Genomic structure of the mouse delta opioid receptor gene

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Current research on opioid receptor function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

Transcriptional and epigenetic regulation of opioid receptor genes: present and future

Three opioid receptors (ORs) are known: μ opioid receptors (MORs), δ opioid receptors (DORs), and κ opioid receptors (KORs). Each is encoded by a distinct gene, and the three OR genes share a highly conserved genomic structure and promoter features, including an absence of TATA boxes and sensitivity to extracellular stimuli and epigenetic regulation. However, each of the genes is differentially expressed. Transcriptional regulation engages both basal and regulated transcriptional machineries and employs activating and silencing mechanisms. In retinoic acid-induced neuronal differentiation, the opioid receptor genes undergo drastically different chromatin remodeling processes and display varied patterns of epigenetic marks. Regulation of KOR expression is distinctly complex, and KOR exerts a unique function in neurite extension, indicating that KOR is not simply a pharmacological cousin of MOR and DOR. As the expression of OR proteins is ultimately controlled by extensive posttranscriptional processing, the pharmacological implication of OR gene regulation at the transcriptional level remains to be determined.

NGF/PI3K signaling-mediated epigenetic regulation of delta opioid receptor gene expression

The G protein-coupled delta opioid receptor gene (dor) has been associated with neuronal survival, differentiation, and neuroprotection. Our previous study identified PI3K/Akt/NF-kappaB signaling is a main downstream signaling pathway in nerve growth factor (NGF)-induced temporal expression of the dor gene in the PC12 cell model. It is still unknown how NGF/PI3K signaling regulates the expression of the dor gene in the nucleus. In the current study, we investigated how PI3K signaling affected epigenetic modifications of histone H3 Lys(9) (H3K9) in the 5'-UTR region of the rat dor gene locus. NGF treatment resulted in the global reversal of H3K9 trimethylation in cells. Moreover, the locus-specific reversal of H3K9 trimethylation and acetylation of H3K9 were dependent upon NGF/PI3K signaling and temporally well correlated with NGF-induced gene expression. These results indicate the importance of epigenetic modifications of H3K9, particularly the reversal of trimethylated H3K9, in the regulation of NGF/PI3K-dependent genes during neuronal differentiation.

Nuclear factor kappaB signaling in opioid functions and receptor gene expression

Opiates are the most powerful of all known analgesics. The prototype opiate morphine has been used as a painkiller for several thousand years. Chronic usage of opiates not only causes drug tolerance, dependence, and addiction, but also suppresses immune functions and affects cell proliferation and cell survival. The diverse functions of opiates underscore the complexity of opioid receptor signaling. Several downstream signaling effector systems, including adenylyl cyclase, mitogen-activated protein kinase, Ca2+ channels, K+ channels, and phosphatidylinositol 3-kinase/Akt, have been identified to be critical in opioid functions. Nuclear factor-kappaB (NF-kappaB), one of the most diverse and critical transcription factors, is one of the downstream molecules that may either directly or indirectly transmit the receptor-mediated upstream signals to the nucleus, resulting in the regulation of the NF-kappaB-dependent genes, which are critical for the opioid-induced biological responses of neuronal and immune cells. In this minireview, we focus on current understanding of the involvement of NF-kappaB signaling in opioid functions and receptor gene expression in cells.

Sustained activation of phosphatidylinositol 3-kinase/Akt/nuclear factor kappaB signaling mediates G protein-coupled delta-opioid receptor gene expression

Expression of the delta-opioid receptor gene (dor) is tightly controlled during neuronal differentiation and developmental stages. Such distinct temporal and spatial expression of dor during development suggests a role for the delta-opioid receptor in early developmental events. However, little is known about intracellular signaling pathways that control dor expression. A well established cell line model for the study of gene expression during neuronal differentiation is the rat adrenal pheochromocytoma PC12 cell line. Here we found that the constitutively activated TrkA/phosphatidylinositol 3-kinase/Akt (protein kinase B)/NF-kappaB survival cascade mediates dor expression during nerve growth factor (NGF)-induced differentiation of PC12h cells. Biochemical experiments showed that constitutive phosphorylation of Akt and IkappaBalpha correlates with NGF-induced dor expression. Overexpression of the transcriptional activator NF-kappaB/p65 increased dor promoter activity. Overexpression of the NF-kappaB signaling super inhibitor mutant IkappaBalpha (S32A/S36A) abolished the effect of p65 and blocked NGF-induced activation of NF-kappaB signaling, resulting in a significant reduction in dor promoter activity. Treatment with SN50, an NF-kappaB-specific nuclear translocation peptide inhibitor, inhibited the translocation of NF-kappaB, resulting in a reduction of dor mRNA. The gel shift assay supported the fact that there exists an NF-kappaB-binding site on the dor promoter. RNA interference experiments using NF-kappaB/p65 small interfering RNA confirmed that NF-kappaB signaling is required for dor expression. Our findings not only provide a new mechanistic explanation for NGF-induced dor expression but also shed some light on the molecular mechanism of the temporal and spatial expression of dor and the roles of the delta-opioid receptor during neuronal differentiation.

DNA Methylation-Related Chromatin Modification in the Regulation of Mouse δ-Opioid Receptor Gene

DNA methylation plays critical roles in gene-silencing through chromatin modification. We reported previously that promoter-region CpG methylation repressed mouse delta-opioid receptor (mDOR) gene expression. In the current study, we demonstrated that the methylation of mDOR gene promoter is correlated with a repressive chromatin structure that has less HaeIII and MspI nuclear accessibility and more deacetylated histone H3 and H4 than that of unmethylated mDOR promoter. Chromatin immunoprecipitation analysis showed the association of a methyl-CpG-binding domain protein 2 (MBD2) with methylated mDOR promoter. Transient expression of MBD2 enhanced the repression of partially methylated mDOR promoter activity, and this repression was partially reversed by treatment of trichostatin A, a specific histone deacetylase inhibitor, indicating that MBD2 may mediate DNA methylation-related chromatin modification through recruiting histone deacetylases to mDOR promoter region. In addition, trichostatin A treatment increased both methylated mDOR promoter activity in a transient transfection assay and endogenous mDOR mRNA level in Neuro2A cells. Taken together, these results demonstrate that the mDOR gene expression is regulated by DNA methylation-related chromatin modification, especially histone acetylation and deacetylation.

Insights into the receptor transcription and signaling: implications in opioid tolerance and dependence

Drug addiction has great social and economical implications. In order to resolve this problem, the molecular and cellular basis for drug addiction must be elucidated. For the past three decades, our research has focused on elucidating the molecular mechanisms behind morphine tolerance and dependence. Although there are many working hypotheses, it is our premise that cellular modulation of the receptor signaling, either via transcriptional or post-translational control of the receptor, is the basis for morphine tolerance and dependence. Thus, in the current review, we will summarize our recent work on the transcriptional and post-translational control of the opioid receptor, with special emphasis on the mu-opioid receptor, which is demonstrated to mediate the in vivo functions of morphine.

Expression and regulation of the rat prostaglandin E2 receptor type 4 (EP4) in pregnant cervical tissue

OBJECTIVE

Prostaglandins play an important role in the regulation of parturition and cervical ripening. Prostaglandin E(2) (PGE(2))-induced tissue remodeling is mediated through activation of prostaglandin E(2) receptor type 4 (EP4). EP4 is known to regulate matrix metalloproteinase secretion and expression and therefore may play a role in cervical ripening. We hypothesized that EP4 expression is regulated in the rat cervix to coincide with cervical ripening. In addition, we analyzed transcriptional regulation of the rat EP4 gene.

STUDY DESIGN

EP4 expression was evaluated in the cervix in timed pregnancy Sprague-Dawley rats by real-time reverse transcriptase polymerase chain reaction and Western blot. The genomic structure of the rat EP4 gene was determined by sequencing rat genomic clones obtained by plaque hybridization. Northern blots were performed to identify the number of different transcripts. The transcriptional start site was identified on the basis of sequence comparison to the human and mouse EP4 gene. The minimal promoter was identified by using reporter constructs containing portions of the 5' flanking region. Reporter activity was evaluated in vitro. Identified regulatory regions were mutated to determine their role in transcription.

RESULTS

Cervical EP4 expression (messenger RNA and protein) peaks on the day of parturition. The rat EP4 gene is structurally similar to other prostaglandin receptors as well as the human EP4 gene and contains three exons separated by two introns. The coding regions are located in the second and third exons separated in the sixth transmembrane spanning region. Northern blot identified a single EP4 transcript with an estimated size of 4 kb, indicating a single transcription initiation site. The first 80 bases of the 5' flanking region were required for constitutive expression of EP4. Expression was lost after mutation of the GC/Sp1 binding site located at position -78 to -66 downstream from the transcription initiation site. Three Sp1 binding sites were identified and appear to cooperate to enhance EP4 transcription.

CONCLUSION

EP4 expression is regulated in the cervix and peaks on the day of parturition. The genomic structure of the human and rat EP4 gene is conserved between these two species. A GC rich/Sp1 binding site located within the first 80 bases of the transcription start site is important in transcription initiation of the EP4 gene.

Transcriptional regulation of mouse delta-opioid receptor gene by CpG methylation: involvement of Sp3 and a methyl-CpG-binding protein, MBD2, in transcriptional repression of mouse delta-opioid receptor gene in Neuro2A cells

Opioid receptors are expressed in a cell type-specific manner. Here we show that the mouse delta-opioid receptor (mDOR) gene is regulated by promoter region CpG methylation. The mDOR promoter containing a putative CpG island is highly methylated in Neuro2A cells, correlating with the repression of this gene in these cells. This is in contrast with the unmethylated state of the mDOR promoter in NS20Y cells, which express a high level of mDOR. Repression of mDOR transcription in Neuro2A cells could be partially relieved by chemically induced demethylation with 5-aza-2'-deoxycytidine. In addition, in vitro methylation of the luciferase reporter gene driven by the mDOR promoter resulted in an inhibition of transcription in NS20Y cells. Methyl-CpG-binding protein complex 1 (MeCP1) has been implicated in methylation-mediated transcriptional repression of several genes. Electrophoretic mobility shift assays showed that fully methylated, but not unmethylated, mDOR promoter fragment formed a MeCP1-like protein complex that contained methyl-CpG-binding domain protein 2 (MBD2) and Sp3. Furthermore, the expression level of Sp3 was decreased when Neuro2A cells were demethylated with 5-aza-2'-deoxycytidine, and increasing Sp3 levels in Schneider's Drosophila line 2 cells led to the repression of mDOR promoter activity when the promoter was methylated. These results demonstrate that Sp3 and MBD2 are involved in the transcriptional repression of mDOR in Neuro2A cells through binding to the methylated CpG sites in the promoter region and may play a role in the cell type-specific expression of mDOR.

Transcriptional regulation of mouse delta-opioid receptor gene. Ikaros-2 and upstream stimulatory factor synergize in trans-activating mouse delta-opioid receptor gene in T cells

Considerable evidence indicates that transcription of the delta-opioid receptor (dor) gene is correlated with both the expression of DOR on T cells and the capacity of DOR agonists to modulate the immunological functions of the T cell. We previously reported that increased Ikaros (Ik) binding activity over an Ik-binding site at -378 to -374 (with the translation start site designated as +1) in the mouse dor promoter was required for the enhanced transcription of dor gene in phytohemagglutinin-activated EL-4 cells, a mouse T cell line that constitutively expresses DOR. In the present study, we have analyzed further the mouse dor promoter in EL-4 cells and have demonstrated that Ik-2 homodimers bind to the -378/-374 Ik-binding site and exerts a position-dependent trans-activation effect on the dor promoter. Moreover, an E box (-185 to -180) that binds upstream stimulatory factor is essential for the dor promoter activity in both resting and phytohemagglutinin-activated T cells. Furthermore, we have demonstrated that Ik-2 and upstream stimulatory factor synergize in trans-activating the dor promoter via the putative Ik-binding site and the E box, respectively.

U50,488 inhibits HIV-1 Tat-induced monocyte chemoattractant protein-1 (CCL2) production by human astrocytes

Kappa-opioid receptor (KOR) ligands have been reported to alter many cell functions and to exert an immunomodulatory role in the CNS. Astrocytes, the predominant brain cell type, have been implicated in the neuropathogenesis of human immunodeficiency virus type 1 (HIV-1). HIV-1 nuclear protein Tat has been reported to induce production of the chemokine monocyte chemoattractant protein-1 (MCP-1 or CCL2) and to activate nuclear factor kappaB (NF-kappaB) in human astrocytes. In the present study, we investigated whether the synthetic KOR ligand trans-3,4-dichloro-N-methyl-N[2-(1-pyrolidinyl)cyclohexyl]benzeneacetamide methanesulfonate (U50,488) would down-regulate MCP-1 production in primary human astrocytes stimulated by Tat. Treatment of astrocytes with U50,488 inhibited Tat-induced MCP-1 production in a concentration-dependent manner. The KOR-selective antagonist nor-binaltrophimine (nor-BNI) completely blocked the inhibitory effect of U50,488, indicating involvement of KOR. While U50,488 alone had a partial inhibitory effect on constituent NF-kappaB activation, it potently suppressed Tat-induced NF-kappaB activation. These findings suggest that KOR ligands could have an anti-inflammatory effect in the CNS and thereby be beneficial in the treatment of HIV-1-associated brain disease.

Orphanin FQ/nociceptin and mu-opioid receptor mRNA levels in human SH-SY5Y neuroblastoma cells: effects of activating the cAMP-PKA signal transduction pathway

Responses to opioid agonists vary, depending on past opioid exposure and the physiological state. The intracellular signaling pathway mediated by cAMP and protein kinase A (PKA) has been linked to regulation of opioid receptor responsiveness. The role of the cAMP-PKA pathway in regulating opioid receptor gene expression is incompletely defined. Mu-opioid receptor (MuOR) and orphanin FQ/nociceptin receptor (ORL(1)) transcripts were measured after activating this pathway in human neuroblastoma cells. Human SH-SY5Y neuroblastoma cells were maintained in continuous monolayer culture. Cells were incubated with combinations of agents which activate the cAMP-PKA signal transduction pathway, including forskolin and choleratoxin (CTX). MuOR and ORL(1) transcript levels were measured by hybridization to specific probes. Activation of the cAMP-PKA signal transduction pathway with forskolin in the presence of phosphodiesterase inhibitors was associated with a time-dependent decrease in the level of MuOR mRNA; partial recovery was observed with prolonged incubations. Forskolin effects were mimicked by CTX, but not by dideoxyforskolin. The PKA inhibitor H89 blunted the actions of forskolin. However, forskolin responses persisted despite coincubation with protein synthesis inhibitors. ORL(1) transcript levels did not significantly change, but vasoactive intestinal polypeptide (VIP) transcripts exhibited substantial increases, in the presence of forskolin or CTX. These observations support a role for cAMP in regulating MuOR responsiveness through actions at the level of receptor gene expression. ORL(1) transcript levels are not effected, suggesting that the cAMP-PKA pathway has differential effects on the expression of mRNA for different, but biochemically closely related, opioid receptor subtypes.

Transcriptional regulation of mouse delta-opioid receptor gene: role of Ets-1 in the transcriptional activation of mouse delta-opioid receptor gene

Previously, we identified a minimum core promoter of the mouse delta-opioid receptor (DOR) gene. The DOR promoter contains an E-box that binds upstream stimulatory factor and is crucial for the DOR promoter activity in NS20Y cells, a mouse neuronal cell line that constitutively expresses DOR. In the present study, we further analyzed the DOR promoter in NS20Y cells and have demonstrated that transcription factor Ets-1 binds to an Ets-1-binding site overlapping the E-box and trans-activates the DOR promoter by synergizing with upstream stimulatory factor in specific DNA binding. In addition, the Ets-1 DNA-binding domain is sufficient to play the functional role of Ets-1 in trans-activating the DOR promoter. Furthermore, through in vivo cross-linking assays and Northern blot analyses, we have demonstrated that Ets-1 binds to the DOR promoter in the neonatal mouse brain and that overexpressed Ets-1 can significantly enhance the expression of DOR mRNA in primary neonatal mouse neuronal cells. Collectively, our data suggest that Ets-1 functions as a trans-activator of the DOR promoter in the neonatal mouse brain and thus may contribute to the development of the mouse brain DOR system.

Ablation of pituitary pro-opiomelanocortin (POMC) cells produces alterations in hypothalamic POMC mRNA levels and midbrain mu opioid receptor binding in a conditional transgenic mouse model

The hypothalamic-pituitary-adrenal (HPA) axis is regulated by stress-related excitatory inputs, and various inhibitory and negative-feedback controls by glucocorticoids and opioids, including pro-opiomelanocortin (POMC)-derived peptides. The role of POMC-derived peptides of pituitary origin in the modulation of brain POMC mRNA expression and opioid receptor binding was investigated using a line of transgenic mice that express a fusion gene composed of the pituitary expression-specific promoter region of the POMC gene driving the herpes simplex viral-1 thymidine kinase (TK). Male adult mice were treated with the antiherpes agent ganciclovir that selectively ablates cells expressing TK. Following treatment, POMC mRNA levels, measured by quantitative solution hybridization/RNase protection assays, were decreased by 48% in the pituitary of the TK+/+ mice, reflecting an expected loss of the pituitary corticotrope POMC cells. This treatment also significantly lowered pituitary beta-endorphin immunoreactivity content and plasma concentrations of corticosterone. In contrast, POMC mRNA levels were increased by 79% in the hypothalamus of the TK+/+ mice with pituitary POMC cell ablation. Binding of [(3)H]DAMGO to mu opioid receptors, as measured by quantitative autoradiography, was significantly reduced in several brain regions including the central grey, median raphe and superficial grey layer of the superior colliculus. These regions are innervated by hypothalamic POMC neurones. No significant differences in binding to either kappa or delta opioid receptors were found in the brain regions studied. These results suggest that POMC-derived peptides of pituitary origin may exert a tonic negative-feedback effect on hypothalamic POMC neurones. In turn, the downregulation of central mu opioid receptors in this model may be mediated through a mechanism related to hypothalamic POMC overexpression.

delta-Opioid receptor gene: effect of Sp1 factor on transcriptional regulation in vivo

delta-Opioid receptor (DOR) promoter exhibited a cell-type-specific expression pattern. Protein-DNA interactions in this promoter were identified by dimethyl sulfate in vivo footprinting analysis of NG108-15 cells, expressing endogenous DOR. Complete protection of the putative Sp1 cis-element and partial protection of the sequence defined as X-NotI in the basal promoter were observed only in the G0/G1 phase of the cell cycle. No protection was detected in Neuro2A cells that do not express DOR. In vivo formaldehyde cross-linking confirmed Sp1 factor binding to its cis-acting element during the G0/G1 phase. The functional significance of these Sp1 and X-NotI sites was evaluated by transient transfection analysis. Northern blot analysis and nuclear run-off assays revealed maximum DOR mRNA level and transcription rate, respectively, during the G0/G1 phase of NG108-15 cells. In summary, the protein-DNA interactions at the Sp1 and X-NotI sites are necessary for cell cycle-dependent and cell-type-specific up-regulated DOR gene expression.

Opioids, opioid receptors, and the immune response

It is now clear that opioid receptors participate in the function of the cells of the immune system, and evidence suggests that opioids modulate both innate and acquired immune responses. We review literature here which establishes that mu-, kappa-, and delta-opioid compounds alter resistance to a variety of infectious agents, including the Human Immunodeficiency Virus (HIV). The nature of the immunomodulatory activity of the opioids has been the subject of a great deal of research over the last ten years. There is increasing evidence that effects of opioids on the immune response are mediated at several levels. Modulation of the inflammatory response appears to be a target of these compounds, including effects on phagocytic activity, as well as the response of cells to various chemoattractant molecules. Moreover, findings from several laboratories have demonstrated the impact of opioid treatment on antibody responses, and the molecular basis for this effect is likely due, at least in part, to the modulation of both cytokine and cytokine receptor expression. Future research should provide a clearer understanding of the cellular and molecular targets of opioid action within the immune system.

Molecular mechanisms and regulation of opioid receptor signaling

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

Regulation of mouse delta-opioid receptor gene transcription: involvement of the transcription factors AP-1 and AP-2

The aim of this study was to examine the effects of the phorbol ester O-tetradecanoylphorbol 13-acetate (TPA) and forskolin on delta-opioid receptor gene transcription. Treatment of NG108-15 cells with TPA (100 nM) for 48 h increased delta-opioid receptor mRNA levels, whereas different concentrations of forskolin induced a transient down-regulation of mRNA 5 h after treatment, followed by increased mRNA levels after 48 h. Reporter gene assays in transiently transfected NG108-15 cells in combination with electrophoretic mobility shift assays indicate that the increase of delta-opioid receptor mRNA after stimulation with TPA is mediated by transcription factor AP-1, which binds 355 bp upstream of the start codon within the gene promoter. The forskolin-induced mRNA increase is mediated neither by a cyclic AMP-response element nor indirectly by AP-1 up-regulation. Reporter gene assays, mutational analysis, and electrophoretic mobility shift assays revealed that delta-opioid receptor gene regulation by forskolin is mediated by transcription factor AP-2, which binds to an element 157 bp upstream of the start codon.

Transcriptional regulation of mouse delta-opioid receptor gene

Three major types of opioid receptors, mu (MOR), delta (DOR), and kappa (KOR), have been cloned and characterized. Each opioid receptor exhibits a distinct pharmacological profile as well as a distinct pattern of temporal and spatial expression in the brain, suggesting the critical role of transcription regulatory elements and their associated factors. Here, we report the identification of a minimum core promoter, in the 5'-flanking region of the mouse DOR gene, containing an E box and a GC box that are crucial for DOR promoter activity in NS20Y cells, a DOR-expressing mouse neuronal cell line. In vitro protein-DNA binding assays and in vivo transient transfection assays indicated that members of both the upstream stimulatory factor and Sp families of transcription factors bound to and trans-activated the DOR promoter via the E box and GC box, respectively. Furthermore, functional and physical interactions between these factors were critical for the basal as well as maximum promoter activity of the DOR gene. Thus, the distinct developmental emergence and brain regional distribution of the delta opioid receptor appear to be controlled, at least in part, by these two regulatory elements and their associated factors.

Opioid receptor gene: cytokine response element and the effect of cytokines

Previous studies demonstrated that cytokines regulate opioid and opioid receptor gene expression in neuronal and immune cells. The gene sequence analysis of opioid receptors revealed that mu-, delta- and kappa-opioid receptor promoter regions contain potential cytokine response elements (NF-IL6 binding sites). It was postulated that the response elements present in opioid receptor promoter regions may have a role in the cytokine effects on opioid receptor gene expression through cis-trans interaction. The present study investigated whether cytokines have an effect on opioid receptor gene expression by cytokine-induced transcription factor, NF-IL6, using a number of immune cell lines which respond to cytokines. Further investigation was made to determine whether the potential cytokine response element DNA sequences on opioid receptor promoter region have functional significance which may be affected by the DNA context of the opioid receptor promoter in immune cell lines. Tandem repeats of conserved cytokine response elements of IL-6 gene fused to a heteropromoter SV40 were utilized as a positive control and expressed 2-fold increased promoter activity after cytokine stimulation. Transient transfection studies in time courses (24-72 h) and different dose treatments (100-500 U/ml for IL-6 and 50-200 U/ml for IL-1 alpha+beta) were also carried out to investigate the possibility that the upregulated gene expression may be transient or cytokine-dose-dependent. Our data demonstrated that there was no significant cytokine-stimulated opioid receptor gene expression in immune cells tested. In addition, the cytokine response elements (NF-IL6 binding sites) in opioid receptor genes are not functional. These results contradict the previous reports in which cytokines modulated the expression of opioid and opioid receptors in neuronal and immune cells. The possible reasons regarding the different results were discussed.

Structure of the human histamine H1 receptor gene

Histamine H1 receptor expression has been reported to change in disorders such as allergic rhinitis, autoimmune myocarditis, rheumatoid arthritis and atherosclerosis. Here we report the isolation and characterization of genomic clones containing the 5' flanking (regulatory) region of the human histamine H1 receptor gene. An intron of approx. 5.8 kb was identified in the 5' untranslated region, which suggests that an entire subfamily of G-protein-coupled receptors may contain an intron immediately upstream of the start codon. The transcription initiation site was mapped by 5' rapid amplification of cDNA ends to a region 6.2 kb upstream of the start codon. Immediately upstream of the transcription start site a fragment of 1.85 kb was identified that showed promoter activity when placed upstream of a luciferase reporter gene and transiently transfected into cells expressing the histamine H1 receptor. The promoter sequence shares a number of characteristics with the promoter sequences of other G-protein-coupled receptor encoding genes, including binding sites for several transcription factors, and the absence of TATA and CAAT sequences at the appropriate locations. The promoter sequence described here differs from that reported previously [Fukui, Fujimoto, Mizuguchi, Sakamoto, Horio, Takai, Yamada and Ito (1994) Biochem. Biophys. Res. Commun. 201, 894-901] because the reported genomic clone was chimaeric. Furthermore our study provides evidence that the 3' untranslated region of the H1 receptor mRNA is much longer than previously accepted. Together, these findings provide a complete view of the structure of the human histamine H1 receptor gene. Both the coding region of the H1 receptor gene and its promoter region were independently mapped to chromosome 3p25.

Cloning, molecular characterization, and distribution of a gene homologous to delta opioid receptor from zebrafish (Danio rerio)

A full-length cDNA, ZFOR1, has been isolated from the teleost zebrafish (Danio rerio) using a probe from rat mu opioid receptor. ZFOR1 encodes a 373 amino acid protein with seven potential transmembrane domains that shows a high degree of homology to mammalian delta opioid receptor. We have also isolated a genomic clone which contains two exons of ZFOR1, homologous to exons 2 and 3 in mouse and human delta opioid receptor. Expression of ZFOR1 appears to be restricted to nervous tissue as assessed by Northern blot. In situ hybridization in zebrafish brain with specific probes revealed several discrete areas of ZFOR1 expression; higher levels are detected in dorsal telencephalic areas, the periventricular layer of the optic tectum, and the granular layer of the cerebellum. This is the first molecular evidence of the presence of the delta opioid receptor in a non-mammalian species and suggests that it has been highly conserved throughout vertebrate evolution.

Functional interference of Sp1 and NF-kappaB through the same DNA binding site

Gene activation by NF-kappaB/Rel transcription factors is modulated by synergistic or antagonistic interactions with other promoter-bound transcription factors. For example, Sp1 sites are often found in NF-kappaB-regulated genes, and Sp1 can activate certain promoters in synergism with NF-kappaB through nonoverlapping binding sites. Here we report that Sp1 acts directly through a subset of NF-kappaB binding sites. The DNA binding affinity of Sp1 to these NF-kappaB sites, as determined by their relative dissociation constants and their relative efficiencies as competitor DNAs or as binding site probes, is in the order of that for a consensus GC box Sp1 site. In contrast, NF-kappaB does not bind to a GC box Sp1 site. Sp1 can activate transcription through immunoglobulin kappa-chain enhancer or P-selectin promoter NF-kappaB sites. p50 homodimers replace Sp1 from the P-selectin promoter by binding site competition and thereby either inhibit basal Sp1-driven expression or, in concert with Bcl-3, stimulate expression. The interaction of Sp1 with NF-kappaB sites thus provides a means to keep an elevated basal expression of NF-kappaB-dependent genes in the absence of activated nuclear NF-kappaB/Rel.

mu Opioid receptor knockout in mice: effects on ligand-induced analgesia and morphine lethality

The mu opioid receptor gene (MOR) was mutated in mice by a gene targeting procedure. In these MOR-knockout mice, the analgesic effects of morphine, its major metabolites, morphine-6-glucuronide (M-6-G) and morphine-6-ethereal sulfate (M-6-S), and endomorphin-2, as well as morphine-induced lethality, were drastically reduced, whereas the effects of DPDPE and U50488 remained unchanged. It is concluded that analgesic effects of mu-specific opioid ligands and acute morphine lethality are mediated by the mu receptor.

Detection of opioid receptor mRNA by RT-PCR reveals alternative splicing for the delta- and kappa-opioid receptors

The three mu-, delta- and kappa-opioid receptors have recently been cloned and characterized at the molecular level. Our analysis of opioid receptor transcripts by RT-PCR revealed two PCR products derived from delta and kappa mRNAs with size higher than expected from the known cDNA sequences. DNA sequencing showed additional nucleotides inserted between the known splice sites, indicating the possible existence of alternative splicing pathways for delta and kappa receptors. The novel delta-opioid receptor transcript is expressed in mouse brain and contains a 243 bp insertion. This additional sequence is located at the splice junction between the first and second coding exons and is encoded by a single exon located 9 kb upstream exon 2 in the mDOR gene. The other alternative transcript occurs in human monocytic and T lymphocytic cell lines and encodes a novel form of the kappa-opioid receptor. The PCR product presents a 23 bp deletion at the 3' end of exon 2 followed by a 246 bp insertion found between exons 2 and 3. In the hKOR gene, this insertion is encoded by two DNA segments. One of them is located 0.4 kb downstream exon 2 while the second is flanking exon 3 on the 5' side. Both novel putative delta and kappa exons present in-frame stop codons that would lead to truncated receptor proteins. A possible functional or regulatory role of these shorter proteins in opioid function remains to be determined.

Quantitative analysis of mu and delta opioid receptor gene expression in rat brain and peripheral ganglia using competitive polymerase chain reaction

Competitive polymerase chain reaction assays following reverse transcription have been developed for quantitative analysis of delta and mu opioid receptor gene expression. The assay was used to obtain quantitative measurements of mu and delta opioid receptor expression levels in different brain regions and sensory and sympathetic ganglia in the rat. The assays provide accurate estimates of the relative levels of receptor messenger RNAs by the inclusion in the assays of known amounts of internal standards with the same sequence, except for a small deletion, as the target complementary DNA. The amplification products of target and competitor can be distinguished by size, and their amounts measured by densitometry. Expression of mu and delta opioid receptor messenger RNAs in different regions of the rat brain, somatic and visceral sensory and sympathetic ganglia was investigated using this method. In the brain the highest density of delta receptor messenger RNA was detected in the olfactory bulb, followed by the striatum. The mu receptor was expressed at highest levels in the midbrain-hypothalamic region. All the sensory ganglia studied expressed both mu and delta opioid receptor messenger RNAs. In the nodose ganglion we observed the highest level of mu receptor messenger RNA of any structure studied; in the trigeminal ganglion the level was about 10 times lower than that in the nodose ganglion. Among the dorsal root ganglia, mu receptor messenger RNA density was highest in the lumbar region, followed by the thoracic and cervical regions. The sympathetic superior cervical ganglion expressed a very low level of mu message. Delta receptor messenger RNA was detected only in the sensory ganglia, at levels that were considerably lower than in the striatum. The reverse transcription polymerase chain reaction assay is quantitatively reliable for comparison of messenger RNA levels between different RNA extracts, and sensitive enough to permit the detection and assay of mu and delta opioid receptor gene expression in a single pair of sensory or autonomic ganglia from the rat.

Regulation of mouse neuropeptide Y Y1 receptor gene transcription: a potential role for nuclear factor-kappa B/Rel proteins

We previously isolated a 1.3-kb genomic fragment in the 5'-flanking region of the murine neuropeptide Y (NPY) Y1 receptor gene, which is able to drive the expression of LacZ reporter gene in neuronal cells. We determined the ability of deletion mutants of this region to modulate transcription of the heterologous luciferase gene in the Y1 receptor-expressing neuroblastoma/ glioma NG108-15 cells and the Y1 receptor-deficient 293 cells. Results suggest the presence of a cell type-specific core promoter (-399 to -218 from the initiator ATG) and, upstream, of two positive and two negative regulatory elements. Sequence analysis of the Y1 receptor promoter identified two decameric sequences corresponding to consensus binding sites for nuclear factor-kappa B/Rel proteins. Gel shift analysis indicated that a 29-bp oligonucleotide comprising the two putative kappa B sites, which we refer to as Y1-kappa B sequence, specifically binds kappa B-related complexes in nuclear extracts from rat brain areas, NG108-15 cells, and the murine T cell clone A.E7. In nuclear extracts from A.E7 and NG108-15 cells, the Y1-kappa B sequence specifically binds an additional complex whose molecular nature remains to be elucidated. Through transient transfection studies, we also demonstrated that the Y1-kappa B sequence acts as an enhancer element, inferring its potential role in regulation of the Y1 receptor gene expression.

Dopaminergic regulation of AP-1 transcription factor DNA binding activity in rat striatum

Dopaminergic modulation of the DNA binding activity of AP-1, Sp1, CREB and AP-2 transcription factors was examined in rat striatal nuclear extracts by gel shift assay. AP-1 binding was selectively increased in the striatum following depletion of dopamine by 6-hydroxydopamine-induced lesion of the nigrostriatal pathway or after reserpine treatment. The D1 agonist SKF 38393 dose-dependently increased AP-1 binding; this effect was significantly increased in reserpine-treated rats and even more markedly enhanced in denervated striatum. The D2/D3 agonist quinpirole, administered alone, did not affect striatal activator protein-1 binding; in combination, quinpirole and SKF 38393 acted synergistically in normal and reserpine-treated rats but not in 6-hydroxydopamine-lesioned rats, suggesting that mechanisms underlying D1-D2/D3 interactions are altered after dopamine denervation. Most, but not all, of the changes in AP-1 binding activity observed in this study are consistent with changes in levels of Fos/Jun family proteins observed after similar treatments. These results support the hypothesis that D1 receptor stimulation activates striatonigral neurons and modulates expression of AP-1-related genes in these neurons, while D2 receptor stimulation mediates tonic inhibition of AP-1 expression and activity in the striatopallidal neurons. Moreover, the findings provide evidence that the loss of dopaminergic input to the striatum, as occurs in Parkinson's disease, induces long-lasting alterations in the regulation of striatal gene expression which may contribute to the disease's progress.

Recent advances in molecular recognition and signal transduction of active peptides: receptors for opioid peptides

1. Opioid peptides are a family of structurally related neuromodulators which play a major role in the control of nociceptive pathways. These peptides act through membrane receptors of the nervous system, defined as mu, delta and kappa and endowed with overlapping but distinct pharmacological, anatomical and functional properties. 2. Recent cloning of an opioid receptor gene family has opened the way to the use of recombinant DNA technology at the receptor level. 3. This review focuses on the molecular cloning and functional characterization of opioid receptors and provides first insights into molecular aspects of opioid peptide recognition and signal transduction mechanisms, using the cloned receptors as investigation tools.

Sequence of kappa-opioid receptor cDNA in the R1.1 thymoma cell line

To our knowledge, this is the first demonstration of kappa-opioid receptor mRNA in cells of the immune system. While the presence of opioid receptors on cells of the immune system has been controversial, cell-binding analysis has indicated that the kappa-opioid receptor is expressed by the immature T cell line R1.1. We have developed a reverse transcriptase-polymerase chain reaction protocol to amplify the mRNA extracted from R1.1 cells with primers derived from the cDNA sequence of the mouse kappa-opioid receptor. Nucleotide sequences of the amplified products were examined and two populations of cDNA were detected which differ in the 5' region upstream of the ATG start codon. Comparison of these sequences to the previously published kappa-opioid receptor cDNA sequence suggests the presence of an intron-exon junction in the 5' non-coding region.

Molecular biology of the opioid receptors: structures, functions and distributions

Opiates like morphine and endogenous opioid peptides exert their pharmacological and physiological effects through binding to their endogenous receptors, opioid receptors. The opioid receptors are classified into at least three types, mu-, delta- and kappa-types. Recently, cDNAs of the opioid receptors have been cloned and have greatly advanced our understanding of their structure, function and expression. This review focuses on the recent advances in the studies on opioid receptors using the cloned cDNAs. We describe the molecular cloning of the opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotide and anatomical distributions of opioid receptors.

Characterization of the murine mu opioid receptor gene

The analgesic and addictive properties of morphine and other opioid drugs are thought to result from their interaction with mu opioid receptors. Using a delta opioid receptor cDNA as a probe, we have isolated a murine mu opioid receptor cDNA clone (mMOR). Stable expression of mMOR in Chinese hamster ovary cells conferred high binding affinity for mu receptor ligands including morphine and [D-Ala2,N-methyl-Phe4,Gly5-ol]-enkephalin and low affinity for delta and kappa preferring ligands. Treatment of these cell lines with morphine and other mu agonists inhibited forskolin-induced cAMP accumulation, demonstrating a functional coupling of mMOR to the inhibition of adenylate cyclase. The predicted amino acid sequence of mMOR shares approximately 55% overall amino acid identity with the delta receptor and approximately 97% identity with the recently reported rat mu opioid receptor. Expression of the mu receptor in mouse brain as revealed by in situ hybridization parallels the reported pattern of distribution of mu-selective ligand binding sites. Chromosomal localization (to mouse chromosome 10) and Southern analysis are consistent with a single mu opioid receptor gene in the mouse genome, suggesting that the various pharmacologically distinct forms of the mu receptor arise from alternative splicing, post-translational events, or from a highly divergent gene(s).