Purification of the D-2 dopamine receptor from bovine striatum

Expression and characterization of human dopamine D2 receptor in baculovirus-infected insect cells

Multiple types of dopamine D2-like receptors (D2, D3, D4) have been identified. Differences in pharmacology among these receptors may have profound clinical ramifications for the treatment of psychosis. Analysis of the structure and function of their binding sites requires a source of large amounts of receptor, uncontaminated by the other types of D2-like receptor. We engineered a recombinant baculovirus containing the human D2 receptor cDNA (DRD2) to express this receptor in insect cells. Spodoptera frugiperda cells (Sf9 and Sf21) and Trichoplusia ni cells (TN-5) were infected with the recombinant baculovirus. Binding of the D2 antagonist [3H]YM-09151-2 to membranes fractions of these cells peaked at a specific activity of 5-8 pmol/mg protein, approximately 40 times that of membranes from bovine striatum. The receptor expressed in Sf9 cells was similar to that of striatum in its affinities for D2 agonists and antagonists. Sodium ion stimulated [3H]YM-09151-2 binding to D2 receptor in infected Sf9 cell membranes. This effect was fit by an allosteric model which predicted the apparent affinity of [3H]YM-09151-2. The D2 receptor expressed in Sf9 and TN-5 cells was photolabeled with N-(p-azido-m-[125I]iodophenylethyl)spiperone. The specifically labeled component(s) ran as a broad band of apparent molecular weight between 54,000 and 60,000. Deglycosylation of the labeled component(s) reduced its molecular weight to 46,000.

Molecular characterization of D2 dopamine-like receptors from brain and from the pituitary gland

D2 dopamine-like receptors have been purified from five bovine brain regions (caudate nucleus, putamen, olfactory tubercle, frontal cortex, cerebellum) and the anterior and neurointermediate lobes of the pituitary gland using a combined ligand-affinity and lectin-affinity chromatography procedure. In all the brain regions except cerebellum and in the neurointermediate lobe of the pituitary gland the purified species appeared as a M(r) 95,000 doublet on SDS-PAGE. In the anterior lobe of the pituitary an additional M(r) 142,000-145,000 species was seen. The M(r) 95,000 species had a low affinity for the lectin wheat germ agglutinin (WGA) whereas the M(r) 142,000-145,000 species had a higher affinity for WGA and additionally showed some affinity for concanavalin A. It is concluded that both the M(r) 95,000 and 142,000-145,000 species are D2 dopamine-like receptors and that the differences between the species are mainly at the oligosaccharide level. Some evidence was also obtained for heterogeneity at the protein level which may correspond to the D2(short) and D2(long) isoforms of these receptors.

Synthesis and characterization of biotinylated and photoactivatable neuroleptics. Novel bifunctional probes for dopamine receptors

We have synthesized and characterized a series of novel derivatives of established antagonists of the neurotransmitter dopamine, i.e. butyrophenones, hexahydrocarbolines and phenothiazines. All derivatives were biotinylated, some of them carried an additional (photoactivatable) azido group. In the case of butyrophenones, the structural modifications were introduced at the aliphatic keto group and/or the heterocyclic ring system, both modifications resulting in significant decreases in binding affinity to dopamine D2 and dopamine D1 receptor subtypes. Biotinylation of hexahydrocarbolines significantly increased their binding affinity to D1 receptors, with the affinity for D2 receptors increasing only slightly, or remaining approximately the same, as compared to the parent compound. As a consequence, the derivatized hexahydrocarbolines behaved as nonselective antagonists of dopamine. Biotinylation of phenothiazines increased their binding affinity to both main subtypes of dopamine receptors by at least one order of magnitude, resulting in binding affinities in the nM range. These derivatives bound to both D1 and D2 receptor subtypes. In three of the biotinylated derivatives the photoactivatable azido group was introduced. These compounds bound to synaptosomal membranes from bovine caudate nuclei with similar affinity and subtype specificity as the biotinylated derivatives, and photoaffinity labelling was shown to proceed under mild conditions and selectively. These novel bifunctional ligands may become useful tools in the purification and characterization of dopamine receptors including their visualization and localization in the central nervous system and in tissue culture.

Coupling of D2 dopamine receptors to G-proteins in solubilized preparations of bovine caudate nucleus

1. The coupling of D2 dopamine receptors and G-proteins has been examined in cholate-solubilized preparations of bovine caudate nucleus. 2. No receptor-G-protein coupling could be detected in solubilized preparations obtained in 0.3% cholate, but if this preparation is diluted 5-fold, coupling is re-established. 3. The dilution process was examined, and it was shown that the change in ionic strength was an important factor in modulating the observed receptor-G-protein interaction. 4. Ionic strength was shown, however, not to be the primary determinant of receptor-G-protein coupling. This is likely to be the formation, upon dilution of the preparation, of vesicles in which receptor and G-protein reassociate. 5. The formation of vesicles upon dilution was examined by a variety of techniques, including thermal-stability studies, gel filtration, centrifugation and electron microscopy.

pH dependence of sulpiride binding to D2 dopamine receptors in bovine brain

In summary then these data suggest that butyrophenones such as spiperone and substituted benzamides such as sulpiride interact with different groups at the active site of the D2 dopamine receptor. The drugs bind in different modes to the receptor which must therefore contain separate but overlapping binding sites for the two classes of drug. Understanding the precise interactions involved that generate this selectivity will be important for drug design.

Vanadate inhibits agonist binding to D2 dopamine receptor

Orthovanadate (in the micromolar range) inhibits the high-affinity binding of the D2 dopamine receptor to specific agonists (apomorphine and N-propylnorapomorphine), while it does not affect the binding to D2 antagonists (spiperone and haloperidol). These effects of vanadate resemble those observed with guanine nucleotides or their analogs. However, in contrast to the guanine nucleotides, vanadate does not induce dissociation of the D2 dopamine receptor from its related G proteins, suggesting that vanadate and guanine nucleotides may exert their effect on the D2 dopamine receptor via different mechanisms. The effect of vanadate on agonist binding was shown to be ATP dependent and correlated with increased protein phosphorylation.

Phosphorylation by cyclic AMP-dependent protein kinase modulates agonist binding to the D2 dopamine receptor

Phosphorylation of striatal membranes by cyclic AMP-dependent protein kinase resulted in a reduction in the affinity of the D2 dopamine receptor toward its agonist N-propylnorapomorphine while the affinity to D2-specific antagonists remained unchanted. The inhibitory effects observed by phosphorylation and guanine nucleotides on agonist binding to the D2 receptor were additive. The purified D2 dopamine receptor from bovine striatum was specifically phosphorylated by cyclic AMP-dependent protein kinase with an apparent stoichiometry of 0.7 mol phosphate/mol receptor. The phosphorylated purified D2 receptor also exhibited a reduced agonist binding activity with no change in antagonist binding. The action of cyclic AMP-dependent protein kinase on both the membrane preparation and the purified D2 receptor was inhibited by a specific inhibitor of the kinase. These data indicate that phosphorylation mediated by cyclic AMP-dependent protein kinase may represent a physiological pathway for modulation of the receptor binding activity.

Molecular biology of adrenergic and dopamine receptors and the study of developmental nephrology

Neurotransmitters convey specific messages by binding to receptors on the cell membrane surface. Receptors are linked to membrane-bound, signal-transducing proteins which act as intermediaries in the generation of second messengers that elicit biological responses. Cell surface receptors could be grouped into families that utilize common systems for their signal transmission. These classes include the growth factor receptors, the transporter receptors which internalize their ligands, ion channels, and G-protein-coupled receptors. In the past few years, the cDNAs and/or genes of a number of G-protein-coupled receptors have been cloned. Structural analysis of the G-protein-coupled receptors, as well as the other classes of receptor, shows that those receptors which use a common signaling pathway have similar topographies and share significant sequence homology. Adrenergic and dopamine receptors are examples of receptors coupled to G proteins. This review outlines some strategies in the study of adrenergic and dopamine receptors using molecular biology techniques and how they relate to investigations in developmental nephrology.

Evidence for the importance of a carboxyl group in the binding of ligands to the D2 dopamine receptor

A series of group specific modifying reagents were tested for their effects on [3H]spiperone binding to brain D2 dopamine receptors to identify amino acid residues at the binding site of the D2 dopamine receptor that are critical for ligand binding. The dependence of ligand binding to the receptor on the pH of the incubation medium was also examined. N-Acetylimidazole, 5,5'-dithiobis(2-nitrobenzoic acid), 1,2-cyclohexanedione, and acetic anhydride had no specific effect on [3H]spiperone binding, indicating the lack of participation of tyrosine, free sulphydryl, arginine, or primary amino groups in ligand binding to the receptor. N,N'-Dicyclohexylcarbodiimide (DCCD) potently reduced the number of [3H]spiperone binding sites, indicating that a carboxyl group is involved in ligand binding to the receptor. The effects of DCCD could be prevented by prior incubation of the receptor with D2 dopamine receptor selective compounds. The pH-binding profile for [3H]spiperone binding indicated the importance of an ionising group of pKa 5.2 for ligand binding which may be the same carboxyl group. Diethyl pyrocarbonate, the histidine modifying reagent, also inhibited [3H]spiperone binding, reducing the affinity of the receptor for this ligand but the effects were not at the ligand binding site. From the effects of pH changes on ligand binding some evidence was obtained for a second ionising group (pKa 7.0) that specifically affects the binding of substituted benzamide drugs to the receptor. It is concluded that the D2 dopamine receptor binding site contains separate but over-lapping binding regions for antagonists such as spiperone and substituted benzamide drugs. The former region contains an important carboxyl group; the latter region contains another group that may be a second carboxyl group or a histidine.

Aspects of the structure of the D2 dopamine receptor

Significant new information on the D2 dopamine receptor has recently become available from a combination of protein chemical and molecular genetic analyses. Molecular genetic studies have shown the receptor to be a member of the family of receptors that are linked to G proteins and that have structures predicted to contain seven transmembrane domains. Two distinct species of D2 dopamine receptor have been found which may differ in their coupling to G proteins; their distributions have been mapped at the nucleic acid level. The D2 dopamine receptor has been purified from brain and anterior pituitary and characterized. Chemical modification of the brain receptor provides evidence for the importance of a carboxyl group that interacts with ligands at the receptor binding site. Here, Philip Strange discusses these points and proposes models of receptor-ligand interaction based on the conservation of several aspartic acid residues in receptors that bind cationic amines.

Purification of D2 dopamine receptor by photoaffinity labelling, high-performance liquid chromatography and preparative sodium dodecyl sulphate polyacrylamide gel electrophoresis

[125I]N-azidophenethylspiperone ([125I]azido-NAPS) was used as a photoaffinity ligand for bovine D2 dopamine receptor. On photolysis, [125I]azido-NAPS was covalently incorporated into a major band of 94 kDa in bovine striatal membrane as assessed by autoradiography after sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) (10% acrylamide gel). The labelled D2 receptor protein from striatal membrane was solubilized and subjected to HPLC using gel filtration (TSK G3000SW) and hydroxyapatite gel (Pentax SH2010C), followed by two steps of preparative SDS-PAGE. The D2 receptor protein could be obtained as a single major polypeptide on SDS-PAGE by either silver staining or autoradiography.

Solubilization and reconstitution of dopamine D1 receptor from bovine striatal membranes: effects of agonist and antagonist pretreatment

The bovine striatal dopamine D1 receptor was solubilized with a combination of sodium cholate and NaCl in the presence of phospholipids, following treatment of membranes with a dopaminergic agonist (SKF-82526-J) or antagonist (SCH-23390). The solubilized receptors were subsequently reconstituted into lipid vesicles by gel-filtration. A comparison of ligand-binding properties shows that the solubilized and reconstituted receptors bound [3H]SCH-23390 to a homogeneous site in a saturable, stereospecific and reversible manner with a Kd of 0.95 and 1.1 nM and a Bmax of 918 and 885 fmol/mg protein respectively for agonist- and antagonist-pretreated preparations. These values are very similar to those obtained for membrane-bound receptors. The competition of antagonists for [3H]SCH-23390 binding exhibited a clear D1 dopaminergic order in the reconstituted preparation obtained from either agonist or antagonist-pretreated membranes, except that (+)butaclamol was about four-fold more potent than cis-flupentixol in displacing [3H]SCH-23390 binding in preparation obtained from agonist-pretreated membranes compared to antagonist-pretreated membranes. The agonist/[3H]SCH-23390 competition studies revealed the presence of a high-affinity component of agonist binding in both the reconstituted receptor preparations. The number of high-affinity agonist binding sites, however, is 40-80% higher in reconstituted preparation obtained from antagonist-treated membrane compared to that obtained from the agonist-treated membrane. In both the preparations, 100 microM guanylylimidodiphosphate (Gpp(NH)p) completely abolished the high-affinity component of agonist binding compared to partial abolition in the native membranes, indicating a close association of a G-protein with the solubilized receptors. Whether the receptor was solubilized following agonist or antagonist preincubation of the membranes, the receptor-detergent complex eluted from a steric-exclusion HPLC column with an apparent molecular size of 360,000. Preincubation of the solubilized preparations with Gpp(NH)p had virtually no effect on the elution profile suggesting a lack of guanine nucleotide-dependent dissociation of G-protein receptor complex.

Preparation of an affinity chromatography matrix for the selective purification of the dopamine D2 receptor from bovine striatal membranes

A ligand affinity matrix has been developed and utilized to purify the dopamine D2 receptor approx. 2100 fold from bovine striatal membranes. 3-[2-Aminoethyl]-8-[3-(4-fluorobenzoyl)propyl]-4-oxo-1-phenyl-1,3,8- triazaspiro[4.5]decan-4-one (AES) was synthesized and used to prepare the affinity matrix by coupling to epoxy-activated Sepharose 6B (AES-Sepharose). AES (Ki approximately 1.7 nM) is similar in potency to the parent compound, spiperone (Ki approximately 0.8 nM), in competing for [3H]spiperone-binding activity. AES has no significant potency in competing for the dopamine D1 receptor as assessed by competition for [3H]SCH23390 binding (Ki greater than 1 microM). Covalent photoaffinity labeling of the dopamine D2 receptor in bovine striatal membranes with N-(p-azido-m-[125I]iodophenethyl)spiperone [( 125I]N3-NAPS) was prevented by AES at nanomolar concentrations. The dopamine D2 receptor was solubilized from bovine striatal membranes using 0.25% cholate in the presence of high ionic strength, followed by precipitation and subsequent treatment with 0.5% digitonin. Nearly 100% of the [3H]spiperone-binding activity in the cholate-digitonin solubilized preparation was absorbed at a receptor-to-resin ratio of 2:1 (v/v). Dopamine D2 receptor was eluted from the affinity resin using a competing dopaminergic antagonist molecule, haloperidol. Recovery of dopamine D2 receptor activity from the affinity matrix was approx. 9% of the activity adsorbed to the resin. The [3H]spiperone-binding activity in AES-Sepharose affinity purified preparations is saturable and of high affinity (0.2 nM). Affinity-purified preparations maintain the ligand-binding characteristics of a dopamine D2 receptor as assessed by agonist and antagonist competition for [3H]spiperone binding.