A novel affinity purification of D-1 dopamine receptors from rat striatum

Nigral-specific increase in ser31 phosphorylation compensates for tyrosine hydroxylase protein and nigrostriatal neuron loss: Implications for delaying parkinsonian signs

Compensatory mechanisms that augment dopamine (DA) signaling are thought to mitigate onset of hypokinesia prior to major loss of tyrosine hydroxylase (TH) in striatum that occurs in Parkinson's disease. However, the identity of such mechanisms remains elusive. In the present study, the rat nigrostriatal pathway was unilaterally-lesioned with 6-hydroxydopamine (6-OHDA) to determine whether differences in DA content, TH protein, TH phosphorylation, or D1 receptor expression in striatum or substantia nigra (SN) aligned with hypokinesia onset and severity at two time points. In striatum, DA and TH loss reached its maximum (>90%) 7 days after lesion induction. However, in SN, no DA loss occurred, despite ∼60% TH loss. Hypokinesia was established at 21 days post-lesion and maintained at 28 days. At this time, DA loss was ∼60% in the SN, but still of lesser magnitude than TH loss. At day 7 and 28, ser31 TH phosphorylation increased only in SN, corresponding to less DA versus TH protein loss. In contrast, ser40 TH phosphorylation was unaffected in either region. Despite DA loss in both regions at day 28, D1 receptor expression increased only in lesioned SN. These results support the concept that augmented components of DA signaling in the SN, through increased ser31 TH phosphorylation and D1 receptor expression, contribute as compensatory mechanisms against progressive nigrostriatal neuron and TH protein loss, and may mitigate hypokinesia severity.

Dopamine D₁-like receptors regulate the α₁A-adrenergic receptor in human renal proximal tubule cells and D₁-like dopamine receptor knockout mice

The homeostatic control of blood pressure hinges upon the delicate balance between prohypertensinogenic and antihypertensinogenic systems. D₁-like dopamine receptors [dopamine D₁ and D₅ receptors (D₁Rs and D₅Rs, respectively)] and the α₁A-adrenergic receptor (α₁A-AR) are expressed in the renal proximal tubule and engender opposing effects on Na(+) transport, i.e., natriuresis (via D₁Rs and D5Rs) or antinatriuresis (via α₁A-ARs). We tested the hypothesis that the D₁R/D₅R regulates the α₁A-AR. D₁-like dopamine receptors coimmunoprecipitated, colocalized, and cofractionated with α₁A-ARs in lipid rafts in immortalized human renal proximal tubule cells. Long-term treatment with the D₁R/D₅R agonist fenoldopam resulted in decreased D₁R and D₅R expression but increased α₁A-AR abundance in the plasma membrane. Short-term fenoldopam treatment stimulated the translocation of Na(+)-K(+)-ATPase from the plasma membrane to the cytosol that was partially reversed by an α₁A-AR agonist, which by itself induced Na(+)-K(+)-ATPase translocation from the cytosol to the plasma membrane. The α₁A-AR-specific agonist A610603 also minimized the ability of fenoldopam to inhibit Na(+)-K(+)-ATPase activity. To determine the interaction among D₁Rs, D₅Rs, and α₁A-ARs in vivo, we used phenylephrine and A610603 to decrease Na(+) excretion in several D1-like dopamine receptor knockout mouse strains. Phenylephrine and A61603 treatment resulted in a partial reduction of urinary Na(+) excretion in wild-type mice and its abolition in D1R knockout, D₅R knockout, and D₁R-D₅R double-knockout mice. Our results demonstrate the ability of the D₁-like dopamine receptors to regulate the expression and activity of α₁A-AR. Elucidating the intricacies of the interaction among these receptors is crucial for a better understanding of the crosstalk between anti- and pro-hypertensive systems.

Differential subcellular distribution of rat brain dopamine receptors and subtype-specific redistribution induced by cocaine

We investigated the subcellular distribution of dopamine D(1), D(2) and D(5) receptor subtypes in rat frontal cortex, and examined whether psychostimulant-induced elevation of synaptic dopamine could alter the receptor distribution. Differential detergent solubilization and density gradient centrifugation were used to separate various subcellular fractions, followed by semi-quantitative determination of the relative abundance of specific receptor proteins in each fraction. D(1) receptors were predominantly localized to detergent-resistant membranes, and a portion of these receptors also floated on sucrose gradients. These properties are characteristic of proteins found in lipid rafts and caveolae. D(2) receptors exhibited variable distribution between cytoplasmic, detergent-soluble and detergent-resistant membrane fractions, yet were not present in buoyant membranes. Most D(5) receptor immunoreactivity was distributed into the cytoplasmic fraction, failing to sediment at forces up to 300,000g, while the remainder was localized to detergent-soluble membranes in cortex. D(5) receptors were undetectable in detergent-resistant fractions or raft-like subdomains. Following daily cocaine administration for seven days, a significant portion of D(1) receptors translocated from detergent-resistant membranes to detergent-soluble membranes and the cytoplasmic fraction. The distributions of D(5) and D(2) receptor subtypes were not significantly altered by cocaine treatment. These data imply that D(5) receptors are predominantly cytoplasmic, D(2) receptors are diffusely distributed within the cell, whereas D(1) receptors are mostly localized to lipid rafts within the rat frontal cortex. Dopamine receptor subtype localization is susceptible to modulation by pharmacological manipulations that elevate synaptic dopamine, however the functional implications of such drug-induced receptor warrant further investigation.

Molecular dynamics of a biophysical model for β2-adrenergic and G protein-coupled receptor activation

This study analyzes 16 molecular dynamic simulations of a biophysical model for beta(2)-adrenergic (B2AR) and G protein-coupled receptor (GPCR) activation. In this model, a highly conserved cysteine residue, C106 (C3.25 or CysIII:01), provides a free sulfhydryl or thiol group in an acid-base equilibrium between uncharged (RSH) and charged (RS(-)) states that functions as an electrostatic molecular switch for receptor activation. The transition of C106 in the B2AR between acid and base states significantly changes the helical/transmembrane (TM) domain interactions and the electrostatic interaction energy differences (DeltaDeltaE(EL)). The DeltaDeltaE(EL) changes correlate well with the experimentally observed ligand efficacies. The TM interaction energies display patterns compatible with those previously recognized as responsible for GPCR activation. Key differences between the agonist, epinephrine, and the antagonist, pindolol, are seen for the TM3 x 6, TM3 x 4, TM6 x 7 and TM1 x 7 interaction energies. Pindolol also produces a weaker DeltaDeltaE(EL) interaction and less TM interaction energy changes, which are important differences between the agonist and antagonist ligands. The D115E mutant with pindolol displays a greater DeltaDeltaE(EL) and TM interactions than for the wild-type B2AR with pindolol. This explains the higher activity of pindolol in the D115E mutant. The constitutively active D130A mutant displays TM interaction patterns similar to those for the activating ligands implying a common pattern for receptor activation. These findings support the broad concept of protean agonism and demonstrate the potential for allosteric modulation. They also demonstrate that this two-state model agrees with many previous experimental and theoretical observations of GPCRs.

Cocaine inhibits NGF-induced PC12 cells differentiation through D(1)-type dopamine receptors

In utero cocaine exposure can adversely affect CNS development. Previous studies showed that cocaine inhibits neuronal differentiation in a dose-dependent fashion in nerve growth factor (NGF)-stimulated PC12 cells. Cocaine binds with high affinity to several neurotransmitter transporters, resulting in elevated neurotransmitter levels in nerve endings. To determine if cocaine inhibits neurite outgrowth through the effects of these neurotransmitters, we applied dopamine, norepinephrine, serotonin, and acetylcholine to NGF-induced PC12 cells. Dopamine was the only neurotransmitter to inhibit neurite outgrowth significantly in a dose-dependent pattern without affecting cell viability. Norepinephrine and acetylcholine did not affect neurite outgrowth, while serotonin enhanced it. Furthermore, GBR 12909, a potent dopamine transporter (DAT) inhibitor, yielded similar effects. We then showed PC12 cells express D(1) and D(2) receptors and DAT proteins. Dopamine uptake measured over time was significantly blocked by cocaine and GBR 12909 which may result in elevated extracellular dopamine. The role of dopamine receptors in PC12 differentiation was further examined by using D(1) and D(2) specific receptor agonists. Only the D(1) agonist, SKF-38393, had a significant dose-dependent inhibitory effect. In addition, a D(1) antagonist produced significant recovery of neurite outgrowth in cocaine-treated cells. These findings suggest that cocaine inhibitory effects on neuronal differentiation are mediated through its binding to the dopamine transporter, resulting in increased dopamine level in the synapses. Subsequently, up regulation of D(1) receptors alters NGF signaling pathways.

Regulation of human D1 dopamine receptor function and gene expression in SK-N-MC neuroblastoma cells

SK-N-MC human neuroblastoma cells express functional D1, but not D5, dopaminergic receptors. Stimulating cells with dopamine or the D1-selective agonist, SKF R-38393, rapidly (t(1/2) = 1 h) resulted in > 95% attenuation of dopamine-mediated accumulation of cyclic AMP, without any change in D1 dopamine receptor levels. Prolonged (> 4 h) exposure of cells to dopamine attenuated D1 receptor levels to 45-50% of control (t(1/2) = 8 h) and was accompanied by a loss of high-affinity binding sites. At the molecular level, the expression of D1 receptor messenger RNA was bimodal: an initial increase (by approximately 60%) of receptor messenger RNA within 2 h of treatment of cells with dopamine was followed by a decline to 50% below control messenger RNA levels. Low concentrations (1-10 nM) of dopamine also potentiated D1 messenger RNA levels (up to 48%), resulting in a twofold increase in receptor levels. Transfection studies with the cloned human D1 promoter construct, pGL-D1P, indicated that the up-regulation of D1 messenger RNA was due to activation of promoter by dopamine. The dopamine-mediated up-regulation of both D1 receptor messenger RNA and promoter was prevented by the D1-selective antagonist, SCH 23390. The results suggest that dopamine regulates D1 receptor gene and protein expression in a bimodal manner, partly through activation of the receptor promoter. Moreover, the effects of dopamine are independent of the second messenger, cyclic AMP.

Photoaffinity labeling of D1 and D5 dopamine receptors

Although human D1 and D5 dopamine receptors are encoded by distinct genes and share only 50% sequence homology at the amino acid level, their pharmacological properties are identical. Using a selective D1 receptor photoaffinity radioligand, (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4-azidophenyl)-2,3,4,5-tetrahyd ro-1H-3-benzazepine ([125I]MAB), we have further probed the molecular properties of these receptors in transfected GH4C1 rat pituitary cells. Under reversible, non-covalent binding conditions, [125I]MAB bound to both the D1 and the D5 receptors with identical affinities, dopaminergic selectivity and stereospecificity. Upon photoactivation of the bound [125I]MAB, the label was incorporated into a approximately 64,000 mol. wt protein corresponding to the D1 dopamine receptor. However, there was no specific photoincorporation of the ligand observed in D5 receptors. The lack of [125I]MAB photolabeling of D5 receptors was independent of the cell line chosen, since similar results were obtained using other transfected cells. The data suggest that although both D1 and D5 receptors share structurally similar binding sites, the protein domains around the sites are different. Thus, although there are currently no specific compounds which bind preferentially to D1 or D5 receptors, these receptors can be distinguished from one another by the inability of [125I]MAB to photolabel D5, but not D1, receptors. Such selective targeting of a specific receptor may be useful in understanding the functional importance and/or interaction between closely related members of the same receptor family when co-expressed in the same cell.

Molecular and structural differences between rat brain D-1 and renal DA-1 dopamine receptors

Renal DA-1 dopamine receptors in proximal tubules (PTs) of the Wistar-Kyoto (WKY) rat display pharmacological binding properties which are different from central nervous system (CNS) striatal D-1 dopamine receptors. In general, the renal DA-1 receptors display affinity binding values of dopaminergic drugs which are 6-36-fold less than those seen for brain D-1 receptors. The renal and brain DA receptors also displayed differential sensitivity toward the alkylating agent, N-ethylmaleimide (NEM). Inactivation of 50% of DA-1 renal receptors was achieved at lower concentrations of NEM (5.2 microM), relative to brain D-1 receptors (140 microM). Western blot analyses of rat pituitary GH4C1 cells, transfected with human CNS D-1 receptor cDNA, with human anti-D-1 dopamine receptor antiserum, detected a single polypeptide with M(r) of 66 kDa. In PTs, a specific polypeptide of higher molecular weight (M(r) = 72 kDa) was seen. Surprisingly, in rat striatal membranes, the D-1 antiserum failed to detect any proteins within this molecular weight range. Photoaffinity labeling studies with a DA-1 selective photoligand, identified the identical protein by autoradiography and Western blots in kidney, but not in striate. Together, these data indicate that renal DA-1 dopamine receptors have distinct molecular properties relative to brain D-1 dopamine receptors.

Differential solubilization of lipids along with membrane proteins by different classes of detergents

Membrane proteins are typically extracted by detergent concentrations of 0.5-2.0%, using detergent/protein ratios of 1:1 to 3:1. We have compared the ability of 14 different detergents from seven different structural and ionic classes, at a concentration of 2.0% and a detergent/protein ratio of 2:1, to extract an integral membrane protein (the serotonin 5-HT1A receptor) in active form and have observed profound differences in both lipids and proteins. All extracts were freed from detergents and dialyzed to form vesicles containing 95-100% of the extracted lipids, prior to [3H]8-hydroxy-2-(N,N-di-n-propylamino)tetralin ([3H]8-OH-DPAT) binding. The most efficient detergents in extracting active 5-HT1A receptor protein were the zwitterionic 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and 3-[(cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO), followed by the neutral n-dodecyl-beta-D-maltoside. Zwitterionic detergents also produced the highest solubilized lipid/protein ratio (3.0 and 2.5, respectively) and in general the relative amounts of extracted lipids and proteins followed inverse profiles. Thus, hydrophobic detergents such as Tritons (with critical micelle concentrations similar to CHAPS) and Thesit (structurally similar to Lubrol) extracted the most protein, but relatively little lipid (ratios of less than 0.2) and very little active 5-HT receptor. Dramatic differences were also observed in the ratios of individual lipids extracted by the same concentrations of different detergents and resolved by high-performance thin-layer chromatography. For example, galactosylceramide (GalCer) content ranged from 2.7% (CHAPSO) to 13.4% (sodium cholate) of the total lipid extract and cholesterol ranged from 0% (digitonin) to 17.9% (Triton X-100). The detergent-extractability profile for phosphatidylethanolamine (PE) (range 15-40% of total lipid) paralleled that of phosphatidylinositol (PI) (range 4-10%), but was inverse to that for GalCer and cholesterol. Detergent-extractability profiles for phosphatidylcholine (PC) and phosphatidylserine (PS) also followed inverse profiles, with zwitterionic detergents giving high PS/PC and high PE/PC ratios (approximately 2:1), whereas the Tritons and digitonin gave ratios of 1:2. We believe that differential solubilization of lipids, as well as proteins, by detergents is important for the biological activity of the extracted proteins, and lipid extractability should be taken into account when purifying membrane proteins.

Coupling of human D-1 dopamine receptors to different guanine nucleotide binding proteins. Evidence that D-1 dopamine receptors can couple to both Gs and G(o)

Coupling between D-1 dopamine receptors and G proteins in cell lines expressing human D-1 receptors and different G proteins was examined. Pertussis toxin (PTX) treatment of rat pituitary GH4C1 cells significantly reduced, but did not abolish, agonist high affinity binding sites of the D-1 dopamine receptor; in SK-N-MC neuroblastoma cells, PTX failed to have any effect on D-1 high affinity sites. Cholera toxin (CTX) treatment of GH4C1 cells reduced but did not abolish the high affinity sites of D-1 receptors, while in SK-N-MC cells, treatment with CTX abolished all the high affinity sites. Western blot analyses with specific antisera indicated that Gs alpha, Gi1 alpha, Gi3 alpha, and Gq alpha were expressed in both cell lines, while Gi2 alpha and G(o) alpha were expressed in GH4C1 but not SK-N-MC cells. Antisera NEI-805 (anti-Gs alpha) and 9072 (anti-G(o) alpha) immunoprecipitated 24 +/- 4.3 and 34.4 +/- 6.9%, respectively, of G protein-associated D-1 dopamine receptors. Antisera 3646 (anti-Gi1 alpha), 1521 (anti-Gi2 alpha), 1518 (anti-Gi3 alpha), and 0941 (anti-Gq alpha) failed to coimmunoprecipitate appreciable levels of soluble receptors. These data indicate that D-1 dopamine receptors are coupled to both Gs alpha and G(o) alpha but not to Gq alpha.

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.