Parkinson's disease (PD) is a neurodegenerative disorder of unknown etiology. The main treatment of PD consists of medication with dopamine-based drugs, which palliate the symptoms but may produce ...adverse effects after chronic administration. Accordingly, there is a need to develop novel neuroprotective therapies. Several studies suggest that omega-3 polyunsaturated fatty acids (
-3 PUFA) might provide protection against brain damage. Here, we studied several experimental models of PD, using striatal neuronal cultures, striatal slices, and mice, to assess the neuroprotective effects of docosahexaenoic acid (DHA), the main
-3 PUFA in the brain, administered in its triglyceride form (TG-DHA). Hence, we determined the beneficial effects of TG-DHA on neural viability following 6-hydroxydopamine (6-OHDA)-induced neurotoxicity, a well-established PD model. We also implemented a novel mouse behavioral test, the beam walking test, to finely assess mouse motor skills following dopaminergic denervation. This test showed potential as a useful behavioral tool to assess novel PD treatments. Our results indicated that TG-DHA-mediated neuroprotection was independent of the net incorporation of PUFA into the striatum, thus suggesting a tight control of brain lipid homeostasis both in normal and pathological conditions.
In the CNS, an antagonistic interaction has been shown between adenosine A2A and dopamine D2 receptors (A2ARs and D2Rs) that may be relevant both in normal and pathological conditions (i.e., ...Parkinson's disease). Thus, the molecular determinants mediating this receptor–receptor interaction have recently been explored, as the fine tuning of this target (namely the A2AR/D2R oligomer) could possibly improve the treatment of certain CNS diseases. Here, we used a fluorescence resonance energy transfer‐based approach to examine the allosteric modulation of the D2R within the A2AR/D2R oligomer and the dependence of this receptor–receptor interaction on two regions rich in positive charges on intracellular loop 3 of the D2R. Interestingly, we observed a negative allosteric effect of the D2R agonist quinpirole on A2AR ligand binding and activation. However, these allosteric effects were abolished upon mutation of specific arginine residues (217–222 and 267–269) on intracellular loop 3 of the D2R, thus demonstrating a major role of these positively charged residues in mediating the observed receptor–receptor interaction. Overall, these results provide structural insights to better understand the functioning of the A2AR/D2R oligomer in living cells.
In the native A2AR/D2R oligomer (left panel), a bidirectional negative allosteric receptor–receptor interaction may outline the final adenylyl cyclase signaling efficacy by balancing Gαs‐ and Gαi/o‐coupling. When the arginine residues located on the third intracellular loop of the D2R are mutated (D2Rmut; right panel), the negative allosteric receptor–receptor interaction is lost, and a predominant D2R‐mediated adenylyl cyclase inhibition is observed.
Caffeine, the most consumed psychoactive substance worldwide, may have beneficial effects on Parkinson’s disease (PD) therapy. The mechanism by which caffeine contributes to its antiparkinsonian ...effects by acting as either an adenosine A2A receptor (A2AR) neutral antagonist or an inverse agonist is unresolved. Here we show that caffeine is an A2AR inverse agonist in cell-based functional studies and in experimental parkinsonism. Thus, we observed that caffeine triggers a distinct mode, opposite to A2AR agonist, of the receptor’s activation switch leading to suppression of its spontaneous activity. These inverse agonist-related effects were also determined in the striatum of a mouse model of PD, correlating well with increased caffeine-mediated motor effects. Overall, caffeine A2AR inverse agonism may be behind some of the well-known physiological effects of this substance both in health and disease. This information might have a critical mechanistic impact for PD pharmacotherapeutic design.
Parkinson's disease (PD) is a dopaminergic-related pathology in which functioning of the basal ganglia is altered. It has been postulated that a direct receptor-receptor interaction - i.e. of ...dopamine D2 receptor (D2R) with adenosine A2A receptor (A2AR) (forming D2R-A2AR oligomers) - finely regulates this brain area. Accordingly, elucidating whether the pathology prompts changes to these complexes could provide valuable information for the design of new PD therapies. Here, we first resolved a long-standing question concerning whether D2R-A2AR assembly occurs in native tissue: by means of different complementary experimental approaches (i.e. immunoelectron microscopy, proximity ligation assay and TR-FRET), we unambiguously identified native D2R-A2AR oligomers in rat striatum. Subsequently, we determined that, under pathological conditions (i.e. in a rat PD model), D2R-A2AR interaction was impaired. Collectively, these results provide definitive evidence for alteration of native D2R-A2AR oligomers in experimental parkinsonism, thus conferring the rationale for appropriate oligomer-based PD treatments.
Background: Several biophysical techniques have been successfully implemented to detect G protein-coupled receptors (GPCRs) heteromerization. Although these approaches have made it possible to ...ascertain the presence of GPCR heteromers in animal models of disease, no success has been accomplished in pathological human post-mortem brains. The AlphaScreen technology has been consistently used to quantify small analyte accumulation or depletion, bimolecular interactions, and post-translational modifications. The high signal-to-background, dynamic range and sensitivity exhibited by this technology support that it may be suitable to detect GPCR heteromers even under non-optimal conditions. Methods: Here, we describe the development of a new AlphaScreen assay to detect GPCR oligomers in human post-mortem brain. Results: Adenosine A2A-dopamine D2 receptor (A2AR/D2R) heteromer formation was monitored in caudate from healthy and Parkinson’s disease (PD) subjects. The approach was first validated using striatal membranes from wild type and A2AR deficient mice. Secondly, we took advantage of the 6-hydroxydopamine hemiparkinsonian rat model to validate previous results. In addition, finally, A2AR/D2R heteromer formation was assessed in caudate membranes from human post-mortem brains. Importantly, our preliminary results revealed an increase in A2AR/D2R heteromer formation in PD brains. Conclusions: The new AlphaScreen assay allowed assessing GPCR heteromers in human post-mortem brains with high sensitivity.
Membrane levels of docosahexaenoic acid (DHA), an essential omega-3 polyunsaturated fatty acid (ω-3 PUFA), are decreased in common neuropsychiatric disorders. DHA modulates key cell membrane ...properties like fluidity, thereby affecting the behaviour of transmembrane proteins like G protein-coupled receptors (GPCRs). These receptors, which have special relevance for major neuropsychiatric disorders have recently been shown to form dimers or higher order oligomers, and evidence suggests that DHA levels affect GPCR function by modulating oligomerisation. In this study, we assessed the effect of membrane DHA content on the formation of a class of protein complexes with particular relevance for brain disease: adenosine A2A and dopamine D2 receptor oligomers. Using extensive multiscale computer modelling, we find a marked propensity of DHA for interaction with both A2A and D2 receptors, which leads to an increased rate of receptor oligomerisation. Bioluminescence resonance energy transfer (BRET) experiments performed on living cells suggest that this DHA effect on the oligomerisation of A2A and D2 receptors is purely kinetic. This work reveals for the first time that membrane ω-3 PUFAs play a key role in GPCR oligomerisation kinetics, which may have important implications for neuropsychiatric conditions like schizophrenia or Parkinson's disease.
► TM domains IV and V of the D2R play a major role in the A2AR–D2R heteromer interface. ► Allosteric modulation found in A2AR–D2R heteromers is blocked by incubation with D2R TM-IV or TM-V. ► ...Mutation of (D401A/D402A/S374A) reduced A2AR–D2R interaction and lost the ability of antagonistic allosteric modulation over the A2AR–D2R interface. ► Bioinformatic analysis proposes that specific AAR, AQE, and VLS protriplets as an important motive in the A2AR–D2LR heteromer interface. ► Molecular dynamic modeling implies D2LR TM-IV/V interacting with A2AR TM-IV/V and D2LR TM-IV/V interacting with A2AR TM-I/VII.
A single serine point mutation (S374A) in the adenosine A2A receptor (A2AR) C-terminal tail reduces A2AR–D2R heteromerization and prevents its allosteric modulation of the dopamine D2 receptor (D2R). By means of site directed mutagenesis of the A2AR and synthetic transmembrane (TM) α-helix peptides of the D2R we further explored the role of electrostatic interactions and TM helix interactions of the A2AR–D2R heteromer interface. We found evidence that the TM domains IV and V of the D2R play a major role in the A2AR–D2R heteromer interface since the incubation with peptides corresponding to these domains significantly reduced the ability of A2AR and D2R to heteromerize. In addition, the incubation with TM-IV or TM-V blocked the allosteric modulation normally found in A2AR–D2R heteromers. The mutation of two negatively charged aspartates in the A2AR C-terminal tail (D401A/D402A) in combination with the S374A mutation drastically reduced the physical A2AR–D2R interaction and lost the ability of antagonistic allosteric modulation over the A2AR–D2R interface, suggesting further evidence for the existence of an electrostatic interaction between the C-terminal tail of A2AR and the intracellular loop 3 (IL3) of D2R. On the other hand, molecular dynamic model and bioinformatic analysis propose that specific AAR, AQE, and VLS protriplets as an important motive in the A2AR–D2LR heteromer interface together with D2LR TM segments IV/V interacting with A2AR TM-IV/V or TM-I/VII.
While the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last fifteen years, the existence of a multi-receptor complex involving direct receptor–receptor ...interactions, called receptor oligomers, begins to be widely accepted. Eventually, it has been postulated that oligomers constitute a distinct functional form of the GPCRs with essential receptorial features. Also, it has been proven, under certain circumstances, that the GPCR oligomerization phenomenon is crucial for the receptor biosynthesis, maturation, trafficking, plasma membrane diffusion, and pharmacology and signalling. Adenosine receptors are GPCRs that mediate the physiological functions of adenosine and indeed these receptors do also oligomerize. Accordingly, adenosine receptor oligomers may improve the molecular mechanism by which extracellular adenosine signals are transferred to the G proteins in the process of receptor transduction. Importantly, these adenosine receptor-containing oligomers may allow not only the control of the adenosinergic function but also the fine-tuning modulation of other neurotransmitter systems (i.e. dopaminergic and glutamatergic transmission). Overall, we underscore here recent significant developments based on adenosine receptor oligomerization that are essential for acquiring a better understanding of neurotransmission in the central nervous system under normal and pathological conditions. This article is part of a Special Issue entitled: “Adenosine Receptors”.
► Adenosine receptors (AR) form oligomers. ► AR oligomers improve the molecular mechanism by which adenosine activates G proteins. ► AR oligomers synchronize wiring transmission vs volume transmission. ► AR oligomers control the dopamine and glutamate neurotransmission.
We have developed a novel methodology for monitoring the σ1 receptor activation switch in living cells. Our assay uncovered the intrinsic nature of σ1 receptor ligands by recording the ...ligand-mediated conformational changes of this chaperone protein. The change triggered by each ligand correlated well with its ability to attenuate formalin induced nociception in an animal model of pain. This tool may assist in predicting the antinociceptive efficacy of σ1 receptor ligands.
Voltage sensitivity has been demonstrated for some GPCRs. At the dopamine D(2S) receptor, this voltage sensitivity is agonist-specific; some agonists, including dopamine, exhibit decreased potency at ...depolarized potentials, whereas others are not significantly affected. In the present study, we examined some of the receptor-agonist interactions contributing to these differences, and investigated how dopamine D(2S) receptor voltage sensitivity affects clinically used dopamine agonists. GIRK channel activation in voltage-clamped Xenopus oocytes was used as readout of receptor activation. Structurally distinct agonists and complementary site-directed mutagenesis of the receptor's binding site were used to investigate the role of agonist-receptor interactions. We also confirmed that the depolarization-induced decrease of dopamine potency in GIRK activation is correlated by decreased binding of radiolabeled dopamine, and by decreased potency in G protein activation. In the mutagenesis experiments, a conserved serine residue as well as the conserved aspartate in the receptor's binding site were found to be important for voltage sensitive potency of dopamine. Furthermore, the voltage sensitivity of the receptor had distinct effects on different therapeutic D(2) agonists. Depolarization decreased the potency of several compounds, whereas for others, efficacy was reduced. For some agonists, both potency and efficacy were diminished, whereas for others still, neither parameter was significantly altered. The present work identifies some of the ligand-receptor interactions which determine agonist-specific effects of voltage at the dopamine D(2S) receptor. The observed differences between therapeutic agonists might be clinically relevant, and make them potential tools for investigating the roles of dopamine D(2) receptor voltage sensitivity in native tissue.
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