Characterization of the A2AR-D2R interface: focus on the role of the C-terminal tail and the transmembrane helices

The World of GPCR dimers - Mapping dopamine receptor D2 homodimers in different activation states and configuration arrangements

G protein-coupled receptors (GPCRs) are known to dimerize, but the molecular and structural basis of GPCR dimers is not well understood. In this study, we developed a computational framework to generate models of symmetric and asymmetric GPCR dimers using different monomer activation states and identified their most likely interfaces with molecular details. We chose the dopamine receptor D2 (D2R) homodimer as a case study because of its biological relevance and the availability of structural information. Our results showed that transmembrane domains 4 and 5 (TM4 and TM5) are mostly found at the dimer interface of the D2R dimer and that these interfaces have a subset of key residues that are mostly nonpolar from TM4 and TM5, which was in line with experimental studies. In addition, TM2 and TM3 appear to be relevant for D2R dimers. In some cases, the inactive configuration is unaffected by the partnered protomer, whereas in others, the active protomer adopts the properties of an inactive receptor. Additionally, the β-arrestin configuration displayed the properties of an active receptor in the absence of an agonist, suggesting that a switch to another meta-state during dimerization occurred. Our findings are consistent with the experimental data, and this method can be adapted to study heterodimers and potentially extended to include additional proteins such as G proteins or β-arrestins. In summary, this approach provides insight into the impact of the conformational status of partnered protomers on the overall quaternary GPCR macromolecular structure and dynamics.

Adenosine A2A Receptor Activation Regulates Niemann-Pick C1 Expression and Localization in Macrophages

Adenosine plays an important role in modulating immune cell function, particularly T cells and myeloid cells, such as macrophages and dendritic cells. Cell surface adenosine A_2A receptors (A_2AR) regulate the production of pro-inflammatory cytokines and chemokines, as well as the proliferation, differentiation, and migration of immune cells. In the present study, we expanded the A_2AR interactome and provided evidence for the interaction between the receptor and the Niemann-Pick type C intracellular cholesterol transporter 1 (NPC1) protein. The NPC1 protein was identified to interact with the C-terminal tail of A_2AR in RAW 264.7 and IPMФ cells by two independent and parallel proteomic approaches. The interaction between the NPC1 protein and the full-length A_2AR was further validated in HEK-293 cells that permanently express the receptor and RAW264.7 cells that endogenously express A_2AR. A_2AR activation reduces the expression of NPC1 mRNA and protein density in LPS-activated mouse IPMФ cells. Additionally, stimulation of A_2AR negatively regulates the cell surface expression of NPC1 in LPS-stimulated macrophages. Furthermore, stimulation of A_2AR also altered the density of lysosome-associated membrane protein 2 (LAMP2) and early endosome antigen 1 (EEA1), two endosomal markers associated with the NPC1 protein. Collectively, these results suggested a putative A_2AR-mediated regulation of NPC1 protein function in macrophages, potentially relevant for the Niemann-Pick type C disease when mutations in NPC1 protein result in the accumulation of cholesterol and other lipids in lysosomes.

Evolutionary Couplings and Molecular Dynamic Simulations Highlight Details of GPCRs Heterodimers' Interfaces

A growing body of evidence suggests that only a few amino acids ("hot-spots") at the interface contribute most of the binding energy in transient protein-protein interactions. However, experimental protocols to identify these hot-spots are highly labor-intensive and expensive. Computational methods, including evolutionary couplings, have been proposed to predict the hot-spots, but they generally fail to provide details of the interacting amino acids. Here we showed that unbiased evolutionary methods followed by biased molecular dynamic simulations could achieve this goal and reveal critical elements of protein complexes. We applied the methodology to selected G-protein coupled receptors (GPCRs), known for their therapeutic properties. We used the structure-prior-assisted direct coupling analysis (SP-DCA) to predict the binding interfaces of A2aR/D2R, CB1R/D2R, A2aR/CB1R, 5HT2AR/D2R, and 5-HT2AR/mGluR2 receptor heterodimers, which all agreed with published data. In order to highlight details of the interactions, we performed molecular dynamic (MD) simulations using the newly developed AWSEM energy model. We found that these receptors interact primarily through critical residues at the C and N terminal domains and the third intracellular loop (ICL3). The MD simulations showed that these residues are energetically necessary for dimerization and revealed their native conformational state. We subsequently applied the methodology to the 5-HT2AR/5-HTR4R heterodimer, given its implication in drug addiction and neurodegenerative pathologies such as Alzheimer's disease (AD). Further, the SP-DCA analysis showed that 5-HT2AR and 5-HTR4R heterodimerize through the C-terminal domain of 5-HT2AR and ICL3 of 5-HT4R. However, elucidating the details of GPCR interactions would accelerate the discovery of druggable sites and improve our knowledge of the etiology of common diseases, including AD.

Structure-function relationship and physiological role of apelin and its G protein coupled receptor

Apelin receptor (APJR) is a class A peptide (apelin) binding G protein-coupled receptor (GPCR) that plays a significant role in regulating blood pressure, cardiac output, and maintenance of fluid homeostasis. It is activated by a wide range of endogenous peptide isoforms of apelin and elabela. The apelin peptide isoforms contain distinct structural features that aid in ligand recognition and activation of the receptor. Site-directed mutagenesis and structure-based studies have revealed the involvement of extracellular and transmembrane regions of the receptor in binding to the peptide isoforms. The structural features of APJR activation of the receptor as well as mediating G-protein and β-arrestin-mediated signaling are delineated by multiple mutagenesis studies. There is increasing evidence that the structural requirements of APJR to activate G-proteins and β-arrestins are different, leading to biased signaling. APJR also responds to mechanical stimuli in a ligand-independent manner. A multitude of studies has focused on developing both peptide and non-peptide agonists and antagonists specific to APJR. Apelin/elabela-activated APJR orchestrates major signaling pathways such as extracellular signal-regulated kinase (ERKs), protein kinase B (PKB/Akt), and p70S. This review focuses on the structural and functional characteristics of apelin, elabela, APJR, and their interactions involved in the binding and activation of the downstream signaling cascade. We also focus on the diverse signaling profile of APJR and its ligands and their involvement in various physiological systems.

The mGlu5 Receptor Protomer-Mediated Dopamine D2 Receptor Trans-Inhibition Is Dependent on the Adenosine A2A Receptor Protomer: Implications for Parkinson's Disease

The adenosine A_2A receptor (A_2AR), dopamine D₂ receptor (D₂R) and metabotropic glutamate receptor type 5 (mGluR₅) form A_2AR-D₂R-mGluR₅ heteroreceptor complexes in living cells and in rat striatal neurons. In the current study, we present experimental data supporting the view that the A_2AR protomer plays a major role in the inhibitory modulation of the density and the allosteric receptor-receptor interaction within the D₂R-mGluR₅ heteromeric component of the A_2AR-D₂R-mGluR₅ complex in vitro and in vivo. The A_2AR and mGluR₅ protomers interact and modulate D₂R protomer recognition and signalling upon forming a trimeric complex from these receptors. Expression of A_2AR in HEK293T cells co-expressing D₂R and mGluR₅ resulted in a significant and marked increase in the formation of the D₂R-mGluR₅ heteromeric component in both bioluminescence resonance energy transfer and proximity ligation assays. A highly significant increase of the the high-affinity component of D₂R (D2R_{Ki High)} values was found upon cotreatment with the mGluR₅ and A_2AR agonists in the cells expressing A_2AR, D₂R and mGluR₅ with a significant effect observed also with the mGluR₅ agonist alone compared to cells expressing only D₂R and mGluR₅. In cells co-expressing A_2AR, D₂R and mGluR₅, stimulation of the cells with an mGluR₅ agonist like or D₂R antagonist fully counteracted the D₂R agonist-induced inhibition of the cAMP levels which was not true in cells only expressing mGluR₅ and D₂R. In agreement, the mGluR₅-negative allosteric modulator raseglurant significantly reduced the haloperidol-induced catalepsy in mice, and in A_2AR knockout mice, the haloperidol action had almost disappeared, supporting a functional role for mGluR₅ and A_2AR in enhancing D₂R blockade resulting in catalepsy. The results represent a relevant example of integrative activity within higher-order heteroreceptor complexes.

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

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

Adenosine Receptors in Neuropsychiatric Disorders: Fine Regulators of Neurotransmission and Potential Therapeutic Targets

Adenosine exerts an important role in the modulation of central nervous system (CNS) activity. Through the interaction with four G-protein coupled receptor (GPCR) subtypes, adenosine subtly regulates neurotransmission, interfering with the dopaminergic, glutamatergic, noradrenergic, serotoninergic, and endocannabinoid systems. The inhibitory and facilitating actions of adenosine on neurotransmission are mainly mediated by A₁ and A_2A adenosine receptors (ARs), respectively. Given their role in the CNS, ARs are promising therapeutic targets for neuropsychiatric disorders where altered neurotransmission represents the most likely etiological hypothesis. Activating or blocking ARs with specific pharmacological agents could therefore restore the balance of altered neurotransmitter systems, providing the rationale for the potential treatment of these highly debilitating conditions. In this review, we summarize and discuss the most relevant studies concerning AR modulation in psychotic and mood disorders such as schizophrenia, bipolar disorders, depression, and anxiety, as well as neurodevelopment disorders such as autism spectrum disorder (ASD), fragile X syndrome (FXS), attention-deficit hyperactivity disorder (ADHD), and neuropsychiatric aspects of neurodegenerative disorders.

The coming together of allosteric and phosphorylation mechanisms in the molecular integration of A2A heteroreceptor complexes in the dorsal and ventral striatal-pallidal GABA neurons

The role of adenosine A2A receptor (A2AR) and striatal-enriched protein tyrosine phosphatase (STEP) interactions in the striatal-pallidal GABA neurons was recently discussed in relation to A2AR overexpression and cocaine-induced increases of brain adenosine levels. As to phosphorylation, combined activation of A2AR and metabotropic glutamate receptor 5 (mGluR5) in the striatal-pallidal GABA neurons appears necessary for phosphorylation of the GluA1 unit of the AMPA receptor to take place. Robert Yasuda (J Neurochem 152: 270–272, 2020) focused on finding a general mechanism by which STEP activation is enhanced by increased A2AR transmission in striatal-pallidal GABA neurons expressing A2AR and dopamine D2 receptor. In his Editorial, he summarized in a clear way the significant effects of A2AR activation on STEP in the dorsal striatal-pallidal GABA neurons which involves a rise of intracellular levels of calcium causing STEP activation through its dephosphorylation. However, the presence of the A2AR in an A2AR-fibroblast growth factor receptor 1 (FGFR1) heteroreceptor complex can be required in the dorsal striatal-pallidal GABA neurons for the STEP activation. Furthermore, Won et al. (Proc Natl Acad Sci USA 116: 8028–8037, 2019) found in mass spectrometry experiments that the STEP splice variant STEP₆₁ can bind to mGluR5 and inactivate it. In addition, A2AR overexpression can lead to increased formation of A2AR-mGluR5 heterocomplexes in ventral striatal-pallidal GABA neurons. It involves enhanced facilitatory allosteric interactions leading to increased Gq-mediated mGluR5 signaling activating STEP. The involvement of both A2AR and STEP in the actions of cocaine on synaptic downregulation was also demonstrated. The enhancement of mGluR5 protomer activity by the A2AR protomer in A2AR-mGluR5 heterocomplexes in the nucleus accumbens shell appears to have a novel significant role in STEP mechanisms by both enhancing the activation of STEP and being a target for STEP₆₁.

In Vitro and In Silico Characterization of G-Protein Coupled Receptor (GPCR) Targets of Phlorofucofuroeckol-A and Dieckol

Phlorotannins are polyphenolic compounds in marine alga, especially the brown algae. Among numerous phlorotannins, dieckol and phlorofucofuroeckol-A (PFF-A) are the major ones and despite a wider biological activity profile, knowledge of the G protein-coupled receptor (GPCR) targets of these phlorotannins is lacking. This study explores prime GPCR targets of the two phlorotannins. In silico proteocheminformatics modeling predicted twenty major protein targets and in vitro functional assays showed a good agonist effect at the α2C adrenergic receptor (α2CAR) and an antagonist effect at the adenosine 2A receptor (A2AR), δ-opioid receptor (δ-OPR), glucagon-like peptide-1 receptor (GLP-1R), and 5-hydroxytryptamine 1A receptor (5-TH1AR) of both phlorotannins. Besides, dieckol showed an antagonist effect at the vasopressin 1A receptor (V1AR) and PFF-A showed a promising agonist effect at the cannabinoid 1 receptor and an antagonist effect at V1AR. In silico molecular docking simulation enabled us to investigate and identify distinct binding features of these phlorotannins to the target proteins. The docking results suggested that dieckol and PFF-A bind to the crystal structures of the proteins with good affinity involving key interacting amino acid residues comparable to reference ligands. Overall, the present study suggests α2CAR, A2AR, δ-OPR, GLP-1R, 5-TH1AR, CB1R, and V1AR as prime receptor targets of dieckol and PFF-A.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Multiple Adenosine-Dopamine (A2A-D2 Like) Heteroreceptor Complexes in the Brain and Their Role in Schizophrenia

In the 1980s and 1990s, the concept was introduced that molecular integration in the Central Nervous System could develop through allosteric receptor–receptor interactions in heteroreceptor complexes presents in neurons. A number of adenosine–dopamine heteroreceptor complexes were identified that lead to the A_2A-D₂ heteromer hypothesis of schizophrenia. The hypothesis is based on strong antagonistic A_2A-D₂ receptor–receptor interactions and their presence in the ventral striato-pallidal GABA anti-reward neurons leading to reduction of positive symptoms. Other types of adenosine A_2A heteroreceptor complexes are also discussed in relation to this disease, such as A_2A-D₃ and A_2A-D₄ heteroreceptor complexes as well as higher order A_2A-D₂-mGluR5 and A_2A-D₂-Sigma1R heteroreceptor complexes. The A_2A receptor protomer can likely modulate the function of the D₄ receptors of relevance for understanding cognitive dysfunction in schizophrenia. A_2A-D₂-mGluR5 complex is of interest since upon A_2A/mGluR5 coactivation they appear to synergize in producing strong inhibition of the D2 receptor protomer. For understanding the future of the schizophrenia treatment, the vulnerability of the current A_2A-D₂like receptor complexes will be tested in animal models of schizophrenia. A_2A-D₂-Simag1R complexes hold the highest promise through Sigma1R enhancement of inhibition of D2R function. In line with this work, Lara proposed a highly relevant role of adenosine for neurobiology of schizophrenia.

Acute cocaine treatment enhances the antagonistic allosteric adenosine A2A-dopamine D2 receptor-receptor interactions in rat dorsal striatum without increasing significantly extracellular dopamine levels

BACKGROUND

Antagonistic adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) receptor-receptor interactions have previously been demonstrated in A2AR-D2R heteroreceptor complexes in the rat dorsal striatum. They mainly involve a reduction of affinity in the high-affinity component of the D2R agonist binding site upon activation in vivo of the A2AR by an A2AR agonist. Upon cocaine self-administration, this antagonistic A2AR-D2R interaction disappeared in the dorsal striatum.

METHODS

In the current experiments, it was tested whether such modifications in the antagonistic A2AR-D2R receptor-receptor interactions can develop also after an acute systemic injection of a low cocaine dose (1 mg/kg; sc).

RESULTS

Microdialysis experiments indicated that acute cocaine did not significantly alter the extracellular dopamine levels in the dorsal striatum of the awake Wistar rats. Competition dopamine receptor binding experiments demonstrated that in the acute cocaine group, the A2AR agonist CGS-21680 produced significantly larger increases in the D2R K_{i, High} values (reduction of high-affinity) versus the saline-injected (i.e. control) group. Furthermore, in the dorsal striatum membrane preparation from acute cocaine-injected rats, CGS-21680 also produced significant increases in the D2R K_{i, Low} values (reduction of low-affinity) and in the proportion of D2Rs in the high-affinity state (RH). Such significant effects were not observed with CGS-21680 in the control group.

CONCLUSIONS

The molecular mechanism involved in the acute cocaine-induced increase in the antagonistic allosteric A2AR-D2R receptor-receptor interactions may be an increased formation of higher-order complexes A2AR-D2R-sigma1R in which cocaine by binding to the sigma1R protomer also allosterically enhances the inhibitory A2AR-D2R interaction in this receptor complex.

Control of glutamate release by complexes of adenosine and cannabinoid receptors

Background

It has been hypothesized that heteromers of adenosine A_2A receptors (A2AR) and cannabinoid CB₁ receptors (CB1R) localized in glutamatergic nerve terminals mediate the integration of adenosine and endocannabinoid signaling involved in the modulation of striatal excitatory neurotransmission. Previous studies have demonstrated the existence of A2AR-CB1R heteromers in artificial cell systems. A dependence of A2AR signaling for the Gi protein-mediated CB1R signaling was described as one of its main biochemical characteristics. However, recent studies have questioned the localization of functionally significant A2AR-CB1R heteromers in striatal glutamatergic terminals.

Results

Using a peptide-interfering approach combined with biophysical and biochemical techniques in mammalian transfected cells and computational modeling, we could establish a tetrameric quaternary structure of the A2AR-CB1R heterotetramer. This quaternary structure was different to the also tetrameric structure of heteromers of A2AR with adenosine A₁ receptors or dopamine D₂ receptors, with different heteromeric or homomeric interfaces. The specific quaternary structure of the A2A-CB1R, which depended on intermolecular interactions involving the long C-terminus of the A2AR, determined a significant A2AR and Gs protein-mediated constitutive activation of adenylyl cyclase. Using heteromer-interfering peptides in experiments with striatal glutamatergic terminals, we could then demonstrate the presence of functionally significant A2AR-CB1R heteromers with the same biochemical characteristics of those studied in mammalian transfected cells. First, either an A2AR agonist or an A2AR antagonist allosterically counteracted Gi-mediated CB1R agonist-induced inhibition of depolarization-induced glutamate release. Second, co-application of both an A2AR agonist and an antagonist cancelled each other effects. Finally, a CB1R agonist inhibited glutamate release dependent on a constitutive activation of A2AR by a canonical Gs-Gi antagonistic interaction at the adenylyl cyclase level.

Conclusions

We demonstrate that the well-established cannabinoid-induced inhibition of striatal glutamate release can mostly be explained by a CB1R-mediated counteraction of the A2AR-mediated constitutive activation of adenylyl cyclase in the A2AR-CB1R heteromer.

Genetic variants in dopamine receptors influence on heterodimerization in the context of antipsychotic drug action

Human dopamine D2 receptor (D₂R) gene has polymorphic variants, three of them alter its amino acid sequence: Val96Ala, Pro310Ser and Ser311Cys. Their functional role never became the object of extensive studies, even though there are some evidence that they correlate with schizophrenia. The present work reviews data indicating that these mutations play a role in dimer formation with dopamine D1 receptor (D₁R), with the strongest effect observed for Ser311Cys variant. Similarly, the affinity for antipsychotic drugs of this genetic variant depends on whether it is expressed together with D₁R or not. Better understanding of altered ability of genetic variants of D₂R to form dimers with D₁R, as well as of altered affinity for antipsychotic drugs, depending on the absence or presence of the second dopamine receptor is of great importance-since these two receptors are not always co-expressed in the same cell. It may well be that targeting new compounds toward the D₁R-D₂R dimers, which the most probably form under conditions of excessive dopamine release, will result in antipsychotic drugs devoid of serious side effects.

Adenosine A2A-dopamine D2 receptor-receptor interaction in neurons and astrocytes: Evidence and perspectives

The discovery of receptor-receptor interactions in the early 1980s, together with a more accurate focusing of allosteric mechanisms in proteins, expanded the knowledge on the G protein-coupled receptor (GPCR)-mediated signaling processes. GPCRs were seen to operate not only as monomers, but also as quaternary structures shaped by allosteric interactions. These integrative mechanisms can change the function of the GPCRs involved, leading to a sophisticated dynamic of the receptor assembly in terms of modulation of recognition and signaling. In this context, the heterodimeric complex formed by the adenosine A_2A and the dopamine D₂ receptors likely represents a prototypical example. The pharmacological evidence obtained, together with the tissue distribution of the A_2A-D₂ heteromeric complexes, suggested they could represent a target for new therapeutic strategies addressing significant disorders of the central nervous system. The research findings and the perspectives they offer from the therapeutic standpoint are the focus of the here presented discussion.

A2AR Transmembrane 2 Peptide Administration Disrupts the A2AR-A2AR Homoreceptor but Not the A2AR-D2R Heteroreceptor Complex: Lack of Actions on Rodent Cocaine Self-Administration

It was previously demonstrated that rat adenosine A2AR transmembrane V peptide administration into the nucleus accumbens enhances cocaine self-administration through disruption of the A2AR-dopamine (D2R) heteroreceptor complex of this region. Unlike human A2AR transmembrane 4 (TM4) and 5 (TM5), A2AR TM2 did not interfere with the formation of the A2AR-D2R heteroreceptor complex in cellular models using BRET¹ assay. A2AR TM2 was proposed to be part of the of the receptor interface of the A2AR homomer instead and was therefore tested in the current article for effects on rat cocaine self-administration using rat A2AR synthetic TM2 peptide bilaterally injected into the nucleus accumbens. The injected A2AR TM2 peptide failed to significantly counteract the inhibitory action of the A2AR agonist CGS 21680 (0.1 mg/Kg) on cocaine self-administration. In line with these results, the microinjected A2AR TM2 peptide did not reduce the number of proximity ligation assay blobs identifying A2AR-D2R heteroreceptor complexes in the nucleus accumbens. In contrast, the A2AR TM2 peptide significantly reduced the number of A2AR-A2AR homoreceptor complexes in the nucleus accumbens. As to effects on the receptor–receptor interactions in the A2AR-D2R heteroreceptor complexes, the A2AR TM2 peptide did not alter the significant increase in the D2R Ki, high values produced by the A2AR agonist CGS 21680 ex vivo in the ventral striatum. The results indicate that the accumbal A2AR-A2AR homomeric complexes are not involved in mediating the A2AR agonist-induced inhibition of cocaine self-administration.

Oligomeric Receptor Complexes and Their Allosteric Receptor-Receptor Interactions in the Plasma Membrane Represent a New Biological Principle for Integration of Signals in the CNS

G protein-coupled receptors (GPCRs) not only exist as monomers but also as homomers and heteromers in which allosteric receptor-receptor interactions take place, modulating the functions of the participating GPCR protomers. GPCRs can also form heteroreceptor complexes with ionotropic receptors and receptor tyrosine kinases modulating their function. Furthermore, adaptor proteins interact with receptor protomers and modulate their interactions. The state of the art is that the allosteric receptor-receptor interactions are reciprocal, highly dynamic and substantially alter the signaling, trafficking, recognition and pharmacology of the participating protomers. The pattern of changes appears to be unique for each heteromer and can favor antagonistic or facilitatory interactions or switch the G protein coupling from e.g., Gi/o to Gq or to beta-arrestin signaling. It lends a new dimension to molecular integration in the nervous system. Future direction should be aimed at determining the receptor interface involving building models of selected heterodimers. This will make design of interface-interfering peptides that specifically disrupt the heterodimer possible. This will help to determine the functional role of the allosteric receptor-receptor interactions as well as the integration of signals at the plasma membrane by the heteroreceptor complexes, vs. integration of the intracellular signaling pathways. Integration of signals also at the plasma membrane seems crucial in view of the hypothesis that learning and memory at a molecular level takes place by reorganization of homo and heteroreceptor complexes in the postsynaptic membrane. Homo and heteroreceptor complexes are in balance with each other, and their disbalance is linked to disease. Targeting heteroreceptor complexes represents a novel strategy for the treatment of brain disorders.

Functional and Neuroprotective Role of Striatal Adenosine A2A Receptor Heterotetramers

In the striatum, adenosine A_2A receptors (A_2AR) are mainly expressed within the soma and dendrites of the striatopallidal neuron. A predominant proportion of these striatal postsynaptic A_2AR form part of the macromolecular complexes that include A_2AR-dopamine D₂ receptor (D₂R) heteromers, G_olf and G_i/o proteins, and the effector adenylyl cyclase (AC), subtype AC5. The A_2AR-D₂R heteromers have a tetrameric structure, constituted by A_2AR and D₂R homomers. By means of reciprocal antagonistic allosteric interactions and antagonistic interactions at the effector level between adenosine and dopamine, the A_2AR-D₂R heterotetramer-AC5 complex acts an integrative molecular device, which determines a switch between the adenosine-facilitated activation and the dopamine-facilitated inhibition of the striatopallidal neuron. Striatal adenosine also plays an important presynaptic modulatory role, driving the function of corticostriatal terminals. This control is mediated by adenosine A₁ receptors (A₁R) and A_2AR, which establish intermolecular interactions forming A₁R-A_2AR heterotetramers. Here, we review the functional role of both presynaptic and postsynaptic striatal A_2AR heterotetramers as well as their possible neuroprotective role. We hypothesize that alterations in the homomer/heteromer stoichiometry (i.e., increase or decrease in the proportion of A_2AR forming homomers or heteromers) are pathogenetically involved in neurological disorders, specifically in Parkinson's disease and restless legs syndrome.

Conventional and Novel Pharmacological Approaches to Treat Dopamine-Related Disorders: Focus on Parkinson's Disease and Schizophrenia

The dopaminergic system integrated by cell groups distributed in several brain regions exerts a modulatory role in brain. Particularly important for this task are the mesencephalic dopamine neurons, which from the substantia nigra and ventral tegmental area project to the dorsal striatum and the cortical/subcortical limbic systems, respectively. Dopamine released from these neurons operates mainly via the short distance extrasynaptic volume transmission and activates five different dopaminergic receptor subtypes modulating synaptic GABA and glutamate transmission. To accomplish this task dopaminergic neurons keep mutual modulating interactions with neurons of other neurotransmitter systems, including allosteric receptor-receptor interactions in heteroreceptor complexes. As a result of its modulatory role dopaminergic mechanisms are involved in either the etiology or physiopathology of many brain diseases such as Parkinsońs disease and schizophrenia. The aim of this work is to review some novel and conventional approaches that either have been used or are currently employed to treat these diseases. Particular attention is paid to the approaches derived from the knowledge recently acquired in the realm of receptor-receptor interactions taking place through multiple dopamine heteroreceptor complexes in the plasma membrane. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Adenosine heteroreceptor complexes in the basal ganglia are implicated in Parkinson's disease and its treatment

The adenosine homo, iso and heteroreceptor complexes in the basal ganglia play a highly significant role in modulating the indirect and direct pathways and the striosomal projections to the nigro-striatal DA system. The major adenosine receptor complexes in the striato-pallidal GABA neurons can be the A2AR-D2R and A2AR-D2R-mGluR5 receptor complexes, in which A2AR protomers and mGluR5 protomers can allosterically interact to inhibit D2R protomer signaling. Through a reorganization of these heteroreceptor complexes upon chronic dopaminergic treatment a pathological and prolonged inhibition of D2R receptor protomer signaling can develop with motor inhibition and wearing off of the therapeutic effects of levodopa and dopamine receptor agonists. The direct pathway is enriched in D1R in and around glutamate synapses enhancing the ability of these GABA neurons to be activated and increase motor initiation. The brake on these GABA neurons is in this case exerted by A1R forming A1R-D1R heteroreceptor complexes in which they allosterically inhibit D1R signaling and thereby reduce motor initiation. Upon chronic levodopa treatment a reorganization of the D1R heteroreceptor complexes develops with the formation of putative A1R-D1R-D3 in addition to D1R-D3R complexes in which D3R enhances D1R protomer signaling and may make the A1R protomer brake less effective. Alpha-synuclein monomers-dimers are postulated to form complexes with A2AR homo and heteroprotomers in the plasma membrane enhancing alpha-synuclein aggregation and toxicity. The alpha-synuclein fibrils formed in the A2AR enriched dendritic spines of the striato-pallidal GABA neurons may reach the surrounding DA terminals via extracellular-vesicle-mediated volume transmission involving internalization of the vesicles and their cargo (alpha-synuclein fibrils) into the vulnerable DA terminals, enhancing their degeneration followed by retrograde flow of these fibrils in the DA axons to the vulnerable nigral DA nerve cells.

Therapeutic targeting of HER2-CB2R heteromers in HER2-positive breast cancer

Although human epidermal growth factor receptor 2 (HER2)-targeted therapies have dramatically improved the clinical outcome of HER2-positive breast cancer patients, innate and acquired resistance remains an important clinical challenge. New therapeutic approaches and diagnostic tools for identification, stratification, and treatment of patients at higher risk of resistance and recurrence are therefore warranted. Here, we unveil a mechanism controlling the oncogenic activity of HER2: heteromerization with the cannabinoid receptor CB₂R. We show that HER2 physically interacts with CB₂R in breast cancer cells, and that the expression of these heteromers correlates with poor patient prognosis. The cannabinoid Δ⁹-tetrahydrocannabinol (THC) disrupts HER2-CB₂R complexes by selectively binding to CB₂R, which leads to (i) the inactivation of HER2 through disruption of HER2-HER2 homodimers, and (ii) the subsequent degradation of HER2 by the proteasome via the E3 ligase c-CBL. This in turn triggers antitumor responses in vitro and in vivo. Selective targeting of CB₂R transmembrane region 5 mimicked THC effects. Together, these findings define HER2-CB₂R heteromers as new potential targets for antitumor therapies and biomarkers with prognostic value in HER2-positive breast cancer.

Pharmacology of Adenosine Receptors: The State of the Art

Adenosine is a ubiquitous endogenous autacoid whose effects are triggered through the enrollment of four G protein-coupled receptors: A₁, A_2A, A_2B, and A₃. Due to the rapid generation of adenosine from cellular metabolism, and the widespread distribution of its receptor subtypes in almost all organs and tissues, this nucleoside induces a multitude of physiopathological effects, regulating central nervous, cardiovascular, peripheral, and immune systems. It is becoming clear that the expression patterns of adenosine receptors vary among cell types, lending weight to the idea that they may be both markers of pathologies and useful targets for novel drugs. This review offers an overview of current knowledge on adenosine receptors, including their characteristic structural features, molecular interactions and cellular functions, as well as their essential roles in pain, cancer, and neurodegenerative, inflammatory, and autoimmune diseases. Finally, we highlight the latest findings on molecules capable of targeting adenosine receptors and report which stage of drug development they have reached.

A2AR-D2R Heteroreceptor Complexes in Cocaine Reward and Addiction

The concept of allosteric receptor-receptor interactions in G protein-coupled receptor homo- and heteroreceptor complexes in which they physically interact provides a new dimension to molecular integration in the brain. The receptor-receptor interactions dynamically change recognition, pharmacology, signaling, and trafficking of the participating receptors. Among the receptor complexes, disruption of the A2A receptor-dopamine D2 receptor (A2AR-D2R) complex by an A2AR agonist has been shown to fully block the inhibition of cocaine self-administration. Cocaine induced pathological A2AR-D2R-Sigma1R complexes may form a long-term memory with a strong and permanent D2R brake, leading to cocaine addiction. These heteroreceptor complexes can potentially be targeted for future pharmacotherapy of cocaine addiction by using heterobivalent compounds or A2AR-D2R receptor interface-interfering peptides that disrupt the A2AR-D2R-Sigma1R complexes.

G protein-coupled receptor heteromers are key players in substance use disorder

G protein-coupled receptors (GPCR) represent the largest family of membrane proteins in the human genome. Physical association between two different GPCRs is linked to functional interactions which generates a novel entity, called heteromer, with specific ligand binding and signaling properties. Heteromerization is increasingly recognized to take place in the mesocorticolimbic pathway and to contribute to various aspects related to substance use disorder. This review focuses on heteromers identified in brain areas relevant to drug addiction. We report changes at the molecular and cellular levels that establish specific functional impact and highlight behavioral outcome in preclinical models. Finally, we briefly discuss selective targeting of native heteromers as an innovative therapeutic option.

Mapping the Interface of a GPCR Dimer: A Structural Model of the A2A Adenosine and D2 Dopamine Receptor Heteromer

The A_2A adenosine (A_2AR) and D₂ dopamine (D₂R) receptors form oligomers in the cell membrane and allosteric interactions across the A_2AR–D₂R heteromer represent a target for development of drugs against central nervous system disorders. However, understanding of the molecular determinants of A_2AR–D₂R heteromerization and the allosteric antagonistic interactions between the receptor protomers is still limited. In this work, a structural model of the A_2AR–D₂R heterodimer was generated using a combined experimental and computational approach. Regions involved in the heteromer interface were modeled based on the effects of peptides derived from the transmembrane (TM) helices on A_2AR–D₂R receptor–receptor interactions in bioluminescence resonance energy transfer (BRET) and proximity ligation assays. Peptides corresponding to TM-IV and TM-V of the A_2AR blocked heterodimer interactions and disrupted the allosteric effect of A_2AR activation on D₂R agonist binding. Protein–protein docking was used to construct a model of the A_2AR–D₂R heterodimer with a TM-IV/V interface, which was refined using molecular dynamics simulations. Mutations in the predicted interface reduced A_2AR–D₂R interactions in BRET experiments and altered the allosteric modulation. The heterodimer model provided insights into the structural basis of allosteric modulation and the technique developed to characterize the A_2AR–D₂R interface can be extended to study the many other G protein-coupled receptors that engage in heteroreceptor complexes.

Disruption of A2AR-D2R Heteroreceptor Complexes After A2AR Transmembrane 5 Peptide Administration Enhances Cocaine Self-Administration in Rats

Antagonistic allosteric A2AR-D2R receptor-receptor interactions in heteroreceptor complexes counteract cocaine self-administration and cocaine seeking in rats as seen in biochemical and behavioral experiments. It was shown that the human A2AR transmembrane five (TM5) was part of the interface of the human A2AR-D2R receptor heteromer. In the current paper, the rat A2AR synthetic TM5 (synthTM5) peptide disrupts the A2AR-D2R heteroreceptor complex in HEK293 cells as shown by the bioluminescence resonance energy transfer method. Rat A2AR synthTM5 peptide, microinjected into the nucleus accumbens, produced a complete counteraction of the inhibitory effects of the A2AR agonist CGS21680 on cocaine self-administration. It was linked to a disappearance of the accumbal A2AR-D2R heteroreceptor complexes and the A2AR agonist induced inhibition of D2R recognition using proximity ligation assay and biochemical binding techniques. However, possible effects of the A2AR synthTM5 peptide on accumbal A2AR-D3R and A2AR-D4R heteroreceptor complexes remain to be excluded. Evidence is provided that accumbal A2AR-D2R-like heteroreceptor complexes with their antagonistic receptor-receptor interactions can be major targets for treatment of cocaine use disorder.

Brain Dopamine Transmission in Health and Parkinson's Disease: Modulation of Synaptic Transmission and Plasticity Through Volume Transmission and Dopamine Heteroreceptors

This perspective article provides observations supporting the view that nigro-striatal dopamine neurons and meso-limbic dopamine neurons mainly communicate through short distance volume transmission in the um range with dopamine diffusing into extrasynaptic and synaptic regions of glutamate and GABA synapses. Based on this communication it is discussed how volume transmission modulates synaptic glutamate transmission onto the D1R modulated direct and D2R modulated indirect GABA pathways of the dorsal striatum. Each nigro-striatal dopamine neuron was first calculated to form large numbers of neostriatal DA nerve terminals and then found to give rise to dense axonal arborizations spread over the neostriatum, from which dopamine is released. These neurons can through DA volume transmission directly influence not only the striatal GABA projection neurons but all the striatal cell types in parallel. It includes the GABA nerve cells forming the island-/striosome GABA pathway to the nigral dopamine cells, the striatal cholinergic interneurons and the striatal GABA interneurons. The dopamine modulation of the different striatal nerve cell types involves the five dopamine receptor subtypes, D1R to D5R receptors, and their formation of multiple extrasynaptic and synaptic dopamine homo and heteroreceptor complexes. These features of the nigro-striatal dopamine neuron to modulate in parallel the activity of practically all the striatal nerve cell types in the dorsal striatum, through the dopamine receptor complexes allows us to understand its unique and crucial fine-tuning of movements, which is lost in Parkinson's disease. Integration of striatal dopamine signals with other transmitter systems in the striatum mainly takes place via the receptor-receptor interactions in dopamine heteroreceptor complexes. Such molecular events also participate in the integration of volume transmission and synaptic transmission. Dopamine modulation of the glutamate synapses on the dorsal striato-pallidal GABA pathway involves D2R heteroreceptor complexes such as D2R-NMDAR, A2AR-D2R, and NTSR1-D2R heteroreceptor complexes. The dopamine modulation of glutamate synapses on the striato-entopeduncular/nigral pathway takes place mainly via D1R heteroreceptor complexes such as D1R-NMDAR, A2R-D1R, and D1R-D3R heteroreceptor complexes. Dopamine modulation of the island/striosome compartment of the dorsal striatum projecting to the nigral dopamine cells involve D4R-MOR heteroreceptor complexes. All these receptor-receptor interactions have relevance for Parkinson's disease and its treatment.

Receptor⁻Receptor Interactions in Multiple 5-HT1A Heteroreceptor Complexes in Raphe-Hippocampal 5-HT Transmission and Their Relevance for Depression and Its Treatment

Due to the binding to a number of proteins to the receptor protomers in receptor heteromers in the brain, the term "heteroreceptor complexes" was introduced. A number of serotonin 5-HT1A heteroreceptor complexes were recently found to be linked to the ascending 5-HT pathways known to have a significant role in depression. The 5-HT1A⁻FGFR1 heteroreceptor complexes were involved in synergistically enhancing neuroplasticity in the hippocampus and in the dorsal raphe 5-HT nerve cells. The 5-HT1A protomer significantly increased FGFR1 protomer signaling in wild-type rats. Disturbances in the 5-HT1A⁻FGFR1 heteroreceptor complexes in the raphe-hippocampal 5-HT system were found in a genetic rat model of depression (Flinders sensitive line (FSL) rats). Deficits in FSL rats were observed in the ability of combined FGFR1 and 5-HT1A agonist cotreatment to produce antidepressant-like effects. It may in part reflect a failure of FGFR1 treatment to uncouple the 5-HT1A postjunctional receptors and autoreceptors from the hippocampal and dorsal raphe GIRK channels, respectively. This may result in maintained inhibition of hippocampal pyramidal nerve cell and dorsal raphe 5-HT nerve cell firing. Also, 5-HT1A⁻5-HT2A isoreceptor complexes were recently demonstrated to exist in the hippocampus and limbic cortex. They may play a role in depression through an ability of 5-HT2A protomer signaling to inhibit the 5-HT1A protomer recognition and signaling. Finally, galanin (1⁻15) was reported to enhance the antidepressant effects of fluoxetine through the putative formation of GalR1⁻GalR2⁻5-HT1A heteroreceptor complexes. Taken together, these novel 5-HT1A receptor complexes offer new targets for treatment of depression.

Glutamate heteroreceptor complexes in the brain

The existence of mGluR, NMDAR, AMPAR and putative KAR heteroreceptor complexes in synaptic and extrasynaptic regions of brain glutamate synapses represents a major integrative mechanism. Our aim in the current article is to analyze if the formation of the different types glutamate hetereceptor complexes involves the contribution of triplet amino acid homologies (protriplets) in a postulated receptor interface based on the triplet puzzle theory. Seven main sets (lists) of receptor pairs in databases were used containing various sets (lists) of human receptor heteromers and nonheteromers obtained from the available scientific publications including the publically available GPCR-hetnet database. Brain mGluR1-mGluR5 and mGluR2-mGluR4 isoreceptor complexes were demonstrated with a predominant extrasynaptic localization at a post- and prejunctional localization. The existence of putative mGluR4-mGluR7 heteroreceptor complexes in the basal ganglia is proposed. Metabotropic glutamate receptor subtypes also participated in the formation of a large number of heteroreceptor complexes like mGluR1-A1R, mGluR5-A2AR, mGluR5-D2R and D2R-A2AR-mGluR5, located in relation to glutamate synapses, especially in the basal ganglia. A putative mGluR1-GABAB1/2 heterocomplex may also exist. NMDAR heteroreceptor complexes were also demonstrated as a fundamental integrative mechanism in the glutamate synapse and its extrasynaptic membranes. It represented fundamental work on inter alia NMDAR-mGluR5, NMDAR-D1R and NMDAR-D2R heteroreceptor complexes involving both antagonistic and facilitatory allosteric receptor-receptor interactions. As to AMPA receptors, a heterocomplex was found for the interaction between IFNgR1 and the AMPAR mediated via the subunit GluA1 which may be of relevance for neuroinflammation. AMPAR-D2R heteroreceptor complexes were also demonstrated. Besides glutamate heteroreceptor complexes and their allosteric receptor-receptor interactions, a significant mechanism for the functional crosstalk can also be phosphorylation and/or reorganization of adapter proteins with dynamic binding to the two receptors modulating the allosteric receptor mechanism.

G protein-coupled receptor-receptor interactions give integrative dynamics to intercellular communication

The proposal of receptor-receptor interactions (RRIs) in the early 1980s broadened the view on the role of G protein-coupled receptors (GPCR) in the dynamics of the intercellular communication. RRIs, indeed, allow GPCR to operate not only as monomers but also as receptor complexes, in which the integration of the incoming signals depends on the number, spatial arrangement, and order of activation of the protomers forming the complex. The main biochemical mechanisms controlling the functional interplay of GPCR in the receptor complexes are direct allosteric interactions between protomer domains. The formation of these macromolecular assemblies has several physiologic implications in terms of the modulation of the signaling pathways and interaction with other membrane proteins. It also impacts on the emerging field of connectomics, as it contributes to set and tune the synaptic strength. Furthermore, recent evidence suggests that the transfer of GPCR and GPCR complexes between cells via the exosome pathway could enable the target cells to recognize/decode transmitters and/or modulators for which they did not express the pertinent receptors. Thus, this process may also open the possibility of a new type of redeployment of neural circuits. The fundamental aspects of GPCR complex formation and function are the focus of the present review article.

Capturing the multiscale dynamics of membrane protein complexes with all-atom, mixed-resolution, and coarse-grained models

The structures and dynamics of protein complexes are often challenging to model in heterogeneous environments such as biological membranes. Herein, we meet this fundamental challenge at attainable cost with all-atom, mixed-resolution, and coarse-grained models of vital membrane proteins. We systematically simulated five complex models formed by two distinct G protein-coupled receptors (GPCRs) in the lipid-bilayer membrane on the ns-to-μs timescales. These models, which suggest the swinging motion of an intracellular loop, for the first time, provide the molecular details for the regulatory role of such a loop. For the models at different resolutions, we observed consistent structural stability but various levels of speed-ups in protein dynamics. The mixed-resolution and coarse-grained models show two and four times faster protein diffusion than the all-atom models, in addition to a 4- and 400-fold speed-up in the simulation performance. Furthermore, by elucidating the strengths and challenges of combining all-atom models with reduced resolution models, this study can serve as a guide to simulating other complex systems in heterogeneous environments efficiently.

Adenosine A2A receptor ligand recognition and signaling is blocked by A2B receptors

The adenosine receptor (AR) subtypes A_2A and A_2B are rhodopsin-like G_s protein-coupled receptors whose expression is highly regulated under pathological, e.g. hypoxic, ischemic and inflammatory conditions. Both receptors play important roles in inflammatory and neurodegenerative diseases, are blocked by caffeine, and have now become major drug targets in immuno-oncology. By Förster resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET), bimolecular fluorescence complementation (BiFC) and proximity ligation assays (PLA) we demonstrated A_2A-A_2BAR heteromeric complex formation. Moreover we observed a dramatically altered pharmacology of the A_2AAR when co-expressed with the A_2BAR (A_2B ≥ A_2A) in recombinant as well as in native cells. In the presence of A_2BARs, A_2A-selective ligands lost high affinity binding to A_2AARs and displayed strongly reduced potency in cAMP accumulation and dynamic mass redistribution (DMR) assays. These results have major implications for the use of A_2AAR ligands as drugs as they will fail to modulate the receptor in an A_2A-A_2B heteromer context. Accordingly, A_2A-A_2BAR heteromers represent novel pharmacological targets.

Understanding the Role of Adenosine A2AR Heteroreceptor Complexes in Neurodegeneration and Neuroinflammation

Adenosine is a nucleoside mainly formed by degradation of ATP, located intracellularly or extracellularly, and acts as a neuromodulator. It operates as a volume transmission signal through diffusion and flow in the extracellular space to modulate the activity of both glial cells and neurons. The effects of adenosine are mediated via four adenosine receptor subtypes: A1R, A2AR, A2BR, A3R. The A2AR has a wide-spread distribution but it is especially enriched in the ventral and dorsal striatum where it is mainly located in the striato-pallidal GABA neurons at a synaptic and extrasynaptic location. A number of A2AR heteroreceptor complexes exist in the striatum. The existence of A2AR-D2R heteroreceptor complexes with antagonistic A2AR-D2R interactions in the striato-pallidal GABA neurons is well-known with A2AR activation inhibiting Gi/o mediated signaling of D2Rs. A2AR-mGluR5 heteroreceptor complexes were also found in with synergistic receptor-receptor interactions enhancing the inhibition of the D2R protomer signaling. They are located mainly in extrasynaptic regions of the striato-pallidal GABA neurons. Results recently demonstrated the existence of brain A2AR-A2BR heteroreceptor complexes, in which A2BR protomer constitutively inhibited the function of the A2AR protomer. These adenosine A2AR heteroreceptor complexes may modulate alpha-synuclein aggregation and toxicity through postulated bidirectional direct interactions leading to marked increases in A2AR signaling both in nerve cells and microglia. It is of high interest that formation of A2AR-A2ABR heteroreceptor complexes provides a brake on A2AR recognition and signaling opening up a novel strategy for treatment of A2AR mediated neurodegeneration.

Neurotransmission systems in Parkinson’s disease

Parkinson’s disease (PD) is histologically characterized by the accumulation of α-synuclein particles, known as Lewy bodies. The second most common neurodegenerative disorder, PD is widely known because of the typical motor manifestations of active tremor, rigidity, and postural instability, while several prodromal non-motor symptoms including REM sleep behavior disorders, depression, autonomic disturbances, and cognitive decline are being more extensively recognized. Motor symptoms most commonly arise from synucleinopathy of nigrostriatal pathway. Glutamatergic, γ-aminobutyric acid (GABA)ergic, cholinergic, serotoninergic, and endocannabinoid neurotransmission systems are not spared from the global cerebral neurodegenerative assault. Wide intrabasal and extrabasal of the basal ganglia provide enough justification to evaluate network circuits disturbance of these neurotransmission systems in PD. In this comprehensive review, English literature in PubMed, Science direct, EMBASE, and Web of Science databases were perused. Characteristics of dopaminergic and non-dopaminergic systems, disturbance of these neurotransmitter systems in the pathophysiology of PD, and their treatment applications are discussed.

Pathological overproduction: the bad side of adenosine

Adenosine is an endogenous ubiquitous purine nucleoside, which is increased by hypoxia, ischaemia and tissue damage and mediates a number of physiopathological effects by interacting with four GPCRs, identified as A1 , A2A , A2B and A3 . Physiological and acutely increased adenosine is mostly associated with beneficial effects that include vasodilatation and a decrease in inflammation. In contrast, chronic overproduction of adenosine occurs in important pathological states, where long-lasting increases in the nucleoside levels are responsible for the bad side of adenosine associated with chronic inflammation, fibrosis and organ damage. In this review, we describe and critically discuss the pathological overproduction of adenosine and analyse when, where and how adenosine exerts its detrimental effects throughout the body.

A Molecular Basis for Selective Antagonist Destabilization of Dopamine D3 Receptor Quaternary Organization

The dopamine D3 receptor (D3R) is a molecular target for both first-generation and several recently-developed antipsychotic agents. Following stable expression of this mEGFP-tagged receptor, Spatial Intensity Distribution Analysis indicated that a substantial proportion of the receptor was present within dimeric/oligomeric complexes and that increased expression levels of the receptor favored a greater dimer to monomer ratio. Addition of the antipsychotics, spiperone or haloperidol, resulted in re-organization of D3R quaternary structure to promote monomerization. This action was dependent on ligand concentration and reversed upon drug washout. By contrast, a number of other antagonists with high affinity at the D3R, did not alter the dimer/monomer ratio. Molecular dynamics simulations following docking of each of the ligands into a model of the D3R derived from the available atomic level structure, and comparisons to the receptor in the absence of ligand, were undertaken. They showed that, in contrast to the other antagonists, spiperone and haloperidol respectively increased the atomic distance between reference α carbon atoms of transmembrane domains IV and V and I and II, both of which provide key interfaces for D3R dimerization. These results offer a molecular explanation for the distinctive ability of spiperone and haloperidol to disrupt D3R dimerization.

Angiotensin II type 1/adenosine A 2A receptor oligomers: a novel target for tardive dyskinesia

Tardive dyskinesia (TD) is a serious motor side effect that may appear after long-term treatment with neuroleptics and mostly mediated by dopamine D₂ receptors (D₂Rs). Striatal D₂R functioning may be finely regulated by either adenosine A_2A receptor (A_2AR) or angiotensin receptor type 1 (AT₁R) through putative receptor heteromers. Here, we examined whether A_2AR and AT₁R may oligomerize in the striatum to synergistically modulate dopaminergic transmission. First, by using bioluminescence resonance energy transfer, we demonstrated a physical AT₁R-A_2AR interaction in cultured cells. Interestingly, by protein-protein docking and molecular dynamics simulations, we described that a stable heterotetrameric interaction may exist between AT₁R and A_2AR bound to antagonists (i.e. losartan and istradefylline, respectively). Accordingly, we subsequently ascertained the existence of AT₁R/A_2AR heteromers in the striatum by proximity ligation in situ assay. Finally, we took advantage of a TD animal model, namely the reserpine-induced vacuous chewing movement (VCM), to evaluate a novel multimodal pharmacological TD treatment approach based on targeting the AT₁R/A_2AR complex. Thus, reserpinized mice were co-treated with sub-effective losartan and istradefylline doses, which prompted a synergistic reduction in VCM. Overall, our results demonstrated the existence of striatal AT₁R/A_2AR oligomers with potential usefulness for the therapeutic management of TD.

Cocaine self-administration specifically increases A2AR-D2R and D2R-sigma1R heteroreceptor complexes in the rat nucleus accumbens shell. Relevance for cocaine use disorder

Adenosine 2A receptor (A2AR) agonists were indicated to reduce cocaine reward and cocaine seeking mainly through activation of antagonistic allosteric A2AR-dopamine D2R (D2R) interactions in A2AR-D2R heteroreceptor complexes. Furthermore, it was shown that modulation of cocaine reward involves antagonistic A2AR-D2R interactions in the ventral but not the dorsal striatum in rats. In the current work the proximity ligation assay (PLA) was used to further study the A2AR-D2R heteroreceptor complexes in the nucleus accumbens shell and core as well as the dorsal striatum under the influence of cocaine self-administration in rats. A significant increase in the A2AR-D2R PLA positive clusters was observed in the nucleus accumbens shell but not in the other regions vs yoked saline controls using the duolink software. Additionally, cocaine self-administration evoked a selective and significant increase in the density of D2R-sigma1R positive clusters in the nucleus accumbens shell vs yoked saline controls, while a significant reduction of the density of the D2R-sigma1R positive clusters was found in the dorsal part of the dorsal striatum. The results suggest that cocaine self-administration can reorganize A2AR and D2R into increased A2AR-D2R heteroreceptor complexes in the nucleus accumbens shell associated with increases in the D2R-sigma1R heteroreceptor complexes in this region. This reorganization can contribute to the demonstrated anti-cocaine actions of A2A receptor agonists and the putative formation of A2AR-D2R-sigma1R heterocomplexes.

Understanding the Role of GPCR Heteroreceptor Complexes in Modulating the Brain Networks in Health and Disease

The introduction of allosteric receptor-receptor interactions in G protein-coupled receptor (GPCR) heteroreceptor complexes of the central nervous system (CNS) gave a new dimension to brain integration and neuropsychopharmacology. The molecular basis of learning and memory was proposed to be based on the reorganization of the homo- and heteroreceptor complexes in the postjunctional membrane of synapses. Long-term memory may be created by the transformation of parts of the heteroreceptor complexes into unique transcription factors which can lead to the formation of specific adapter proteins. The observation of the GPCR heterodimer network (GPCR-HetNet) indicated that the allosteric receptor-receptor interactions dramatically increase GPCR diversity and biased recognition and signaling leading to enhanced specificity in signaling. Dysfunction of the GPCR heteroreceptor complexes can lead to brain disease. The findings of serotonin (5-HT) hetero and isoreceptor complexes in the brain over the last decade give new targets for drug development in major depression. Neuromodulation of neuronal networks in depression via 5-HT, galanin peptides and zinc involve a number of GPCR heteroreceptor complexes in the raphe-hippocampal system: GalR1-5-HT1A, GalR1-5-HT1A-GPR39, GalR1-GalR2, and putative GalR1-GalR2-5-HT1A heteroreceptor complexes. The 5-HT1A receptor protomer remains a receptor enhancing antidepressant actions through its participation in hetero- and homoreceptor complexes listed above in balance with each other. In depression, neuromodulation of neuronal networks in the raphe-hippocampal system and the cortical regions via 5-HT and fibroblast growth factor 2 involves either FGFR1-5-HT1A heteroreceptor complexes or the 5-HT isoreceptor complexes such as 5-HT1A-5-HT7 and 5-HT1A-5-HT2A. Neuromodulation of neuronal networks in cocaine use disorder via dopamine (DA) and adenosine signals involve A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complexes in the dorsal and ventral striatum. The excitatory modulation by A2AR agonists of the ventral striato-pallidal GABA anti-reward system via targeting the A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complex holds high promise as a new way to treat cocaine use disorders. Neuromodulation of neuronal networks in schizophrenia via DA, adenosine, glutamate, 5-HT and neurotensin peptides and oxytocin, involving A2AR-D2R, D2R-NMDAR, A2AR-D2R-mGluR5, D2R-5-HT2A and D2R-oxytocinR heteroreceptor complexes opens up a new world of D2R protomer targets in the listed heterocomplexes for treatment of positive, negative and cognitive symptoms of schizophrenia.

Evidence for the heterotetrameric structure of the adenosine A2A-dopamine D2 receptor complex

Heteromers of G-protein-coupled receptors (GPCRs) have emerged as potential novel targets for drug development. Accumulating evidence indicates that GPCRs can form homodimers and heteromers, with homodimers being the predominant species and oligomeric receptors being formed as multiples of dimers. Recently, heterotetrameric structures have been proposed for dopamine D1receptor (D1R)-dopamine D3receptor (D3R) and adenosine A2Areceptor (A2AR)-dopamine D2receptor (D2R) heteromers. The structural model proposed for these complexes is a heteromer constituted by two receptor homodimers. The existence of GPCR homodimers and heteromers provides a structural basis for inter-protomer allosteric mechanisms that might account for a multiplicity of unique pharmacological properties. In this review, we focus on the A2AR-D2R heterotetramer as an example of an oligomeric structure that is key in the modulation of striatal neuronal function. We also review the interfaces involved in this and other recently reported heteromers of GPCRs. Furthermore, we discuss several published studies showing theex vivoexpression of A2AR-D2R heteromers. The ability of A2AR agonists to decrease the affinity of D2R agonists has been reported and, on the basis of this interaction, A2AR antagonists have been proposed as potential drugs for the treatment of Parkinson's disease. The heterotetrameric structure of the A2AR-D2R complex offers a novel model that can provide new clues about how to adjust the drug dosage to the expected levels of endogenous adenosine.

A2A-D2 receptor-receptor interaction modulates gliotransmitter release from striatal astrocyte processes

Evidence for striatal A2A-D2 heterodimers has led to a new perspective on molecular mechanisms involved in schizophrenia and Parkinson's disease. Despite the increasing recognition of astrocytes' participation in neuropsychiatric disease vulnerability, involvement of striatal astrocytes in A2A and D2 receptor signal transmission has never been explored. Here, we investigated the presence of D2 and A2A receptors in isolated astrocyte processes prepared from adult rat striatum by confocal imaging; the effects of receptor activation were measured on the 4-aminopyridine-evoked release of glutamate from the processes. Confocal analysis showed that A2A and D2 receptors were co-expressed on the same astrocyte processes. Evidence for A2A-D2 receptor-receptor interactions was obtained by measuring the release of the gliotransmitter glutamate: D2 receptors inhibited the glutamate release, while activation of A2A receptors, per se ineffective, abolished the effect of D2 receptor activation. The synthetic D2 peptide VLRRRRKRVN corresponding to the receptor region involved in electrostatic interaction underlying A2A-D2 heteromerization abolished the ability of the A2A receptor to antagonize the D2 receptor-mediated effect. Together, the findings are consistent with heteromerization of native striatal astrocytic A2A-D2 receptors that via allosteric receptor-receptor interactions could play a role in the control of striatal glutamatergic transmission. These new findings suggest possible new pathogenic mechanisms and/or therapeutic approaches to neuropsychiatric disorders.

Understanding the Functional Plasticity in Neural Networks of the Basal Ganglia in Cocaine Use Disorder: A Role for Allosteric Receptor-Receptor Interactions in A2A-D2 Heteroreceptor Complexes

Our hypothesis is that allosteric receptor-receptor interactions in homo- and heteroreceptor complexes may form the molecular basis of learning and memory. This principle is illustrated by showing how cocaine abuse can alter the adenosine A2AR-dopamine D2R heterocomplexes and their receptor-receptor interactions and hereby induce neural plasticity in the basal ganglia. Studies with A2AR ligands using cocaine self-administration procedures indicate that antagonistic allosteric A2AR-D2R heterocomplexes of the ventral striatopallidal GABA antireward pathway play a significant role in reducing cocaine induced reward, motivation, and cocaine seeking. Anticocaine actions of A2AR agonists can also be produced at A2AR homocomplexes in these antireward neurons, actions in which are independent of D2R signaling. At the A2AR-D2R heterocomplex, they are dependent on the strength of the antagonistic allosteric A2AR-D2R interaction and the number of A2AR-D2R and A2AR-D2R-sigma1R heterocomplexes present in the ventral striatopallidal GABA neurons. It involves a differential cocaine-induced increase in sigma1Rs in the ventral versus the dorsal striatum. In contrast, the allosteric brake on the D2R protomer signaling in the A2AR-D2R heterocomplex of the dorsal striatopallidal GABA neurons is lost upon cocaine self-administration. This is potentially due to differences in composition and allosteric plasticity of these complexes versus those in the ventral striatopallidal neurons.

Neurochemical evidence supporting dopamine D1-D2 receptor heteromers in the striatum of the long-tailed macaque: changes following dopaminergic manipulation

Although it has long been widely accepted that dopamine receptor types D1 and D2 form GPCR heteromers in the striatum, the presence of D1–D2 receptor heteromers has been recently challenged. In an attempt to properly characterize D1–D2 receptor heteromers, here we have used the in situ proximity ligation assay (PLA) in striatal sections comprising the caudate nucleus, the putamen and the core and shell territories of the nucleus accumbens. Experiments were carried out in control macaques as well as in MPTP-treated animals (with and without dyskinesia). Obtained data support the presence of D1–D2 receptor heteromers within all the striatal subdivisions, with the highest abundance in the accumbens shell. Dopamine depletion by MPTP resulted in an increase of D1–D2 density in caudate and putamen which was normalized by levodopa treatment. Two different sizes of heteromers were consistently found, thus suggesting that besides individual heteromers, D1–D2 receptor heteromers are sometimes organized in macromolecular complexes made of a number of D1–D2 heteromers. Furthermore, the PLA technique was combined with different neuronal markers to properly characterize the identities of striatal neurons expressing D1–D2 heteromers. We have found that striatal projection neurons giving rise to either the direct or the indirect basal ganglia pathways expressed D1–D2 heteromers. Interestingly, macromolecular complexes of D1–D2 heteromers were only found within cholinergic interneurons. In summary, here we provide overwhelming proof that D1 and D2 receptors form heteromeric complexes in the macaque striatum, thus representing a very appealing target for a number of brain diseases involving dopamine dysfunction.

Heterodimerization of the kappa opioid receptor and neurotensin receptor 1 contributes to a novel β-arrestin-2-biased pathway

Together with its endogenous ligands (dynorphin), the kappa opioid receptor (KOR) plays an important role in modulating various physiological and pharmacological responses, with a classical G protein-coupled pathway mediating analgesia and non-G protein-dependent pathway, especially the β-arrestin-dependent pathway, eliciting side effects of dysphoria, aversion, drug-seeking in addicts, or even relapse to addiction. Although mounting evidence has verified a functional overlap between dynorphin/KOR and neurotensin/neurotensin receptor 1 (NTSR1) systems, little is known about direct interaction between the two receptors. Here, we showed that KOR and NTSR1 form a heterodimer that functions as a novel pharmacological entity, and this heterodimer, in turn, brings about a switch in KOR-mediated signal transduction, from G protein-dependent to β-arrestin-2-dependent. This was simultaneously verified by analyzing a KOR mutant (196th residue) that lost the ability to dimerize with NTSR1. We also found that dual occupancy of the heterodimer forced the β-arrestin-2-dependent pathway back into G_i protein-dependent signaling, according to KOR activation. These data provide new insights into the interaction between KOR and NTSR1, and the newly discovered role of NTSR1 acting as a switch between G protein- and β-arrestin-dependent pathways of KOR also suggests a new approach for utilizing pathologically elevated dynorphin/KOR system into full play for its analgesic effect with limited side effects.

Multiple D2 heteroreceptor complexes: new targets for treatment of schizophrenia

The dopamine (DA) neuron system most relevant for schizophrenia is the meso-limbic-cortical DA system inter alia densely innervating subcortical limbic regions. The field of dopamine D2 receptors and schizophrenia changed markedly with the discovery of many types of D2 heteroreceptor complexes in subcortical limbic areas as well as the dorsal striatum. The results indicate that the D2 is a hub receptor which interacts not only with many other G protein-coupled receptors (GPCRs) including DA isoreceptors but also with ion-channel receptors, receptor tyrosine kinases, scaffolding proteins and DA transporters. Disturbances in several of these D2 heteroreceptor complexes may contribute to the development of schizophrenia through changes in the balance of diverse D2 homo- and heteroreceptor complexes mediating the DA signal, especially to the ventral striato-pallidal γ-aminobutyric acid (GABA) pathway. This will have consequences for the control of this pathway of the glutamate drive to the prefrontal cortex via the mediodorsal thalamic nucleus which can contribute to psychotic processes. Agonist activation of the A2A protomer in the A2A-D2 heteroreceptor complex inhibits D2 Gi/o mediated signaling but increases the D2 β-arrestin2 mediated signaling. Through this allosteric receptor-receptor interaction, the A2A agonist becomes a biased inhibitory modulator of the Gi/o mediated D2 signaling, which may the main mechanism for its atypical antipsychotic properties especially linked to the limbic A2A-D2 heterocomplexes. The DA and glutamate hypotheses of schizophrenia come together in the signal integration in D2-N-methyl-d-aspartate (NMDA) and A2A-D2-metabotropic glutamate receptor 5 (mGlu5) heteroreceptor complexes, especially in the ventral striatum. 5-Hydroxytryptamine 2A (5-HT2A)-D2 heteroreceptor complexes are special targets for atypical antipsychotics with high potency to block their 5-HT2A protomer signaling in view of the potential development of pathological allosteric facilitatory 5-HT2A-D2 interaction increasing D2 protomer signaling. Neurotensin (NTS1)-D2 heterocomplexes also exist in the ventral and dorsal striatum, and likely also in midbrain DA nerve cells as NTS1-D2 autoreceptor complexes where neurotensin produces antipsychotic and propsychotic actions, respectively.

The role of transmitter diffusion and flow versus extracellular vesicles in volume transmission in the brain neural-glial networks

Two major types of intercellular communication are found in the central nervous system (CNS), namely wiring transmission (point-to-point communication, the prototype being synaptic transmission with axons and terminals) and volume transmission (VT; communication in the extracellular fluid and in the cerebrospinal fluid (CSF)) involving large numbers of cells in the CNS. Volume and synaptic transmission become integrated inter alia through the ability of their chemical signals to activate different types of receptor protomers in heteroreceptor complexes located synaptically or extrasynaptically in the plasma membrane. The demonstration of extracellular dopamine (DA) and serotonin (5-HT) fluorescence around the DA and 5-HT nerve cell bodies with the Falck-Hillarp formaldehyde fluorescence method after treatment with amphetamine and chlorimipramine, respectively, gave the first indications of the existence of VT in the brain, at least at the soma level. There exist different forms of VT. Early studies on VT only involved spread including diffusion and flow of soluble biological signals, especially transmitters and modulators, a communication called extrasynaptic (short distance) and long distance (paraaxonal and paravascular and CSF pathways) VT. Also, the extracellular vesicle type of VT was demonstrated. The exosomes (endosome-derived vesicles) appear to be the major vesicular carriers for VT but the larger microvesicles also participate. Both mainly originate at the soma-dendritic level. They can transfer lipids and proteins, including receptors, Rab GTPases, tetraspanins, cholesterol, sphingolipids and ceramide. Within them there are also subsets of mRNAs and non-coding regulatory microRNAs. At the soma-dendritic membrane, sets of dynamic postsynaptic heteroreceptor complexes (built up of different types of physically interacting receptors and proteins) involving inter alia G protein-coupled receptors including autoreceptors, ion channel receptors and receptor tyrosine kinases are hypothesized to be the molecular basis for learning and memory. At nerve terminals, the presynaptic heteroreceptor complexes are postulated to undergo plastic changes to maintain the pattern of multiple transmitter release reflecting the firing pattern to be learned by the heteroreceptor complexes in the postsynaptic membrane.

Role of iso-receptors in receptor-receptor interactions with a focus on dopamine iso-receptor complexes

Intercellular and intracellular communication processes consist of signals and recognition/decoding apparatuses of these signals. In humans, the G protein-coupled receptor (GPCR) family represents the largest family of cell surface receptors. More than 30 years ago, it has been proposed that GPCR could form dimers or higher-order oligomers (receptor mosaics [RMs] at the plasma membrane level and receptor-receptor interactions [RRIs] have been proposed as a new integrative mechanism for chemical signals impinging on cell plasma membranes). The basic phenomena involved in RRIs are allostery and cooperativity of membrane receptors, and the present paper provides basic information concerning their relevance for the integrative functions of RMs. In this context, the possible role of iso-receptor RM is discussed (with a special focus on dopamine receptor subtypes and on some of the RMs they form with other dopamine iso-receptors), and it is proposed that two types of cooperativity, namely, homotropic and heterotropic cooperativity, could allow distinguishing two types of functionally different RMs. From a general point of view, the presence of iso-receptors and their topological organization within RMs allow the use of a reduced number of signals for the intercellular communication processes, since the target cells can recognize and decode the same signal in different ways. This theoretical aspect is further analyzed here by means of an analogy with artificial information systems. Thus, it is suggested that the ‘multiplexer’ and ‘demultiplexer’ concepts could, at least in part, model the role of RMs formed by iso-receptors in the information handling by the cell.