Detection of heteromerization of more than two proteins by sequential BRET-FRET

G protein-coupled receptor oligomerization revisited: functional and pharmacological perspectives

Most evidence indicates that, as for family C G protein-coupled receptors (GPCRs), family A GPCRs form homo- and heteromers. Homodimers seem to be a predominant species, with potential dynamic formation of higher-order oligomers, particularly tetramers. Although monomeric GPCRs can activate G proteins, the pentameric structure constituted by one GPCR homodimer and one heterotrimeric G protein may provide a main functional unit, and oligomeric entities can be viewed as multiples of dimers. It still needs to be resolved if GPCR heteromers are preferentially heterodimers or if they are mostly constituted by heteromers of homodimers. Allosteric mechanisms determine a multiplicity of possible unique pharmacological properties of GPCR homomers and heteromers. Some general mechanisms seem to apply, particularly at the level of ligand-binding properties. In the frame of the dimer-cooperativity model, the two-state dimer model provides the most practical method to analyze ligand-GPCR interactions when considering receptor homomers. In addition to ligand-binding properties, unique properties for each GPCR oligomer emerge in relation to different intrinsic efficacy of ligands for different signaling pathways (functional selectivity). This gives a rationale for the use of GPCR oligomers, and particularly heteromers, as novel targets for drug development. Herein, we review the functional and pharmacological properties of GPCR oligomers and provide some guidelines for the application of discrete direct screening and high-throughput screening approaches to the discovery of receptor-heteromer selective compounds.

L-DOPA disrupts adenosine A(2A)-cannabinoid CB(1)-dopamine D(2) receptor heteromer cross-talk in the striatum of hemiparkinsonian rats: biochemical and behavioral studies

Long-term therapy with L-3,4-dihydroxyphenylalanine (L-DOPA), still the most effective treatment in Parkinson's disease (PD), is associated with severe motor complications such as dyskinesia. Experimental and clinical data have indicated that adenosine A2A receptor antagonists can provide symptomatic improvement by potentiating L-DOPA efficacy and minimizing its side effects. It is known that the G-protein-coupled adenosine A2A, cannabinoid CB1 and dopamine D2 receptors may interact and form functional A2A-CB1-D2 receptor heteromers in co-transfected cells as well as in rat striatum. These data suggest that treatment with a combination of drugs or a single compound selectively acting on A2A-CB1-D2 heteromers may represent an alternative therapeutic treatment of PD. We investigated the expression of A2A-CB1-D2 receptor heteromers in the striatum of both naïve and hemiparkinsonian rats (HPD-rats) bearing a unilateral 6-hydroxydopamine (6-OHDA) lesion, and assessed how receptor heteromer expression and biochemical properties were affected by L-DOPA treatment. Radioligand binding data showed that A2A-CB1-D2 receptor heteromers are present in the striatum of both naïve and HPD-rats. However, behavioral results indicated that the combined administration of A2A (MSX-3 or SCH58261) and CB1 (rimonabant) receptor antagonists, in the presence of L-DOPA does not produce a response different from administration of the A2A receptor antagonist alone. These behavioral results prompted identification of heteromers in L-DOPA-treated animals. Interestingly, the radioligand binding results in samples from lesioned animals suggest that the heteromer is lost following acute or chronic treatment with L-DOPA.

Dopamine D2 heteroreceptor complexes and their receptor-receptor interactions in ventral striatum: novel targets for antipsychotic drugs

This review is focused on the D2 heteroreceptor complexes within the ventral striatum with their receptor-receptor interactions and relevance for the treatment of schizophrenia. A "guide-and-clasp" manner for receptor-receptor interactions is proposed where "adhesive guides" may be amino acid triplet homologies, which were determined for different kinds of D2 heteroreceptor complexes. The first putative D2 heteroreceptor complex to be discovered in relation to schizophrenia was the A2A-D2 heteroreceptor complex where antagonistic A2A-D2 receptor-receptor interactions were demonstrated after A2A agonist treatment in the ventral striatum. The A2A agonist CGS 21680 with atypical antipsychotic properties may at least in part act by increasing β-arrestin2 signaling over the D2 protomer in the A2A-D2 heteroreceptor complex in the ventral striatum. The antagonistic NTS1-D2 interactions in the NTS1-D2 heteroreceptor complex in the ventral striatum are proposed as one molecular mechanism for the potential antipsychotic effects of NT. Indications were obtained that the psychotic actions of the 5-HT2AR hallucinogens LSD and DOI can involve enhancement of D2R protomer signaling via a biased agonist action at the 5-HT2A protomer in the D2-5-HT2A heteroreceptor complex in the ventral striatum. Facilitatory allosteric D2likeR-OTR interactions in heteroreceptor complexes in nucleus accumbens may have a role in social and emotional behaviors. By blocking the D2 protomers of these heteroreceptor complexes, antipsychotics can fail to reduce the negative symptoms of schizophrenia. The discovery of different types of D2 heteroreceptor complexes gives an increased understanding of molecular mechanisms involved in causing schizophrenia and new strategies for its treatment and understanding the side effects of antipsychotics.

Diversity and Bias through Receptor-Receptor Interactions in GPCR Heteroreceptor Complexes. Focus on Examples from Dopamine D2 Receptor Heteromerization

Allosteric receptor-receptor interactions in GPCR heteromers appeared to introduce an intermolecular allosteric mechanism contributing to the diversity and bias in the protomers. Examples of dopamine D2R heteromerization are given to show how such allosteric mechanisms significantly change the receptor protomer repertoire leading to diversity and biased recognition and signaling. In 1980s and 1990s, it was shown that neurotensin (NT) through selective antagonistic NTR-D2 like receptor interactions increased the diversity of DA signaling by reducing D2R-mediated dopamine signaling over D1R-mediated dopamine signaling. Furthermore, D2R protomer appeared to bias the specificity of the NTR orthosteric binding site toward neuromedin N vs. NT in the heteroreceptor complex. Complex CCK2R-D1R-D2R interactions in possible heteroreceptor complexes were also demonstrated further increasing receptor diversity. In D2R-5-HT2AR heteroreceptor complexes, the hallucinogenic 5-HT2AR agonists LSD and DOI were recently found to exert a biased agonist action on the orthosteric site of the 5-HT2AR protomer leading to the development of an active conformational state different from the one produced by 5-HT. Furthermore, as recently demonstrated allosteric A2A-D2R receptor-receptor interaction brought about not only a reduced affinity of the D2R agonist binding site but also a biased modulation of the D2R protomer signaling in A2A-D2R heteroreceptor complexes. A conformational state of the D2R was induced, which moved away from Gi/o signaling and instead favored β-arrestin2-mediated signaling. These examples on allosteric receptor-receptor interactions obtained over several decades serve to illustrate the significant increase in diversity and biased recognition and signaling that develop through such mechanisms.

Moonlighting proteins and protein-protein interactions as neurotherapeutic targets in the G protein-coupled receptor field

There is serious interest in understanding the dynamics of the receptor-receptor and receptor-protein interactions in space and time and their integration in GPCR heteroreceptor complexes of the CNS. Moonlighting proteins are special multifunctional proteins because they perform multiple autonomous, often unrelated, functions without partitioning into different protein domains. Moonlighting through receptor oligomerization can be operationally defined as an allosteric receptor-receptor interaction, which leads to novel functions of at least one receptor protomer. GPCR-mediated signaling is a more complicated process than previously described as every GPCR and GPCR heteroreceptor complex requires a set of G protein interacting proteins, which interacts with the receptor in an orchestrated spatio-temporal fashion. GPCR heteroreceptor complexes with allosteric receptor-receptor interactions operating through the receptor interface have become major integrative centers at the molecular level and their receptor protomers act as moonlighting proteins. The GPCR heteroreceptor complexes in the CNS have become exciting new targets for neurotherapeutics in Parkinson's disease, schizophrenia, drug addiction, and anxiety and depression opening a new field in neuropsychopharmacology.

Biophysical Detection of Diversity and Bias in GPCR Function

Guanine nucleotide binding protein (G protein)-coupled receptors (GPCRs) function in complexes with a range of molecules and proteins including ligands, G proteins, arrestins, ubiquitin, and other receptors. Elements of these complexes may interact constitutively or dynamically, dependent upon factors such as ligand binding, phosphorylation, and dephosphorylation. They may also be allosterically modulated by other proteins in a manner that changes temporally and spatially within the cell. Elucidating how these complexes function has been greatly enhanced by biophysical technologies that are able to monitor proximity and/or binding, often in real time and in live cells. These include resonance energy transfer approaches such as bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET). Furthermore, the use of fluorescent ligands has enabled novel insights into allosteric interactions between GPCRs. Consequently, biophysical approaches are helping to unlock the amazing diversity and bias in G protein-coupled receptor signaling.

Development and experimental testing of an optical micro-spectroscopic technique incorporating true line-scan excitation

Multiphoton micro-spectroscopy, employing diffraction optics and electron-multiplying CCD (EMCCD) cameras, is a suitable method for determining protein complex stoichiometry, quaternary structure, and spatial distribution in living cells using Förster resonance energy transfer (FRET) imaging. The method provides highly resolved spectra of molecules or molecular complexes at each image pixel, and it does so on a timescale shorter than that of molecular diffusion, which scrambles the spectral information. Acquisition of an entire spectrally resolved image, however, is slower than that of broad-bandwidth microscopes because it takes longer times to collect the same number of photons at each emission wavelength as in a broad bandwidth. Here, we demonstrate an optical micro-spectroscopic scheme that employs a laser beam shaped into a line to excite in parallel multiple sample voxels. The method presents dramatically increased sensitivity and/or acquisition speed and, at the same time, has excellent spatial and spectral resolution, similar to point-scan configurations. When applied to FRET imaging using an oligomeric FRET construct expressed in living cells and consisting of a FRET acceptor linked to three donors, the technique based on line-shaped excitation provides higher accuracy compared to the point-scan approach, and it reduces artifacts caused by photobleaching and other undesired photophysical effects.

(R,R')-4'-methoxy-1-naphthylfenoterol targets GPR55-mediated ligand internalization and impairs cancer cell motility

(R,R')-4'-Methoxy-1-naphthylfenoterol (MNF) promotes growth inhibition and apoptosis of human HepG2 hepatocarcinoma cells via cannabinoid receptor (CBR) activation. The synthetic CB1R inverse agonist, AM251, has been shown to block the anti-mitogenic effect of MNF in these cells; however, AM251 is also an agonist of the recently deorphanized, lipid-sensing receptor, GPR55, whose upregulation contributes to carcinogenesis. Here, we investigated the role of MNF in GPR55 signaling in human HepG2 and PANC-1 cancer cell lines in culture by focusing first on internalization of the fluorescent ligand Tocrifluor 1117 (T1117). Initial results indicated that cell pretreatment with GPR55 agonists, including the atypical cannabinoid O-1602 and l-α-lysophosphatidylinositol, dose-dependently reduced the rate of cellular T1117 uptake, a process that was sensitive to MNF inhibition. GPR55 internalization and signaling mediated by O-1602 was blocked by MNF in GPR55-expressing HEK293 cells. Pretreatment of HepG2 and PANC-1 cells with MNF significantly abrogated the induction of ERK1/2 phosphorylation in response to AM251 and O-1602. Moreover, MNF exerted a coordinated negative regulation of AM251 and O-1602 inducible processes, including changes in cellular morphology and cell migration using scratch wound healing assay. This study shows for the first time that MNF impairs GPR55-mediated signaling and, therefore, may have therapeutic potential in the management of cancer.

Disease-specific heteromerization of G-protein-coupled receptors that target drugs of abuse

Drugs of abuse such as morphine or marijuana exert their effects through the activation of G-protein-coupled receptors (GPCRs), the opioid and cannabinoid receptors, respectively. Moreover, interactions between either of these receptors have been shown to be involved in the rewarding effects of drugs of abuse. Recent advances in the field, using a variety of approaches, have demonstrated that many GPCRs, including opioid, cannabinoid, and dopamine receptors, can form associations between different receptor subtypes or with other GPCRs to form heteromeric complexes. The formation of these complexes, in turn, leads to the modulation of the properties of individual protomers. The development of tools that can selectively disrupt GPCR heteromers as well as monoclonal antibodies that can selectively block signaling by specific heteromer pairs has indicated that heteromers involving opioid, cannabinoid, or dopamine receptors may play a role in various disease states. In this review, we describe evidence for opioid, cannabinoid, and dopamine receptor heteromerization and the potential role of GPCR heteromers in pathophysiological conditions.

L-DOPA-treatment in primates disrupts the expression of A(2A) adenosine-CB(1) cannabinoid-D(2) dopamine receptor heteromers in the caudate nucleus

The molecular basis of priming for L-DOPA-induced dyskinesias in Parkinson's disease (PD), which depends on the indirect pathway of motor control, is not known. In rodents, the indirect pathway contains striatopallidal GABAergic neurons that express heterotrimers composed of A(2A) adenosine, CB(1) cannabinoid and D(2) dopamine receptors that regulate dopaminergic neurotransmission. The present study was designed to investigate the expression of these heteromers in the striatum of a primate model of Parkinson's disease and to determine whether their expression and pharmacological properties are altered upon L-DOPA treatment. By using the recently developed in situ proximity ligation assay and by identification of a biochemical fingerprint, we discovered a regional distribution of A(2A)/CB(1) /D(2) receptor heteromers that predicts differential D(2)-mediated neurotransmission in the caudate-putamen of Macaca fascicularis. Whereas heteromers were abundant in the caudate nucleus of both naïve and MPTP-treated monkeys, L-DOPA treatment blunted the biochemical fingerprint and led to weak heteromer expression. These findings constitute the first evidence of altered receptor heteromer expression in pathological conditions and suggest that drugs targeting A(2A)-CB(1) -D(2) receptor heteromers may be successful to either normalize basal ganglia output or prevent L-DOPA-induced side effects.

Evolution of BRET Biosensors from Live Cell to Tissue-Scale In vivo Imaging

Development of bioluminescence resonance energy transfer (BRET) based genetic sensors for sensing biological functions such as protein-protein interactions (PPIs) in vivo has a special value in measuring such dynamic events at their native environment. Since its inception in the late nineties, BRET related research has gained significant momentum in terms of adding versatility to the assay format and wider applicability where it has been suitably used. Beyond the scope of quantitative measurement of PPIs and protein dimerization, molecular imaging applications based on BRET assays have broadened its scope for screening pharmacologically important compounds by in vivo imaging as well. In this mini-review we focus on an in-depth analysis of engineered BRET systems developed and their successful application to cell-based assays as well as in vivo non-invasive imaging in live subjects.

Electrochemiluminescence energy transfer-promoted ultrasensitive immunoassay using near-infrared-emitting CdSeTe/CdS/ZnS quantum dots and gold nanorods

The marriage of energy transfer with electrochemiluminescence has produced a new technology named electrochemiluminescence energy transfer (ECL-ET), which can realize effective and sensitive detection of biomolecules. To obtain optimal ECL-ET efficiency, perfect energy overlapped donor/acceptor pair is of great importance. Herein, we present a sensitive ECL-ET based immunosensor for the detection of tumor markers, using energy tunable CdSeTe/CdS/ZnS double shell quantum dots (QDs) and gold nanorods (GNRs) as the donor and acceptor, respectively. Firstly a facile microwave-assisted strategy for the synthesis of green- to near-infrared-emitting CdSeTe/CdS/ZnS QDs with time- and component-tunable photoluminescence was proposed. And, on the basis of the adjustable optical properties of both CdSeTe/CdS/ZnS QDs and GNRs, excellent overlap between donor emission and acceptor absorption can be obtained to ensure effective ECL-ET quenching, thus improving the sensing sensitivity. This method represents a novel approach for versatile detection of biomolecules at low concentrations.

HOMOGENEITY TESTS FOR MICHAELIS–MENTEN CURVES WITH APPLICATION TO FLUORESCENCE RESONANCE ENERGY TRANSFER DATA

Resonance energy transfer methods are widely used for evaluating protein–protein interactions and protein conformational changes. Sensitized emission fluorescence resonance energy transfer (FRET) measures energy transfer as a function of the acceptor-to-donor ratio, generating FRET saturation curves. To reduce sampling variability effects, several replications (statistical samples) of the saturation curve are generated in the same biological conditions. Here we study procedures to determine whether these statistical samples are homogeneous, in the sense that they are extracted from the same underlying regression model (Michaelis–Menten kinetics). We used three methods to test the homogeneity of the samples: two hypothesis testing procedures (an F-test and bootstrap resampling) and model selection. The performance of the three methods was compared in a Monte Carlo study and through analysis in living cells of FRET saturation curves for dimeric CXCR4 complexes. This analysis shows that the F-test, the bootstrap procedure and the model selection method lead in general to similar conclusions, although the latter gave the best results when sample sizes were small, whereas the F-test and the bootstrap method were more appropriate for large samples. In practice, all three methods are easy to use simultaneously and show consistency.

Determination of the quaternary structure of a bacterial ATP-binding cassette (ABC) transporter in living cells

Pseudomonas aeruginosa is a pathogenic Gram-negative bacterium that affects patients with cystic fibrosis and immunocompromised individuals. This bacterium coexpresses two unique forms of lipopolysaccharides (LPSs) on its surface, the A- and B-band LPS, which are among the main virulence factors that contribute to its pathogenicity. The polysaccharides in A-band LPSs are synthesized in the cytoplasm and translocated into the periplasm via an ATP-binding cassette (ABC) transporter consisting of a transmembrane protein, Wzm, and a cytoplasmic nucleotide-binding protein, Wzt. Most of the biochemical studies of A-band PSs in Pseudomonas aeruginosa are focused on the stages of the synthesis and ligation of PS, leaving the export stage involving the ABC transporter mostly unexplored. This difficulty is compounded by the fact that the subunit composition and structure of this bi-component ABC transporter are still unknown. Here we propose a simple but powerful method, based on Förster Resonance Energy Transfer (FRET) and optical micro-spectroscopy technology, to probe the structure of dynamic (as opposed to static) protein complexes in living cells. We use this method to determine the association stoichiometry and quaternary structure of the Wzm-Wzt complex in living cells. It is found that Wzt forms a rhombus-shaped homo-tetramer which becomes a square upon co-expression with Wzm, and that Wzm forms a square-shaped homo-tetramer both in the presence and absence of Wzt. Based on these results, we propose a structural model for the double-tetramer complex formed by the bi-component ABC transporter in living cells. An understanding of the structure and behavior of this ABC transporter will help develop antibiotics targeting the biosynthesis of the A-band LPS endotoxin.

Novel multistep BRET-FRET energy transfer using nanoconjugates of firefly proteins, quantum dots, and red fluorescent proteins

Sequential bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) from firefly luciferase to red fluorescent proteins using quantum dot or rod acceptor/donor linkers is described. The effect of morphology and tuned optical properties on the efficiency of this unique BRET-FRET system was evaluated.

Cocaine inhibits dopamine D2 receptor signaling via sigma-1-D2 receptor heteromers

Under normal conditions the brain maintains a delicate balance between inputs of reward seeking controlled by neurons containing the D1-like family of dopamine receptors and inputs of aversion coming from neurons containing the D2-like family of dopamine receptors. Cocaine is able to subvert these balanced inputs by altering the cell signaling of these two pathways such that D1 reward seeking pathway dominates. Here, we provide an explanation at the cellular and biochemical level how cocaine may achieve this. Exploring the effect of cocaine on dopamine D2 receptors function, we present evidence of σ1 receptor molecular and functional interaction with dopamine D2 receptors. Using biophysical, biochemical, and cell biology approaches, we discovered that D2 receptors (the long isoform of the D2 receptor) can complex with σ1 receptors, a result that is specific to D2 receptors, as D3 and D4 receptors did not form heteromers. We demonstrate that the σ1-D2 receptor heteromers consist of higher order oligomers, are found in mouse striatum and that cocaine, by binding to σ1 -D2 receptor heteromers, inhibits downstream signaling in both cultured cells and in mouse striatum. In contrast, in striatum from σ1 knockout animals these complexes are not found and this inhibition is not seen. Taken together, these data illuminate the mechanism by which the initial exposure to cocaine can inhibit signaling via D2 receptor containing neurons, destabilizing the delicate signaling balance influencing drug seeking that emanates from the D1 and D2 receptor containing neurons in the brain.

Current advanced bioluminescence technology in drug discovery

BACKGROUND

Bioluminescence technology is based on the luciferin-luciferase reaction and is generally well known as a reporter gene assay system that uses firefly luciferase. It has revolutionized the field of transcriptional analysis owing to its usability and quantitative capability. Several methods for transcription analysis have emerged in the past two decades. Recently, novel bioluminescence techniques that differ from typical approaches were developed for the detection of transcriptional regulation or direct protein-protein interactions.

OBJECTIVE

As each method has its own characteristics, this review summarizes the latest bioluminescence methods that are applicable to the field of drug discovery research.

METHODS

Considering the diversity of related techniques, this review covers several aspects that have been divided into the following classes: variation of reporter gene assays, secretion properties, protein-protein interaction assays in living cells and bioluminescence imaging of living cells.

RESULTS/CONCLUSIONS

The practical application of several luciferins and/or luciferases and the generation of novel applications by incorporating fluorescent molecules into bioluminescence techniques will become increasingly important because bioluminescence technology has a significant potential depending on how we use it.

Design, synthesis and biological evaluation of bivalent ligands against A(1)-D(1) receptor heteromers

AIM

To design and synthesize bivalent ligands for adenosine A1-dopamine D1 receptor heteromers (A1-D1R), and evaluate their pharmacological activities.

METHODS

Bivalent ligands and their corresponding A1R monovalent ligands were designed and synthesized. The affinities of the bivalent ligands for A1R and D1R in rat brain membrane preparation were examined using radiolabeled binding assays. To demonstrate the formation of A1-D1R, fluorescence resonance energy transfer (FRET) was conducted in HEK293 cells transfected with D1-CFP and A1-YFP. Molecular modeling was used to analyze the possible mode of protein-protein and protein-ligand interactions.

RESULTS

Two bivalent ligands for A1R and D1R (20a, 20b), as well as the corresponding A1R monovalent ligands (21a, 21b) were synthesized. In radiolabeled binding assays, the bivalent ligands showed affinities for A1R 10-100 times higher than those of the corresponding monovalent ligands. In FRET experiments, the bivalent ligands significantly increased the heterodimerization of A1R and D1R compared with the corresponding monovalent ligands. A heterodimer model with the interface of helixes 3, 4, 5 of A1R and helixes 1, 6, 7 from D1R was established with molecular modeling. The distance between the two ligand binding sites in the heterodimer model was approximately 48.4 Å, which was shorter than the length of the bivalent ligands.

CONCLUSION

This study demonstrates the existence of A1-D1R in situ and a simultaneous interaction of bivalent ligands with both the receptors.

Anomalous statistics of random relaxations in random environments

We comprehensively analyze the emergence of anomalous statistics in the context of the random relaxation (RARE) model [Eliazar and Metzler, J. Chem. Phys. 137, 234106 (2012)], a recently introduced versatile model of random relaxations in random environments. The RARE model considers excitations scattered randomly across a metric space around a reaction center. The excitations react randomly with the center, the reaction rates depending on the excitations' distances from this center. Relaxation occurs upon the first reaction between an excitation and the center. Addressing both the relaxation time and the relaxation range, we explore when these random variables display anomalous statistics, namely, heavy tails at zero and at infinity that manifest, respectively, exceptionally high occurrence probabilities of very small and very large outliers. A cohesive set of closed-form analytic results is established, determining precisely when such anomalous statistics emerge.

Investigation of stable and transient protein-protein interactions: Past, present, and future

This article presents an overview of the literature and a review of recent advances in the analysis of stable and transient protein-protein interactions (PPIs) with a focus on their function within cells, organs, and organisms. The significance of PTMs within the PPIs is also discussed. We focus on methods to study PPIs and methods of detecting PPIs, with particular emphasis on electrophoresis-based and MS-based investigation of PPIs, including specific examples. The validation of PPIs is emphasized and the limitations of the current methods for studying stable and transient PPIs are discussed. Perspectives regarding PPIs, with focus on bioinformatics and transient PPIs are also provided.

Detection of receptor heteromers involving dopamine receptors by the sequential BRET-FRET technology

Until very recently, dopamine receptors, like other G-protein-coupled receptors, were believed to function as individual units on the cell surface. Now it has been described by several groups including ours that dopamine receptors not only function as homomers but also form heteromers with other receptors at the membrane level. Bioluminescence and fluorescence resonance energy transfer (BRET and FRET) based techniques have been very useful to determine the interaction between two receptors, but to demonstrate the existence of higher-order complexes involving more than two molecules requires more sophisticated techniques. Combining BRET and FRET in the Sequential BRET-FRET (SRET) technique permits heteromers formed by three different proteins to be identified. In SRET experiments, the oxidation of a Renilla Luciferase substrate triggers acceptor excitation by BRET and subsequent energy transfer to a FRET acceptor. Using this methodology here we describe the heteromerization between adenosine A(2A), dopamine D(2), and cannabinoids CB(1) receptors in living cells.

G protein-coupled receptor heterocomplexes in neuropsychiatric disorders

G protein-coupled receptors (or GPCRs) represent the largest family of membrane proteins in the human genome and are the target of approximately half of all therapeutic drugs. GPCRs contain a conserved structure of seven transmembrane domains. Their amino terminus is located extracellularly, whereas the carboxy terminus extends into the cytoplasm. Accumulating evidence suggests that GPCRs exist and function as monomeric entities. Nevertheless, more recent findings indicate that GPCRs can also form dimers or even higher order oligomers. The differential pharmacological and signaling properties of GPCR heteromeric complexes hint that their physiological effects may be different as compared to those obtained in tissue cultures that express a particular GPCR. In this chapter, we review current data on the role of GPCR heteromerization in receptor signaling, as well as its potential implication in neuropsychiatric disorders such as schizophrenia, depression, and Parkinson's disease.

Endocannabinoid signaling and synaptic function

Endocannabinoids are key modulators of synaptic function. By activating cannabinoid receptors expressed in the central nervous system, these lipid messengers can regulate several neural functions and behaviors. As experimental tools advance, the repertoire of known endocannabinoid-mediated effects at the synapse, and their underlying mechanism, continues to expand. Retrograde signaling is the principal mode by which endocannabinoids mediate short- and long-term forms of plasticity at both excitatory and inhibitory synapses. However, growing evidence suggests that endocannabinoids can also signal in a nonretrograde manner. In addition to mediating synaptic plasticity, the endocannabinoid system is itself subject to plastic changes. Multiple points of interaction with other neuromodulatory and signaling systems have now been identified. In this Review, we focus on new advances in synaptic endocannabinoid signaling in the mammalian brain. The emerging picture not only reinforces endocannabinoids as potent regulators of synaptic function but also reveals that endocannabinoid signaling is mechanistically more complex and diverse than originally thought.

Nanophotonic control of the Förster resonance energy transfer efficiency

We have studied the influence of the local density of optical states (LDOS) on the rate and efficiency of Förster resonance energy transfer (FRET) from a donor to an acceptor. The donors and acceptors are dye molecules that are separated by a short strand of double-stranded DNA. The LDOS is controlled by carefully positioning the FRET pairs near a mirror. We find that the energy transfer efficiency changes with LDOS, and that, in agreement with theory, the energy transfer rate is independent of the LDOS, which allows one to quantitatively control FRET systems in a new way. Our results imply a change in the characteristic Förster distance, in contrast to common lore that this distance is fixed for a given FRET pair.

Identification of three residues essential for 5-hydroxytryptamine 2A-metabotropic glutamate 2 (5-HT2A·mGlu2) receptor heteromerization and its psychoactive behavioral function

Serotonin and glutamate G protein-coupled receptor (GPCR) neurotransmission affects cognition and perception in humans and rodents. GPCRs are capable of forming heteromeric complexes that differentially alter cell signaling, but the role of this structural arrangement in modulating behavior remains unknown. Here, we identified three residues located at the intracellular end of transmembrane domain four that are necessary for the metabotropic glutamate 2 (mGlu2) receptor to be assembled as a GPCR heteromer with the serotonin 5-hydroxytryptamine 2A (5-HT(2A)) receptor in the mouse frontal cortex. Substitution of these residues (Ala-677(4.40), Ala-681(4.44), and Ala-685(4.48)) leads to absence of 5-HT(2A)·mGlu2 receptor complex formation, an effect that is associated with a decrease in their heteromeric ligand binding interaction. Disruption of heteromeric expression with mGlu2 attenuates the psychosis-like effects induced in mice by hallucinogenic 5-HT(2A) agonists. Furthermore, the ligand binding interaction between the components of the 5-HT(2A)·mGlu2 receptor heterocomplex is up-regulated in the frontal cortex of schizophrenic subjects as compared with controls. Together, these findings provide structural evidence for the unique behavioral function of a GPCR heteromer.

Bioluminescence based in vivo screening technologies

Bioluminescence is the biologically active luminescence light producing event encountered in nature. In recent years several new screening methods utilizing bioluminescent cell-based biosensors have been designed demonstrating their utility towards dynamic monitoring of a variety of cellular functions. Because luciferase is unnatural to mammalian physiology, assays utilizing specific substrates to yield a luminescent signal are attractive and serve the purpose with high sensitivity and specificity. Often genetic or chemical modifications in different luciferase-substrate system in use have afforded new functionalities making these assays even more robust. Finally, in the evolving paradigm of molecular imaging, in vivo bioluminescence imaging (BLI) has evolved as a very attractive tool for interrogating human cellular biology in rodent models. In this short review we explore various bioluminescence screening strategies developed and analyze their scope in future drug screening processes.

Circadian-related heteromerization of adrenergic and dopamine D₄ receptors modulates melatonin synthesis and release in the pineal gland

Dopamine and adrenergic receptor complexes form under a circadian-regulated cycle and directly modulate melatonin synthesis and release from the pineal gland.

The role of the pineal gland is to translate the rhythmic cycles of night and day encoded by the retina into hormonal signals that are transmitted to the rest of the neuronal system in the form of serotonin and melatonin synthesis and release. Here we describe that the production of both melatonin and serotonin by the pineal gland is regulated by a circadian-related heteromerization of adrenergic and dopamine D₄ receptors. Through α_1B-D₄ and β₁-D₄ receptor heteromers dopamine inhibits adrenergic receptor signaling and blocks the synthesis of melatonin induced by adrenergic receptor ligands. This inhibition was not observed at hours of the day when D₄ was not expressed. These data provide a new perspective on dopamine function and constitute the first example of a circadian-controlled receptor heteromer. The unanticipated heteromerization between adrenergic and dopamine D₄ receptors provides a feedback mechanism for the neuronal hormone system in the form of dopamine to control circadian inputs.

Animals respond to cycles of light and dark with patterns in sleeping, feeding, body temperature alterations, and other biological functions. The pineal gland translates these light signals received from the retina into a language understandable to the rest of the body through the rhythmic synthesis and release of melatonin in response to the light and dark cycle. This process is controlled by adrenergic receptors. One impressive and mysterious aspect of the system is the rapid ability of rhythmic melatonin production and/or degradation to respond to changes in the cycle. In this study, we demonstrate that part of this response is due to the formation of receptor-receptor complexes (heteromers) between the adrenergic receptors α_1B or β₁ and the D₄ dopamine receptor. Using both biochemical and biophysical methods in transfected cells and in ex vivo tissue we show that dopamine, a neurotransmitter, inhibits adrenergic receptor signaling through these heteromers. This inhibition causes a dramatic decrease in melatonin production of the pineal gland. We postulate that these heteromers provide a rapid feedback mechanism for the neuronal hormone system to modulate circadian-controlled outputs.

Dopamine D4 receptor, but not the ADHD-associated D4.7 variant, forms functional heteromers with the dopamine D2S receptor in the brain

Polymorphic variants of the dopamine D(4) receptor have been consistently associated with attention-deficit hyperactivity disorder (ADHD). However, the functional significance of the risk polymorphism (variable number of tandem repeats in exon 3) is still unclear. Here, we show that whereas the most frequent 4-repeat (D(4.4)) and the 2-repeat (D(4.2)) variants form functional heteromers with the short isoform of the dopamine D(2) receptor (D(2S)), the 7-repeat risk allele (D(4.7)) does not. D(2) receptor activation in the D(2S)-D(4) receptor heteromer potentiates D(4) receptor-mediated MAPK signaling in transfected cells and in the striatum, which did not occur in cells expressing D(4.7) or in the striatum of knockin mutant mice carrying the 7 repeats of the human D(4.7) in the third intracellular loop of the D(4) receptor. In the striatum, D(4) receptors are localized in corticostriatal glutamatergic terminals, where they selectively modulate glutamatergic neurotransmission by interacting with D(2S) receptors. This interaction shows the same qualitative characteristics than the D(2S)-D(4) receptor heteromer-mediated mitogen-activated protein kinase (MAPK) signaling and D(2S) receptor activation potentiates D(4) receptor-mediated inhibition of striatal glutamate release. It is therefore postulated that dysfunctional D(2S)-D(4.7) heteromers may impair presynaptic dopaminergic control of corticostriatal glutamatergic neurotransmission and explain functional deficits associated with ADHD.

Heterotropic cooperativity within and between protomers of an oligomeric M(2) muscarinic receptor

At least four allosteric sites have been found to mediate the dose-dependent effects of gallamine on the binding of [(3)H]quinuclidinylbenzilate (QNB) and N-[(3)H]methylscopolamine (NMS) to M(2) muscarinic receptors in membranes and solubilized preparations from porcine atria, CHO cells, and Sf9 cells. The rate of dissociation of [(3)H]QNB was affected in a bell-shaped manner with at least one Hill coefficient (n(H)) greater than 1, indicating that at least three allosteric sites are involved. The level of binding of [(3)H]QNB was decreased in a biphasic manner, revealing at least two allosteric sites; binding of [(3)H]NMS was affected in a triphasic, serpentine manner, revealing at least three sites, and values of n(H) >1 pointed to at least four sites. Several lines of evidence indicate that all effects of gallamine were allosteric in nature and could be observed at equilibrium. The rates of equilibration and dissociation suggest that the receptor was predominately oligomeric, and the heterogeneity revealed by gallamine can be attributed to differences in its affinity for the constituent protomers of a tetramer. Those differences appear to arise from inter- and intramolecular cooperativity between gallamine and the radioligand.

Adenosine A(2A)-cannabinoid CB(1) receptor interaction: an integrative mechanism in striatal glutamatergic neurotransmission

The striatum is a subcortical area involved in sensorimotor, cognitive and emotional processes. Adenosine A(2A) receptors (A(2A)Rs) are highly expressed in the striatum, and their ability to establish functional and molecular interactions with many other receptors attributes to a pivotal role in the modulation and integration of striatal neurotransmission. This review will focus on the interaction between A(2A)Rs and cannabinoid CB(1) receptors (CB(1)Rs), taking it as a paradigmatic example of synaptic integration. Indeed, A(2A)Rs can exert an opposite (permissive vs. inhibitory) influence on CB1-dependent synaptic effect. These apparently irreconcilable functions could depend on a different role of pre- vs. postsynaptic A(2A)Rs, on their interaction with other receptors (namely adenosine A(1), metabotropic glutamate 5 and dopamine D2 receptors), and on whether A(2A)Rs form or not heteromers with CB(1)Rs. Besides providing a good example of the intricate pattern of events taking place in striatal synapses, the A(2A)/CB(1)R interaction proves very informative to understand the physiology of the basal ganglia and the mechanisms of related diseases. This article is part of a Special Issue entitled: Brain Integration.

Cannabinoid receptors CB1 and CB2 form functional heteromers in brain

Exploring the role of cannabinoid CB(2) receptors in the brain, we present evidence of CB(2) receptor molecular and functional interaction with cannabinoid CB(1) receptors. Using biophysical and biochemical approaches, we discovered that CB(2) receptors can form heteromers with CB(1) receptors in transfected neuronal cells and in rat brain pineal gland, nucleus accumbens, and globus pallidus. Within CB(1)-CB(2) receptor heteromers expressed in a neuronal cell model, agonist co-activation of CB(1) and CB(2) receptors resulted in a negative cross-talk in Akt phosphorylation and neurite outgrowth. Moreover, one specific characteristic of CB(1)-CB(2) receptor heteromers consists of both the ability of CB(1) receptor antagonists to block the effect of CB(2) receptor agonists and, conversely, the ability of CB(2) receptor antagonists to block the effect of CB(1) receptor agonists, showing a bidirectional cross-antagonism phenomenon. Taken together, these data illuminate the mechanism by which CB(2) receptors can negatively modulate CB(1) receptor function.

NCS-1 associates with adenosine A(2A) receptors and modulates receptor function

Modulation of G protein-coupled receptor (GPCR) signaling by local changes in intracellular calcium concentration is an established function of Calmodulin (CaM) which is known to interact with many GPCRs. Less is known about the functional role of the closely related neuronal EF-hand Ca²⁺-sensor proteins that frequently associate with CaM targets with different functional outcome. In the present study we aimed to investigate if a target of CaM—the A_2A adenosine receptor is able to associate with two other neuronal calcium binding proteins (nCaBPs), namely NCS-1 and caldendrin. Using bioluminescence resonance energy transfer (BRET) and co-immunoprecipitation experiments we show the existence of A_2A—NCS-1 complexes in living cells whereas caldendrin did not associate with A_2A receptors under the conditions tested. Interestingly, NCS-1 binding modulated downstream A_2A receptor intracellular signaling in a Ca²⁺-dependent manner. Taken together this study provides further evidence that neuronal Ca²⁺-sensor proteins play an important role in modulation of GPCR signaling.

Decoding the signaling of a GPCR heteromeric complex reveals a unifying mechanism of action of antipsychotic drugs

Atypical antipsychotic drugs, such as clozapine and risperidone, have a high affinity for the serotonin 5-HT(2A) G protein-coupled receptor (GPCR), the 2AR, which signals via a G(q) heterotrimeric G protein. The closely related non-antipsychotic drugs, such as ritanserin and methysergide, also block 2AR function, but they lack comparable neuropsychological effects. Why some but not all 2AR inhibitors exhibit antipsychotic properties remains unresolved. We now show that a heteromeric complex between the 2AR and the G(i)-linked GPCR, metabotropic glutamate 2 receptor (mGluR2), integrates ligand input, modulating signaling output and behavioral changes. Serotonergic and glutamatergic drugs bind the mGluR2/2AR heterocomplex, which then balances Gi- and Gq-dependent signaling. We find that the mGluR2/2AR-mediated changes in Gi and Gq activity predict the psychoactive behavioral effects of a variety of pharmocological compounds. These observations provide mechanistic insight into antipsychotic action that may advance therapeutic strategies for disorders including schizophrenia and dementia.

Mathematical models for quantitative assessment of bioluminescence resonance energy transfer: application to seven transmembrane receptors oligomerization

The idea that seven transmembrane receptors (7TMRs; also designated G-protein coupled receptors, GPCRs) might form dimers or higher order oligomeric complexes was formulated more than 20 years ago and has been intensively studied since then. In the last decade, bioluminescence resonance energy transfer (BRET) has been one of the most frequently used biophysical methods for studying 7TMRs oligomerization. This technique enables monitoring physical interactions between protein partners in living cells fused to donor and acceptor moieties. It relies on non-radiative transfer of energy between donor and acceptor, depending on their intermolecular distance (1-10 nm) and relative orientation. Results derived from BRET-based techniques are very persuasive; however, they need appropriate controls and critical interpretation. To overcome concerns about the specificity of BRET-derived results, a set of experiments has been proposed, including negative control with a non-interacting receptor or protein, BRET dilution, saturation, and competition assays. This article presents the theoretical background behind BRET assays, then outlines mathematical models for quantitative interpretation of BRET saturation and competition assay results, gives examples of their utilization and discusses the possibilities of quantitative analysis of data generated with other RET-based techniques.

BRET and Time-resolved FRET strategy to study GPCR oligomerization: from cell lines toward native tissues

The concept of oligomerization of G protein-coupled receptor (GPCR) opens new perspectives regarding physiological function regulation. The capacity of one GPCR to modify its binding and coupling properties by interacting with a second one can be at the origin of regulations unsuspected two decades ago. Although the concept is interesting, its validation at a physiological level is challenging and probably explains why receptor oligomerization is still controversial. Demonstrating direct interactions between two proteins is not trivial since few techniques present a spatial resolution allowing this precision. Resonance energy transfer (RET) strategies are actually the most convenient ones. During the last two decades, bioluminescent resonance energy transfer and time-resolved fluorescence resonance energy transfer (TR-FRET) have been widely used since they exhibit high signal-to-noise ratio. Most of the experiments based on GPCR labeling have been performed in cell lines and it has been shown that all GPCRs have the propensity to form homo- or hetero-oligomers. However, whether these data can be extrapolated to GPCRs expressed in native tissues and explain receptor functioning in real life, remains an open question. Native tissues impose different constraints since GPCR sequences cannot be modified. Recently, a fluorescent ligand-based GPCR labeling strategy combined to a TR-FRET approach has been successfully used to prove the existence of GPCR oligomerization in native tissues. Although the RET-based strategies are generally quite simple to implement, precautions have to be taken before concluding to the absence or the existence of specific interactions between receptors. For example, one should exclude the possibility of collision of receptors diffusing throughout the membrane leading to a specific FRET signal. The advantages and the limits of different approaches will be reviewed and the consequent perspectives discussed.

The functional size of GPCRs - monomers, dimers or tetramers?

In almost 16 years since the word "dimer" was used in a publication to describe the organization of G protein-coupled receptors (GPCRs), a large number of studies have since weighed in on this notion. Are native, functional GPCRs monomers, dimers or as some would suggest even higher order structures? Here, we review some of the latest evidence regarding the organization of these receptors in both homo- and hetero-oligomeric formats, with a particular focus on β-adrenergic receptors. This is particularly important for understanding the allosteric nature of receptor/receptor interactions. It is likely that, over the course of evolution, mechanisms have come into play using all of the possible variations in receptor/receptor stoichiometry, depending on the cell and the physiological context in question. Finally, we provide some data that suggests that higher order structures of GPCRs, as with dimers themselves are probably assembled in the ER.

A new D₂ dopamine receptor agonist allosterically modulates A(2A) adenosine receptor signalling by interacting with the A(2A)/D₂ receptor heteromer

The structural and functional interaction between D₂ dopamine receptor (DR) and A(2A) adenosine receptor (AR) has suggested these two receptors as a pharmacological target in pathologies associated with dopamine dysfunction, such as Parkinson's disease. In transfected cell lines it has been demonstrated the activation of D₂DR induces a significant negative regulation of A(2A)AR-mediated responses, whereas few data are at now available about the regulation of A(2A)AR by D₂DR agonists at receptor recognition site. In this work we confirmed that in A(2A)AR/D₂DR co-transfected cells, these receptors exist as homo- and hetero-dimers. The classical D₂DR agonists were able to negatively modulate both A(2A)AR affinity and functionality. These effects occurred even if any significant changes in A(2A)AR/D₂DR energy transfer interaction could be detected in BRET experiments. Since the development of new molecules able to target A(2A)/D₂ dimers may represent an attractive tool for innovative pharmacological therapy, we also identified a new small molecule, 3-(3,4-dimethylphenyl)-1-(2-piperidin-1-yl)ethyl)piperidine (compound 1), full agonist of D₂DR and modulator of A(2A)-D₂ receptor dimer. This compound was able to negatively modulate A(2A)AR binding properties and functional responsiveness in a manner comparable to classical D₂R agonists. In contrast to classical agonists, compound 1 led to conformational changes in the quaternary structure in D₂DR homomers and heteromers and induced A(2A)AR/D₂DR co-internalization. These results suggest that compound 1 exerts a high control of the function of heteromers and could represent a starting point for the development of new drugs targeting A(2A)AR/D₂ DR heteromers.

G protein-coupled receptors: walking hand-in-hand, talking hand-in-hand?

Most cells express a panel of different G protein-coupled receptors (GPCRs) allowing them to respond to at least a corresponding variety of extracellular ligands. In order to come to an integrative well-balanced functional response these ligand-receptor pairs can often cross-regulate each other. Although most GPCRs are fully capable to induce intracellular signalling upon agonist binding on their own, many GPCRs, if not all, appear to exist and function in homomeric and/or heteromeric assemblies for at least some time. Such heteromeric organization offers unique allosteric control of receptor pharmacology and function between the protomers and might even unmask 'new' features. However, it is important to realize that some functional consequences that are proposed to originate from heteromeric receptor interactions may also be observed due to intracellular crosstalk between signalling pathways of non-associated GPCRs.

Real-time G-protein-coupled receptor imaging to understand and quantify receptor dynamics

Understanding the trafficking of G-protein-coupled receptors (GPCRs) and their regulation by agonists and antagonists is fundamental to develop more effective drugs. Optical methods using fluorescent-tagged receptors and spinning disk confocal microscopy are useful tools to investigate membrane receptor dynamics in living cells. The aim of this study was to develop a method to characterize receptor dynamics using this system which offers the advantage of very fast image acquisition with minimal cell perturbation. However, in short-term assays photobleaching was still a problem. Thus, we developed a procedure to perform a photobleaching-corrected image analysis. A study of short-term dynamics of the long isoform of the dopamine type 2 receptor revealed an agonist-induced increase in the mobile fraction of receptors with a rate of movement of 0.08 μm/s For long-term assays, the ratio between the relative fluorescence intensity at the cell surface versus that in the intracellular compartment indicated that receptor internalization only occurred in cells co-expressing G protein-coupled receptor kinase 2. These results indicate that the lateral movement of receptors and receptor internalization are not directly coupled. Thus, we believe that live imaging of GPCRs using spinning disk confocal image analysis constitutes a powerful tool to study of receptor dynamics.

Post-translational membrane insertion of tail-anchored transmembrane EF-hand Ca2+ sensor calneurons requires the TRC40/Asna1 protein chaperone

Calneuron-1 and -2 are neuronal EF-hand-type calcium sensor proteins that are prominently targeted to trans-Golgi network membranes and impose a calcium threshold at the Golgi for phosphatidylinositol 4-OH kinase IIIβ activation and the regulated local synthesis of phospholipids that are crucial for TGN-to-plasma membrane trafficking. In this study, we show that calneurons are nonclassical type II tail-anchored proteins that are post-translationally inserted into the endoplasmic reticulum membrane via an association of a 23-amino acid-long transmembrane domain (TMD) with the TRC40/Asna1 chaperone complex. Following trafficking to the Golgi, calneurons are probably retained in the TGN because of the length of the TMD and phosphatidylinositol 4-phosphate lipid binding. Both calneurons rapidly self-associate in vitro and in vivo via their TMD and EF-hand containing the N terminus. Although dimerization and potentially multimerization precludes TRC40/Asna1 binding and thereby membrane insertion, we found no evidence for a cytosolic pool of calneurons and could demonstrate that self-association of calneurons is restricted to membrane-inserted protein. The dimerization properties and the fact that they, unlike every other EF-hand calmodulin-like Ca(2+) sensor, are always associated with membranes of the secretory pathway, including vesicles and plasma membrane, suggests a high degree of spatial segregation for physiological target interactions.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

Sequential bioluminescence resonance energy transfer-fluorescence resonance energy transfer-based ratiometric protease assays with fusion proteins of firefly luciferase and red fluorescent protein

We report here the preparation of ratiometric luminescent probes that contain two well-separated emission peaks produced by a sequential bioluminescence resonance energy transfer (BRET)-fluorescence resonance energy transfer (FRET) process. The probes are single soluble fusion proteins consisting of a thermostable firefly luciferase variant that catalyze yellow-green (560nm maximum) bioluminescence and a red fluorescent protein covalently labeled with a near-infrared fluorescent dye. The two proteins are connected by a decapeptide containing a protease recognition site specific for factor Xa, thrombin, or caspase 3. The rates of protease cleavage of the fusion protein substrates were monitored by recording emission spectra and plotting the change in peak ratios over time. Detection limits of 0.41nM for caspase 3, 1.0nM for thrombin, and 58nM for factor Xa were realized with a scanning fluorometer. Our results demonstrate for the first time that an efficient sequential BRET-FRET energy transfer process based on firefly luciferase bioluminescence can be employed to assay physiologically important protease activities.

Pharmacological evidence for different populations of postsynaptic adenosine A2A receptors in the rat striatum

Adenosine A(2A) receptors (A(2A)Rs) are highly concentrated in the striatum. Two pharmacological different functional populations of A(2A)Rs have been recently described based on their different affinities for the A(2A)R antagonist SCH-442416. This compound has high affinity for A(2A)Rs not forming heteromers or forming heteromers with adenosine A(1) receptors (A(1)Rs) while showing very low affinity for A(2A)Rs forming heteromers with dopamine D(2) receptors (D(2)Rs). It has been widely described that striatal A(1)R-A(2A)R heteromers are preferentially localized presynaptically in the glutamatergic terminals that contact GABAergic dynorphinergic neurons, and that A(2A)R-D(2)R heteromers are localized postsynaptically in GABAergic enkephalinergic neurons. In the present study we provide evidence suggesting that SCH-442416 also targets postsynaptic A(2A)R not forming heteromers with D(2)R, which are involved in the motor depressant effects induced by D(2)R antagonists. SCH-442416 counteracted motor depression in rats induced by the D(2)R antagonist raclopride at a dose that did not produce motor activation or that blocked motor depression induced by an A(2A)R agonist. Furthermore, we re-evaluated the recently suggested key role of cannabinoid CB(1) receptors (CB(1)Rs) in the effects of A(2A)R antagonists acting at postsynaptic A(2A)Rs. By recording locomotor activity and monitoring striatal glutamate release induced by cortical electrical stimulation in rats after administration of A(2A)R and CB(1)R antagonists, we did not find evidence for any significant role of endocannabinoids in the post- or presynaptic effects of A(2A)R antagonists. The present results further suggest the existence of at least two functionally and pharmacologically different populations of striatal postsynaptic A(2A)Rs.

Adenosine A(2A) Receptors and A(2A) Receptor Heteromers as Key Players in Striatal Function

A very significant density of adenosine A_2A receptors (A_2ARs) is present in the striatum, where they are preferentially localized postsynaptically in striatopallidal medium spiny neurons (MSNs). In this localization A_2ARs establish reciprocal antagonistic interactions with dopamine D₂ receptors (D₂Rs). In one type of interaction, A_2AR and D₂R are forming heteromers and, by means of an allosteric interaction, A_2AR counteracts D₂R-mediated inhibitory modulation of the effects of NMDA receptor stimulation in the striatopallidal neuron. This interaction is probably mostly responsible for the locomotor depressant and activating effects of A_2AR agonist and antagonists, respectively. The second type of interaction involves A_2AR and D₂R that do not form heteromers and takes place at the level of adenylyl cyclase (AC). Due to a strong tonic effect of endogenous dopamine on striatal D₂R, this interaction keeps A_2AR from signaling through AC. However, under conditions of dopamine depletion or with blockade of D₂R, A_2AR-mediated AC activation is unleashed with an increased gene expression and activity of the striatopallidal neuron and with a consequent motor depression. This interaction is probably the main mechanism responsible for the locomotor depression induced by D₂R antagonists. Finally, striatal A_2ARs are also localized presynaptically, in cortico-striatal glutamatergic terminals that contact the striato-nigral MSN. These presynaptic A_2ARs heteromerize with A₁ receptors (A₁Rs) and their activation facilitates glutamate release. These three different types of A_2ARs can be pharmacologically dissected by their ability to bind ligands with different affinity and can therefore provide selective targets for drug development in different basal ganglia disorders.