Basal signaling activity of human dopamine D2L receptor demonstrated with an ecdysone-inducible mammalian expression system

Critical Impact of Different Conserved Endoplasmic Retention Motifs and Dopamine Receptor Interacting Proteins (DRIPs) on Intracellular Localization and Trafficking of the D2 Dopamine Receptor (D2-R) Isoforms

The type 2 dopamine receptor D₂ (D₂-R), member of the G protein-coupled receptor (GPCR) superfamily, exists in two isoforms, short (D_2S-R) and long (D_2L-R). They differ by an additional 29 amino acids (AA) in the third cytoplasmic loop (ICL3) of the D_2L-R. These isoforms differ in their intracellular localization and trafficking functionality, as D_2L-R possesses a larger intracellular pool, mostly in the endoplasmic reticulum (ER). This review focuses on the evolutionarily conserved motifs in the ICL3 of the D₂-R and proteins interacting with the ICL3 of both isoforms, specifically with the 29 AA insert. These motifs might be involved in D₂-R exit from the ER and have an impact on cell-surface and intracellular localization and, therefore, also play a role in the function of dopamine receptor signaling, ligand binding and possible homo/heterodimerization. Our recent bioinformatic data on potential new interaction partners for the ICL3 of D₂-Rs are also presented. Both are highly relevant, and have clinical impacts on the pathophysiology of several diseases such as Parkinson’s disease, schizophrenia, Tourette’s syndrome, Huntington’s disease, manic depression, and others, as they are connected to a variety of essential motifs and differences in communication with interaction partners.

Inducible expression of G protein-coupled receptors in transfected cells

Biochemical or pharmacological studies of G protein-coupled receptors (GPCRs) are widely conducted in transfected mammalian cells. A variety of commercially available systems allow the generation of stable cell-lines in which expression of the recombinant receptor can be induced on addition of a defined chemical to the culture medium, which operates as a control switch for the transcription of the cloned sequence. Such systems offer the possibility to induce graded levels of receptor expression in the experimental model, or to induce an abrupt downregulation of receptor expression during the maintenance of the cell-line. This chapter provides an overview of the different systems available and provides methods for the generation and validation of stably transfected cell-lines expressing the GPCR of choice.

Partial agonist actions of aripiprazole and the candidate antipsychotics S33592, bifeprunox, N-desmethylclozapine and preclamol at dopamine D2Lreceptors are modified by co-transfection of D3receptors: potential role of heterodimer formation

Aripiprazole and the candidate antipsychotics, S33592, bifeprunox, N-desmethylclozapine (NDMC) and preclamol, are partial agonists at D(2) receptors. Herein, we examined their actions at D(2L) and D(3) receptors expressed separately or together in COS-7 cells. In D(2L) receptor-expressing cells co-transfected with (D(3) receptor-insensitive) chimeric adenylate cyclase-V/VI, drugs reduced forskolin-stimulated cAMP production by approximately 20% versus quinpirole (48%). Further, quinpirole-induced inhibition was blunted by aripiprazole and S33592, confirming partial agonist properties. In cells co-transfected with equal amounts of D(2L)and D(3) receptors (1 : 1), efficacies of aripiprazole and S33592 were attenuated. Further, in cells co-transfected with D(2L) and an excess of D(3) receptors (1 : 3), aripiprazole and S33592 were completely inactive, and they abolished the actions of quinpirole. Likewise, bifeprunox, NDMC and preclamol lost agonist properties in cells co-transfected with D(2L)and D(3) receptors. Accordingly, at split D(2trunk)/D(3tail) and D(3trunk)/D(2tail) chimeras, agonist actions of quinpirole were blocked by aripiprazole and S33592 that, like bifeprunox, NDMC and preclamol, were inactive alone. Conversely, when a 12 amino acid sequence in the third intracellular loop of D(3) receptors was replaced by the homologous sequence of D(2L) receptors, aripiprazole, S33592, bifeprunox, NDMC and preclamol inhibited cAMP formation by approximately 20% versus quinpirole (42%). Moreover, at D(2L) receptor-expressing cells co-transfected with modified D(3i3(D2)) receptors, drugs behaved as partial agonists. To summarize, low efficacy agonist actions of aripiprazole, S33592, bifeprunox, NDMC and preclamol at D(2L) receptors are abrogated upon co-expression of D(3) receptors, probably due to physical association and weakened coupling efficacy. These findings have implications for the functional profiles of antipsychotics.

Dopamine D(2) receptor-induced COX-2-mediated production of prostaglandin E(2) in D(2)-transfected Chinese hamster ovary cells without simultaneous administration of a Ca(2+)-mobilizing agent

We have earlier demonstrated that dopamine stimulates the liberation of the prostaglandin E(2) (PGE(2)) precursor, arachidonic acid, in Chinese hamster ovary cells transfected with the rat dopamine D(2) receptor (long isoform), also without concomitant administration of a Ca(2+)-releasing agent [Nilsson et al., Br J Pharmacol 1998;124:1651-8]. In the present report, we show that dopamine, under the same conditions, also induces a concentration-dependent increase in the production of PGE(2), with a maximal effect of 235% at approximately 100 microM, and with an EC(50) of 794 nM. The effect was counteracted by the D(2) antagonist eticlopride, pertussis toxin, the inhibitor of intracellular Ca(2+) release TMB-8, incubation in Ca(2+)-free experimental medium, and PKC desensitization obtained by chronic pretreatment with the phorbol ester TPA. It was also antagonized by the non-specific cyclooxygenase (COX) inhibitor, indomethacin, and by the selective COX-2 inhibitor, NS-398, but not by the specific COX-1 inhibitor, valeryl salicylate. Both the non-specific phospholipase A(2) inhibitor, quinacrine, and an inhibitor of cPLA(2) and iPLA(2), AACOF3, counteracted the effect; in contrast, a selective iPLA(2) inhibitor, BEL, and a selective sPLA(2) inhibitor, TAPC, were ineffective. No effects of dopamine were obtained in control cells mock-transfected with the p3C vector only. The results reinforce previous assumptions that dopamine may interact with eicosanoid metabolism by means of D(2) receptor activation, and implicate an involvement of cPLA(2) and COX-2 in this effect. It is suggested that measurement of dopamine-induced PGE(2) production may serve as a convenient way to study D(2) receptor function in vitro.

Differential signalling of both wild-type and Thr(343)Arg dopamine D(2short) receptor by partial agonists in a G-protein-dependent manner

G-protein activation and Ca(2+) responses by the wild-type D(2short) receptor and a mutation Thr(343)Arg, in the distal BBXXB motif of its third intracellular loop, were investigated in CHO-K1 cells in terms of ligand:receptor:G-protein interactions. No evidence was obtained for constitutive, agonist-independent receptor activation, but differences in the ligand-mediated activation profiles of both the wild-type and mutant Thr(343)Arg D(2short) receptor were observed. Most of the partial agonists, but not bromocriptine, displayed an enhanced response at the mutant D(2short) receptor, suggesting that the mutation brings the receptor in a more active state. This enhancement was apparent both at the level of G-protein activation ([35S]GTPgammaS binding) and at the effector (Ca(2+) response) and occurred with different G(alpha)-proteins. Partial agonists were also found to act differently via the wild-type D(2short) receptor depending on the involved G(alpha)-protein. Compared with higher efficacy agonists, partial agonists displayed Ca(2+) responses with slower and dissimilar kinetic properties. Lisuride and in particular bromocriptine produced a more potent response in the co-presence of a G(alphao) protein instead of a chimeric G(alphaq/o)- or a promiscuous G(alpha15)-protein. S(+)-propylnorapomorphine showed a similar partial response irrespective of the combined G(alpha)-protein. Bromerguride and (+)-UH 232 induced weak (16 to 21% versus dopamine) intrinsic activity in the co-presence of a G(alphaq/o)-protein in contrast to their silent properties with a G(alpha15)- or a G(alphao)Cys(351)Ile-protein. In conclusion, the present data strongly suggest that multiple activation binding sites are involved with these ligands at the D(2short) receptor, and that their activation may be unravelled by either the mutation or co-expressed G(alpha)-proteins being investigated.

Real-time analysis of dopamine: antagonist interactions at recombinant human D2long receptor upon modulation of its activation state

1. Antipsychotic drugs may mediate their therapeutic effects not only by preventing the binding of dopamine but also by decreasing the propensity of the dopamine receptor to assume an active R* state. Ligand-mediated activation and blockade of the recombinant human D(2long) receptor was investigated in CHO-K1 cells upon modulation of its R* state. 2. Both the Ala(371)Lys (A371K) and Thr(372)Arg (T372R) D2long receptor mutants could be activated in a ligand-dependent manner via a chimeric G(alphaq/o) protein, and more efficaciously so than with the promiscuous G(alpha15) protein. 3. Dopamine and partial agonists (E(max): lisuride >> (+)-UH 232 approximately bromerguride) displayed dissimilar Ca(2+) kinetic properties at wild-type and mutant receptors. A371K and T372R D2long receptor mutants demonstrated an attenuated and enhanced maximal response to these partial agonists, respectively. 4. Dopamine antagonists were unable to block the transient high-magnitude Ca(2+) phase at the wild-type D2long receptor upon simultaneous exposure to antagonist and dopamine, while full blockade of the low-magnitude Ca(2+) phase did occur at a later time (onset-time: haloperidol < bromerguride < (+)-butaclamol). A similar, though more efficacious, antagonist profile was also found at the A371K mutant receptor. Conversely, the blockade of the low-magnitude Ca(2+) phase was attenuated (haloperidol) or almost absent [(+)-butaclamol and bromerguride] at the T372R mutant receptor. 5. In conclusion, mutagenesis of the Ala(371) and Thr(372) positions affects in an opposite way the ligand-dependent activation and blockade of the D2long receptor. The observed attenuation of dopamine-mediated Ca(2+) signal generation with different decay-times may underlie distinct properties of the dopaminergic ligands.