Development of a Novel σ1 Receptor Biosensor Based on Its Heterodimerization with Binding Immunoglobulin Protein in Living Cells

The σ₁ receptor (S1R) is a ligand-regulated non-opioid intracellular receptor involved in several pathological conditions. The development of S1R-based drugs as therapeutic agents is a challenge due to the lack of simple functional assays to identify and classify S1R ligands. We have developed a novel nanoluciferase binary technology (NanoBiT) assay based on the ability of S1R to heteromerize with the binding immunoglobulin protein (BiP) in living cells. The S1R-BiP heterodimerization biosensor allows for rapid and accurate identification of S1R ligands by monitoring the dynamics of association-dissociation of S1R and BiP. Acute treatment of cells with the S1R agonist PRE-084 produced rapid and transient dissociation of the S1R-BiP heterodimer, which was blocked by haloperidol. The effect of PRE-084 was enhanced by calcium depletion, leading to a higher reduction in heterodimerization even in the presence of haloperidol. Prolonged incubation of cells with S1R antagonists (haloperidol, NE-100, BD-1047, and PD-144418) increased the formation of S1R-BiP heteromers, while agonists (PRE-084, 4-IBP, and pentazocine) did not alter heterodimerization under the same experimental conditions. The newly developed S1R-BiP biosensor is a simple and effective tool for exploring S1R pharmacology in an easy cellular setting. This biosensor is suitable for high-throughput applications and a valuable resource in the researcher's toolkit.

Unique effect of clozapine on adenosine A2A-dopamine D2 receptor heteromerization

The striatal dopamine D₂ receptor (D₂R) is generally accepted to be involved in positive symptoms of schizophrenia and is a main target for clinically used antipsychotics. D₂R are highly expressed in the striatum, where they form heteromers with the adenosine A_2A receptor (A_2AR). Changes in the density of A_2AR-D₂R heteromers have been reported in postmortem tissue from patients with schizophrenia, but the degree to which A₂R are involved in schizophrenia and the effect of antipsychotic drugs is unknown. Here, we examine the effect of exposure to three prototypical antipsychotic drugs on A_2AR-D₂R heteromerization in mammalian cells using a NanoBiT assay. After 16 h of exposure, a significant increase in the density of A_2AR-D₂R heteromers was found with haloperidol and aripiprazole, but not with clozapine. On the other hand, clozapine, but not haloperidol or aripiprazole, was associated with a significant decrease in A_2AR-D₂R heteromerization after 2 h of treatment. Computational binding models of these compounds revealed distinctive molecular signatures that explain their different influence on heteromerization. The bulky tricyclic moiety of clozapine displaces TM 5 of D₂R, inducing a clash with A_2AR, while the extended binding mode of haloperidol and aripiprazole stabilizes a specific conformation of the second extracellular loop of D₂R that enhances the interaction with A_2AR. It is proposed that an increase in A_2AR-D₂R heteromerization is involved in the extrapyramidal side effects (EPS) of antipsychotics and that the specific clozapine-mediated destabilization of A_2AR-D₂R heteromerization can explain its low EPS liability.

Dopamine-induced arrestin recruitment and desensitization of the dopamine D4 receptor is regulated by G protein-coupled receptor kinase-2

The dopamine D₄ receptor (D₄R) is expressed in the retina, prefrontal cortex, and autonomic nervous system and has been implicated in attention deficit hyperactivity disorder (ADHD), substance use disorders, and erectile dysfunction. D₄R has also been investigated as a target for antipsychotics due to its high affinity for clozapine. As opposed to the closely related dopamine D₂ receptor (D₂R), dopamine-induced arrestin recruitment and desensitization at the D₄R have not been studied in detail. Indeed, some earlier investigations could not detect arrestin recruitment and desensitization of this receptor upon its activation by agonist. Here, we used a novel nanoluciferase complementation assay to study dopamine-induced recruitment of β-arrestin2 (βarr2; also known as arrestin3) and G protein-coupled receptor kinase-2 (GRK2) to the D₄R in HEK293T cells. We also studied desensitization of D₄R-evoked G protein-coupled inward rectifier potassium (GIRK; also known as Kir3) current responses in Xenopus oocytes. Furthermore, the effect of coexpression of GRK2 on βarr2 recruitment and GIRK response desensitization was examined. The results suggest that coexpression of GRK2 enhanced the potency of dopamine to induce βarr2 recruitment to the D₄R and accelerated the rate of desensitization of D₄R-evoked GIRK responses. The present study reveals new details about the regulation of arrestin recruitment to the D₄R and thus increases our understanding of the signaling and desensitization of this receptor.

Decreased striatal adenosine A2A-dopamine D2 receptor heteromerization in schizophrenia

According to the adenosine hypothesis of schizophrenia, the classically associated hyperdopaminergic state may be secondary to a loss of function of the adenosinergic system. Such a hypoadenosinergic state might either be due to a reduction of the extracellular levels of adenosine or alterations in the density of adenosine A_2A receptors (A_2ARs) or their degree of functional heteromerization with dopamine D₂ receptors (D₂R). In the present study, we provide preclinical and clinical evidences for this latter mechanism. Two animal models for the study of schizophrenia endophenotypes, namely the phencyclidine (PCP) mouse model and the A_2AR knockout mice, were used to establish correlations between behavioural and molecular studies. In addition, a new AlphaLISA-based method was implemented to detect native A_2AR-D₂R heteromers in mouse and human brain. First, we observed a reduction of prepulse inhibition in A_2AR knockout mice, similar to that observed in the PCP animal model of sensory gating impairment of schizophrenia, as well as a significant upregulation of striatal D₂R without changes in A_2AR expression in PCP-treated animals. In addition, PCP-treated animals showed a significant reduction of striatal A_2AR-D₂R heteromers, as demonstrated by the AlphaLISA-based method. A significant and pronounced reduction of A_2AR-D₂R heteromers was next demonstrated in postmortem caudate nucleus from schizophrenic subjects, even though both D₂R and A_2AR were upregulated. Finally, in PCP-treated animals, sub-chronic administration of haloperidol or clozapine counteracted the reduction of striatal A_2AR-D₂R heteromers. The degree of A_2AR-D₂R heteromer formation in schizophrenia might constitute a hallmark of the illness, which indeed should be further studied to establish possible correlations with chronic antipsychotic treatments.

Involvement of adenosine A1 and A2A receptors on guanosine-mediated anti-tremor effects in reserpinized mice

Parkinson's disease (PD) signs and symptoms regularly include tremor. Interestingly, the nucleoside guanosine (GUO) has already proven to be effective in reducing reserpine-induced tremulous jaw movements (TJMs) in rodent models, thus becoming a promising antiparkinsonian drug. Here, we aimed at revealing the mechanism behind GUO antiparkinsonian efficacy by assessing the role of adenosine A₁ and A_2A receptors (A₁R and A_2AR) on GUO-mediated anti-tremor effects in the reserpinized mouse model of PD. Reserpinized mice showed elevated reactive oxygen species (ROS) production and cellular membrane damage in striatal slices assessed ex vivo and GUO treatment reversed ROS production. Interestingly, while the simultaneous administration of sub-effective doses of GUO (5 mg/kg) and SCH58261 (0.01 mg/kg), an A_2AR antagonist, precluded reserpine-induced TJMs, these were ineffective on reverting ROS production in ex vivo experiments. Importantly, GUO was able to reduce TJM and ROS production in reserpinized mouse lacking the A_2AR, thus suggesting an A_2AR-independent mechanism of GUO-mediated effects. Conversely, the administration of DPCPX (0.75 mg/kg), an A₁R antagonist, completely abolished both GUO-mediated anti-tremor effects and blockade of ROS production. Overall, these results indicated that GUO anti-tremor and antioxidant effects in reserpinized mice were A₁R dependent but A_2AR independent, thus suggesting a differential participation of adenosine receptors in GUO-mediated effects.

Striatal Dopamine D2-Muscarinic Acetylcholine M1 Receptor-Receptor Interaction in a Model of Movement Disorders

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor control deficits, which is associated with the loss of striatal dopaminergic neurons from the substantia nigra. In parallel to dopaminergic denervation, there is an increase of acetylcholine within the striatum, resulting in a striatal dopaminergic–cholinergic neurotransmission imbalance. Currently, available PD pharmacotherapy (e.g., prodopaminergic drugs) does not reinstate the altered dopaminergic–cholinergic balance. In addition, it can eventually elicit cholinergic-related adverse effects. Here, we investigated the interplay between dopaminergic and cholinergic systems by assessing the physical and functional interaction of dopamine D₂ and muscarinic acetylcholine M₁ receptors (D₂R and M₁R, respectively), both expressed at striatopallidal medium spiny neurons. First, we provided evidence for the existence of D₂R–M₁R complexes via biochemical (i.e., co-immunoprecipitation) and biophysical (i.e., BRET¹ and NanoBiT^®) assays, performed in transiently transfected HEK293T cells. Subsequently, a D₂R–M₁R co-distribution in the mouse striatum was observed through double-immunofluorescence staining and AlphaLISA^® immunoassay. Finally, we evaluated the functional interplay between both receptors via behavioral studies, by implementing the classical acute reserpine pharmacological animal model of experimental parkinsonism. Reserpinized mice were administered with a D₂R-selective agonist (sumanirole) and/or an M₁R-selective antagonist (VU0255035), and alterations in PD-related behavioral tasks (i.e., locomotor activity) were evaluated. Importantly, VU0255035 (10 mg/kg) potentiated the antiparkinsonian-like effects (i.e., increased locomotor activity and decreased catalepsy) of an ineffective sumanirole dose (3 mg/kg). Altogether, our data suggest the existence of putative striatal D₂R/M₁R heteromers, which might be a relevant target to manage PD motor impairments with fewer adverse effects.

Pridopidine Reverses Phencyclidine-Induced Memory Impairment

Pridopidine is in clinical trials for Huntington's disease treatment. Originally developed as a dopamine D₂ receptor (D₂R) ligand, pridopidine displays about 100-fold higher affinity for the sigma-1 receptor (sigma-1R). Interestingly, pridopidine slows disease progression and improves motor function in Huntington's disease model mice and, in preliminarily reports, Huntington's disease patients. The present study examined the anti-amnesic potential of pridopidine. Thus, memory impairment was produced in mice by administration of phencyclidine (PCP, 10 mg/kg/day) for 10 days, followed by 14 days' treatment with pridopidine (6 mg/kg/day), or saline. Finally, novel object recognition performance was assessed in the animals. Mice receiving PCP and saline exhibited deficits in novel object recognition, as expected, while pridopidine treatment counteracted PCP-induced memory impairment. The effect of pridopidine was attenuated by co-administration of the sigma receptor antagonist, NE-100 (10 mg/kg). Our results suggest that pridopidine exerts anti-amnesic and potentially neuroprotective actions. These data provide new insights into the therapeutic potential of pridopidine as a pro-cognitive drug.

Behavioral control by striatal adenosine 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 A_2A receptors (A_2A R) and dopamine D₂ receptors (D₂ R) predominantly form A_2A R-D₂ R heteromers in the striatum. The aim of the present study was evaluating the behavioral effects of pharmacological manipulation and genetic blockade of A_2A R and D₂ R within the frame of such a predominant striatal heteromeric population. First, in order to avoid possible strain-related differences, a new D₂ R-deficient mouse with the same genetic background (CD-1) than the A_2A R knock-out mouse was generated. Locomotor activity, pre-pulse inhibition (PPI) and drug-induced catalepsy were then evaluated in wild-type, A_2A R and D₂ R knock-out mice, with and without the concomitant administration of either the D₂ R agonist sumanirole or the A_2A R antagonist SCH442416. SCH442416-mediated locomotor effects were demonstrated to be dependent on D₂ R signaling. Similarly, a significant dependence on A_2A R 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 A_2A R-D₂ R heteromers, which may constitute a relevant target for the treatment of Parkinson's disease and other neuropsychiatric disorders.

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 A_2A receptor (A_2AR) has been associated to D₂R agonist-induced β-arrestin recruitment. Accordingly, here we aimed to demonstrate the A_2AR 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/A_2AR homomers and heteromers, we demonstrated that the D₂R agonists quinpirole and UNC9994 could promote β-arrestin-2 recruitment only when A_2AR/D₂R heteromers were expressed. Subsequently, the role of A_2AR in the antipsychotic-like activity of UNC9994 was assessed in wild-type and A_2AR^-/- mice administered with phencyclidine (PCP) or amphetamine (AMPH). Interestingly, while UNC9994 reduced hyperlocomotion in wild-type animals treated either with PCP or AMPH, in A_2AR^-/- 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/A_2AR heteromerization facilitates D₂R β-arrestin recruitment, and furthermore, reveal a pivotal role for A_2AR in the antipsychotic-like activity of the β-arrestin-biased D₂R ligand, UNC9994.