GPCR-G Protein-β-Arrestin Super-Complex Mediates Sustained G Protein Signaling

Conformational dynamics underlying atypical chemokine receptor 3 activation

Atypical Chemokine Receptor 3 (ACKR3) belongs to the G protein-coupled receptor family but it does not signal through G proteins. The structural properties that govern the functional selectivity and the conformational dynamics of ACKR3 activation are poorly understood. Here, we combined hydrogen/deuterium exchange mass spectrometry, site-directed mutagenesis, and molecular dynamics simulations to examine the binding mode and mechanism of action of ACKR3 ligands of different efficacies. Our results show that activation or inhibition of ACKR3 is governed by intracellular conformational changes of helix 6, intracellular loop 2, and helix 7, while the DRY motif becomes protected during both processes. Moreover, we identified the binding sites and the allosteric modulation of ACKR3 upon β-arrestin 1 binding. In summary, this study highlights the structure-function relationship of small ligands, the binding mode of β-arrestin 1, the activation dynamics, and the atypical dynamic features in ACKR3 that may contribute to its inability to activate G proteins.

Functional consequences of spatial, temporal and ligand bias of G protein-coupled receptors

G protein-coupled receptors (GPCRs) regulate every aspect of kidney function by mediating the effects of various endogenous and exogenous substances. A key concept in GPCR function is biased signalling, whereby certain ligands may selectively activate specific pathways within the receptor's signalling repertoire. For example, different agonists may induce biased signalling by stabilizing distinct active receptor conformations - a concept that is supported by advances in structural biology. However, the processes underlying functional selectivity in receptor signalling are extremely complex, involving differences in subcellular compartmentalization and signalling dynamics. Importantly, the molecular mechanisms of spatiotemporal bias, particularly its connection to ligand binding kinetics, have been detailed for GPCRs critical to kidney function, such as the AT1 angiotensin receptor (AT1R), V2 vasopressin receptor (V2R) and the parathyroid hormone 1 receptor (PTH1R). This expanding insight into the multifaceted nature of biased signalling paves the way for innovative strategies for targeting GPCR functions; the development of novel biased agonists may represent advanced pharmacotherapeutic approaches to the treatment of kidney diseases and related systemic conditions, such as hypertension, diabetes and heart failure.

Role of the V2R-βarrestin-Gβγ complex in promoting G protein translocation to endosomes

Classically, G protein-coupled receptors (GPCRs) promote signaling at the plasma membrane through activation of heterotrimeric Gαβγ proteins, followed by the recruitment of GPCR kinases and βarrestin (βarr) to initiate receptor desensitization and internalization. However, studies demonstrated that some GPCRs continue to signal from internalized compartments, with distinct cellular responses. Both βarr and Gβγ contribute to such non-canonical endosomal G protein signaling, but their specific roles and contributions remain poorly understood. Here, we demonstrate that the vasopressin V2 receptor (V2R)-βarr complex scaffolds Gβγ at the plasma membrane through a direct interaction with βarr, enabling its transport to endosomes. Gβγ subsequently potentiates Gαs endosomal translocation, presumably to regenerate an endosomal pool of heterotrimeric Gs. This work shines light on the mechanism underlying G protein subunits translocation from the plasma membrane to the endosomes and provides a basis for understanding the role of βarr in mediating sustained G protein signaling.

Engineered mini-G proteins block the internalization of cognate GPCRs and disrupt downstream intracellular signaling

Mini-G proteins are engineered, thermostable variants of Gα subunits designed to stabilize G protein-coupled receptors (GPCRs) in their active conformations. Because of their small size and ease of use, they are popular tools for assessing GPCR behaviors in cells, both as reporters of receptor coupling to Gα subtypes and for cellular assays to quantify compartmentalized signaling at various subcellular locations. Here, we report that overexpression of mini-G proteins with their cognate GPCRs disrupted GPCR endocytic trafficking and associated intracellular signaling. In cells expressing the Gαs-coupled GPCR glucagon-like peptide 1 receptor (GLP-1R), coexpression of mini-Gs, a mini-G protein derived from Gαs, blocked β-arrestin 2 recruitment and receptor internalization and disrupted endosomal GLP-1R signaling. These effects did not involve changes in receptor phosphorylation or lipid nanodomain segregation. Moreover, we found that mini-G proteins derived from Gαi and Gαq also inhibited the internalization of GPCRs that couple to them. Finally, we developed an alternative intracellular signaling assay for GLP-1R using a nanobody specific for active Gαs:GPCR complexes (Nb37) that did not affect GLP-1R internalization. Our results have important implications for designing methods to assess intracellular GPCR signaling.

Reduced incretin receptor trafficking upon activation enhances glycemic control and reverses obesity in diet-induced obese mice

Activation of incretin receptors by their cognate agonist augments sustained cAMP generation both from the plasma membrane as well as from the endosome. To address the functional outcome of this spatiotemporal signaling, we developed a nonacylated glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor dual agonist I-M-150847 that reduced receptor internalization following activation of the incretin receptors. The incretin receptor dual agonist I-M-150847 was developed by replacing the tryptophan cage of exendin-4 tyrosine substituted at the amino terminus with the C-terminal undecapeptide sequence of oxyntomodulin that placed lysine 30 of I-M-150847 in frame with the corresponding lysine residue of GIP. The peptide I-M-150847 is a partial agonist of GLP-1R and GIPR; however, the receptors, upon activation by I-M-150847, undergo reduced internalization that promotes agonist-mediated iterative cAMP signaling and augments glucose-stimulated insulin exocytosis in pancreatic β cells. Chronic administration of I-M-150847 improved glycemic control, enhanced insulin sensitivity, and provided profound weight loss in diet-induced obese (DIO) mice. Our results demonstrated that despite being a partial agonist, I-M-150847, by reducing the receptor internalization upon activation, enhanced the incretin effect and reversed obesity.NEW & NOTEWORTHY Replacement of the tryptophan cage (Trp-cage) with the C-terminal oxyntomodulin undecapeptide along with the tyrosine substitution at the amino terminus converts the selective glucagon-like peptide-1 receptor (GLP-1R) agonist exendin-4 to a novel GLP-1R and GIPR dual agonist I-M-150847. Reduced internalization of incretin receptors upon activation by the GLP-1R and GIPR dual agonist I-M-150847 promotes iterative receptor signaling that enhances the incretin effect and reverses obesity.

Identification of protein kinase A signalling molecules in renal collecting ducts

Body water homeostasis is maintained by the correct balance between water intake and water loss through urine, faeces, sweat and breath. It is known that elevated circulating levels of the antidiuretic hormone vasopressin decrease urine volume to prevent excessive water loss from the body. Vasopressin/cAMP/protein kinase A (PKA) signalling is the canonical pathway in renal collecting ducts for phosphorylating aquaporin-2 (AQP2) water channels, which leads to the reabsorption of water from urine via AQP2. Although recent omics data have verified various downstream targets of PKA, crucial regulators that mediate PKA-induced AQP2 phosphorylation remain unknown, mainly because vasopressin is usually used to activate PKA as a positive control. Vasopressin is extremely potent and phosphorylates various PKA substrates non-specifically, making it difficult to narrow down the candidate mediators responsible for AQP2 phosphorylation. The intracellular localization of PKA is tightly regulated by its scaffold proteins, also known as A-kinase anchoring proteins (AKAPs). Furthermore, each AKAP has a target domain that determines its intracellular localization, enabling the creation of a local PKA signalling network. Although vasopressin activates most PKAs independently of their intracellular localization, some chemical compounds preferentially act on PKAs localized on AQP2-containing vesicles while simultaneously phosphorylating AQP2 and its surrounding PKA substrates. Immunoprecipitation with antibodies against phosphorylated PKA substrates followed by mass spectrometry analysis revealed that the PKA substrate in proximity to AQP2 was lipopolysaccharide-responsive and beige-like anchor (LRBA). Furthermore, Lrba knockout studies revealed that LRBA was required for vasopressin-induced AQP2 phosphorylation.

Constitutive internalisation of EP2 differentially regulates G protein signalling

The prostanoid G protein-coupled receptor (GPCR) EP2 is widely expressed and implicated in endometriosis, osteoporosis, obesity, pre-term labour and cancer. Internalisation and intracellular trafficking are critical for shaping GPCR activity, yet little is known regarding the spatial programming of EP2 signalling and whether this can be exploited pharmacologically. Using three EP2-selective ligands that favour activation of different EP2 pathways, we show that EP2 undergoes limited agonist-driven internalisation but is constitutively internalised via dynamin-dependent, β-arrestin-independent pathways. EP2 was constitutively trafficked to early and very early endosomes (VEE), which was not altered by ligand activation. APPL1, a key adaptor and regulatory protein of the VEE, did not impact EP2 agonist-mediated cAMP. Internalisation was required for ~70% of the acute butaprost- and AH13205-mediated cAMP signalling, yet PGN9856i, a Gαs-biased agonist, was less dependent on receptor internalisation for its cAMP signalling, particularly in human term pregnant myometrial cells that endogenously express EP2. Inhibition of EP2 internalisation partially reduced calcium signalling activated by butaprost or AH13205 and had no effect on PGE2 secretion. This indicates an agonist-dependent differential spatial requirement for Gαs and Gαq/11 signalling and a role for plasma membrane-initiated Gαq/11-Ca2+-mediated PGE2 secretion. These findings reveal a key role for EP2 constitutive internalisation in its signalling and potential spatial bias in mediating its downstream functions. This, in turn, could highlight important considerations for future selective targeting of EP2 signalling pathways.

G Protein-Coupled Receptors: A Century of Research and Discovery

GPCRs (G protein-coupled receptors), also known as 7 transmembrane domain receptors, are the largest receptor family in the human genome, with ≈800 members. GPCRs regulate nearly every aspect of human physiology and disease, thus serving as important drug targets in cardiovascular disease. Sharing a conserved structure comprised of 7 transmembrane α-helices, GPCRs couple to heterotrimeric G-proteins, GPCR kinases, and β-arrestins, promoting downstream signaling through second messengers and other intracellular signaling pathways. GPCR drug development has led to important cardiovascular therapies, such as antagonists of β-adrenergic and angiotensin II receptors for heart failure and hypertension, and agonists of the glucagon-like peptide-1 receptor for reducing adverse cardiovascular events and other emerging indications. There continues to be a major interest in GPCR drug development in cardiovascular and cardiometabolic disease, driven by advances in GPCR mechanistic studies and structure-based drug design. This review recounts the rich history of GPCR research, including the current state of clinically used GPCR drugs, and highlights newly discovered aspects of GPCR biology and promising directions for future investigation. As additional mechanisms for regulating GPCR signaling are uncovered, new strategies for targeting these ubiquitous receptors hold tremendous promise for the field of cardiovascular medicine.

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

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

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.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

Ubiquitin-driven G protein-coupled receptor inflammatory signaling at the endosome

G protein-coupled receptors (GPCRs) are ubiquitously expressed cell surface receptors that mediate numerous physiological responses and are highly druggable. Upon activation, GPCRs rapidly couple to heterotrimeric G proteins and are then phosphorylated and internalized from the cell surface. Recent studies indicate that GPCRs not only localize at the plasma membrane but also exist in intracellular compartments where they are competent to signal. Intracellular signaling by GPCRs is best described to occur at endosomes. Several studies have elegantly documented endosomal GPCR-G protein and GPCR-β-arrestin signaling. Besides phosphorylation, GPCRs are also posttranslationally modified with ubiquitin. GPCR ubiquitination has been studied mainly in the context of receptor endosomal-lysosomal trafficking. However, new studies indicate that ubiquitination of endogenous GPCRs expressed in endothelial cells initiates the assembly of an intracellular p38 mitogen-activated kinase signaling complex that promotes inflammatory responses from endosomes. In this mini-review, we discuss emerging discoveries that provide critical insights into the function of ubiquitination in regulating GPCR inflammatory signaling at endosomes.

Adrenoceptor Desensitization: Current Understanding of Mechanisms

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of β-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, β-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, β-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the β adrenoceptors and highlights the role of β-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

TRAF6-mediated ubiquitination of AKT in the nucleus is a critical event underlying the desensitization of G protein-coupled receptors

BACKGROUND

Desensitization of G protein-coupled receptors (GPCRs) refers to the attenuation of receptor responsiveness by prolonged or intermittent exposure to agonists. The binding of β-arrestin to the cytoplasmic cavity of the phosphorylated receptor, which competes with the G protein, has been widely accepted as an extensive model for explaining GPCRs desensitization. However, studies on various GPCRs, including dopamine D2-like receptors (D2R, D3R, D4R), have suggested the existence of other desensitization mechanisms. The present study employed D2R/D3R variants with different desensitization properties and utilized loss-of-function approaches to uncover the mechanisms underlying GPCRs homologous desensitization, focusing on the signaling cascade that regulates the ubiquitination of AKT.

RESULTS

AKT undergoes K8/14 ubiquitination by TRAF6, which occurs in the nucleus and promotes its membrane recruitment, phosphorylation and activation under receptor desensitization conditions. The nuclear entry of TRAF6 relies on the presence of the importin complex. Src regulates the nuclear entry of TRAF6 by mediating the interaction between TRAF6 and importin β1. Ubiquitinated AKT translocates to the plasma membrane where it associates with Mdm2 to phosphorylate it at the S166 and S186 residues. Thereafter, phosphorylated Mdm2 is recruited to the nucleus, resulting in the deubiquitination of β-Arr2. The deubiquitinated β-Arr2 then forms a complex with Gβγ, which serves as a biomarker for GPCRs desensitization. Like in D3R, ubiquitination of AKT is also involved in the desensitization of β2 adrenoceptors.

CONCLUSION

Our study proposed that the property of a receptor that causes a change in the subcellular localization of TRAF6 from the cytoplasm to the nucleus to mediate AKT ubiquitination could initiate the desensitization of GPCRs.

Rational Design of Drugs Targeting G-Protein-Coupled Receptors: A Structural Biology Perspective

G protein-coupled receptors (GPCRs) play a key role in the transduction of extracellular signals to cells and regulation of many biological processes, which makes these membrane proteins one of the most important targets for pharmacological agents. A significant increase in the number of resolved atomic structures of GPCRs has opened the possibility of developing pharmaceuticals targeting these receptors via structure-based drug design (SBDD). SBDD employs information on the structure of receptor-ligand complexes to search for selective ligands without the need for an extensive high-throughput experimental ligand screening and can significantly expand the chemical space for ligand search. In this review, we describe the process of deciphering GPCR structures using X-ray diffraction analysis and cryoelectron microscopy as an important stage in the rational design of drugs targeting this receptor class. Our main goal was to present modern developments and key features of experimental methods used in SBDD of GPCR-targeting agents to a wide range of specialists.

Oxytocin and vasopressin signaling in health and disease

Neurohypophysial peptides are ancient and evolutionarily highly conserved neuropeptides that regulate many crucial physiological functions in vertebrates and invertebrates. The human neurohypophysial oxytocin/vasopressin (OT/VP) signaling system with its four receptors has become an attractive drug target for a variety of diseases, including cancer, pain, cardiovascular indications, and neurological disorders. Despite its promise, drug development faces hurdles, including signaling complexity, selectivity and off-target concerns, translational interspecies differences, and inefficient drug delivery. In this review we dive into the complexity of the OT/VP signaling system in health and disease, provide an overview of relevant pharmacological probes, and discuss the latest trends in therapeutic lead discovery and drug development.

Computational Modeling and Characterization of Peptides Derived from Nanobody Complementary-Determining Region 2 (CDR2) Targeting Active-State Conformation of the β2-Adrenergic Receptor (β2AR)

This study assessed the suitability of the complementarity-determining region 2 (CDR2) of the nanobody (Nb) as a template for the derivation of nanobody-derived peptides (NDPs) targeting active-state β2-adrenergic receptor (β2AR) conformation. Sequences of conformationally selective Nbs favoring the agonist-occupied β2AR were initially analyzed by the informational spectrum method (ISM). The derived NDPs in complex with β2AR were subjected to protein-peptide docking, molecular dynamics (MD) simulations, and metadynamics-based free-energy binding calculations. Computational analyses identified a 25-amino-acid-long CDR2-NDP of Nb71, designated P4, which exhibited the following binding free-energy for the formation of the β2AR:P4 complex (ΔG = -6.8 ± 0.8 kcal/mol or a Ki = 16.5 μM at 310 K) and mapped the β2AR:P4 amino acid interaction network. In vitro characterization showed that P4 (i) can cross the plasma membrane, (ii) reduces the maximum isoproterenol-induced cAMP level by approximately 40% and the isoproterenol potency by up to 20-fold at micromolar concentration, (iii) has a very low affinity to interact with unstimulated β2AR in the cAMP assay, and (iv) cannot reduce the efficacy and potency of the isoproterenol-mediated β2AR/β-arrestin-2 interaction in the BRET2-based recruitment assay. In summary, the CDR2-NDP, P4, binds preferentially to agonist-activated β2AR and disrupts Gαs-mediated signaling.

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.

Functional diversification of cell signaling by GPCR localization

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and a critical class of regulators of mammalian physiology. Also known as seven transmembrane receptors (7TMs), GPCRs are ubiquitously expressed and versatile, detecting a diverse set of endogenous stimuli, including odorants, neurotransmitters, hormones, peptides, and lipids. Accordingly, GPCRs have emerged as the largest class of drug targets, accounting for upward of 30% of all prescription drugs. The view that ligand-induced GPCR responses originate exclusively from the cell surface has evolved to reflect accumulating evidence that receptors can elicit additional waves of signaling from intracellular compartments. These events in turn shape unique cellular and physiological outcomes. Here, we discuss our current understanding of the roles and regulation of compartmentalized GPCR signaling.

GPCR kinases differentially modulate biased signaling downstream of CXCR3 depending on their subcellular localization

Some G protein-coupled receptors (GPCRs) demonstrate biased signaling such that ligands of the same receptor exclusively or preferentially activate certain downstream signaling pathways over others. This phenomenon may result from ligand-specific receptor phosphorylation by GPCR kinases (GRKs). GPCR signaling can also exhibit location bias because GPCRs traffic to and signal from subcellular compartments in addition to the plasma membrane. Here, we investigated whether GRKs contributed to location bias in GPCR signaling. GRKs translocated to endosomes after stimulation of the chemokine receptor CXCR3 or other GPCRs in cultured cells. GRK2, GRK3, GRK5, and GRK6 showed distinct patterns of recruitment to the plasma membrane and to endosomes depending on the identity of the biased ligand used to activate CXCR3. Analysis of engineered forms of GRKs that localized to either the plasma membrane or endosomes demonstrated that biased CXCR3 ligands elicited different signaling profiles that depended on the subcellular location of the GRK. Each GRK exerted a distinct effect on the regulation of CXCR3 engagement of β-arrestin, internalization, and activation of the downstream effector kinase ERK. Our work highlights a role for GRKs in location-biased GPCR signaling and demonstrates the complex interactions between ligands, GRKs, and cellular location that contribute to biased signaling.

Hypothalamic tanycytes as mediators of maternally programmed seasonal plasticity

In mammals, maternal photoperiodic programming (MPP) provides a means whereby juvenile development can be matched to forthcoming seasonal environmental conditions.1,2,3,4 This phenomenon is driven by in utero effects of maternal melatonin5,6,7 on the production of thyrotropin (TSH) in the fetal pars tuberalis (PT) and consequent TSH receptor-mediated effects on tanycytes lining the 3rd ventricle of the mediobasal hypothalamus (MBH).8,9,10 Here we use LASER capture microdissection and transcriptomic profiling to show that TSH-dependent MPP controls the attributes of the ependymal region of the MBH in juvenile animals. In Siberian hamster pups gestated and raised on a long photoperiod (LP) and thereby committed to a fast trajectory for growth and reproductive maturation, the ependymal region is enriched for tanycytes bearing sensory cilia and receptors implicated in metabolic sensing. Contrastingly, in pups gestated and raised on short photoperiod (SP) and therefore following an over-wintering developmental trajectory with delayed sexual maturation, the ependymal region has fewer sensory tanycytes. Post-weaning transfer of SP-gestated pups to an intermediate photoperiod (IP), which accelerates reproductive maturation, results in a pronounced shift toward a ciliated tanycytic profile and formation of tanycytic processes. We suggest that tanycytic plasticity constitutes a mechanism to tailor metabolic development for extended survival in variable overwintering environments.

Endosome positioning coordinates spatially selective GPCR signaling

G-protein-coupled receptors (GPCRs) can initiate unique functional responses depending on the subcellular site of activation. Efforts to uncover the mechanistic basis of compartmentalized GPCR signaling have concentrated on the biochemical aspect of this regulation. Here we assess the biophysical positioning of receptor-containing endosomes as an alternative salient mechanism. We devise a strategy to rapidly and selectively redistribute receptor-containing endosomes 'on command' in intact cells without perturbing their biochemical composition. Next, we present two complementary optical readouts that enable robust measurements of bulk- and gene-specific GPCR/cyclic AMP (cAMP)-dependent transcriptional signaling with single-cell resolution. With these, we establish that disruption of native endosome positioning inhibits the initiation of the endosome-dependent transcriptional responses. Finally, we demonstrate a prominent mechanistic role of PDE-mediated cAMP hydrolysis and local protein kinase A activity in this process. Our study, therefore, illuminates a new mechanism regulating GPCR function by identifying endosome positioning as the principal mediator of spatially selective receptor signaling.

ACKR3 Proximity Labeling Identifies Novel G protein- and β-arrestin-independent GPCR Interacting Proteins

The canonical paradigm of GPCR signaling recognizes G proteins and β-arrestins as the two primary transducers that promote GPCR signaling. Recent evidence suggests the atypical chemokine receptor 3 (ACKR3) does not couple to G proteins, and β-arrestins are dispensable for some of its functions. Here, we employed proximity labeling to identify proteins that interact with ACKR3 in cells devoid of β-arrestin. We identified proteins involved in the endocytic machinery and evaluated a subset of proteins conserved across several GPCR-based proximity labeling experiments. We discovered that the bone morphogenic protein 2-inducible kinase (BMP2K) interacts with many different GPCRs with varying dependency on β-arrestin. Together, our work highlights the existence of modulators that can act independently of G proteins and β-arrestins to regulate GPCR signaling and provides important evidence for other targets that may regulate GPCR signaling.

G Protein-Coupled Receptor Signaling: New Insights Define Cellular Nanodomains

G protein-coupled receptors are the largest and pharmacologically most important receptor family and are involved in the regulation of most cell functions. Most of them reside exclusively at the cell surface, from where they signal via heterotrimeric G proteins to control the production of second messengers such as cAMP and IP3 as well as the activity of several ion channels. However, they may also internalize upon agonist stimulation or constitutively reside in various intracellular locations. Recent evidence indicates that their function differs depending on their precise cellular localization. This is because the signals they produce, notably cAMP and Ca2+, are mostly bound to cell proteins that significantly reduce their mobility, allowing the generation of steep concentration gradients. As a result, signals generated by the receptors remain confined to nanometer-sized domains. We propose that such nanometer-sized domains represent the basic signaling units in a cell and a new type of target for drug development.

New insights into sperm physiology regulation: Enlightenment from G-protein-coupled receptors

BACKGROUND

G-protein-coupled receptors are critical in many physiological and pathological processes in various organs. Serving as the control panel for sensing extracellular stimuli, G-protein-coupled receptors recognise various ligands, including light, temperature, odours, pheromones, hormones, neurotransmitters, chemokines, etc. Most recently, G-protein-coupled receptors residing in spermatozoa have been found to be indispensable for sperm function.

OBJECTIVE

Here, we have summarised cutting-edge findings on the functional mechanisms of G-protein-coupled receptors that are known to be associated with sperm functions and the activation of their downstream effectors, providing new insights into the roles of G-protein-coupled receptors in sperm physiology.

RESULTS

Emerging studies hint that alterations in G-protein-coupled receptors could affect sperm function, implicating their role in fertility, but solid evidence needs to be continuing excavated with various means. Several members of the G-protein-coupled receptor superfamily, including olfactory receptors, opsins, orphan G-protein-coupled receptors, CXC chemokine receptor 4, CC chemokine receptor 5 and CC chemokine receptor 6 as well as their downstream effector β-arrestins, etc., were suggested to be essential for sperm motility, capacitation, thermotaxis, chemotaxis, Ca2+ influx through CatSper channel and fertilisation capacity.

CONCLUSION

The present review provides a comprehensive overview of studies describing G-protein-coupled receptors and their potential action in sperm function. We also present a critical discussion of these issues, and a possible framework for future investigations on the diverse ligands, biological functions and cell signalling of G-protein-coupled receptors in spermatozoa. Here, the G-protein-coupled receptors and their related G proteins that specifically were identified in spermatozoa were summarised, and provided references valuable for further illumination, despite the evidence that is not overwhelming in most cases.

Investigating G-protein coupled receptor signalling with light-emitting biosensors

G protein-coupled receptors (GPCRs) are the most frequent target of currently approved drugs and play a central role in both physiological and pathophysiological processes. Beyond the canonical understanding of GPCR signal transduction, the importance of receptor conformation, beta-arrestin (β-arr) biased signalling, and signalling from intracellular locations other than the plasma membrane is becoming more apparent, along with the tight spatiotemporal compartmentalisation of downstream signals. Fluorescent and bioluminescent biosensors have played a pivotal role in elucidating GPCR signalling events in live cells. To understand the mechanisms of action of the GPCR-targeted drugs currently available, and to develop new and better GPCR-targeted therapeutics, understanding these novel aspects of GPCR signalling is critical. In this review, we present some of the tools available to interrogate each of these features of GPCR signalling, we illustrate some of the key findings which have been made possible by these tools and we discuss their limitations and possible developments.

From membrane to nucleus: A three-wave hypothesis of cAMP signaling

For many decades, our understanding of G protein-coupled receptor (GPCR) activity and cyclic AMP (cAMP) signaling was limited exclusively to the plasma membrane. However, a growing body of evidence has challenged this view by introducing the concept of endocytosis-dependent GPCR signaling. This emerging paradigm emphasizes not only the sustained production of cAMP but also its precise subcellular localization, thus transforming our understanding of the spatiotemporal organization of this process. Starting from this alternative point of view, our recent work sheds light on the role of an endocytosis-dependent calcium release from the endoplasmic reticulum in the control of nuclear cAMP levels. This is achieved through the activation of local soluble adenylyl cyclase, which in turn regulates the activation of local protein kinase A (PKA) and downstream transcriptional events. In this review, we explore the dynamic evolution of research on cyclic AMP signaling, including the findings that led us to formulate the novel three-wave hypothesis. We delve into how we abandoned the paradigm of cAMP generation limited to the plasma membrane and the changing perspectives on the rate-limiting step in nuclear PKA activation.

Catecholamine-induced hypertensive crises: current insights and management

Phaeochromocytomas and paragangliomas (PPGLs) release catecholamines leading to catecholamine-induced hypertensive (CIH) crises, with blood pressure greater than or equal to 180/120 mm Hg. CIH crises can be complicated by tachyarrhythmias, hypotension, or life-threatening target organ damage while treatment remains undefined, often requiring co-management between endocrinologists and cardiologists. Furthermore, biochemical diagnosis of a PPGL as a cause of a CIH crisis can be difficult to identify or confounded by comorbid conditions, potentially resulting in misdiagnosis. Here, we combine relevant evidence, 60 years of collective clinical experience, insights derived from assessing over 2600 patients with PPGL, and supplementary outcomes from 100 patients (treated at the National Institutes of Health) with a CIH crisis to inform diagnosis and treatment of CIH crises. Recognising that disparities exist between availability, cost, and familiarity of various agents, flexible approaches are delineated allowing for customisation, given institutional availability and provider preference. A CIH crisis and its complications are readily treatable with available drugs, with effective intervention defining an avenue for mitigating consequent morbidity and mortality in patients with PPGL.

Distinct activation mechanisms of β-arrestin-1 revealed by 19F NMR spectroscopy

β-Arrestins (βarrs) are functionally versatile proteins that play critical roles in the G-protein-coupled receptor (GPCR) signaling pathways. While it is well established that the phosphorylated receptor tail plays a central role in βarr activation, emerging evidence highlights the contribution from membrane lipids. However, detailed molecular mechanisms of βarr activation by different binding partners remain elusive. In this work, we present a comprehensive study of the structural changes in critical regions of βarr1 during activation using 19F NMR spectroscopy. We show that phosphopeptides derived from different classes of GPCRs display different βarr1 activation abilities, whereas binding of the membrane phosphoinositide PIP2 stabilizes a distinct partially activated conformational state. Our results further unveil a sparsely-populated activation intermediate as well as complex cross-talks between different binding partners, implying a highly multifaceted conformational energy landscape of βarr1 that can be intricately modulated during signaling.

Stepwise phosphorylation of BLT1 defines complex assemblies with β-arrestin serving distinct functions

G protein-coupled receptors (GPCRs) utilize complex cellular systems to respond to diverse ligand concentrations. By taking BLT1, a GPCR for leukotriene B4 (LTB4 ), as a model, our previous work elucidated that this system functions through the modulation of phosphorylation status on two specific residues: Thr308 and Ser310 . Ser310 phosphorylation occurs at a lower LTB4 concentration than Thr308 , leading to a shift in ligand affinity from a high-to-low state. However, the implications of BLT1 phosphorylation in signal transduction processes or the underlying mechanisms have remained unclear. Here, we identify the sequential BLT1-engaged conformations of β-arrestin and subsequent alterations in signal transduction. Stimulation of the high-affinity BLT1 with LTB4 induces phosphorylation at Ser310 via the ERK1/2-GRK pathway, resulting in a β-arrestin-bound low-affinity state. This configuration, referred to as the "low-LTB4 -induced complex," necessitates the finger loop region and the phosphoinositide-binding motif of β-arrestins to interact with BLT1 and deactivates the ERK1/2 signaling. Under high LTB4 concentrations, the low-affinity BLT1 again binds to the ligand and triggers the generation of the low-LTB4 -induced complex into a different form termed "high-LTB4 -induced complex." This change is propelled by The308 -phosphorylation-dependent basal phosphorylation by PKCs. Within the high-LTB4 -induced complex, β-arrestin adapts a unique configuration that involves additional N domain interaction to the low-affinity BLT1 and stimulates the PI3K/AKT pathway. We propose that the stepwise phosphorylation of BLT1 defines the formation of complex assemblies, wherein β-arrestins perform distinct functions.

Location bias: A "Hidden Variable" in GPCR pharmacology

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors and primarily signal through two main effector proteins: G proteins and β-arrestins. Many agonists of GPCRs promote "biased" responses, in which different cellular signaling pathways are activated with varying efficacies. The mechanisms underlying biased signaling have not been fully elucidated, with many potential "hidden variables" that regulate this behavior. One contributor is "location bias," which refers to the generation of unique signaling cascades from a given GPCR depending upon the cellular location at which the receptor is signaling. Here, we review evidence that GPCRs are expressed at and traffic to various subcellular locations and discuss how location bias can impact the pharmacologic properties and characterization of GPCR agonists. We also evaluate how differences in subcellular environments can modulate GPCR signaling, highlight the physiological significance of subcellular GPCR signaling, and discuss the therapeutic potential of exploiting GPCR location bias.

Src tyrosine kinase promotes cardiac remodeling induced by chronic sympathetic activation

Cardiac remodeling serves as the underlying pathological basis for numerous cardiovascular diseases and represents a pivotal stage for intervention. The excessive activation of β-adrenergic receptors (β-ARs) assumes a crucial role in cardiac remodeling. Nonetheless, the underlying molecular mechanisms governing β-AR-induced cardiac remodeling remain largely unresolved. In the present study, we identified Src tyrosine kinase as a key player in the cardiac remodeling triggered by excessive β-AR activation. Our findings demonstrated that Src mediates isoproterenol (ISO)-induced cardiac hypertrophy, fibrosis, and inflammation in vivo. Furthermore, Src facilitates β-AR-mediated proliferation and transdifferentiation of cardiac fibroblasts, and hypertrophy and cardiomyocytes in vitro. Subsequent investigations have substantiated that Src mediates β-AR induced the extracellular signal-regulated protein kinase (ERK1/2) signaling pathway activated by β-AR. Our research presents compelling evidence that Src promotes β-AR-induced cardiac remodeling in both in vivo and in vitro settings. It establishes the promoting effect of the β-AR/Src/ERK signaling pathway on overall cardiac remodeling in cardiac fibroblasts and underscores the potential of Src as a therapeutic target for cardiac remodeling.

β-Arrestin-dependent and -independent endosomal G protein activation by the vasopressin type 2 receptor

The vasopressin type 2 receptor (V2R) is an essential G protein-coupled receptor (GPCR) in renal regulation of water homeostasis. Upon stimulation, the V2R activates Gαs and Gαq/11, which is followed by robust recruitment of β-arrestins and receptor internalization into endosomes. Unlike canonical GPCR signaling, the β-arrestin association with the V2R does not terminate Gαs activation, and thus, Gαs-mediated signaling is sustained while the receptor is internalized. Here, we demonstrate that this V2R ability to co-interact with G protein/β-arrestin and promote endosomal G protein signaling is not restricted to Gαs, but also involves Gαq/11. Furthermore, our data imply that β-arrestins potentiate Gαs/Gαq/11 activation at endosomes rather than terminating their signaling. Surprisingly, we found that the V2R internalizes and promote endosomal G protein activation independent of β-arrestins to a minor degree. These new observations challenge the current model of endosomal GPCR signaling and suggest that this event can occur in both β-arrestin-dependent and -independent manners.

GLP-1R signaling neighborhoods associate with the susceptibility to adverse drug reactions of incretin mimetics

G protein-coupled receptors are important drug targets that engage and activate signaling transducers in multiple cellular compartments. Delineating therapeutic signaling from signaling associated with adverse events is an important step towards rational drug design. The glucagon-like peptide-1 receptor (GLP-1R) is a validated target for the treatment of diabetes and obesity, but drugs that target this receptor are a frequent cause of adverse events. Using recently developed biosensors, we explored the ability of GLP-1R to activate 15 pathways in 4 cellular compartments and demonstrate that modifications aimed at improving the therapeutic potential of GLP-1R agonists greatly influence compound efficacy, potency, and safety in a pathway- and compartment-selective manner. These findings, together with comparative structure analysis, time-lapse microscopy, and phosphoproteomics, reveal unique signaling signatures for GLP-1R agonists at the level of receptor conformation, functional selectivity, and location bias, thus associating signaling neighborhoods with functionally distinct cellular outcomes and clinical consequences.

Interplay between G protein-coupled receptors and nanotechnology

G protein-coupled receptors (GPCRs), as the largest family of membrane receptors, actively modulate plasma membrane and endosomal signalling. Importantly, GPCRs are naturally nanosized, and spontaneously formed nanoaggregates of GPCRs (natural nano-GPCRs) may enhance GPCR-related signalling and functions. Although GPCRs are the molecular targets of the majority of marketed drugs, the poor pharmacokinetics and physicochemical properties of GPCR ligands greatly limit their clinical applicability. Nanotechnology, as versatile techniques, can encapsulate GPCR ligands to assemble synthetic nano-GPCRs to overcome their obstacles, robustly elevating drug efficacy and safety. Moreover, endosomal delivery of GPCR ligands by nanoparticles can precisely initiate sustained endosomal signal transduction, while nanotechnology has been widely utilized for isolation, diagnosis, and detection of GPCRs. In turn, due to overexpression of GPCRs on the surface of various types of cells, GPCR ligands can endow nanoparticles with active targeting capacity for specific cells via ligand-receptor binding and mediate receptor-dependent endocytosis of nanoparticles. This significantly enhances the potency of nanoparticle delivery systems. Therefore, emerging evidence has revealed the interplay between GPCRs and nanoparticles, although investigations into their relationship have been inadequate. This review aims to summarize the interaction between GPCRs and nanotechnology for understanding their mutual influences and utilizing their interplay for biomedical applications. It will provide a fundamental platform for developing powerful and safe GPCR-targeted drugs and nanoparticle systems. STATEMENT OF SIGNIFICANCE: GPCRs as molecular targets for the majority of marketed drugs are naturally nanosized, and even spontaneously form nano aggregations (nano-GPCRs). Nanotechnology has also been applied to construct synthetic nano-GPCRs or detect GPCRs, while endosomal delivery of GPCR ligands by nanoparticles can magnify endosomal signalling. Meanwhile, molecular engineering of nanoparticles with GPCRs or their ligands can modulate membrane binding and endocytosis, powerfully improving the efficacy of nanoparticle system. However, there are rare summaries on the interaction between GPCRs and nanoparticles. This review will not only provide a versatile platform for utilizing nanoparticles to modulate or detect GPCRs, but also facilitate better understanding of the designated value of GPCRs for molecular engineering of biomaterials with GPCRs in therapeutical application.

Gαq signalling from endosomes: A new conundrum

G-protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors, and are involved in the transmission of a variety of extracellular stimuli such as hormones, neurotransmitters, light and odorants into intracellular responses. They regulate every aspect of physiology and, for this reason, about one third of all marketed drugs target these receptors. Classically, upon binding to their agonist, GPCRs are thought to activate G-proteins from the plasma membrane and to stop signalling by subsequent desensitisation and endocytosis. However, accumulating evidence indicates that, upon internalisation, some GPCRs can continue to activate G-proteins in endosomes. Importantly, this signalling from endomembranes mediates alternative cellular responses other than signalling at the plasma membrane. Endosomal G-protein signalling and its physiological relevance have been abundantly documented for Gαs - and Gαi -coupled receptors. Recently, some Gαq -coupled receptors have been reported to activate Gαq on endosomes and mediate important cellular processes. However, several questions relative to the series of cellular events required to translate endosomal Gαq activation into cellular responses remain unanswered and constitute a new conundrum. How are these responses in endosomes mediated in the quasi absence of the substrate for the canonical Gαq -activated effector? Is there another effector? Is there another substrate? If so, how does this alternative endosomal effector or substrate produce a downstream signal? This review aims to unravel and discuss these important questions, and proposes possible routes of investigation.

β-Arrestins: Structure, Function, Physiology, and Pharmacological Perspectives

The two β-arrestins, β-arrestin-1 and -2 (systematic names: arrestin-2 and -3, respectively), are multifunctional intracellular proteins that regulate the activity of a very large number of cellular signaling pathways and physiologic functions. The two proteins were discovered for their ability to disrupt signaling via G protein-coupled receptors (GPCRs) via binding to the activated receptors. However, it is now well recognized that both β-arrestins can also act as direct modulators of numerous cellular processes via either GPCR-dependent or -independent mechanisms. Recent structural, biophysical, and biochemical studies have provided novel insights into how β-arrestins bind to activated GPCRs and downstream effector proteins. Studies with β-arrestin mutant mice have identified numerous physiologic and pathophysiological processes regulated by β-arrestin-1 and/or -2. Following a short summary of recent structural studies, this review primarily focuses on β-arrestin-regulated physiologic functions, with particular focus on the central nervous system and the roles of β-arrestins in carcinogenesis and key metabolic processes including the maintenance of glucose and energy homeostasis. This review also highlights potential therapeutic implications of these studies and discusses strategies that could prove useful for targeting specific β-arrestin-regulated signaling pathways for therapeutic purposes. SIGNIFICANCE STATEMENT: The two β-arrestins, structurally closely related intracellular proteins that are evolutionarily highly conserved, have emerged as multifunctional proteins able to regulate a vast array of cellular and physiological functions. The outcome of studies with β-arrestin mutant mice and cultured cells, complemented by novel insights into β-arrestin structure and function, should pave the way for the development of novel classes of therapeutically useful drugs capable of regulating specific β-arrestin functions.

β-arrestin-dependent and -independent endosomal G protein activation by the vasopressin type 2 receptor

The vasopressin type 2 receptor (V2R) is an essential GPCR in renal regulation of water homeostasis. Upon stimulation, the V2R activates Gαs and Gαq/11, which is followed by robust recruitment of β-arrestins and receptor internalization into endosomes. Unlike canonical GPCR signaling, the β-arrestin association with the V2R does not terminate Gαs activation, and thus, Gαs-mediated signaling is sustained while the receptor is internalized. Here, we demonstrate that this V2R ability to co-interact with G protein/β-arrestin and promote endosomal G protein signaling is not restricted to Gαs, but also involves Gαq/11. Furthermore, our data implies that β-arrestins potentiate Gαs/Gαq/11 activation at endosomes rather than terminating their signaling. Surprisingly, we found that the V2R internalizes and promote endosomal G protein activation independent of β-arrestins to a minor degree. These new observations challenge the current model of endosomal GPCR signaling and suggest that this event can occur in both β-arrestin-dependent and -independent manners.

Tail engagement of arrestin at the glucagon receptor

Arrestins have pivotal roles in regulating G protein-coupled receptor (GPCR) signalling by desensitizing G protein activation and mediating receptor internalization1,2. It has been proposed that the arrestin binds to the receptor in two different conformations, 'tail' and 'core', which were suggested to govern distinct processes of receptor signalling and trafficking3,4. However, little structural information is available for the tail engagement of the arrestins. Here we report two structures of the glucagon receptor (GCGR) bound to β-arrestin 1 (βarr1) in glucagon-bound and ligand-free states. These structures reveal a receptor tail-engaged binding mode of βarr1 with many unique features, to our knowledge, not previously observed. Helix VIII, instead of the receptor core, has a major role in accommodating βarr1 by forming extensive interactions with the central crest of βarr1. The tail-binding pose is further defined by a close proximity between the βarr1 C-edge and the receptor helical bundle, and stabilized by a phosphoinositide derivative that bridges βarr1 with helices I and VIII of GCGR. Lacking any contact with the arrestin, the receptor core is in an inactive state and loosely binds to glucagon. Further functional studies suggest that the tail conformation of GCGR-βarr governs βarr recruitment at the plasma membrane and endocytosis of GCGR, and provides a molecular basis for the receptor forming a super-complex simultaneously with G protein and βarr to promote sustained signalling within endosomes. These findings extend our knowledge about the arrestin-mediated modulation of GPCR functionalities.

Altered Signaling and Desensitization Responses in PTH1R Mutants Associated with Eiken Syndrome

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that plays key roles in regulating calcium homeostasis and skeletal development via binding the ligands, PTH and PTH-related protein (PTHrP), respectively. Eiken syndrome is a rare disease of delayed bone mineralization caused by homozygous PTH1R mutations. Of the three mutations identified so far, R485X, truncates the PTH1R C-terminal tail, while E35K and Y134S alter residues in the receptor's amino-terminal extracellular domain. Here, using a variety of cell-based assays, we show that R485X increases the receptor's basal rate of cAMP signaling and decreases its capacity to recruit β-arrestin2 upon ligand stimulation. The E35K and Y134S mutations each weaken the binding of PTHrP leading to impaired β-arrestin2 recruitment and desensitization of cAMP signaling response to PTHrP but not PTH. Our findings support a critical role for interaction with β-arrestin in the mechanism by which the PTH1R regulates bone formation.

Detecting and measuring of GPCR signaling - comparison of human induced pluripotent stem cells and immortal cell lines

G protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that play a major role in many physiological processes, and thus GPCR-targeted drug development has been widely promoted. Although research findings generated in immortal cell lines have contributed to the advancement of the GPCR field, the homogenous genetic backgrounds, and the overexpression of GPCRs in these cell lines make it difficult to correlate the results with clinical patients. Human induced pluripotent stem cells (hiPSCs) have the potential to overcome these limitations, because they contain patient specific genetic information and can differentiate into numerous cell types. To detect GPCRs in hiPSCs, highly selective labeling and sensitive imaging techniques are required. This review summarizes existing resonance energy transfer and protein complementation assay technologies, as well as existing and new labeling methods. The difficulties of extending existing detection methods to hiPSCs are discussed, as well as the potential of hiPSCs to expand GPCR research towards personalized medicine.

Pathophysiology and pharmacology of G protein-coupled receptors in the heart

G protein-coupled receptors (GPCRs), comprising the largest superfamily of cell surface receptors, serve as fundamental modulators of cardiac health and disease owing to their key roles in the regulation of heart rate, contractile dynamics, and cardiac function. Accordingly, GPCRs are heavily pursued as drug targets for a wide variety of cardiovascular diseases ranging from heart failure, cardiomyopathy, and arrhythmia to hypertension and coronary artery disease. Recent advancements in understanding the signalling mechanisms, regulation, and pharmacological properties of GPCRs have provided valuable insights that will guide the development of novel therapeutics. Herein, we review the cellular signalling mechanisms, pathophysiological roles, and pharmacological developments of the major GPCRs in the heart, highlighting the β-adrenergic, muscarinic, and angiotensin receptors as exemplar subfamilies.

Plasma membrane preassociation drives β-arrestin coupling to receptors and activation

β-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-β-arrestin interactions at the plasma membrane of living cells remain elusive. Here, we combine single-molecule microscopy with molecular dynamics simulations to dissect the complex sequence of events involved in β-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that β-arrestin spontaneously inserts into the lipid bilayer and transiently interacts with receptors via lateral diffusion on the plasma membrane. Moreover, they indicate that, following receptor interaction, the plasma membrane stabilizes β-arrestin in a longer-lived, membrane-bound state, allowing it to diffuse to clathrin-coated pits separately from the activating receptor. These results expand our current understanding of β-arrestin function at the plasma membrane, revealing a critical role for β-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

Model of ligand-triggered information transmission in G-protein coupled receptor complexes

We present a model for the effects of ligands on information transmission in G-Protein Coupled Receptor (GPCR) complexes. The model is built ab initio entirely on principles of statistical mechanics and tenets of information transmission theory and was validated in part using agonist-induced effector activity and signaling bias for the angiotensin- and adrenergic-mediated signaling pathways, with in vitro observations of phosphorylation sites on the C tail of the GPCR complex, and single-cell information-transmission experiments. The model extends traditional kinetic models that form the basis for many existing models of GPCR signaling. It is based on maximizing the rates of entropy production and information transmission through the GPCR complex. The model predicts that (1) phosphatase-catalyzed reactions, as opposed to kinase-catalyzed reactions, on the C-tail and internal loops of the GPCR are responsible for controlling the signaling activity, (2) signaling favors the statistical balance of the number of switches in the ON state and the number in the OFF state, and (3) biased-signaling response depends discontinuously on ligand concentration.

G protein-coupled receptors in neurodegenerative diseases and psychiatric disorders

Neuropsychiatric disorders are multifactorial disorders with diverse aetiological factors. Identifying treatment targets is challenging because the diseases are resulting from heterogeneous biological, genetic, and environmental factors. Nevertheless, the increasing understanding of G protein-coupled receptor (GPCR) opens a new possibility in drug discovery. Harnessing our knowledge of molecular mechanisms and structural information of GPCRs will be advantageous for developing effective drugs. This review provides an overview of the role of GPCRs in various neurodegenerative and psychiatric diseases. Besides, we highlight the emerging opportunities of novel GPCR targets and address recent progress in GPCR drug development.

A visual opsin from jellyfish enables precise temporal control of G protein signalling

Phototransduction is mediated by distinct types of G protein cascades in different animal taxa: bilateral invertebrates typically utilise the Gαq pathway whereas vertebrates typically utilise the Gαt(i/o) pathway. By contrast, photoreceptors in jellyfish (Cnidaria) utilise the Gαs intracellular pathway, similar to olfactory transduction in mammals¹. How this habitually slow pathway has adapted to support dynamic vision in jellyfish remains unknown. Here we study a light-sensing protein (rhodopsin) from the box jellyfish Carybdea rastonii and uncover a mechanism that dramatically speeds up phototransduction: an uninterrupted G protein-coupled receptor - G protein complex. Unlike known G protein-coupled receptors (GPCRs), this rhodopsin constitutively binds a single downstream Gαs partner to enable G-protein activation and inactivation within tens of milliseconds. We use this GPCR in a viral gene therapy to restore light responses in blind mice.

Subcellular location defines GPCR signal transduction

Intracellular G protein-coupled receptors (GPCRs) can be activated by permeant ligands, which contributes to agonist selectivity. Opioid receptors (ORs) provide a notable example, where opioid drugs rapidly activate ORs in the Golgi apparatus. Our knowledge on intracellular GPCR function remains incomplete, and it is unknown whether OR signaling in plasma membrane (PM) and Golgi apparatus differs. Here, we assess the recruitment of signal transducers to mu- and delta-ORs in both compartments. We find that Golgi ORs couple to Gαi/o probes and are phosphorylated but, unlike PM receptors, do not recruit β-arrestin or a specific Gα probe. Molecular dynamics simulations with OR-transducer complexes in bilayers mimicking PM or Golgi composition reveal that the lipid environment promotes the location-selective coupling. We then show that delta-ORs in PM and Golgi have distinct effects on transcription and protein phosphorylation. The study reveals that the subcellular location defines the signaling effects of opioid drugs.

Pepducin ICL1-9-Mediated β2-Adrenergic Receptor-Dependent Cardiomyocyte Contractility Occurs in a Gi Protein/ROCK/PKD-Sensitive Manner

PURPOSE

β-Adrenergic receptors (βAR) are essential targets for the treatment of heart failure (HF); however, chronic use of βAR agonists as positive inotropes to increase contractility in a Gs protein-dependent manner is associated with increased mortality. Alternatively, we previously reported that allosteric modulation of β2AR with the pepducin intracellular loop (ICL)1-9 increased cardiomyocyte contractility in a β-arrestin (βarr)-dependent manner, and subsequently showed that ICL1-9 activates the Ras homolog family member A (RhoA). Here, we aimed to elucidate both the proximal and downstream signaling mediators involved in the promotion of cardiomyocyte contractility in response to ICL1-9.

METHODS

We measured adult mouse cardiomyocyte contractility in response to ICL1-9 or isoproterenol (ISO, as a positive control) alone or in the presence of inhibitors of various potential components of βarr- or RhoA-dependent signaling. We also assessed the contractile effects of ICL1-9 on cardiomyocytes lacking G protein-coupled receptor (GPCR) kinase 2 (GRK2) or 5 (GRK5).

RESULTS

Consistent with RhoA activation by ICL1-9, both Rho-associated protein kinase (ROCK) and protein kinase D (PKD) inhibition were able to attenuate ICL1-9-mediated contractility, as was inhibition of myosin light chain kinase (MLCK). While neither GRK2 nor GRK5 deletion impacted ICL1-9-mediated contractility, pertussis toxin attenuated the response, suggesting that ICL1-9 promotes downstream RhoA-dependent signaling in a Gi protein-dependent manner.

CONCLUSION

Altogether, our study highlights a novel signaling modality that may offer a new approach to the promotion, or preservation, of cardiac contractility during HF via the allosteric regulation of β2AR to promote Gi protein/βarr-dependent activation of RhoA/ROCK/PKD signaling.

The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

GPR176 Promotes Cancer Progression by Interacting with G Protein GNAS to Restrain Cell Mitophagy in Colorectal Cancer

GPR176 belongs to the G protein-coupled receptor superfamily, which responds to external stimuli and regulates cancer progression, but its role in colorectal cancer (CRC) remains unclear. In the present study, expression analyses of GPR176 are performed in patients with colorectal cancer. Genetic mouse models of CRC coupled with Gpr176-deficiency are investigated, and in vivo and in vitro treatments are conducted. A positive correlation between GPR176 upregulation and the proliferation and poor overall survival of CRC is demonstrated. GPR176 is confirmed to activate the cAMP/PKA signaling pathway and modulate mitophagy, promoting CRC oncogenesis and development. Mechanistically, the G protein GNAS is recruited intracellularly to transduce and amplify extracellular signals from GPR176. A homolog model tool confirmed that GPR176 recruits GNAS intracellularly via its transmembrane helix 3-intracellular loop 2 domain. The GPR176/GNAS complex inhibits mitophagy via the cAMP/PKA/BNIP3L axis, thereby promoting the tumorigenesis and progression of CRC.