Agonist-induced cell surface trafficking of an intracellularly sequestered D1 dopamine receptor homo-oligomer

Distinct Dopamine D₂ Receptor Antagonists Differentially Impact D₂ Receptor Oligomerization

Dopamine D₂ receptors (D₂R) are known to form transient homodimer complexes, of which the increased formation has already been associated with development of schizophrenia. Pharmacological targeting and modulation of the equilibrium of these receptor homodimers might lead to a better understanding of the critical role played by these complexes in physiological and pathological conditions. Whereas agonist addition has shown to prolong the D₂R dimer lifetime and increase the level of dimer formation, the possible influence of D₂R antagonists on dimerization has remained rather unexplored. Here, using a live-cell reporter assay based on the functional complementation of a split Nanoluciferase, a panel of six D₂R antagonists were screened for their ability to modulate the level of D_2LR dimer formation. Incubation with the D₂R antagonist spiperone decreased the level of D_2LR dimer formation significantly by 40–60% in real-time and after long-term (≥16 h) incubations. The fact that dimer formation of the well-studied A_2a–D_2LR dimer was not altered following incubation with spiperone supports the specificity of this observation. Other D₂R antagonists, such as clozapine, risperidone, and droperidol did not significantly evoke this dissociation event. Furthermore, molecular modeling reveals that spiperone presents specific Tyr199^5.48 and Phe390^6.52 conformations, compared to clozapine, which may determine D₂R homodimerization.

Discovery of D1 Dopamine Receptor Positive Allosteric Modulators: Characterization of Pharmacology and Identification of Residues that Regulate Species Selectivity

The present studies represent the first published report of a dopamine D1 positive allosteric modulator (PAM). D1 receptors have been proposed as a therapeutic target for the treatment of cognitive deficits associated with schizophrenia. However, the clinical utility of orthosteric agonist compounds is limited by cardiovascular side effects, poor pharmacokinetics, lack of D1 selectivity, and an inverted dose response. A number of these challenges may be overcome by utilization of a selective D1 PAM. The current studies describe two chemically distinct D1 PAMs: Compound A [1-((rel-1S,3R,6R)-6-(benzo[d][1,3]dioxol-5-yl)bicyclo[4.1.0]heptan-3-yl)-4-(2-bromo-5-chlorobenzyl)piperazine] and Compound B [rel-(9R,10R,12S)-N-(2,6-dichloro-3-methylphenyl)-12-methyl-9,10-dihydro-9,10-ethanoanthracene-12-carboxamide]. Compound A shows pure PAM activity, with an EC50 of 230 nM and agonist activity at the D2 receptor in D2-expressing human embryonic kidney cells. Compound B shows superior potency (EC50 of 43 nM) and selectivity for D1 versus D2 dopamine receptors. Unlike Compound A, Compound B is selective for human and nonhuman primate D1 receptors, but lacks activity at the rodent (rat and mouse) D1 receptors. Using molecular biology techniques, a single amino acid was identified at position 130, which mediates the species selectivity of Compound B. These data represent the first described D1-selective PAMs and define critical amino acids that regulate species selectivity.

Heteromeric dopamine receptor signaling complexes: emerging neurobiology and disease relevance

The pharmacological modification of dopamine transmission has long been employed as a therapeutic tool in the treatment of many mental health disorders. However, as many of the pharmacotherapies today are not without significant side effects, or they alleviate only a particular subset of symptoms, the identification of novel therapeutic targets is imperative. In light of these challenges, the recognition that dopamine receptors can form heteromers has significantly expanded the range of physiologically relevant signaling complexes as well as potential drug targets. Furthermore, as the physiology and disease relevance of these receptor heteromers is further understood, their ability to exhibit pharmacological and functional properties distinct from their constituent receptors, or modulate the function of endogenous homomeric receptor complexes, may allow for the development of alternate therapeutic strategies and provide new avenues for drug design. In this review, we describe the emerging neurobiology of the known dopamine receptor heteromers, their physiological relevance in brain, and discuss the potential role of these receptor complexes in neuropsychiatric disease. We highlight their value as targets for future drug development and discuss innovative research strategies designed to selectively target these dopamine receptor heteromers in the search for novel and clinically efficacious pharmacotherapies.

Deletion of the NMDA-NR1 receptor subunit gene in the mouse nucleus accumbens attenuates apomorphine-induced dopamine D1 receptor trafficking and acoustic startle behavior

The nucleus accumbens (Acb) contains subpopulations of neurons defined by their receptor content and potential involvement in sensorimotor gating and other behaviors that are dysfunctional in schizophrenia. In Acb neurons, the NMDA NR1 (NR1) subunit is coexpressed not only with the dopamine D1 receptor (D1R), but also with the µ-opioid receptor (µ-OR), which mediates certain behaviors that are adversely impacted by schizophrenia. The NMDA-NR1 subunit has been suggested to play a role in the D1R trafficking and behavioral dysfunctions resulting from systemic administration of apomorphine, a D1R and dopamine D2 receptor agonist that impacts prepulse inhibition to auditory-evoked startle (AS). Together, this evidence suggests that the NMDA receptor may regulate D1R trafficking in Acb neurons, including those expressing µ-OR, in animals exposed to auditory startle and apomorphine. We tested this hypothesis by combining spatial-temporal gene deletion technology, dual labeling immunocytochemistry, and behavioral analysis. Deleting NR1 in Acb neurons prevented the increase in the dendritic density of plasma membrane D1Rs in single D1R and dual (D1R and µ-OR) labeled dendrites in the Acb in response to apomorphine and AS. Deleting NR1 also attenuated the decrease in AS induced by apomorphine. In the absence of apomorphine and startle, deletion of Acb NR1 diminished social interaction, without affecting novel object recognition, or open field activity. These results suggest that NR1 expression in the Acb is essential for apomorphine-induced D1R surface trafficking, as well as auditory startle and social behaviors that are impaired in multiple psychiatric disorders.

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

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

Probing the steric space at the floor of the D1 dopamine receptor orthosteric binding domain: 7α-, 7β-, 8α-, and 8β-methyl substituted dihydrexidine analogues

To probe the space at the floor of the orthosteric ligand binding site in the dopamine D(1) receptor, four methylated analogues of dihydrexidine (DHX) were synthesized with substitutions at the 7 and 8 positions. The 8α-axial, 8β-equatorial, and 7α-equatorial were synthesized by photochemical cyclization of appropriately substituted N-benzoyl enamines, and the 7β-axial analogue was prepared by an intramolecular Henry reaction. All of the methylated analogues displayed losses in affinity when compared to DHX (20 nM): 8β-Me(ax)-DHX (270 nM), 8α-Me(eq)-DHX (920 nM), 7β-Me(eq)-DHX (6540 nM), and 7α-Me(ax)-DHX (>10000 nM). Molecular modeling studies suggest that although the disruption of an aromatic interaction between Phe203(5.47) and Phe288(6.51) is the cause for the 14-fold loss in affinity associated with 8β-axial substitution, unfavorable steric interactions with Ser107(3.36) result in the more dramatic decreases in binding affinity suffered by the rest of the analogues.

Characterization of in vivo pharmacokinetic properties of the dopamine D1 receptor agonist DAR-0100A in nonhuman primates using PET with [11C] NNC112 and [11C] raclopride

DAR-0100A, the active enantiomer of dihydrexidine, is a potent dopamine D1 agonist under investigation for treatment of cognitive impairment and negative symptoms of schizophrenia. We measured the dose-occupancy relationship for DAR-0100A at D1 receptors using positron emission tomography (PET) imaging in baboons with [(11)C] NNC112 and its binding to D2 with [(11)C] raclopride. Two baboons were scanned with [(11)C] NNC112 at baseline and after three different doses of DAR-0100A. Two baboons were scanned with [(11)C] raclopride at baseline and after one dose of DAR-0100A. Occupancy (ΔBP(ND)) was computed in the striatum and cortex. A clear relationship was observed between plasma concentration of DAR-0100A and ΔBP(ND). ΔBP(ND) was larger in the striatum than in the cortex, consistent with reports showing that 25% of [(11)C] NNC112 BP(ND) in the cortex is attributed to 5-HT(2A). Plasma EC(50) estimates ranged from 150 to 550 ng/mL according to the constraints on the model. There was no detectable effect of DAR-0100A on [(11)C] raclopride BP(ND). These data suggest that at doses likely to be administered to patients, occupancy will not be detectable with [(11)C] NNC112 PET and binding of DAR-0100A to D2 will be negligible. This is the first demonstration with PET of a significant occupancy by a full D1 agonist in vivo.

Dopamine receptor homooligomers and heterooligomers in schizophrenia

Over the past two decades the dopamine D2 receptor has been undoubtedly the most widely studied dopamine receptor for the therapeutic treatment of schizophrenia, as the majority of antipsychotics exhibit antagonism at this receptor. However, the cognitive symptoms of the disorder are mostly resistant to the majority of available antipsychotic treatments and, as a result, there is a critical need to develop novel therapies that ameliorate all symptoms. The recognition that dopamine receptors, such as all G protein-coupled receptors (GPCRs), exist as oligomeric complexes has provided new avenues for drug design in the search for novel therapies. Furthermore, that it is now known that dopamine receptors can form heteromers, such as the dopamine D1-D2 receptor heteromer, with pharmacology and function distinct from its constituent receptors, has significantly expanded the range of potential drug targets. The aim of this review is to discuss the therapeutic relevance of these dopamine receptor oligomers to schizophrenia and to address the potential value of dopamine receptor heteromers in the search for new therapeutic strategies.

Direct involvement of sigma-1 receptors in the dopamine D1 receptor-mediated effects of cocaine

It is well known that cocaine blocks the dopamine transporter. This mechanism should lead to a general increase in dopaminergic neurotransmission, and yet dopamine D(1) receptors (D(1)Rs) play a more significant role in the behavioral effects of cocaine than the other dopamine receptor subtypes. Cocaine also binds to σ-1 receptors, the physiological role of which is largely unknown. In the present study, D(1)R and σ(1)R were found to heteromerize in transfected cells, where cocaine robustly potentiated D(1)R-mediated adenylyl cyclase activation, induced MAPK activation per se and counteracted MAPK activation induced by D(1)R stimulation in a dopamine transporter-independent and σ(1)R-dependent manner. Some of these effects were also demonstrated in murine striatal slices and were absent in σ(1)R KO mice, providing evidence for the existence of σ(1)R-D(1)R heteromers in the brain. Therefore, these results provide a molecular explanation for which D(1)R plays a more significant role in the behavioral effects of cocaine, through σ(1)R-D(1)R heteromerization, and provide a unique perspective toward understanding the molecular basis of cocaine addiction.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

A hybrid indoloquinolizidine peptide as allosteric modulator of dopamine D1 receptors

The indoloquinolizidine-peptide 28 [(3S,12bR)-N-((S)-1-((S)-1-((S)-2-carbamoylpyrrolidin-1-yl)-3-(4-fluorophenyl)-1-oxopropan-2-ylamino)-4-cyclohexyl-1-oxobutan-2-yl)-1,2,3,4,6,7,12, 12b-octahydroindolo[2,3-a]quinolizine-3-carboxamide], a trans-indoloquinolizidine-peptide hybrid obtained by a combinatorial approach, behaved as an orthosteric ligand of all dopamine D(2)-like receptors (D(2), D(3), and D(4)) and dopamine D(5) receptors, but as a negative allosteric modulator of agonist and antagonist binding to striatal dopamine D(1) receptors. Indoloquinolizidine-peptide 28 induced a concentration-dependent hyperbolic increase in the antagonist apparent equilibrium dissociation constant values and altered the dissociation kinetics of dopamine D(1) receptor antagonists. The negative allosteric modulation was also found when agonist binding to D(1) receptors was assayed. Indoloquinolizidine-peptide 28 was a weak ago-allosteric modulator but markedly led to a decreased potency without decreasing the maximum partial/full agonist-mediated effect on cAMP levels. Compounds able to decrease the potency while preserving the efficacy of D(1) receptor agonists are promising for exploration in psychotic pathologies.

Useful pharmacological parameters for G-protein-coupled receptor homodimers obtained from competition experiments. Agonist-antagonist binding modulation

Many G-protein-coupled receptors (GPCRs) are expressed on the plasma membrane as dimers. Since drug binding data are currently fitted using equations developed for monomeric receptors, the interpretation of the pharmacological data are equivocal in many cases. As reported here, GPCR dimer models account for changes in competition curve shape as a function of the radioligand concentration used, something that cannot be explained by monomeric receptor models. Macroscopic equilibrium dissociation constants for the agonist and homotropic cooperativity index reflecting the intramolecular communication within the dopamine D1 or adenosine A2A receptor homodimer as well as hybrid equilibrium dissociation constant, which reflects the antagonist/agonist modulation may be calculated by fitting binding data from antagonist/agonist competition experiments to equations developed from dimer receptor models. Comparing fitting the data by assuming a classical monomeric receptor model or a dimer model, it is shown that dimer receptor models provide more clues useful in drug discovery than monomer-based models.

Marked changes in signal transduction upon heteromerization of dopamine D1 and histamine H3 receptors

BACKGROUND AND PURPOSE

Functional interactions between the G protein-coupled dopamine D1 and histamine H3 receptors have been described in the brain. In the present study we investigated the existence of D1-H3 receptor heteromers and their biochemical characteristics.

EXPERIMENTAL APPROACH

D1-H3 receptor heteromerization was studied in mammalian transfected cells with Bioluminescence Resonance Energy Transfer and binding assays. Furthermore, signalling through mitogen-activated protein kinase (MAPK) and adenylyl cyclase pathways was studied in co-transfected cells and compared with cells transfected with either D1 or H3 receptors.

KEY RESULTS

Bioluminescence Resonance Energy Transfer and binding assays confirmed that D1 and H3 receptors can heteromerize. Activation of histamine H3 receptors did not lead to signalling towards the MAPK pathway unless dopamine D1 receptors were co-expressed. Also, dopamine D1 receptors, usually coupled to G(s) proteins and leading to increases in cAMP, did not couple to G(s) but to G(i) in co-transfected cells. Furthermore, signalling via each receptor was blocked not only by a selective antagonist but also by an antagonist of the partner receptor.

CONCLUSIONS AND IMPLICATIONS

D1-H3 receptor heteromers constitute unique devices that can direct dopaminergic and histaminergic signalling towards the MAPK pathway in a G(s)-independent and G(i)-dependent manner. An antagonist of one of the receptor units in the D1-H3 receptor heteromer can induce conformational changes in the other receptor unit and block specific signals originating in the heteromer. This gives rise to unsuspected therapeutic potentials for G protein-coupled receptor antagonists.

Pharmacological analysis demonstrates dramatic alteration of D1 dopamine receptor neuronal distribution in the rat analog of L-DOPA-induced dyskinesia

We have associated behavioral, pharmacological, and quantitative immunohistochemical study in a rat analog of l-DOPA-induced dyskinesia to understand whether alterations in dopamine receptor fate in striatal neurons may be involved in mechanisms leading to movement abnormalities. Detailed analysis at the ultrastructural level demonstrates specific alterations of dopamine D(1) receptor (D(1)R) subcellular localization in striatal medium spiny neurons in l-DOPA-treated 6-hydroxydopamine-lesioned rats with abnormal involuntary movements (AIMs). This includes exaggerated D(1)R expression at the plasma membrane. However, D(1)R retains ability of internalization, as a challenge with the potent D(1)R agonist SKF-82958 induces a strong decrease of labeling at membrane in animals with AIMs. Since a functional cross talk between D(1)R and D(3)R has been suggested, we hypothesized that their coactivation by dopamine derived from l-DOPA might anchor D(1)R at the membrane. Accordingly, cotreatment with l-DOPA and the D(3)R antagonist ST 198 restores normal level of membrane-bound D(1)R. Together, these results demonstrate that AIMs are related to abnormal D(1)R localization at the membrane and intraneuronal trafficking dysregulation, and suggest that strategies aiming at disrupting the D(1)R-D(3)R cross talk might reduce l-DOPA-induced dyskinesia by reducing D(1)R availability at the membrane.

Pharmacological analysis of the novel, rapid, and potent inactivation of the human 5-Hydroxytryptamine7 receptor by risperidone, 9-OH-Risperidone, and other inactivating antagonists

In a previous publication, using human 5-hydroxytryptamine(7) (h5-HT(7)) receptor-expressing human embryonic kidney (HEK) 293 cells, we reported the rapid, potent inactivation of the h5-HT(7) receptor stimulation of cAMP production by three antagonists: risperidone, 9-OH-risperidone, and methiothepin (Smith et al., 2006). To better understand the drug-receptor interaction producing the inactivation, we 1) expanded the list of inactivating drugs, 2) determined the inactivating potencies and efficacies by performing concentration-response experiments, and 3) determined the potencies and efficacies of the inactivators as irreversible binding site inhibitors. Three new drugs were found to fully inactivate the h5-HT(7) receptor: lisuride, bromocryptine, and metergoline. As inactivators, these drugs displayed potencies of 1, 80, and 321 nM, respectively. Pretreatment of 5-HT(7)-expressing HEK cells with increasing concentrations of the inactivating drugs risperidone, 9-OH-risperidone, methiothepin, lisuride, bromocriptine, and metergoline potently inhibited radiolabeling of the h5-HT(7) receptor, with IC(50) values of 9, 5.5, 152, 3, 73, and 10 nM, respectively. We were surprised to find that maximal concentrations of risperidone and 9-OH-risperidone inhibited only 50% of the radiolabeling of h5-HT(7) receptors. These results indicate that risperidone and 9-OH risperidone may be producing 5-HT(7) receptor inactivation by different mechanisms than lisuride, bromocryptine, metergoline, and methiothepin. These results are not interpretable using the conventional model of G-protein-coupled receptor function. The complex seems capable of assuming a stable inactive conformation as a result of the interaction of certain antagonists. The rapid, potent inactivation of the receptor-G-protein complex by antagonists implies a constitutive, pre-existing complex between the h5-HT(7) receptor and a G-protein.

The integrity of the glycine co-agonist binding site of N-methyl-D-aspartate receptors is a functional quality control checkpoint for cell surface delivery

N-Methyl-D-aspartate receptors are a subclass of ligand-gated, heteromeric glutamatergic neurotransmitter receptors whose cell surface expression is regulated by quality control mechanisms. Functional quality control checkpoints are known to contribute to cell surface trafficking of non-N-methyl-D-aspartate glutamate receptors. Here we investigated if similar mechanisms operate for the surface delivery of NMDA receptors. Point mutations in the glycine binding domain of the NR1-1a subunit were generated: D732A, a mutation that results in an approximately 3 x 10(4) decrease in glycine binding affinity; D732E, a conservative change; and D723A, a residue in the same NR1-1a domain that has no effect on glycine binding affinity. Each NR1-1a subunit was co-expressed with NR2A in mammalian cells. Immunoblotting and immunoprecipitations showed that all mutants were expressed to similar levels as wild-type NR1-1a and associated with NR2A. Cell surface expression measured by an enzyme-linked immunosorbent assay found that whereas NR1-1a (D732E)/NR2A and NR1-1a (D723A)/NR2A trafficked as efficiently as NR1-1a/NR2A, there was a 90% decrease in surface expression for NR1-1a (D732A)/NR2A. This was confirmed by confocal microscopy imaging and cell surface biotinylation. Further imaging showed that NR1-1a (D732A) and co-transfected NR2A co-localized with an endoplasmic reticulum marker. Dichlorokynurenic acid, a competitive glycine site antagonist, partially rescued surface expression. Mutation of the NR1-1a ER retention motif showed that the ligand binding checkpoint is an early event preceding endoplasmic reticulum sorting mechanisms. These findings demonstrate that integrity of the glycine co-agonist binding site is a functional checkpoint requisite for efficient cell surface trafficking of assembled NMDA receptors.