GPCR oligomerization and receptor trafficking

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor

The melanocortin-4 receptor (MC4R) is a class A G protein-coupled receptor (GPCR), essential for regulation of appetite and metabolism. Pathogenic inactivating MC4R mutations are the most frequent cause of monogenic obesity, a growing medical and socioeconomic problem worldwide. The MC4R mediates either ligand-independent or ligand-dependent signaling. Agonists such as α-melanocyte-stimulating hormone (α-MSH) induce anorexigenic effects, in contrast to the endogenous inverse agonist agouti-related peptide (AgRP), which causes orexigenic effects by suppressing high basal signaling activity. Agonist action triggers the binding of different subtypes of G proteins and arrestins, leading to concomitant induction of diverse intracellular signaling cascades. An increasing number of experimental studies have unraveled molecular properties and mechanisms of MC4R signal transduction related to physiological and pathophysiological aspects. In addition, the MC4R crystal structure was recently determined at 2.75 Å resolution in an inactive state bound with a peptide antagonist. Underpinned by structural homology models of MC4R complexes simulating a presumably active-state conformation compared to the structure of the inactive state, we here briefly summarize the current understanding and key players involved in the MC4R switching process between different activity states. Finally, these perspectives highlight the complexity and plasticity in MC4R signaling regulation and identify gaps in our current knowledge.

FLIM-FRET-Based Structural Characterization of a Class-A GPCR Dimer in the Cell Membrane

Class-A G protein-coupled receptors (GPCRs) are known to homo-dimerize in the membrane. Yet, methods to characterize the structure of GPCR dimer in the native environment are lacking. Accordingly, the molecular basis and functional relevance of the class-A GPCR dimerization remain unclear. Here, we present the dimeric structural model of GPR17 in the cell membrane. The dimer mainly involves transmembrane helix 5 (TM5) at the interface, with F229 in TM5, a critical residue. An F229A mutation makes GPR17 monomeric regardless of the expression level of the receptor. Monomeric mutants of GPR17 display impaired ERK1/2 activation and cannot be properly internalized upon agonist treatment. Conversely, the F229C mutant is cross-linked as a dimer and behaves like wild-type. Importantly, the GPR17 dimer structure has been modeled using sparse inter-protomer FRET distance restraints obtained from fluorescence lifetime imaging microscopy. The same approach can be applied to characterizing the interactions of other important membrane proteins in the cell.

Fluorogen activating protein toolset for protein trafficking measurements

Throughout the past decade the use of fluorogen activating proteins (FAPs) has expanded with several unique reporter dyes that support a variety of methods to specifically quantify protein trafficking events. The platform's capabilities have been demonstrated in several systems and shared for widespread use. This review will highlight the current FAP labeling techniques for protein traffic measurements and focus on the use of the different designed fluorogenic dyes for selective and specific labeling applications.

Arrestin recruitment and signaling by G protein-coupled receptor heteromers

G protein-coupled receptors (GPCR) have a long history of being considered a prime target for drug development to treat a plethora of diseases and disorders. In fact in 1827, the first approved therapeutic in the United States was morphine, a drug that targets a GPCR, namely the mu opioid receptor. However, with the rise in biologics over the last two decades, the market share of small molecules targeting GPCRs has declined. Still, two phenomena concerning GPCR pharmacology, specifically heteromerization and biased signaling, have bolstered new interests in this particular class of drug targets. Heteromerization, the process by which two distinct GPCRs come together to form a unique signaling complex, has been demonstrated between many different GPCRs and has spurred efforts to discover heteromer selective drugs. Additionally, the discovery of biased signaling, a concept by which a GPCR can transduce intracellular signaling by favoring a specific pathway (e.g. G-protein) over another pathway (e.g. arrestin), has led to the development of signal-biased drugs with potentially fewer side effects. Our goal for this review is to highlight studies that have investigated the interplay of these two phenomena by providing an overview of the current literature describing instances where GPCR heteromers have distinct arrestin recruitment profiles when compared to the individual GPCRs, with a focus on those GPCRs expressed in the central nervous system. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Role of mGlu2 in the 5-HT2A receptor-dependent antipsychotic activity of clozapine in mice

BACKGROUND

Serotonin 5-HT_2A and metabotropic glutamate 2 (mGlu2) are neurotransmitter G protein-coupled receptors (GPCRs) involved in the signaling mechanisms underlying psychosis and schizophrenia treatment. Previous findings in mGlu2 knockout (KO) mice suggested that mGlu2 is necessary for head-twitch behavior, a rodent phenotype characteristic of hallucinogenic 5-HT_2A receptor agonists. However, the role of mGlu2 in the behavioral effects induced by antipsychotic drugs remains poorly understood. Here, we tested antipsychotic-like behavioral phenotypes induced by the atypical antipsychotic clozapine in mGlu2-KO mice and wild-type control littermates.

METHODS

Locomotor activity was tested in mGlu2-KO mice and control littermates injected (i.p.) with clozapine (1.5 mg/kg) or vehicle followed by MK801 (0.5 mg/kg), PCP (7.5 mg/kg), amphetamine (6 mg/kg), scopolamine (2 mg/kg), or vehicle. Using a virally (HSV) mediated transgene expression approach, the role of frontal cortex mGlu2 in the modulation of MK801-induced locomotor activity by clozapine treatment was also evaluated.

RESULTS

The effect of clozapine on hyperlocomotor activity induced by the dissociative drugs MK801 and phencyclidine (PCP) was decreased in mGlu2-KO mice as compared to controls. Clozapine treatment, however, reduced hyperlocomotor activity induced by the stimulant drug amphetamine and the deliriant drug scopolamine in both wild-type and mGlu2-KO mice. Virally mediated over-expression of mGlu2 in the frontal cortex of mGlu2-KO mice rescued the ability of clozapine to reduce MK801-induced hyperlocomotion.

CONCLUSION

These findings further support the existence of a functionally relevant crosstalk between 5-HT_2A and mGlu2 receptors in different preclinical models of antipsychotic activity.

Dimerization of sortilin regulates its trafficking to extracellular vesicles

Extracellular vesicles (EVs) play a critical role in intercellular communication by transferring microRNAs, lipids, and proteins to neighboring cells. Sortilin, a sorting receptor that directs target proteins to the secretory or endocytic compartments of cells, is found in both EVs and cells. In many human diseases, including cancer and cardiovascular disorders, sortilin expression levels are atypically high. To elucidate the relationship between cardiovascular disease, particularly vascular calcification, and sortilin expression levels, we explored the trafficking of sortilin in both the intracellular and extracellular milieu. We previously demonstrated that sortilin promotes vascular calcification via its trafficking of tissue-nonspecific alkaline phosphatase to EVs. Although recent reports have noted that sortilin is regulated by multiple post-translational modifications, the precise mechanisms of sortilin trafficking still need to be determined. Here, we show that sortilin forms homodimers with an intermolecular disulfide bond at the cysteine 783 (Cys⁷⁸³) residue, and because Cys⁷⁸³ can be palmitoylated, it could be shared via palmitoylation and an intermolecular disulfide bond. Formation of this intermolecular disulfide bond leads to trafficking of sortilin to EVs by preventing palmitoylation, which further promotes sortilin trafficking to the Golgi apparatus. Moreover, we found that sortilin-derived propeptide decreased sortilin homodimers within EVs. In conclusion, sortilin is transported to EVs via the formation of homodimers with an intermolecular disulfide bond, which is endogenously regulated by its own propeptide. Therefore, we propose that inhibiting dimerization of sortilin acts as a new therapeutic strategy for the treatment of EV-associated diseases, including vascular calcification and cancer.

Biased signalling: the instinctive skill of the cell in the selection of appropriate signalling pathways

GPCRs (G-protein-coupled receptors) are members of a family of proteins which are generally regarded as the largest group of therapeutic drug targets. Ligands of GPCRs do not usually activate all cellular signalling pathways linked to a particular seven-transmembrane receptor in a uniform manner. The fundamental idea behind this concept is that each ligand has its own ability, while interacting with the receptor, to activate different signalling pathways (or a particular set of signalling pathways) and it is this concept which is known as biased signalling. The importance of biased signalling is that it may selectively activate biological responses to favour therapeutically beneficial signalling pathways and to avoid adverse effects. There are two levels of biased signalling. First, bias can arise from the ability of GPCRs to couple to a subset of the available G-protein subtypes: Gαs, Gαq/11, Gαi/o or Gα12/13. These subtypes produce the diverse effects of GPCRs by targeting different effectors. Secondly, biased GPCRs may differentially activate G-proteins or β-arrestins. β-Arrestins are ubiquitously expressed and function to terminate or inhibit classic G-protein signalling and initiate distinct β-arrestin-mediated signalling processes. The interplay of G-protein and β-arrestin signalling largely determines the cellular consequences of the administration of GPCR-targeted drugs. In the present review, we highlight the particular functionalities of biased signalling and discuss its biological effects subsequent to GPCR activation. We consider that biased signalling is potentially allowing a choice between signalling through ‘beneficial’ pathways and the avoidance of ‘harmful’ ones.

Regulation of oligomeric organization of the serotonin 5-hydroxytryptamine 2C (5-HT2C) receptor observed by spatial intensity distribution analysis

The questions of whether G protein-coupled receptors exist as monomers, dimers, and/or oligomers and if these species interconvert in a ligand-dependent manner are among the most contentious current issues in biology. When employing spatial intensity distribution analysis to laser scanning confocal microscope images of cells stably expressing either a plasma membrane-associated form of monomeric enhanced green fluorescent protein (eGFP) or a tandem version of this fluorophore, the eGFP tandem was identified as a dimer. Similar studies on cells stably expressing an eGFP-tagged form of the epidermal growth factor receptor demonstrated that, although largely a monomer in the basal state, this receptor rapidly became predominantly dimeric upon the addition of its ligand epidermal growth factor. In cells induced to express an eGFP-tagged form of the serotonin 5-hydroxytryptamine 2C (5-HT2C) receptor, global analysis of construct quantal brightness was consistent with the predominant form of the receptor being dimeric. However, detailed spatial intensity distribution analysis demonstrated the presence of multiple forms ranging from monomers to higher-order oligomers. Furthermore, treatment with chemically distinct 5-HT2C receptor antagonists resulted in a time-dependent change in the quaternary organization to one in which there was a preponderance of receptor monomers. This antagonist-mediated effect was reversible, because washout of the ligand resulted in the regeneration of many of the oligomeric forms of the receptor.

Multiplex detection of homo- and heterodimerization of g protein-coupled receptors by proximity biotinylation

Dimerization of G protein-coupled receptors (GPCRs) represents a potential mechanism by which GPCR functions are regulated. Several resonance energy transfer (RET)-based methods have revealed GPCR homo- and heterodimerization. However, interpretation of an increase in FRET efficiency could be attributed to either dimerization/oligomerization events or conformational changes within an already dimerized/oligomerized receptor complex. Furthermore, RET-based methods can only measure pairwise dimerization, and cannot easily achieve multiplex detection. In this study, we applied proximity-based biotinylation for detecting receptor dimerization by utilizing a specific enzyme-substrate pair that are fused to GPCRs. The biotin ligase BirA is fused to CXCR4 and site-specifically biotinylates an acceptor peptide (AP) in the presence of biotin. As a test case for our newly developed assay, we have characterized the homo-dimerization of chemokine receptor CXCR4 and heterodimerization of CXCR4 with CCR2 or CCR5. The degree of biotinylation varies with the amount of GPCR-AP as well as biotinylation time. Using enzyme/substrate receptor pairs and measuring receptor biotinylation, we demonstrate that CXCR4 can homo-dimerize and hetero-dimerize with CCR2 and CCR5. The effect of CXCL12, agonist for CXCR4, was found to decrease surface biotinylation of CXCR4-AP. This effect is due to a combination of CXCR4 endocytosis and stabilization of CXCR4 homodimers. Finally, when CXCR4-AP, CCR2-AP, and CCR5-AP were expressed together, we observed CXCR4-CXCR4 homodimers and CXCR4-CCR2 and CXCR4-CCR5 heterodimers. The newly developed assay opens new opportunity for multiplex detection for GPCR homo- and heterodimerization within the same cellular context.

Structure and physiological actions of ghrelin

Ghrelin is a gastric peptide hormone, discovered as being the endogenous ligand of growth hormone secretagogue receptor. Ghrelin is a 28 amino acid peptide presenting a unique n-octanoylation modification on its serine in position 3, catalyzed by ghrelin O-acyl transferase. Ghrelin is mainly produced by a subset of stomach cells and also by the hypothalamus, the pituitary, and other tissues. Transcriptional, translational, and posttranslational processes generate ghrelin and ghrelin-related peptides. Homo- and heterodimers of growth hormone secretagogue receptor, and as yet unidentified receptors, are assumed to mediate the biological effects of acyl ghrelin and desacyl ghrelin, respectively. Ghrelin exerts wide physiological actions throughout the body, including growth hormone secretion, appetite and food intake, gastric secretion and gastrointestinal motility, glucose homeostasis, cardiovascular functions, anti-inflammatory functions, reproductive functions, and bone formation. This review focuses on presenting the current understanding of ghrelin and growth hormone secretagogue receptor biology, as well as the main physiological effects of ghrelin.

Taking two to tango: a role for ghrelin receptor heterodimerization in stress and reward

The gut hormone, ghrelin, is the only known peripherally derived orexigenic signal. It activates its centrally expressed receptor, the growth hormone secretagogue receptor (GHS-R1a), to stimulate food intake. The ghrelin signaling system has recently been suggested to play a key role at the interface of homeostatic control of appetite and the hedonic aspects of food intake, as a critical role for ghrelin in dopaminergic mesolimbic circuits involved in reward signaling has emerged. Moreover, enhanced plasma ghrelin levels are associated with conditions of physiological stress, which may underline the drive to eat calorie-dense "comfort-foods" and signifies a role for ghrelin in stress-induced food reward behaviors. These complex and diverse functionalities of the ghrelinergic system are not yet fully elucidated and likely involve crosstalk with additional signaling systems. Interestingly, accumulating data over the last few years has shown the GHS-R1a receptor to dimerize with several additional G-protein coupled receptors (GPCRs) involved in appetite signaling and reward, including the GHS-R1b receptor, the melanocortin 3 receptor (MC3), dopamine receptors (D1 and D2), and more recently, the serotonin 2C receptor (5-HT2C). GHS-R1a dimerization was shown to affect downstream signaling and receptor trafficking suggesting a potential novel mechanism for fine-tuning GHS-R1a receptor mediated activity. This review summarizes ghrelin's role in food reward and stress and outlines the GHS-R1a dimer pairs identified to date. In addition, the downstream signaling and potential functional consequences of dimerization of the GHS-R1a receptor in appetite and stress-induced food reward behavior are discussed. The existence of multiple GHS-R1a heterodimers has important consequences for future pharmacotherapies as it significantly increases the pharmacological diversity of the GHS-R1a receptor and has the potential to enhance specificity of novel ghrelin-targeted drugs.

Genetic loss or pharmacological blockade of testes-expressed taste genes causes male sterility

TAS1R taste receptors and their associated heterotrimeric G protein gustducin are involved in sugar and amino acid sensing in taste cells and in the gastrointestinal tract. They are also strongly expressed in testis and sperm, but their functions in these tissues were previously unknown. Using mouse models, we show that the genetic absence of both TAS1R3, a component of sweet and amino acid taste receptors, and the gustducin α-subunit GNAT3 leads to male-specific sterility. To gain further insight into this effect, we generated a mouse model that expressed a humanized form of TAS1R3 susceptible to inhibition by the antilipid medication clofibrate. Sperm formation in animals without functional TAS1R3 and GNAT3 is compromised, with malformed and immotile sperm. Furthermore, clofibrate inhibition of humanized TAS1R3 in the genetic background of Tas1r3(-/-), Gnat3(-/-) doubly null mice led to inducible male sterility. These results indicate a crucial role for these extraoral "taste" molecules in sperm development and maturation. We previously reported that blocking of human TAS1R3, but not mouse TAS1R3, can be achieved by common medications or chemicals in the environment. We hypothesize that even low levels of these compounds can lower sperm count and negatively affect human male fertility, which common mouse toxicology assays would not reveal. Conversely, we speculate that TAS1R3 and GNAT3 activators may help infertile men, particularly those that are affected by some of the mentioned inhibitors and/or are diagnosed with idiopathic infertility involving signaling pathway of these receptors.