Human D2 Research Articles

Potent activation of dopamine D3/D2 heterodimers by the antiparkinsonian agents, S32504, pramipexole and ropinirole.

Recombinant, human dopamine D3 and D2 receptors form functional heterodimers upon co-expression in COS-7 cells. Herein, actions of the antiparkinsonian agents, S32504, ropinirole and pramipexole, at D3/D2L heterodimers were compared to their effects at the respective monomers and at split, chimeric D3trunk/D2tail and D2trunk/D3tail receptors: the trunk incorporated transmembrane domains (TDs) I-V and the tail TDs VI and VII. In binding assays with the antagonist [3H]nemonapride, all agonists were potent ligands of D3 receptors showing, respectively, 100-, 18- and 56-fold lower affinity at D2L receptors, mimicking the selective D3 receptor antagonist, S33084 (100-fold). At D3trunk/D2tail receptors, except for ropinirole, all drugs showed lower affinities than at D3 sites, whereas for D2trunk/D3tail receptors, affinities of all drugs were higher than at D2L sites. The proportion of high affinity binding sites recognized by S32504, pramipexole and ropinirole in membranes derived from cells co-expressing D3 and D2L sites was higher than in an equivalent mixture of membranes from cells expressing D3 or D2L sites, consistent with the promotion of heterodimer formation. In contrast, the percentage of high and low affinity sites (biphasic isotherms) recognized by S33084 was identical. Functional actions were determined by co-transfection of a chimeric adenylyl cyclase (AC)-V/VI insensitive to D3 receptors. Accordingly, D3 receptor-transfected cells were irresponsive whereas, in D2L receptor-transfected cells, agonists suppressed forskolin-stimulated cAMP production with modest potencies. In cells co-transfected with D3 and D2L receptors, S32504, ropinirole and pramipexole potently suppressed AC-V/VI with EC50s 33-, 19- and 11-fold lower than at D2L receptors, respectively. S32504 also suppressed AC-V/VI activity at split D3trunk/D2tail and D2trunk/D3tail chimeras transfected into COS-7 cells. In conclusion, antiparkinson agents behave as potent agonists at D3/D2'heterodimers', though any role in their actions in vivo remains to be demonstrated.

Constitutive oligomerization of human D2 dopamine receptors expressed in Spodoptera frugiperda 9 (Sf9) and in HEK293 cells. Analysis using co-immunoprecipitation and time-resolved fluorescence resonance energy transfer.

Human D2Long (D2L) and D2Short (D2S) dopamine receptor isoforms were modified at their N-terminus by the addition of a human immunodeficiency virus (HIV) or a FLAG epitope tag. The receptors were then expressed in Spodoptera frugiperda 9 (Sf9) cells using the baculovirus system, and their oligomerization was investigated by means of co-immunoprecipitation and time-resolved fluorescence resonance energy transfer (FRET). [3H]Spiperone labelled D2 receptors in membranes prepared from Sf9 cells expressing epitope-tagged D2L or D2S receptors, with a pKd value of approximately 10. Co-immunoprecipitation using antibodies specific for the tags showed constitutive homo-oligomerization of D2L and D2S receptors in Sf9 cells. When the FLAG-tagged D2S and HIV-tagged D2L receptors were co-expressed, co-immunoprecipitation showed that the two isoforms can also form hetero-oligomers in Sf9 cells. Time-resolved FRET with europium and XL665-labelled antibodies was applied to whole Sf9 cells and to membranes from Sf9 cells expressing epitope-tagged D2 receptors. In both cases, constitutive homo-oligomers were revealed for D2L and D2S isoforms. Time-resolved FRET also revealed constitutive homo-oligomers in HEK293 cells expressing FLAG-tagged D2S receptors. The D2 receptor ligands dopamine, R-(-)propylnorapomorphine, and raclopride did not affect oligomerization of D2L and D2S in Sf9 and HEK293 cells. Human D2 dopamine receptors can therefore form constitutive oligomers in Sf9 cells and in HEK293 cells that can be detected by different approaches, and D2 oligomerization in these cells is not regulated by ligands.

Molecular basis for the substrate selectivity of cat type I iodothyronine deiodinase.

The type I iodothyronine deiodinase (D1) catalyzes the activation of T4 to T3 as well as the degradation of T3 (rT3) and sulfated iodothyronines. A comparison of the catalytic activities of D1 in liver microsomal preparations from several species revealed a remarkable difference between cat D1 on one hand and rat/human D1 on the other hand. The Michaelis constant (Km) of cat D1 for rT3 (11 microm) is 30-fold higher than that of rat and human D1 (0.2-0.5 microm). Deiodination of rT3 by cat D1 is facilitated by sulfation [maximal velocity (Vmax)/Km rT3 = 3 and Vmax/Km rT3S = 81]. To understand the molecular basis for the difference in substrate interaction the cat D1 cDNA was cloned, and the deduced amino acid sequence was compared with rat/human D1 protein. In the region between amino acid residues 40 and 70 of cat D1, various differences with rat/human D1 are concentrated. By site-directed mutagenesis of cat D1 it was found that a combination of mutations was necessary to improve the deiodination of rT3 by cat D1 enzyme. For efficient rT3 deiodination, a Phe at position 65 and the insertion of the Thr-Gly-Met-Thr-Arg48-52 sequence as well as the amino acids Gly and Glu at position 45-46 are essential. Either of these changes alone resulted in only a limited improvement of rT3 deiodination. At the same time the combination of the described mutations did not affect the already quite efficient outer ring deiodination of rT3S nor the inner ring deiodination of T3S, whereas each of the described changes alone did affect rT3S deiodination. Our findings suggest great flexibility of the active site in D1 that adapts to its various substrates. The active site of wild-type cat D1 is less flexible than the active site of rat/human D1 and favors sulfated iodothyronines.

Endoplasmic reticulum-associated degradation of the human type 2 iodothyronine deiodinase (D2) is mediated via an association between mammalian UBC7 and the carboxyl region of D2.

The type 2 iodothyronine selenodeiodinase (D2) is an endoplasmic reticulum (ER)-resident selenoprotein that activates T4 to T3, playing a critical role in thyroid homeostasis. D2 has an approximately 45-min half-life due to selective ubiquitin-mediated ER-associated degradation (ERAD), a process of particular interest because it is accelerated by exposure to D2 substrates, T4 or rT3. The present in vitro binding studies indicate that glutathione-S-transferase (GST)-human D2 fusion proteins specifically associate with a mammalian homolog of the ubiquitin conjugase UBC7 (MmUBC7), with localization to amino acids 169-234 of D2. Coexpression of D2 with an inactive D2 mutant or a truncated version containing amino acids 169-234 stabilizes D2 half-life, supporting the importance of the carboxyl region of D2 for ERAD. Mammalian UBC6 (MmUBC6) does not directly associate with D2 but can associate with a complex containing UBC7 and D2. At the same time, functional studies in human embryonic kidney-293 cells indicate that D2 activity half-life and protein levels are stabilized only when inactive mutants of both UBC6 and UBC7 are overexpressed with D2, suggesting that redundancy may exist at the level of the E2 for both basal and substrate-accelerated D2 ERAD. In conclusion, D2 ERAD in human cells proceeds via an association between UBC7 and the carboxyl region of D2, a unique mechanism for the control of thyroid hormone activation.

Alternative mammary oncogenic pathways are induced by D-type cyclins; MMTV-cyclin D3 transgenic mice develop squamous cell carcinoma.

The three human D-type cyclins, cyclin D1, D2 and D3 share the ability to bind to and activate cdk4 and 6. MMTV-cyclin D1 transgenic mice develop mainly adenocarcinoma, while MMTV-cyclin D2 mice show a lack of alveologenesis during pregnancy and only develop carcinoma at low frequency. The effect of cyclin D3 overexpression in mammary glands remains hitherto unknown. We generated MMTV-cyclin D3 transgenic mice and report here that they develop exclusively squamous cell carcinoma. We show that although cyclin D3 transgene expression was detected early in puberty, postnatal development and mammary gland proliferation were normal in virgin animals. In contrast, multiparous mice develop multiple foci of abnormal growth that correspond to various stages of squamous metaplasia. Therefore, our results support a role for cyclin D3 in squamous differentiation. In addition, we found that p16 expression during involution is abolished, while p27 expression increased in MMTV-cyclin D3 mice, two modifications that have been reported in the other MMTV-D-type cyclin transgenic models. Our observations indicate that despite biochemical redundancy in vitro and in vivo, D-type cyclins promote distinct oncogenic pathways.

Functional coupling of the human dopamine D2 receptor with G alpha i1, G alpha i2, G alpha i3 and G alpha o G proteins: evidence for agonist regulation of G protein selectivity.

(1) The human dopamine D(2long) (D(2L)) receptor was expressed with four different G proteins in Sf9 cells using the baculovirus expression system. When co-expressed with G(i)/G(o) G proteins (G(i1)alpha, G(i2)alpha, G(i3)alpha, or G(o)alpha, plus Gbeta(1) and Ggamma(2)), the receptor displayed a high-affinity binding site for the agonists (dopamine and NPA), which was sensitive to GTP (100 micro M), demonstrating interaction between the receptor and the different G proteins. (2) The receptor to G protein ratio (R : G ratio) was evaluated using [(3)H]-spiperone saturation binding (R) and [(35)S]-GTPgammaS saturation binding (G). R : G ratios of 1 : 12, 1 : 3, 1 : 14 and 1 : 5 were found for G(i1), G(i2), G(i3), and G(o) preparations, respectively. However, when R : G ratios of 1 : 2 and 1 : 12 were compared for G(i2) and G(o), no difference was found for the stimulation of [(35)S]-GTPgammaS binding. (3) Several agonists were tested for their ability to stimulate [(35)S]-GTPgammaS binding to membranes co-expressing the receptor and various G proteins. All the compounds tested showed agonist activity in preparations expressing G(i3) and G(o). However, for G(i2) and G(i1) preparations, compounds such as S-(-)-3-PPP and p-tyramine were unable to stimulate [(35)S]-GTPgammaS binding. (4) Most of the compounds showed higher relative efficacies (compared to dopamine) and higher potencies in the preparation expressing G(o). Comparison of the effects of different agonists in the different preparations showed that each agonist differentially activates the four G proteins. (5) We conclude that the degree of selectivity of G protein activation by the D(2L) receptor can depend on the conformation of the receptor stabilised by an agonist.

Synonymous mutations in the human dopamine receptor D2 (DRD2) affect mRNA stability and synthesis of the receptor.

Although changes in nucleotide sequence affecting the composition and the structure of proteins are well known, functional changes resulting from nucleotide substitutions cannot always be inferred from simple analysis of DNA sequence. Because a strong synonymous codon usage bias in the human DRD2 gene, suggesting selection on synonymous positions, was revealed by the relative independence of the G+C content of the third codon positions from the isochoric G+C frequencies, we chose to investigate functional effects of the six known naturally occurring synonymous changes (C132T, G423A, T765C, C939T, C957T, and G1101A) in the human DRD2. We report here that some synonymous mutations in the human DRD2 have functional effects and suggest a novel genetic mechanism. 957T, rather than being 'silent', altered the predicted mRNA folding, led to a decrease in mRNA stability and translation, and dramatically changed dopamine-induced up-regulation of DRD2 expression. 1101A did not show an effect by itself but annulled the above effects of 957T in the compound clone 957T/1101A, demonstrating that combinations of synonymous mutations can have functional consequences drastically different from those of each isolated mutation. C957T was found to be in linkage disequilibrium in a European-American population with the -141C Ins/Del and TaqI 'A' variants, which have been reported to be associated with schizophrenia and alcoholism, respectively. These results call into question some assumptions made about synonymous variation in molecular population genetics and gene-mapping studies of diseases with complex inheritance, and indicate that synonymous variation can have effects of potential pathophysiological and pharmacogenetic importance.

Synonymous mutations in the human dopamine receptor D2 (DRD2) affect mRNA stability and synthesis of the receptor

Although changes in nucleotide sequence affecting the composition and the structure of proteins are well known, functional changes resulting from nucleotide substitutions cannot always be inferred from simple analysis of DNA sequence. Because a strong synonymous codon usage bias in the human DRD2 gene, suggesting selection on synonymous positions, was revealed by the relative independence of the G+C content of the third codon positions from the isochoric G+C frequencies, we chose to investigate functional effects of the six known naturally occurring synonymous changes (C132T, G423A, T765C, C939T, C957T, and G1101A) in the human DRD2. We report here that some synonymous mutations in the human DRD2 have functional effects and suggest a novel genetic mechanism. 957T, rather than being ‘silent’, altered the predicted mRNA folding, led to a decrease in mRNA stability and translation, and dramatically changed dopamine-induced up-regulation of DRD2 expression. 1101A did not show an effect by itself but annulled the above effects of 957T in the compound clone 957T/1101A, demonstrating that combinations of synonymous mutations can have functional consequences drastically different from those of each isolated mutation. C957T was found to be in linkage disequilibrium in a European-American population with the -141C Ins/Del and TaqI ‘A’ variants, which have been reported to be associated with schizophrenia and alcoholism, respectively. These results call into question some assumptions made about synonymous variation in molecular population genetics and gene-mapping studies of diseases with complex inheritance, and indicate that synonymous variation can have effects of potential pathophysiological and pharmacogenetic importance.

Cyclin D3 Is a Cofactor of Retinoic Acid Receptors, Modulating Their Activity in the Presence of Cellular Retinoic Acid-binding Protein II

Ligand-induced transcription activation of retinoic acid (RA) target genes by nuclear receptors (retinoic acid (RAR) and retinoid X (RXR) receptors) depends on the recruitment of coactivators. We have previously demonstrated that the small 15-kDa cellular RA-binding protein II (CRABPII) is a coactivator present in the RA-dependent nuclear complex. As identifying cell-specific partners of CRABPII might help to understand the novel control of RA signaling, we performed a yeast two-hybrid screen of a hematopoietic HL-60 cDNA library using human CRABPII as bait and have subsequently identified human cyclin D3 as a partner of CRABPII. Cyclin D3 interacted with CRABPII in a ligand-independent manner and equally bound RAR alpha, but not RXR alpha, and only in the presence of RA. We further show that cyclin D3 positively modulated RA-mediated transcription through CRABPII. Therefore, cyclin D3 may be part of a ternary complex with CRABPII and RAR. Finally, we show that cyclin D3 expression paralleled HL-60 differentiation and arrest of cell growth. These findings led us to speculate that control of cell proliferation during induction of differentiation may directly involve, at the transcriptional level, nuclear receptors, coactivators, and proteins of the cell cycle in a cell- and nuclear receptor-specific manner.

Human Type 3 Iodothyronine Selenodeiodinase Is Located in the Plasma Membrane and Undergoes Rapid Internalization to Endosomes

The type 3 iodothyronine selenodeiodinase (D3) is an integral membrane protein that inactivates thyroid hormones. By using immunofluorescence cytochemistry confocal microscopy of live or fixed cells transiently expressing FLAG-tagged human D3 or monkey hepatocarcinoma cells expressing endogenous D3, we identified D3 in the plasma membrane. It co-localizes with Na,K-ATPase alpha, with the early endosomal marker EEA-1 and clathrin, but not with two endoplasmic reticulum resident proteins. Most of the D3 molecule is extracellular and can be biotinylated with a cell-impermeant probe. There is constant internalization of D3 that is blocked by sucrose or methyl-beta-cyclodextrin-containing medium. Exposing cells to a weak base such as primaquine increases the pool of internalized D3, suggesting that D3 is recycled between plasma membrane and early endosomes. Such recycling could account for the much longer half-life of D3 (12 h) than the thyroxine activating members of the selenodeiodinase family, type 1 (D1; 8 h) or type 2 (D2; 2 h) deiodinase. The extracellular location of D3 gives ready access to circulating thyroid hormones, explaining its capacity for rapid inactivation of circulating thyroxine and triiodothyronine in patients with hemangiomas and its blockade of the access of maternal thyroid hormones to the human fetus.

Pharmacological Survey of Medicinal Plants for Activity at Dopamine Receptor Subtypes. II. Screen for Binding Activity at the D1 and D2 Dopamine Receptor Subtypes

Aqueous, organic and alcoholic extracts of 163 Bolivian, Chinese and Pakistani medicinal plants were evaluated for their ability to inhibit the binding of radioligands selective for the D1 and D2 dopamine receptor subtypes expressed in Sf9 cells. Based upon the results of the initial screen, 25 extracts were selected for further evaluation. Concentrationresponse competitive radioligand binding studies were performed to obtain IC 50 values for the inhibition of the binding at D1 or D2 receptors by the selected extracts. The D1:D2 receptor ratio of IC 50 values for the extracts ranged from approximately two-fold selective for the D1 receptor to threefold selective for the D2 receptor. D1 and D2 dopamine receptors are known to be linked to the activation and inhibition of adenylyl cyclase activity, respectively. Therefore, the selected extracts were tested for the ability to either a) stimulate adenylyl cyclase activity using stably transfected HEK 293 cells expressing human D1 receptors (hD1-HEK) or b) inhibit forskolin-dependent stimulation of adenylyl cyclase activity in stably transfected HEK 293 cells expressing rat D2 dopamine receptors (rD2-HEK). Two extracts were found to stimulate adenylyl cyclase activity in hD1-HEK cells. However, similar results were observed when untransfected HEK cells were used, suggesting that the ability of these two extracts to increase cAMP levels in hD1-HEK cells was not mediated through a D1-like dopamine receptor. While the majority of extracts did not have an effect on forskolin-dependent stimulation in rD2-HEK cells, six extracts were capable of inhibiting adenylyl cyclase activity in these cells. For five of these extracts, the inhibitory effect appeared to be dependent upon the expression of D2 receptors because no inhibitory effect was observed when untransfected HEK 293 cells were used. In summary, the results of this preliminary radioligand binding and functional activity screen of extracts from medicinal plants for pharmacologic activity at D1 and D2 dopamine receptors indicated that the majority of components capable of binding to the D1 and D2 dopamine receptor subtypes appeared to be essentially nonselective. While none of the selected extracts were found to have agonist activity at D1 receptors, several of the extracts exhibited inverse agonist activity at D1 dopamine receptors. Five of the extracts appeared to be agonists at D2 dopamine receptors. Therefore, several classes of extracts were identified which appeared to contained unique combinations of the following pharmacologic properties: D1 dopamine receptor antagonist, D2 dopamine receptor antagonist, D1 receptor inverse agonists and D2 receptor agonists. In addition, two of the extracts tested appeared to modulate adenylyl cyclase activity, but this activity did not appear to be mediated through activation of a dopamine receptor subtype.

Sequence variants in the human 25-hydroxyvitamin D3 1-alpha-hydroxylase (CYP27B1) gene are not associated with prostate cancer risk.

1,25-dihydroxyvitamin D(3) has been shown to have antiproliferative properties on normal and neoplastic prostatic cells. 25-hydroxyvitamin D(3) 1-alpha-hydroxylase, the enzyme that catalyzes the final step of vitamin D synthesis, converting 25-hydroxyvitamin D(3) to 1,25-dihydroxyvitamin D(3), is expressed in the prostate. The human 25-hydroxyvitamin D(3) 1-alpha-hydroxylase gene (CYP27B1) was resequenced in a case/control panel consisting of 64 individuals (48 Caucasians and 16 African Americans), with equal numbers of hereditary prostate cancer cases, sporadic cases, and unaffected controls. Three frequent single nucleotide polymorphisms (SNPs) were genotyped in 245 prostate cancer cases and 222 controls. Six noncoding SNPs were identified in the CYP27B1 gene. No significant difference was found in allele and genotype frequencies between sporadic cases and unaffected controls for the three genotyped SNPs. This study suggests that the CYP27B1 gene does not play a major role as a prostate cancer susceptibility gene.

Mechanisms of ligand binding and efficacy at the human D2(short) dopamine receptor.

Mechanisms of ligand binding and receptor activation for the human D2(short) dopamine receptor have been probed using two homologous series of monohydroxylated and dihydroxylated agonists (phenylethylamines and 2-dipropylaminotetralins). In ligand binding studies, the majority of compounds exhibited competition curves versus [3H]spiperone that were best fitted using a two site binding model. The compounds had different abilities (potencies and maximal effects) to stimulate [35S]GTPgammaS binding and to inhibit forskolin-stimulated cAMP accumulation. From the data it can be concluded that: (i) the ability of an agonist to stabilize receptor/G protein coupling can be used to predict agonist efficacy for some groups of compounds (2-dipropylaminotetralins) but not for others (phenylethylamines); (ii) the receptor may be activated by unhydroxylated compounds; (iii) single hydroxyl groups or pairs of hydroxyl groups on the agonist may contribute to binding affinity, potency and efficacy; and (iv) for the 2-dipropylaminotetralin series two modes of agonist/receptor interaction have been identified associated with different relative efficacy.

Desensitization of human renal D1 dopamine receptors by G protein-coupled receptor kinase 4

The D1 dopamine receptor, expressed in several nephron segments, participates in the regulation of water and electrolyte transport. Because the renal D1 receptor is desensitized in genetic hypertension, we sought to determine the mechanism(s) of the desensitization of D1 receptors endogenously expressed in renal proximal tubules. The mechanisms involved in the homologous desensitization of the D1 receptor in human renal proximal tubule cells were studied by measuring the production of cAMP in response to stimulation or inhibition of G protein-coupled receptor kinase (GRK) activity and expression. Protein expression was assessed by immunoblotting. In human renal proximal tubule cells, the D1 agonist, fenoldopam, increased cAMP accumulation (73 +/- 2%). Fenoldopam pre-treatment decreased the responsiveness to subsequent fenoldopam stimulation (t(1/2) approximately equal to 20 min) with complete desensitization at 30 minutes. Recovery occurred gradually (t(1/2) approximately equal to 20 min) with full recovery at 60 minutes. Forskolin pretreatment minimally affected the fenoldopam effect, indicating a minor involvement of protein kinase A in the homologous desensitization process. Because GRKs are involved in the homologous desensitization process, we determined the consequences of inhibition of GRK expression and activity. Heparin, an inhibitor of GRK activity, decreased the expression of GRK2 and GRK4 and attenuated the desensitization of the D1 receptor (85 +/- 1%). Antisense oligonucleotides (GRK4> GRK2) blunted the D1 receptor desensitization. However, the first 20 minutes of homologous desensitization were not affected by either heparin or GRK antisense oligonucleotides. These studies document the critical role of GRK4, relative to GRK2, in the homologous desensitization of D1 receptors in renal proximal tubule cells. However, the early phase of homologous desensitization is regulated by a non-GRK-mediated pathway.

Cloning of the gene and complete cDNA encoding a type 2 deiodinase from Fundulus heteroclitus

Recently, we reported the cloning of a cDNA fragment from Fundulus heteroclitus liver encoding the open reading frame of type 2 deiodinase (FhD2). We here report the cloning of 14 kb of genomic sequence from F. heteroclitus that includes the previously reported coding region of the F. heteroclitus Dio2 gene ( FhDio2), the 5 ′ and 3 ′ untranslated regions, and flanking regions and introns. This FhDio2 gene comprises two exons divided by a 4.8-kb intron. The position of the intron is similar to that of introns in other Dio2 genes. The analysis of approximately 1.3 kb of genomic sequence upstream of the mRNA start site revealed that, in contrast to mammalian Dio2 genes, there were no apparent TATA or CRE sequences. Nevertheless, a putative Sp1 site was found, similar to that in other F. heteroclitus TATA-less promoters. We have also cloned the complete FhD2 cDNA, which spans 4652 bp and contains a sequence adjacent to its poly(A) tail that is highly similar to the selenocysteine insertion sequence (SECIS) found in human D2 cDNA. The expression of a construct containing the FhD2 ORF plus the native SECIS resulted in a protein with deiodinase activity similar to that of the native FhD2. Analysis of the regulation of this gene, combined with ongoing studies of the F. heteroclitus D1 gene, will allow us to elucidate the functions of the colocalized deiodinases in teleost liver.

Ubc6p and ubc7p are required for normal and substrate-induced endoplasmic reticulum-associated degradation of the human selenoprotein type 2 iodothyronine monodeiodinase.

The type 2 monodeiodinase (D2) is an endoplasmic reticulum-resident membrane selenoprotein responsible for catalyzing the first step in thyroid hormone action, T(4) deiodination to T(3). Its short half-life is due to ubiquitination and proteolysis by proteasomes, a mechanism that is accelerated by D2 interaction with T(4). To identify proteins involved in D2 ubiquitination, a FLAG-tagged selenocystine133-to-Cys mutation of the human D2 (CysD2) was created and expressed in Saccharomyces cerevisiae using the GAL1 gene promoter. CysD2 activity was detected in the microsomes, indistinguishable from transiently expressed CysD2 in vertebrate cells. Treatment with 100 mg/ml cycloheximide or 30 micro M T(4) caused rapid loss of CysD2 (t(1/2) = approximately 30 min). Clasto-lactacystin beta-lactone not only increased galactose-inducible CysD2 but also stabilized CysD2 in the presence of cycloheximide or T(4). Immunoprecipitation with anti-FLAG antibody combined with Western analysis with antiubiquitin revealed that CysD2 is heavily ubiquitinated. Expression of CysD2 in yeast strains that lack the ubiquitin conjugases Ubc6p or Ubc7p stabilized CysD2 half-life by markedly reducing CysD2 ubiquitination, whereas no difference was detected in Ubc1p-deficient mutants. Similarly, expression of CysD2 in UBC6 and UBC7 mutants also impaired the substrate-induced loss of CysD2 activity and protein. In conclusion, Ubc6p and Ubc7p are required for normal and substrate-induced ubiquitination and proteolysis of D2.

The mRNA Structure Has Potent Regulatory Effects on Type 2 Iodothyronine Deiodinase Expression

Type 2 deiodinase (D2) is a selenoenzyme catalyzing the activation of T4 to T3. D2 activity/mRNA ratios are often low, suggesting that there is significant posttranscriptional regulation. The D2 mRNA in higher vertebrates is more than 6 kb, containing long 5′ and 3′ untranslated regions (UTRs). The D2 5′UTRs are greater than 600 nucleotides and contain 3–5 short open reading frames. These full-length 5′UTRs reduce the D2 translation efficiency approximately 5-fold. The inhibition by human D2 5′UTR is localized to a region containing the first short open reading frame encoding a tripeptide—MKG. This inhibition was abolished by mutating the AUG start codon and weakened by modification of the essential purine of the Kozak consensus. Deletion of the 3.7-kb 3′UTR of the chicken D2 mRNA increased D2 activity approximately 3.8-fold due to an increase in D2 mRNA half-life. In addition, alternatively spliced D2 mRNA transcripts similar in size to the major 6- to 7-kb D2 mRNAs but not encoding an active enzyme are present in both human and chicken tissues. Our results indicate that a number of factors reduce the D2 protein levels. These mechanisms, together with the short half-life of the protein, ensure limited expression of this key regulator of T4 activation.

The mRNA structure has potent regulatory effects on type 2 iodothyronine deiodinase expression.

Type 2 deiodinase (D2) is a selenoenzyme catalyzing the activation of T(4) to T(3). D2 activity/mRNA ratios are often low, suggesting that there is significant posttranscriptional regulation. The D2 mRNA in higher vertebrates is more than 6 kb, containing long 5' and 3' untranslated regions (UTRs). The D2 5'UTRs are greater than 600 nucleotides and contain 3-5 short open reading frames. These full-length 5'UTRs reduce the D2 translation efficiency approximately 5-fold. The inhibition by human D2 5'UTR is localized to a region containing the first short open reading frame encoding a tripeptide-MKG. This inhibition was abolished by mutating the AUG start codon and weakened by modification of the essential purine of the Kozak consensus. Deletion of the 3.7-kb 3'UTR of the chicken D2 mRNA increased D2 activity approximately 3.8-fold due to an increase in D2 mRNA half-life. In addition, alternatively spliced D2 mRNA transcripts similar in size to the major 6- to 7-kb D2 mRNAs but not encoding an active enzyme are present in both human and chicken tissues. Our results indicate that a number of factors reduce the D2 protein levels. These mechanisms, together with the short half-life of the protein, ensure limited expression of this key regulator of T(4) activation.

Aripiprazole, a novel antipsychotic, is a high-affinity partial agonist at human dopamine D2 receptors.

Aripiprazole is the first next-generation atypical antipsychotic with a mechanism of action that differs from currently marketed typical and atypical antipsychotics. Aripiprazole displays properties of an agonist and antagonist in animal models of dopaminergic hypoactivity and hyperactivity, respectively. This study examined the interactions of aripiprazole with a single population of human D2 receptors to clarify further its pharmacologic properties. In membranes prepared from Chinese hamster ovary cells that express recombinant D2L receptors, aripiprazole bound with high affinity to both the G protein-coupled and uncoupled states of receptors. Aripiprazole potently activated D2 receptor-mediated inhibition of cAMP accumulation. Partial receptor inactivation using the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) significantly reduced the maximum effect of aripiprazole on inhibition of cAMP accumulation. This effect was seen with concentrations of EEDQ that did not alter the maximal inhibitory effect of dopamine. Consistent with the expected effects of a partial agonist, increasing concentrations of aripiprazole blocked the action of dopamine with maximal blockade equal to the agonist effect of aripiprazole alone. The efficacy of aripiprazole relative to that of dopamine varied from 25% in cells that lacked spare receptors for dopamine to 90% in cells with receptor reserve. These results, together with previous studies demonstrating partial agonist activity at serotonin 5-hydroxytryptamine (5-HT)1A receptors and antagonist activity at 5-HT2A receptors, support the identification of aripiprazole as a dopamine-serotonin system stabilizer. The receptor activity profile may underlie the unique activity of aripiprazole in animals and its antipsychotic activity in humans.

Functional Polymorphism (Ser311→Cys311) in the Dopamine D2 Receptor Gene and Alcohol Drinking Habits in Siberia

The human dopamine receptor D2 gene is an important candidate gene for drug addiction, alcohol- ism and/or for the modification of its severity. In the present study, the distribution of DRD2 polymorphism (Ser311/Cys311) among a random Siberian female popula- tion sample was examnined and correlated with self- reported alcohol drinking and alcohol-induced flushing response. The Cys311 variant was detected by DNA am- plification using PCR followed by digestion with the restriction enzyme Sau961. The Ser 311/Cys311 heterozy- gous genotype was present in 4% of the samples and the frequency of the Cys 311-encoding allele was found to be 0.021. Subjects with Cy311/Cys311 allele consumed al- cohol less frequently than subjects with Ser 311/Cys311 al- lele.