Jaume Taura

Untangling dopamine-adenosine receptor-receptor assembly in experimental parkinsonism in rats

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.

Uncovering caffeine's adenosine a2A receptor inverse agonism in experimental parkinsonism

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.

Ras-association domain of sorting Nexin 27 is critical for regulating expression of GIRK potassium channels.

G protein-gated inwardly rectifying potassium (GIRK) channels play an important role in regulating neuronal excitability. Sorting nexin 27b (SNX27b), which reduces surface expression of GIRK channels through a PDZ domain interaction, contains a putative Ras-association (RA) domain with unknown function. Deleting the RA domain in SNX27b (SNX27b-ΔRA) prevents the down-regulation of GIRK2c/GIRK3 channels. Similarly, a point mutation (K305A) in the RA domain disrupts regulation of GIRK2c/GIRK3 channels and reduces H-Ras binding in vitro. Finally, the dominant-negative H-Ras (S17N) occludes the SNX27b-dependent decrease in surface expression of GIRK2c/GIRK3 channels. Thus, the presence of a functional RA domain and the interaction with Ras-like G proteins comprise a novel mechanism for modulating SNX27b control of GIRK channel surface expression and cellular excitability.

Photomodulation of G Protein-Coupled Adenosine Receptors by a Novel Light-Switchable Ligand

The adenosinergic system operates through G protein-coupled adenosine receptors, which have become promising therapeutic targets for a wide range of pathological conditions. However, the ubiquity of adenosine receptors and the eventual lack of selectivity of adenosine-based drugs have frequently diminished their therapeutic potential. Accordingly, here we aimed to develop a new generation of light-switchable adenosine receptor ligands that change their intrinsic activity upon irradiation, thus allowing the spatiotemporal control of receptor functioning (i.e., receptor activation/inactivation dependent on location and timing). Therefore, we synthesized an orthosteric, photoisomerizable, and nonselective adenosine receptor agonist, nucleoside derivative MRS5543 containing an aryl diazo linkage on the N6 substituent, which in the dark (relaxed isomer) behaved as a full adenosine A3 receptor (A3R) and partial adenosine A2A receptor (A2AR) agonist. Conversely, upon photoisomerization with blue light (460 nm), it remained a full A3R agonist but became an A2AR antagonist. Interestingly, molecular modeling suggested that structural differences encountered within the third extracellular loop of each receptor could modulate the intrinsic, receptor subtype-dependent, activity. Overall, the development of adenosine receptor ligands with photoswitchable activity expands the pharmacological toolbox in support of research and possibly opens new pharmacotherapeutic opportunities.

Visualizing G Protein‐Coupled Receptor‐Receptor Interactions in Brain Using Proximity Ligation In Situ Assay

G protein‐coupled receptors (GPCRs) constitute the largest family of plasma membrane receptors, thus representing the more investigated drug targets in the design of new therapeutic strategies. The existence of receptor‐receptor interactions has revolutionized the field, since GPCR oligomerization might confer new intervention opportunities in pharmacotherapy. However, demonstrating the existence of such receptor‐receptor interactions in native tissue has been a bottleneck in GPCR pharmacology. Here, we discuss an experimental approach, the proximity ligation in situ assay (P‐LISA), which provides enough sensitivity to evaluate a receptors close proximity within a named GPCR oligomer. Indeed, we provide a detailed step‐by‐step protocol for P‐LISA experiments visualizing receptor‐receptor interactions in brain slices. Additionally, we provide instructions for slide observation, data acquisition and quantification. Finally, we also discuss these critical aspects determining the success of the technique, namely the fixation process and the validation of the primary antibodies used. Overall, the P‐LISA is a powerful and straightforward technique to visualize receptor‐receptor interactions when performed under optimal conditions.

Antipsychotic-Like Efficacy of Dopamine D2 Receptor-Biased Ligands is Dependent on Adenosine A2A Receptor Expression

Dopamine D2 receptor (D2R) activation triggers both G protein- and β-arrestin-dependent signaling. Biased D2R ligands activating β-arrestin pathway have been proposed as potential antipsychotics. The ability of D2R to heteromerize with adenosine A2A receptor (A2AR) has been associated to D2R agonist-induced β-arrestin recruitment. Accordingly, here we aimed to demonstrate the A2AR dependence of D2R/β-arrestin signaling. By combining bioluminescence resonance energy transfer (BRET) between β-arrestin-2 tagged with yellow fluorescent protein and bimolecular luminescence complementation (BiLC) of D2R/A2AR homomers and heteromers, we demonstrated that the D2R agonists quinpirole and UNC9994 could promote β-arrestin-2 recruitment only when A2AR/D2R heteromers were expressed. Subsequently, the role of A2AR in the antipsychotic-like activity of UNC9994 was assessed in wild-type and A2AR−/− mice administered with phencyclidine (PCP) or amphetamine (AMPH). Interestingly, while UNC9994 reduced hyperlocomotion in wild-type animals treated either with PCP or AMPH, in A2AR−/− mice, it failed to reduce PCP-induced hyperlocomotion or produced only a moderate reduction of AMPH-mediated hyperlocomotion. Overall, the results presented here reinforce the notion that D2R/A2AR heteromerization facilitates D2R β-arrestin recruitment, and furthermore, reveal a pivotal role for A2AR in the antipsychotic-like activity of the β-arrestin-biased D2R ligand, UNC9994.

Behavioral control by striatal A2A-dopamine D2 receptor heteromers

G protein‐coupled receptors (GPCR) exhibit the ability to form receptor complexes that include molecularly different GPCR (ie, GPCR heteromers), which endow them with singular functional and pharmacological characteristics. The relative expression of GPCR heteromers remains a matter of intense debate. Recent studies support that adenosine A2A receptors (A2AR) and dopamine D2 receptors (D2R) predominantly form A2AR‐D2R heteromers in the striatum. The aim of the present study was evaluating the behavioral effects of pharmacological manipulation and genetic blockade of A2AR and D2R within the frame of such a predominant striatal heteromeric population. First, in order to avoid possible strain‐related differences, a new D2R‐deficient mouse with the same genetic background (CD‐1) than the A2AR knock‐out mouse was generated. Locomotor activity, pre‐pulse inhibition (PPI) and drug‐induced catalepsy were then evaluated in wild‐type, A2AR and D2R knock‐out mice, with and without the concomitant administration of either the D2R agonist sumanirole or the A2AR antagonist SCH442416. SCH442416‐mediated locomotor effects were demonstrated to be dependent on D2R signaling. Similarly, a significant dependence on A2AR signaling was observed for PPI and for haloperidol‐induced catalepsy. The results could be explained by the existence of one main population of striatal postsynaptic A2AR‐D2R heteromers, which may constitute a relevant target for the treatment of Parkinsons disease and other neuropsychiatric disorders.

Essential control of the function of the striatopallidal neuron by pre-coupled complexes of adenosine A2A-dopamine D2 receptor heterotetramers and adenylyl cyclase

The central adenosine system and adenosine receptors play a fundamental role in the modulation of dopaminergic neurotransmission. This is mostly achieved by the strategic co-localization of different adenosine and dopamine receptor subtypes in the two populations of striatal efferent neurons, striatonigral and striatopallidal, that give rise to the direct and indirect striatal efferent pathways, respectively. With optogenetic techniques it has been possible to dissect a differential role of the direct and indirect pathways in mediating “Go” responses upon exposure to reward-related stimuli and “NoGo” responses upon exposure to non-rewarded or aversive-related stimuli, respectively, which depends on their different connecting output structures and their differential expression of dopamine and adenosine receptor subtypes. The striatopallidal neuron selectively expresses dopamine D2 receptors (D2R) and adenosine A2A receptors (A2AR), and numerous experiments using multiple genetic and pharmacological in vitro, in situ and in vivo approaches, demonstrate they can form A2AR-D2R heteromers. It was initially assumed that different pharmacological interactions between dopamine and adenosine receptor ligands indicated the existence of different subpopulations of A2AR and D2R in the striatopallidal neuron. However, as elaborated in the present essay, most evidence now indicates that all interactions can be explained with a predominant population of striatal A2AR-D2R heteromers forming complexes with adenylyl cyclase subtype 5 (AC5). The A2AR-D2R heteromer has a tetrameric structure, with two homodimers, which allows not only multiple allosteric interactions between different orthosteric ligands, agonists, and antagonists, but also the canonical Gs-Gi antagonistic interaction at the level of AC5. We present a model of the function of the A2AR-D2R heterotetramer-AC5 complex, which acts as an integrative device of adenosine and dopamine signals that determine the excitability and gene expression of the striatopallidal neurons. The model can explain most behavioral effects of A2AR and D2R ligands, including the psychostimulant effects of caffeine. The model is also discussed in the context of different functional striatal compartments, mainly the dorsal and the ventral striatum. The current accumulated knowledge of the biochemical properties of the A2AR-D2R heterotetramer-AC5 complex offers new therapeutic possibilities for Parkinson’s disease, schizophrenia, SUD and other neuropsychiatric disorders with dysfunction of dorsal or ventral striatopallidal neurons.

Remote control of movement disorders using a photoactive adenosine A 2A receptor antagonist

&NA; G protein‐coupled adenosine receptors are promising therapeutic targets for a wide range of neuropathological conditions, including Parkinsons disease (PD). However, the ubiquity of adenosine receptors and the ultimate lack of selectivity of certain adenosine‐based drugs have frequently diminished their therapeutic use. Photopharmacology is a novel approach that allows the spatiotemporal control of receptor function, thus circumventing some of these limitations. Here, we aimed to develop a light‐sensitive caged adenosine A2A receptor (A2AR) antagonist to photocontrol movement disorders. We synthesized MRS7145 by blocking with coumarin the 5‐amino position of the selective A2AR antagonist SCH442416, which could be photoreleased upon violet light illumination (405 nm). First, the light‐dependent pharmacological profile of MRS7145 was determined in A2AR‐expressing cells. Upon photoactivation, MRS7145 precluded A2AR ligand binding and agonist‐induced cAMP accumulation. Next, the ability of MRS7145 to block A2AR in a light‐dependent manner was assessed in vivo. To this end, A2AR antagonist‐mediated locomotor activity potentiation was evaluated in brain (striatum) fiber‐optic implanted mice. Upon irradiation (405 nm) of the dorsal striatum, MRS7145 induced significant hyperlocomotion and counteracted haloperidol‐induced catalepsy and pilocarpine‐induced tremor. Finally, its efficacy in reversing motor impairment was evaluated in a PD animal model, namely the hemiparkinsonian 6‐hydroxydopamine (6‐OHDA)‐lesioned mouse. Photo‐activated MRS7145 was able to potentiate the number of contralateral rotations induced by L‐3,4‐dihydroxyphenylalanine (l‐DOPA). Overall, MRS7145 is a new light‐operated A2AR antagonist with potential utility to manage movement disorders, including PD. Graphical abstract Figure. No Caption available.

PBF509, an Adenosine A2A Receptor Antagonist With Efficacy in Rodent Models of Movement Disorders

Adenosine A2A receptor (A2AR) antagonists have emerged as complementary non-dopaminergic drugs to alleviate Parkinson’s disease (PD) symptomatology. Here, we characterize a novel non-xhantine non-furan A2AR antagonist, PBF509, as a potential pro-dopaminergic drug for PD management. First, PBF509 was shown to be a highly potent ligand at the human A2AR, since it antagonized A2AR agonist-mediated cAMP accumulation and impedance responses with KB values of 72.8 ± 17.4 and 8.2 ± 4.2 nM, respectively. Notably, these results validated our new A2AR-based label-free assay as a robust and sensitive approach to characterize A2AR ligands. Next, we evaluated the efficacy of PBF509 reversing motor impairments in several rat models of movement disorders, including catalepsy, tremor, and hemiparkinsonism. Thus, PBF509 (orally) antagonized haloperidol-mediated catalepsy, reduced pilocarpine-induced tremulous jaw movements and potentiated the number of contralateral rotations induced by L-3,4-dihydroxyphenylalanine (L-DOPA) in unilaterally 6-OHDA-lesioned rats. Moreover, PBF509 (3 mg/kg) inhibited L-DOPA-induced dyskinesia (LID), showing not only its efficacy on reversing parkinsonian motor impairments but also acting as antidyskinetic agent. Overall, here we describe a new orally selective A2AR antagonist with potential utility for PD treatment, and for some of the side effects associated to the current pharmacotherapy (i.e., dyskinesia).