Purification and characterization of the D2-dopamine receptor from bovine anterior pituitary

Structure of a D2 dopamine receptor-G-protein complex in a lipid membrane

The D₂ dopamine receptor (DRD2) is a therapeutic target for Parkinson’s disease¹ and antipsychotic drugs². DRD2 is activated by the endogenous neurotransmitter dopamine and synthetic agonist drugs such as bromocriptine³, leading to stimulation of G_i and inhibition of adenylyl cyclase. We used cryo-electron microscopy to elucidate the structure of an agonist-bound activated DRD2-G_i complex reconstituted into a phospholipid membrane. The extracellular ligand binding site of DRD2 is remodeled in response to agonist binding, with conformational changes in extracellular loop 2 (ECL2), transmembrane domain 5 (TM5), TM6, and TM7 propagating to opening of the intracellular G_i binding site. The DRD2-G_i structure represents the first experimental model of a GPCR-G protein complex embedded in a phospholipid bilayer, which serves as a benchmark to validate the interactions seen in previous detergent-bound structures. The structure also reveals interactions that are unique to the membrane-embedded complex, including helix 8 burial in the inner leaflet, ordered lysine and arginine sidechains in the membrane interfacial regions, and lipid anchoring of the G protein in the membrane. Our model of the activated DRD2 will help inform the design of subtype-selective DRD2 ligands for multiple human CNS disorders.

Dopaminergic receptor-ligand binding assays based on molecularly imprinted polymers on quartz crystal microbalance sensors

Molecularly imprinted polymers (MIPs) have been successfully applied as selective materials for assessing the binding activity of agonist and antagonist of dopamine D1 receptor (D1R) by using quartz crystal microbalance (QCM). In this study, D1R derived from rat hypothalamus was used as a template and thus self-organized on stamps. Those were pressed into an oligomer film consisting of acrylic acid: N-vinylpyrrolidone: N,N'-(1,2-dihydroxyethylene) bis-acrylamide in a ratio of 2:3:12 spin coated onto a dual electrode QCM. Such we obtained one D1R-MIP-QCM electrode, whereas the other electrode carried the non-imprinted control polymer (NIP) that had remained untreated. Successful imprinting of D1R was confirmed by AFM. The polymer can re-incorporate D1R leading to frequency responses of 100-1200Hz in a concentration range of 5.9-47.2µM. In a further step such frequency changes proved inherently useful for examining the binding properties of test ligands to D1R. The resulting mass-sensitive measurements revealed Kd of dopamine∙HCl, haloperidol, and (+)-SCH23390 at 0.874, 25.6, and 0.004nM, respectively. These results correlate well with the values determined in radio ligand binding assays. Our experiments revealed that D1R-MIP sensors are useful for estimating the strength of ligand binding to the active single site. Therefore, we have developed a biomimetic surface imprinting strategy for QCM studies of D1R-ligand binding and presented a new method to ligand binding assay for D1R.

Cell-free protein synthesis and purification of human dopamine D2 receptor long isoform

The human dopamine D2 receptor long isoform (D2L) has significant implications in neurological and neuropsychiatric disorders such as Parkinson's disease and schizophrenia. Detailed structural knowledge of this receptor is limited owing to its highly hydrophobic nature, which leads to protein aggregation and host toxicity when expressed in cellular systems. The newly emerging field of cell-free protein expression presents numerous advantages to overcome these challenges. This system utilizes protein synthesis machinery and exogenous DNA to synthesize functional proteins outside of intact cells. This study utilizes two different cell-free systems for the synthesis of human dopamine D2L receptor. These include the Escherichia coli lysate-based system and the wheat-germ lysate-based system. The bacterial cell-free method used pET 100/D-TOPO vector to synthesize hexa-histidine-tagged D2L receptor using a dialysis bag system; the resulting protein was purified using nickel-nitrilotriacetic acid affinity resin. The wheat germ system used pEU-glutathione-S-transferase (GST) vector to synthesize GST-tagged D2L receptor using a bilayer translation method; the resulting protein was purified using a GST affinity resin. The presence and binding capacity of the synthesized D2L receptor was confirmed by immunoblotting and radioligand competition assays, respectively. Additionally, in-gel protein sequencing via Nano LC-MS/MS was used to confirm protein synthesis via the wheat germ system. The results showed both systems to synthesize microgram quantities of the receptor. Improved expression of this highly challenging protein can improve research and understanding of the human dopamine D2L receptor.

Discovery and characterization of orthosteric and allosteric muscarinic M2 acetylcholine receptor ligands by affinity selection-mass spectrometry

Screening assays using target-based affinity selection coupled with high-sensitivity detection technologies to identify small-molecule hits from chemical libraries can provide a useful discovery approach that complements traditional assay systems. Affinity selection-mass spectrometry (AS-MS) is one such methodology that holds promise for providing selective and sensitive high-throughput screening platforms. Although AS-MS screening platforms have been used to discover small-molecule ligands of proteins from many target families, they have not yet been used routinely to screen integral membrane proteins. The authors present a proof-of-concept study using size exclusion chromatography coupled to AS-MS to perform a primary screen for small-molecule ligands of the purified muscarinic M2 acetylcholine receptor, a G-protein-coupled receptor. AS-MS is used to characterize the binding mechanisms of 2 newly discovered ligands. NGD-3350 is a novel M2-specific orthosteric antagonist of M2 function. NGD-3366 is an allosteric ligand with binding properties similar to the allosteric antagonist W-84, which decreases the dissociation rate of N-methyl-scopolamine from the M2 receptor. Binding properties of the ligands discerned from AS-MS assays agree with those from in vitro biochemical assays. The authors conclude that when used with appropriate small-molecule libraries, AS-MS may provide a useful high-throughput assay system for the discovery and characterization of all classes of integral membrane protein ligands, including allosteric modulators.

Purification and reconstitution of a recombinant human neurokinin-1 receptor

Recombinant human neurokinin-1 receptors expressed in insect cells have been purified to near homogeneity by sequential metal-chelating chromatography and gel filtration chromatography. The purified receptor consists of a single polypeptide with an apparent molecular weight of 50 kD as revealed by SDS gel electrophoresis, and exhibits a specific activity of 19 nmol of L-703,606 bound per mg of protein. Immunoblot experiments further confirm the identity of the stained protein band. The purified receptor binds the antagonist L-703,606 with an affinity similar to that of native human neurokinin-1 receptor, and binds the agonist substance P with an affinity similar to that of the low affinity state of uncoupled native receptor. The purified receptor can be reconstituted with membranes from uninfected insect cells, and the reconstitution results in an increased affinity for substance P, consistent with the reappearance of the high affinity state of the receptor for agonist in the presence of endogenous G proteins. These data indicate that the purified neurokinin-1 receptor is functional with respect to agonist and antagonist binding and G protein coupling.

Dopamine receptors and brain function

In the central nervous system (CNS), dopamine is involved in the control of locomotion, cognition, affect and neuroendocrine secretion. These actions of dopamine are mediated by five different receptor subtypes, which are members of the large G-protein coupled receptor superfamily. The dopamine receptor subtypes are divided into two major subclasses: the D1-like and D2-like receptors, which typically couple to Gs and Gj mediated transduction systems. In the CNS, the various receptor subtypes display specific anatomical distributions, with D1-like receptors being mainly post-synaptic and D2-like receptors being both pre- and post-synaptic. D1 and D2 dopamine receptors, the most abundant subtypes in the CNS, appear to be expressed largely in distinct neurons. Substance P and dynorphin, which are expressed in D1 receptor-containing neurons, as well as pre-proenkephalin in D2 receptor-containing neurons, have been used as monitors of dopaminergic activity in the CNS. Expression of immediate early genes, in particular fos, has also been found to correlate with dopaminergic transmission. Dopamine released from the hypothalamus controls the synthesis and secretion of prolactin from the anterior pituitary via D2 dopamine receptors. As yet none of the dopamine receptor subtypes have been associated with the etiology of psychotic disorders, such as schizophrenia. However, the recent characterization of D3 and D4 receptors which are, interestingly, expressed in areas of the CNS mediating cognition and affect or showing increased affinity for certain neuroleptics, have renewed the interest and hope of finding effective neuroleptics devoid of side effects. Finally, the recent production of genetically-derived animals lacking several of these receptor genes should help elucidate which specific physiological paradigms the receptors mediate.

Purification of recombinant porcine m2 muscarinic acetylcholine receptor from Chinese hamster ovary cells. Circular dichroism spectra and ligand binding properties

The recombinant porcine m2 muscarinic acetylcholine receptor (rPm2R) from Chinese hamster ovary cells has been purified to homogeneity. Two mg of purified rPm2R, with a specific activity of 12 nmol of R-(-)-quinuclidinyl benzilate/mg of protein, were obtained from 30 ml of packed Chinese hamster ovary cells. The apparent molecular mass (78.5 kDa) and specific activity for the rPm2R preparation were the same as that for the Pm2R purified from atrial tissue, but the yield was 100 times greater. Purified rPm2R bound agonist and antagonist with the same affinities and coupled to the inhibitory guanine nucleotide-binding protein with the same efficiency as the purified native atrial Pm2R. Ligand binding studies were consistent with a single class of antagonist binding sites but two subclasses of agonist binding sites. The fraction of rPm2R having high affinity for agonists was increased by mM Mg2+, low detergent concentration, and low temperature. Circular dichroism spectra obtained for the purified rPm2R with and without agonists were indistinguishable, but spectra for the antagonist-occupied receptor showed reproducibly deeper characteristic negative deflections at 208 and 220 nm. Secondary structure analysis of the CD spectra predicted 53% alpha-helix for the free receptor and 49% alpha-helix for the R-(-)-quinuclidinyl benzilate-receptor complex.

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.

Gene expression in cells of the central nervous system

This review summarized a part of our studies over a long period of time, relating them to the literature on the same topics. We aimed our research toward an understanding of the genetic origin of brain specific proteins, identified by B. W. Moore and of the high complexity of the nucleotide sequence of brain mRNA, originally investigated by W. E. Hahn, but have not completely achieved the projected goal. According to our studies, the reason for the high complexity in the RNA of brain nuclei might be the high complexity in neuronal nuclear RNA as described in the Introduction. Although one possible explanation is that it results from the summation of RNA complexities of several neuronal types, our saturation hybridization study with RNA from the isolated nuclei of granule cells showed an equally high sequence complexity as that of brain. It is likely that this type of neuron also contains numerous rare proteins and peptides, perhaps as many as 20,000 species which were not detectable even by two-dimensional PAGE. I was possible to gain insight into the reasons for the high sequence complexity of brain RNA by cloning the cDNA and genomic DNA of the brain-specific proteins as described in the previous sections. These data provided evidence for the long 3'-noncoding regions in the cDNA of the brain-specific proteins which caused the mRNA of brain to be larger than that from other tissues. During isolation of such large mRNAs, a molecule might be split into a 3'-poly(A)+RNA and 5'-poly(A)-RNA. In the studies on genomic DNA, genes with multiple transcription initiation sites were found in brain, such as CCK, CNP and MAG, in addition to NSE which was a housekeeping gene, and this may contribute to the high sequence complexity of brain RNA. Our studies also indicated the presence of genes with alternative splicing in brain, such as those for CNP, MAG and NGF, suggesting a further basis for greater RNA nucleotide sequence complexity. It is noteworthy that alternative splicing of the genes for MBP and PLP also produced multiple mRNAs. Such a mechanism may be a general characteristic of the genes for the myelin-specific proteins produced by oligodendrocytes. In considering the high nucleotide sequence complexity, it is interesting that MAG and S-100 beta genes etc. possess two additional sites for poly(A).(ABSTRACT TRUNCATED AT 400 WORDS)

An anti-peptide antibody that recognizes the dopamine D2 receptor from bovine striatum

The bovine dopamine D2 receptor was purified by wheat-germ-agglutinin-Sepharose chromatography and affinity chromatography, using the D2-receptor-specific agonist N-0434. Purification yields a preparation with a major protein band of 95 kDa. In order to ascertain the identity of this protein, polyclonal antibodies against the dopamine D2 receptor have been raised using synthetic peptides based on the predicted amino acid sequence of the cloned D2 receptor. For the initial screening of these antibodies, three fusion proteins consisting of beta-galactosidase and receptor fragments were constructed. One antiserum reacted strongly with the corresponding D2 receptor fusion protein, both on Western blots and in immunoprecipitation experiments. In each case, recognition was inhibited by competition with free peptide. On Western blots of partially purified receptor preparations from bovine striatum, the antiserum specifically recognized a 95-kDa glycoprotein. From similar preparations, the antiserum precipitated a substantial proportion of active D2 receptor, as determined by a decrease in [3H]spiperone binding in the supernatant. Active receptor could be released from the immunoprecipitate by addition of free peptide. Immunocytochemical analysis of cells transiently transfected with DNA coding for the D2 receptor showed specific staining of transfected cells. The antibody raised against a sequence in the third intracellular loop is able to shift the affinity of the receptor for dopamine from high to low, indicating that the antiserum may be interfering with receptor-GTP-binding-protein interactions.

Production and characterization of a monoclonal antibody to dopamine D2 receptor: Comparison with a polyclonal antibody to a different epitope

A monoclonal antibody (Mab) that recognizes the rat dopamine D2 receptor (DAR) has been generated using DAR specific peptide. The Mab, IgM isotype recognizes five proteins (Mr 220, 145, 95, 66 and 47 kDa) in striatal membrane on Western blot. Preincubation of Mab with free peptide blocked the labeling of all five bands. A polyclonal antibody against peptide from a different region of the DAR, reacted with three out of five proteins (220, 66, and 47 kDa) in these membranes. The DAR antagonist NAPS-biotinyl binds to a 220 kDa protein in striatal membrane on ligand blotts; the labeling can be blocked by the addition of 2 microM sulpride. The 220 kDa Mab reactive protein was less in cerebellum and was absent in the liver. Neither the Mab nor polyclonal antibody inhibited binding of a DAR antagonist, [3H]YM09151-2, to the striatal membranes. These antibodies will enable us to study the structure/function and regulation of the synthesis of DAR protein.

Post-transcriptional regulation of loss of rat striatal D2 dopamine receptor during aging

The mechanism(s) underlying age-associated diminutions in the rat striatal D2 dopamine receptor (D2-receptor) number was investigated. The levels of D2-receptor mRNA in 4-, 12- and 18-month-old rat striata were found not to change. In contrast, the levels of 110 kDa protein, labeled with a D2-receptor specific antibody, decreased in parallel with [3H]YM-09151-2 binding to striatal membranes. These data suggest a role for post-transcriptional mechanism(s) in age-associated decrease in D2-receptor.

Inhibition of prolactin release by gonadotropin-releasing hormone-associated Peptide in benign, dopamine-sensitive and in malignant, dopamine-resistant pituitary tumors

Since the gonadotropin-releasing hormone-associated peptide (GAP) has been reported to be capable of inhibiting prolactin release from normal lactotrophs, with the present study we have examined the in vitro effects of GAP on prolactin release in an estrone-induced, dopamine-sensitive rat pituitary adenoma and two malignant, transplantable and dopamine-resistant rat pituitary tumors, 7315a and MtTW15. Enzymatically dispersed cells obtained from the three types of tumor were cultured in multiwell dishes for 4 days. On the fifth day, the cells were exposed for 4 h to human GAP 1-56 or to the analog GAP 42-56 or to rat GAP 1-53, at various concentrations. In some experiments, the effect of a pretreatment of the cells for 16 h with pertussis toxin before exposure to human GAP was also evaluated. In the three tissues, rat GAP was able to inhibit prolactin release in a dose-dependent manner. Human GAP 1-56 and GAP 42-56 were able to inhibit prolactin release in a dose-dependent manner in all cells except those of the MtTW15 tumor. Furthermore, in adenomatous cells, the inhibitory effects of these peptides were suppressed by pretreatment of the cells with pertussis toxin. These findings indicate that GAP is capable of inhibiting prolactin release even in dopamine-resistant pituitary tumors. This inhibition is exerted through a pertussis toxin-sensitive G-protein-dependent signaling mechanism in adenomatous cells.

Molecular biology of the dopamine receptors

Because of their importance in pathophysiology, the dopamine receptors have been the subjects of intense pharmacological and physiological research. Their structures have remained mostly unknown until recently with the application of molecular biological approaches. The cloning of the first dopamine receptor, the D2 receptor opened a new era in dopamine receptor research. It has led not only to new studies of its own biology but also to the characterization of the other dopamine receptors. The most striking conclusion of this fast moving research is that the dopamine receptors are more diverse than expected from their pharmacological characterizations. We discuss here the history of the cloning of the dopamine receptors and the impact that this research had on our understanding of the dopamine system.

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.

Characterization of D2 receptors and dopamine levels in the thalamus of the rat

We kinetically characterized D2 receptors in thalami pooled from a group of Sprague-Dawley rats and then determined thalamic levels of dopamine (DA), homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC), and norepinephrine (NE) in relation to a measure of thalamic DA D2 receptor densities in another group of rats. The equilibrium dissociation constant (kd) was estimated as 0.1 nM by three independent methods, while the Bmax for thalamic D2 receptors was found to be 6.4 fmol/mg p using 3H-spiperone as ligand and ketanserin to occlude 5HT2 binding. Kinetic constants were in agreement with previously reported kinetic data from rodent caudate-putamen. This suggests that thalamic D2 receptors are similar to D2 receptors from other brain areas. Mean thalamic levels of DA (22.6 ng/mg p), DOPAC (1.19 ng/mg p) and HVA (0.31 ng/mg p) concur with previous reports of a sparse distribution of thalamic DA neurons. D2 receptor densities were positively correlated with DA metabolites DOPAC (P less than .05; r = 0.423) and HVA (P less than .05; r = 0.368), but not DA or NE. These results establish fundamental characteristics of thalamic DA neurotransmission to assist in the investigation of behavioral pharmacology of this area.

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.

Purification and characterization of bovine cerebral cortex A1 adenosine receptor

A1 adenosine receptors (A1AR) acting via the inhibitory guanine nucleotide binding protein inhibit adenylate cyclase activity in brain, cardiac, and adipose tissue. We now report the purification of the A1AR from bovine cerebral cortex. This A1AR is distinct from other A1ARs in that it displays an agonist potency series of N6-R-phenylisopropyladenosine (R-PIA) greater than N6-S-phenylisopropyladenosine greater than (S-PIA) greater than 5'-N-ethylcarboxamidoadenosine (NECA) compared to the traditional potency series of R-PIA greater than NECA greater than S-PIA. The A1AR was solubilized in 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps) and then purified by chromatography on an antagonist [xanthine amine congener (XAC)]-coupled Affi-Gel 10 followed by hydroxylapatite chromatography. Following purification, sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single protein of Mr 36,000 by silver staining, Na125I iodination with chloramine T and photoaffinity labeling with [125I]8-[4-[[[[2-(4-aminophenyl acetylamino) ethyl] carbonyl] methyl] oxy]-phenyl]-1,3- dipropylxanthine. This single protein displayed all the characteristics of the A1AR, including binding an antagonist radioligand [( 3H]XAC) with high affinity (Kd = 0.7 nM) and in a saturable manner (Bmax greater than 4500 pmol/mg). Agonist competition curves demonstrated the expected bovine brain A1AR pharmacology: R-PIA greater than S-PIA greater than NECA. The overall yield from soluble preparation was 7%. The glycoprotein nature of the purified A1AR was determined with endo- and exoglycosidases. Deglycosylation with endoglycosidase F increased the mobility of the A1AR from Mr 36,000 to Mr 32,000 in a single step. The A1AR was sensitive to neuraminidase but resistant to alpha-mannosidase, suggesting the single carbohydrate chain was of the complex type. This makes the bovine brain A1AR similar to rat brain and fat A1AR in terms of its carbohydrate chains yet the purified A1AR retains its unique agonist potency series observed in membranes.

Control of phospholipid turnover and prolactin release in a dopamine-sensitive, prolactin-secreting rat pituitary adenoma and in two dopamine-resistant, prolactin-secreting rat pituitary tumors

Abstract The secretion of prolactin by the pituitary gland is under a tonic inhibitory control exerted by tubero-infundibular dopamine. Recently, it has been suggested that dopamine may exert its action by inhibiting production of inositol phosphates and mobilization of intracellular Ca(2+). To study the effects of dopamine on the production of inositol phosphates and prolactin release, we have utilized an estrone-induced, dopamine-sensitive rat pituitary adenoma and two transplantable and dopamine-resistant rat pituitary tumors, 7315a and MtTW15. Purified cells, obtained from the three tissues, were incubated for 30 min in media with drugs (thyrotropin-releasing hormone or angiotensin II) stimulating inositol phosphates and prolactin release, in the presence or the absence of dopamine. Basal production of inositol phosphates and prolactin release by adenomatous lactotrophs were inhibited by dopamine. Thyrotropin-releasing hormone and angiotensin II stimulated inositol phosphates by adenomatous and 7315a cells. This effect was antagonized by dopamine in adenomatous cells. Prolactin release by adenomatous cells only was stimulated by thyrotropin-releasing hormone and angiotensin II. This stimulation was inhibited by dopamine. The results show differences, in the mechanisms of regulation of prolactin release, between adenoma and transplantable pituitary tumors as well as between the two tumors themselves. These differences may be responsible, in part, for the resistance of the two transplantable pituitary tumors to the inhibitory effects of dopamine on prolactin release and tumor size. Our results obtained both with adenoma and tumoral cells also suggest that inositol phosphates probably intervene only in the late phases of dopamine inhibition of prolactin release and only in the presence of a normal Ca(2+) signaling system.

Solubilization and hydrodynamic characteristics of the erythropoietin receptor. Evidence for a multimeric complex

In order to study the erythropoietin receptor in its native state, we solubilized erythropoietin-receptor complexes from spleen cell membranes of mice infected with the anemia strain of Friend virus using mild detergents. Among 11 tested detergents, Triton X-100 and Lubrol PX were the most effective. Triton X-100 was therefore selected for this study. The solubilized complexes appeared to be well representative of the total membrane receptor population as indicated by cross-linking experiments and affinity measurements. The hydrodynamic characteristics of the complexes were determined by gel filtration chromatography and ultracentrifugation through sucrose gradients prepared with H2O or D2O. Although erythropoietin-receptor-detergent complexes exhibited some heterogeneity, we determined the following minimal hydrodynamic values: sedimentation coefficient (s20,w): 11.7 +/- 0.8 S, Stokes radius: 7.7 +/- 0.2 nm, partial specific volume: 0.774 +/- 0.017 ml/g, giving a molecular mass of 458 +/- 66 kDa. The contribution of the detergent was estimated to be 28% from the measured partial specific volume, giving an estimated molecular mass of 330 +/- 48 kDa for the erythropoietin-receptor complex. The minimal molecular mass value was significantly greater than those obtained by polyacrylamide gel electrophoresis under denaturing conditions, strongly suggesting that the erythropoietin receptors were present as multimeric complexes. The nature of these complexes is discussed. Beside this major component our results revealed the presence of higher-molecular-mass erythropoietin binding components. We also demonstrated that erythropoietin-receptor complexes could be precipitated with anti-erythropoietin antibodies. This property should greatly improve the purification of erythropoietin receptors.

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.

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.

Presence of D2-dopamine receptors in human term placenta

We studied the binding of [3H]-spiperone on human term placental membranes. This binding reached plateau level after 30 min incubation at 37 degrees C and was reversed (t1/2 approximately 5 min) by addition of an excess of unlabeled spiperone. Scatchard analysis of saturation experiments with increasing doses of [3H]-spiperone (0-25 nM) showed one class of high affinity binding sites with a dissociation constant (Kd) of 14 +/- 2 nM and a maximal binding capacity (Bmax) of 222 +/- 9 fmoles/mg protein. The affinity of 5 competitors was determined in competitive binding assays. The D2-dopamine antagonists were the most potent inhibitors: Ki for spiperone and haloperidol were 8 +/- 2 and 56 +/- 22 nM respectively. Dopamine inhibited [3H]-spiperone binding with a Ki of 570 +/- 50 microM whereas Schering 23390 (D1 antagonist) and propranolol (beta-adrenergic antagonist) were without effect. The binding was also inhibited by 100 microM GTP gamma S (38 +/- 8% inhibition), indicating that the dopamine receptor is coupled with a GTP binding protein. These results demonstrate for the first time the presence of D2-dopamine receptors in human placenta.

Multiple D2 dopamine receptors produced by alternative RNA splicing

Dopamine receptor belong to a large class of neurotransmitter and hormone receptors that are linked to their signal transduction pathways through guanine nucleotide binding regulatory proteins (G proteins). Pharmacological, biochemical and physiological criteria have been used to define two subcategories of dopamine receptors referred to as D1 and D2. D1 receptors activate adenylyl cyclase and are coupled with the Gs regulatory protein. By contrast, activation of D2 receptors results in various responses including inhibition of adenylyl cyclase, inhibition of phosphatidylinositol turnover, increase in K+ channel activity and inhibition of Ca2+ mobilization. The G protein(s) linking the D2 receptors to these responses have not been identified, although D2 receptors have been shown to both copurify and functionally reconstitute with both Gi and Go related proteins. The diversity of responses elicited by D2-receptor activation could reflect the existence of multiple D2 receptor subtypes, the identification of which is facilitated by the recent cloning of a complementary DNA encoding a rat D2 receptor. This receptor exhibits considerable amino-acid homology with other members of the G protein-coupled receptor superfamily. Here we report the identification and cloning of a cDNA encoding an RNA splice variant of the rat D2 receptor cDNA. This cDNA codes for a receptor isoform which is predominantly expressed in the brain and contains an additional 29 amino acids in the third cytoplasmic loop, a region believed to be involved in G protein coupling.

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.