings reveal the molecular basis of the final committed step of G protein activation.

A set of common movements within GPCR-G-protein complexes from variability analysis of cryo-EM datasets

G-protein coupled receptors (GPCRs) are among the most versatile signal transducers in the cell. Once activated, GPCRs sample a large conformational space and couple to G-proteins to initiate distinct signaling pathways. The dynamical behavior of GPCR-G-protein complexes is difficult characterize structurally, and it might hinder obtaining routine high-resolution density maps in single-particle reconstructions. Here, we used variability analysis on the rhodopsin-G_i-Fab16 complex cryo-EM dataset, and the results provide insights into the dynamic nature of the receptor-complex interaction. We compare the outcome of this analysis with recent results obtained on the cannabinoid-G_i- and secretin-G_s-receptor complexes. Despite differences related to the biochemical compositions of the three samples, a set of consensus movements emerges. We anticipate that systematic variability analysis on GPCR-G-protein complexes may provide useful information not only at the biological level, but also for improving the preparation of more stable samples for cryo-EM single-particle analysis.

The Old and New Visions of Biased Agonism Through the Prism of Adenosine Receptor Signaling and Receptor/Receptor and Receptor/Protein Interactions

Biased signaling is a concept that has arisen in the G protein-coupled receptor (GCPR) research field, and holds promise for the development of new drug development strategies. It consists of different signaling outputs depending on the agonist's chemical structure. Here we review the most accepted mechanisms for explaining biased agonism, namely the induced fit hypothesis and the key/lock hypothesis, but we also consider how bias can be produced by a given agonist. In fact, different signaling outputs may originate at a given receptor when activated by, for instance, the endogenous agonist. We take advantage of results obtained with adenosine receptors to explain how such mechanism of functional selectivity depends on the context, being receptor-receptor interactions (heteromerization) one of the most relevant and most studied mechanisms for mammalian homeostasis. Considering all the possible mechanisms underlying functional selectivity is essential to optimize the selection of biased agonists in the design of drugs targeting GPCRs.

Conformational switch that induces GDP release from Gi

Heterotrimeric guanine nucleotide-binding proteins (G proteins) are composed of α, β, and γ subunits. Gα switches between guanosine diphosphate (GDP)-bound inactive and guanosine triphosphate (GTP)-bound active states, and Gβγ interacts with the GDP-bound state. The GDP-binding regions are composed of two sites: the phosphate-binding and guanine-binding regions. The turnover of GDP and GTP is induced by guanine nucleotide-exchange factors (GEFs), including G protein-coupled receptors (GPCRs), Ric8A, and GIV/Girdin. However, the key structural factors for stabilizing the GDP-bound state of G proteins and the direct structural event for GDP release remain unclear. In this study, we investigated structural factors affecting GDP release by introducing point mutations in selected, conserved residues in Gαi3. We examined the effects of these mutations on the GDP/GTP turnover rate and the overall conformation of Gαi3 as well as the binding free energy between Gαi3 and GDP. We found that dynamic changes in the phosphate-binding regions are an immediate factor for the release of GDP.

G protein-coupled receptor-G protein interactions: a single-molecule perspective

G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.

Compartmentalized GPCR Signaling from Intracellular Membranes

G protein-coupled receptors (GPCRs) are integral membrane proteins that transduce a wide array of inputs including light, ions, hormones, and neurotransmitters into intracellular signaling responses which underlie complex processes ranging from vision to learning and memory. Although traditionally thought to signal primarily from the cell surface, GPCRs are increasingly being recognized as capable of signaling from intracellular membrane compartments, including endosomes, the Golgi apparatus, and nuclear membranes. Remarkably, GPCR signaling from these membranes produces functional effects that are distinct from signaling from the plasma membrane, even though often the same G protein effectors and second messengers are activated. In this review, we will discuss the emerging idea of a "spatial bias" in signaling. We will present the evidence for GPCR signaling through G protein effectors from intracellular membranes, and the ways in which this signaling differs from canonical plasma membrane signaling with important implications for physiology and pharmacology. We also highlight the potential mechanisms underlying spatial bias of GPCR signaling, including how intracellular membranes and their associated lipids and proteins affect GPCR activity and signaling.

Structural mechanism underlying primary and secondary coupling between GPCRs and the Gi/o family

Heterotrimeric G proteins are categorized into four main families based on their function and sequence, Gs, Gi/o, Gq/11, and G12/13. One receptor can couple to more than one G protein subtype, and the coupling efficiency varies depending on the GPCR-G protein pair. However, the precise mechanism underlying different coupling efficiencies is unknown. Here, we study the structural mechanism underlying primary and secondary Gi/o coupling, using the muscarinic acetylcholine receptor type 2 (M2R) as the primary Gi/o-coupling receptor and the β₂-adrenergic receptor (β₂AR, which primarily couples to Gs) as the secondary Gi/o-coupling receptor. Hydrogen/deuterium exchange mass spectrometry and mutagenesis studies reveal that the engagement of the distal C-terminus of Gαi/o with the receptor differentiates primary and secondary Gi/o couplings. This study suggests that the conserved hydrophobic residue within the intracellular loop 2 of the receptor (residue 34.51) is not critical for primary Gi/o-coupling; however, it might be important for secondary Gi/o-coupling.

G protein-coupled receptors (GPCRs) can couple to more than one G protein subtype, and the coupling efficiency varies depending on the GPCR-G protein pair. Here authors use hydrogen/deuterium exchange mass spectrometry and mutagenesis to study the structural mechanism underlying primary and secondary Gi/o coupling.

Structural insights into emergent signaling modes of G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest family of cell membrane proteins, with >800 GPCRs in humans alone, and recognize highly diverse ligands, ranging from photons to large protein molecules. Very important to human medicine, GPCRs are targeted by about 35% of prescription drugs. GPCRs are characterized by a seven-transmembrane α-helical structure, transmitting extracellular signals into cells to regulate major physiological processes via heterotrimeric G proteins and β-arrestins. Initially viewed as receptors whose signaling via G proteins is delimited to the plasma membrane, it is now recognized that GPCRs signal also at various intracellular locations, and the mechanisms and (patho)physiological relevance of such signaling modes are actively investigated. The propensity of GPCRs to adopt different signaling modes is largely encoded in the structural plasticity of the receptors themselves and of their signaling complexes. Here, we review emerging modes of GPCR signaling via endosomal membranes and the physiological implications of such signaling modes. We further summarize recent structural insights into mechanisms of GPCR activation and signaling. We particularly emphasize the structural mechanisms governing the continued GPCR signaling from endosomes and the structural aspects of the GPCR resensitization mechanism and discuss the recently uncovered and important roles of lipids in these processes.

Structure of a D2 dopamine receptor-G-protein complex in a lipid membrane

The D₂ dopamine receptor (DRD2) is a therapeutic target for Parkinson’s disease¹ and antipsychotic drugs². DRD2 is activated by the endogenous neurotransmitter dopamine and synthetic agonist drugs such as bromocriptine³, leading to stimulation of G_i and inhibition of adenylyl cyclase. We used cryo-electron microscopy to elucidate the structure of an agonist-bound activated DRD2-G_i complex reconstituted into a phospholipid membrane. The extracellular ligand binding site of DRD2 is remodeled in response to agonist binding, with conformational changes in extracellular loop 2 (ECL2), transmembrane domain 5 (TM5), TM6, and TM7 propagating to opening of the intracellular G_i binding site. The DRD2-G_i structure represents the first experimental model of a GPCR-G protein complex embedded in a phospholipid bilayer, which serves as a benchmark to validate the interactions seen in previous detergent-bound structures. The structure also reveals interactions that are unique to the membrane-embedded complex, including helix 8 burial in the inner leaflet, ordered lysine and arginine sidechains in the membrane interfacial regions, and lipid anchoring of the G protein in the membrane. Our model of the activated DRD2 will help inform the design of subtype-selective DRD2 ligands for multiple human CNS disorders.

Differential activation of G protein-mediated signaling by synthetic cannabinoid receptor agonists

Synthetic cannabinoid receptor agonists (SCRAs) are new psychoactive substances associated with acute intoxication and even death. However, the molecular mechanisms through which SCRAs may exert their toxic effects remain unclear-including the potential differential activation of G protein subtypes by cannabinoid receptor type 1 (CB1), a major target of SCRA. We measured CB1-mediated activation of Gαs and Gαi/o proteins by SCRAs by examining stimulation (pertussis toxin, PTX treated) as well as inhibition (non-PTX treated) of forskolin (FSK)-induced cyclic adenosine monophosphate (cAMP) accumulation in human embryonic kidney (HEK) cells stably expressing CB1. Real-time measurements of stimulation and inhibition of cAMP levels were made using a BRET biosensor. We found that the maximum concentration of SCRAs tested (10 µmol L-1 ), increased cAMP levels 12%-45% above that produced by FSK alone, while the phytocannabinoid THC did not significantly alter cAMP levels in PTX-treated HEK-CB1 cells. All SCRAs had greater potency to inhibit FSK-induced cAMP levels than to stimulate cAMP levels. The rank order of potencies for SCRA stimulation of cAMP (Gαs ) was PB-22 > 5F-MDMB-PICA > JWH-018 ≈ AB-FUBINACA > XLR-11. By contrast, the potency of SCRAs for inhibition of cAMP (Gαi/o ) was 5F-MDMB-PICA > AB-FUBINACA > PB-22 > JWH-018 > XLR-11. The different rank order of potency and EMax  of the SCRAs to stimulate Gαs -like signaling compared to Gαi/o signaling suggests differences in G protein preference between SCRAs. Understanding the apparent differences among these drugs may contribute to unravelling their complex effects in humans.

Structures of Gα Proteins in Complex with Their Chaperone Reveal Quality Control Mechanisms

Many chaperones promote nascent polypeptide folding followed by substrate release through ATP-dependent conformational changes. Here we show cryoEM structures of Gα subunit folding intermediates in complex with full-length Ric-8A, a unique chaperone-client system in which substrate release is facilitated by guanine nucleotide binding to the client G protein. The structures of Ric-8A-Gαi and Ric-8A-Gαq complexes reveal that the chaperone employs its extended C-terminal region to cradle the Ras-like domain of Gα, positioning the Ras core in contact with the Ric-8A core while engaging its switch2 nucleotide binding region. The C-terminal α5 helix of Gα is held away from the Ras-like domain through Ric-8A core domain interactions, which critically depend on recognition of the Gα C terminus by the chaperone. The structures, complemented with biochemical and cellular chaperoning data, support a folding quality control mechanism that ensures proper formation of the C-terminal α5 helix before allowing GTP-gated release of Gα from Ric-8A.

Structure of the G protein chaperone and guanine nucleotide exchange factor Ric-8A bound to Gαi1

Ric-8A is a cytosolic Guanine Nucleotide exchange Factor (GEF) that activates heterotrimeric G protein alpha subunits (Gα) and serves as an essential Gα chaperone. Mechanisms by which Ric-8A catalyzes these activities, which are stimulated by Casein Kinase II phosphorylation, are unknown. We report the structure of the nanobody-stabilized complex of nucleotide-free Gα bound to phosphorylated Ric-8A at near atomic resolution by cryo-electron microscopy and X-ray crystallography. The mechanism of Ric-8A GEF activity differs considerably from that employed by G protein-coupled receptors at the plasma membrane. Ric-8A engages a specific conformation of Gα at multiple interfaces to form a complex that is stabilized by phosphorylation within a Ric-8A segment that connects two Gα binding sites. The C-terminus of Gα is ejected from its beta sheet core, thereby dismantling the GDP binding site. Ric-8A binds to the exposed Gα beta sheet and switch II to stabilize the nucleotide-free state of Gα.

Adreno-melatonin receptor complexes control ion homeostasis and intraocular pressure - their disruption contributes to hypertensive glaucoma

BACKGROUND AND PURPOSE

Often, glaucoma presents with elevated eye hydrostatic pressure, which is regulated by endogenous melatonin. Phenylephrine increases cytoplasmic [Ca²⁺ ], via α₁ -adrenoceptor activation, that is detrimental in glaucoma. The aims of this study were (a) to elucidate the role of melatonin receptors in humour production and intraocular pressure (IOP) maintenance and (b) to identify glaucoma-relevant melatonin-adrenoceptor interactions.

EXPERIMENTAL APPROACH

Biophysical and proximity ligation assays were performed to identify interactions in heterologous expression systems, in cell lines and in human eyes. G_s /G_i /G_q signalling was investigated in these systems and in cells producing aqueous humour. IOP was determined in a mice model of glaucoma. Retinography and topically pharmacological treatment were performed in control and in glaucomatous mice.

KEY RESULTS

α₁ -adreno- and melatonin receptors form functional complexes in which the C-terminal tail of the adrenoceptor plays a role. Remarkably, activation of α₁ -adrenoceptors in these complexes did not lead to cytosolic Ca²⁺ increases, suggesting G_s instead of G_q coupling is involved. The number of these complexes significantly decreased in models of glaucoma and, importantly, in human samples from glaucoma patients. This has led to hypothesize that melatonin, a hypotensive agent, plus blockade of α₁ -adrenoceptors could normalize pressure in glaucoma. Remarkably, co-instillation of melatonin and prazosin, an α₁ -adrenoceptor antagonist, resulted in long-term decreases in IOP in a well-established animal model of glaucoma.

CONCLUSIONS AND IMPLICATIONS

The findings are instrumental to understand the physiological function of melatonin in the eye and its potential to address eye pathologies by targeting melatonin receptors and their complexes.

G Protein–Coupled Receptor Pharmacology at the Single-Molecule Level

G protein–coupled receptors (GPCRs) mediate the effects of numerous hormones and neurotransmitters and are major pharmacological targets. Classical studies with crude cell lysates or membrane preparations have identified the main biochemical steps involved in GPCR signaling. Moreover, recent studies on purified proteins have provided astounding details at the atomic level of the 3-D structures of receptors in multiple conformations, including in complex with G proteins and β-arrestins. However, several fundamental questions remain regarding the highly specific effects and rapid nature of GPCR signaling. Recent developments in single-molecule microscopy are providing important contributions to answering these questions. Overall, single-molecule studies have revealed unexpected levels of complexity, with receptors existing in different conformations and dynamically interacting among themselves, their signaling partners, and structural elements of the plasma membrane to produce highly localized signals in space and time. These findings may provide a new basis to develop innovative strategies to modulate GPCR function for pharmacological purposes.

Development of "Plug and Play" Fiducial Marks for Structural Studies of GPCR Signaling Complexes by Single-Particle Cryo-EM

"Universal" synthetic antibody (sAB)-based fiducial marks have been generated by customized phage display selections to facilitate the rapid structure determination of G protein-coupled receptor (GPCR) signaling complexes by single-particle cryo-electron microscopy (SP cryo-EM). sABs were generated to the two major G protein subclasses: trimeric G_i and G_s, as well as mini-G_s, and were tested to ensure binding in the context of their cognate GPCRs. Epitope binning revealed that multiple distinct epitopes exist for each G(αβγ) protein. Several Gβγ-specific sABs, cross-reactive between trimeric G_i and G_s, were identified suggesting they could be used across all subclasses in a "plug and play" fashion. sABs were also generated to a representative of another class of GPCR signaling partner, G protein receptor kinase 1 (GRK1) and evaluated further, supporting the generalizability of the approach. EM data suggested that the subclass-specific sABs provide effective single and dual fiducials for multiple GPCR signaling complexes.

Structures of the Rhodopsin-Transducin Complex: Insights into G-Protein Activation

Rhodopsin (Rho), a prototypical G-protein-coupled receptor (GPCR) in vertebrate vision, activates the G-protein transducin (G_T) by catalyzing GDP-GTP exchange on its α subunit (Gα_T). To elucidate the determinants of G_T coupling and activation, we obtained cryo-EM structures of a fully functional, light-activated Rho-G_T complex in the presence and absence of a G-protein-stabilizing nanobody. The structures illustrate how G_T overcomes its low basal activity by engaging activated Rho in a conformation distinct from other GPCR-G-protein complexes. Moreover, the nanobody-free structures reveal native conformations of G-protein components and capture three distinct conformers showing the Gα_T helical domain (αHD) contacting the Gβγ subunits. These findings uncover the molecular underpinnings of G-protein activation by visual rhodopsin and shed new light on the role played by Gβγ during receptor-catalyzed nucleotide exchange.

Assembly of a GPCR-G Protein Complex

The activation of G proteins by G protein-coupled receptors (GPCRs) underlies the majority of transmembrane signaling by hormones and neurotransmitters. Recent structures of GPCR-G protein complexes obtained by crystallography and cryoelectron microscopy (cryo-EM) reveal similar interactions between GPCRs and the alpha subunit of different G protein isoforms. While some G protein subtype-specific differences are observed, there is no clear structural explanation for G protein subtype-selectivity. All of these complexes are stabilized in the nucleotide-free state, a condition that does not exist in living cells. In an effort to better understand the structural basis of coupling specificity, we used time-resolved structural mass spectrometry techniques to investigate GPCR-G protein complex formation and G-protein activation. Our results suggest that coupling specificity is determined by one or more transient intermediate states that serve as selectivity filters and precede the formation of the stable nucleotide-free GPCR-G protein complexes observed in crystal and cryo-EM structures.

Vitamin E-based glycoside amphiphiles for membrane protein structural studies

Membrane proteins play critical roles in a variety of cellular processes. For a detailed molecular level understanding of their biological functions and roles in disease, it is necessary to extract them from the native membranes. While the amphipathic nature of these bio-macromolecules presents technical challenges, amphiphilic assistants such as detergents serve as useful tools for membrane protein structural and functional studies. Conventional detergents are limited in their ability to maintain the structural integrity of membrane proteins and thus it is essential to develop novel agents with enhanced properties. Here, we designed and characterized a novel class of amphiphiles with vitamin E (i.e., α-tocopherol) as the hydrophobic tail group and saccharide units as the hydrophilic head group. Designated vitamin E-based glycosides (VEGs), these agents were evaluated for their ability to solubilize and stabilize a set of membrane proteins. VEG representatives not only conferred markedly enhanced stability to a diverse range of membrane proteins compared to conventional detergents, but VEG-3 also showed notable efficacy toward stabilization and visualization of a membrane protein complex. In addition to hydrophile-lipophile balance (HLB) of detergent molecules, the chain length and molecular geometry of the detergent hydrophobic group seem key factors in determining detergent efficacy for membrane protein (complex) stability.

Assemblies of lauryl maltose neopentyl glycol (LMNG) and LMNG-solubilized membrane proteins

Laurylmaltose neopentylglycol (LMNG) bears two linked hydrophobic chains of equal length and two hydrophilic maltoside groups. It arouses a strong interest in the field of membrane protein biochemistry, since it was shown to efficiently solubilize and stabilize membrane proteins often better than the commonly used dodecylmaltopyranoside (DDM), and to allow structure determination of some challenging membrane proteins. However, LMNG was described to form large micelles, which could be unfavorable for structural purposes. We thus investigated its auto-assemblies and the association state of different membrane proteins solubilized in LMNG by analytical ultracentrifugation, size exclusion chromatography coupled to light scattering, centrifugation on sucrose gradient and/or small angle scattering. At high concentrations (in the mM range), LMNG forms long rods, and it stabilized the membrane proteins investigated herein, i.e. a bacterial multidrug transporter, BmrA; a prokaryotic analogous of the eukaryotic NADPH oxidases, SpNOX; an E. coli outer membrane transporter, FhuA; and the halobacterial bacteriorhodopsin, bR. BmrA, in the Apo and the vanadate-inhibited forms showed reduced kinetics of limited proteolysis in LMNG compared to DDM. Both SpNOX and BmrA display an increased specific activity in LMNG compared to DDM. The four proteins form LMNG complexes with their usual quaternary structure and with usual amount of bound detergent. No heterogeneous complexes related to the large micelle size of LMNG alone were observed. In conditions where LMNG forms assemblies of large size, FhuA crystals diffracting to 4.0 Å were obtained by vapor diffusion. LMNG large micelle size thus does not preclude membrane protein homogeneity and crystallization.

Structure of a Signaling Cannabinoid Receptor 1-G Protein Complex

Cannabis elicits its mood-enhancing and analgesic effects through the cannabinoid receptor 1 (CB1), a G protein-coupled receptor (GPCR) that signals primarily through the adenylyl cyclase-inhibiting heterotrimeric G protein G_i. Activation of CB1-G_i signaling pathways holds potential for treating a number of neurological disorders and is thus crucial to understand the mechanism of G_i activation by CB1. Here, we present the structure of the CB1-G_i signaling complex bound to the highly potent agonist MDMB-Fubinaca (FUB), a recently emerged illicit synthetic cannabinoid infused in street drugs that have been associated with numerous overdoses and fatalities. The structure illustrates how FUB stabilizes the receptor in an active state to facilitate nucleotide exchange in G_i. The results compose the structural framework to explain CB1 activation by different classes of ligands and provide insights into the G protein coupling and selectivity mechanisms adopted by the receptor.

Tracking Higher Order Protein Structure by Hydrogen-Deuterium Exchange Mass Spectrometry

BACKGROUND

Structural biology has provided a fundamental understanding of protein structure and mechanistic insight into their function. However, high-resolution structures alone are insufficient for a complete understanding of protein behavior. Higher energy conformations, conformational changes, and subtle structural fluctuations that underlie the proper function of proteins are often difficult to probe using traditional structural approaches. Hydrogen/Deuterium Exchange with Mass Spectrometry (HDX-MS) provides a way to probe the accessibility of backbone amide protons under native conditions, which reports on local structural dynamics of solution protein structure that can be used to track complex structural rearrangements that occur in the course of a protein's function.

CONCLUSION

In the last 20 years the advances in labeling techniques, sample preparation, instrumentation, and data analysis have enabled HDX to gain insights into very complex biological systems. Analysis of challenging targets such as membrane protein complexes is now feasible and the field is paving the way to the analysis of more and more complex systems.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

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

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

Rules of Engagement: GPCRs and G Proteins

G protein-coupled receptors (GPCRs) are a key drug target class. They account for over one-third of current pharmaceuticals, and both drugs that inhibit and promote receptor function are important therapeutically; in some cases, the same GPCR can be targeted with agonists and inhibitors, depending upon disease context. There have been major breakthroughs in understanding GPCR structure and drug binding through advances in X-ray crystallography, and membrane protein stabilization. Nonetheless, these structures have predominately been of inactive receptors bound to inhibitors. Efforts to capture structures of fully active GPCRs, in particular those in complex with the canonical, physiological transducer G protein, have been limited via this approach. Very recently, advances in cryo-electron microscopy have provided access to agonist:GPCR:G protein complex structures. These promise to revolutionize our understanding of GPCR:G protein engagement and provide insight into mechanisms of efficacy and coupling selectivity and how these might be controlled by biased agonists. Here we review what we have currently learned from the new GPCR:Gs and GPCR:Gi/o complex structures.

To sense or not to sense-new insights from GPCR-based and arrestin-based biosensors

Advances in resolving crystal structures of GPCRs and their binding partners as well as improvements in live-cell microscopy and the fluorescent proteins pallet has greatly driven new ideas for designing optical sensors for the same. Sensors have been developed to monitor ligand binding as well as the ensuing ligand-induced conformational changes in GPCRs, G-proteins and arrestins. In this review we will highlight the functionality of such sensor designs starting from monitoring ligand binding to receptor activation and interaction with arrestins. Furthermore, we will highlight the importance of sensor designs to monitor receptor-dependent arrestin conformations and give an idea about the various detected arrestin conformations and their possible implications.

Single-molecule labeling for studying trafficking of renal transporters

The ability to detect and track single molecules presents the advantage of visualizing the complex behavior of transmembrane proteins with a time and space resolution that would otherwise be lost with traditional labeling and biochemical techniques. Development of new imaging probes has provided a robust method to study their trafficking and surface dynamics. This mini-review focuses on the current technology available for single-molecule labeling of transmembrane proteins, their advantages, and limitations. We also discuss the application of these techniques to the study of renal transporter trafficking in light of recent research.