Identification and characterization of functional D1 dopamine receptors in a human neuroblastoma cell line

2-(Fluoromethoxy)-4'-(S-methanesulfonimidoyl)-1,1'-biphenyl (UCM-1306), an Orally Bioavailable Positive Allosteric Modulator of the Human Dopamine D1 Receptor for Parkinson's Disease

Tolerance development caused by dopamine replacement with l-DOPA and therapeutic drawbacks upon activation of dopaminergic receptors with orthosteric agonists reveal a significant unmet need for safe and effective treatment of Parkinson’s disease. In search for selective modulators of the D₁ receptor, the screening of a chemical library and subsequent medicinal chemistry program around an identified hit resulted in new synthetic compound 26 [UCM-1306, 2-(fluoromethoxy)-4′-(S-methanesulfonimidoyl)-1,1′-biphenyl] that increases the dopamine maximal effect in a dose-dependent manner in human and mouse D₁ receptors, is inactive in the absence of dopamine, modulates dopamine affinity for the receptor, exhibits subtype selectivity, and displays low binding competition with orthosteric ligands. The new allosteric modulator potentiates cocaine-induced locomotion and enhances l-DOPA recovery of decreased locomotor activity in reserpinized mice after oral administration. The behavior of compound 26 supports the interest of a positive allosteric modulator of the D₁ receptor as a promising therapeutic approach for Parkinson’s disease.

Protein kinase D1-dependent phosphorylation of dopamine D1 receptor regulates cocaine-induced behavioral responses

The dopamine (DA) D1 receptor (D1R) is critically involved in reward and drug addiction. Phosphorylation-mediated desensitization or internalization of D1R has been extensively investigated. However, the potential for upregulation of D1R function through phosphorylation remains to be determined. Here we report that acute cocaine exposure induces protein kinase D1 (PKD1) activation in the rat striatum, and knockdown of PKD1 in the rat dorsal striatum attenuates cocaine-induced locomotor hyperactivity. Moreover, PKD1-mediated phosphorylation of serine 421 (S421) of D1R promotes surface localization of D1R and enhances downstream extracellular signal-regulated kinase signaling in D1R-transfected HEK 293 cells. Importantly, injection of the peptide Tat-S421, an engineered Tat fusion-peptide targeting S421 (Tat-S421), into the rat dorsal striatum inhibits cocaine-induced locomotor hyperactivity and injection of Tat-S421 into the rat hippocampus or the shell of the nucleus accumbens (NAc) also inhibits cocaine-induced conditioned place preference (CPP). However, injection of Tat-S421 into the rat NAc shell does not establish CPP by itself and injection of Tat-S421 into the hippocampus does not influence spatial learning and memory. Thus, targeting S421 of D1R represents a promising strategy for the development of pharmacotherapeutic treatments for drug addiction and other disorders that result from DA imbalances.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Long-term cultivation of a neuroblastoma cell line in medium with reduced serum content as a model system for neuronal aging?

Impaired energy metabolism and increased vulnerability to additional stress are some of the pivotal characteristics of the aging brain. This study was designed to establish a cell culture model for long-term investigations of some mechanisms underlying the process of aging using the neuroblastoma cell line SK-N-MC. As high serum concentrations in the culture medium are a major disadvantage for the investigation of regulatory or toxic influences, the effects of serum reduction in the culture medium on growth, viability and energy metabolism during long-term cultivation were determined. Serum reduction resulted in a decrease in the proliferation rate and in increased vulnerability of the cells, measured as release of lactate dehydrogenase into the culture medium. The rates of glucose consumption and lactate production were elevated, whereas the energetic state was markedly compromised, as was obvious from a 40% reduction of creatine phosphate. The observed increased vulnerability and the decreased energy state of the SK-N-MC cells were even more pronounced after induction of free radicals by addition of FeSO(4) to the medium with reduced serum content. Increased oxidative stress was indicated by elevated cellular contents of glutathione both after serum reduction and after incubation with FeSO(4). It is concluded that the SK-N-MC cells cultured chronically in medium with low serum content display biochemical characteristics that are similar to those observed in aging studies with animal models.

CK2 negatively regulates Galphas signaling

We present evidence, using biochemical and cellular approaches, that the kinase, CK2, negatively controls signaling via Galpha(s) (or Galpha(olf)) coupled to dopamine D1 and adenosine A2A receptors. Pharmacological inhibition of CK2 or CK2 knockdown by RNAi lead to elevated cAMP levels in dopamine D1 receptor-activated neuroblastoma cells. Phosphorylation levels of protein kinase A substrates were increased in the presence of CK2 inhibitors in mouse striatal slices. The effect of D1 receptor and A2A receptor agonists on the phosphorylation of protein kinase A sites was potentiated upon CK2 inhibition. Furthermore, in cell lines, we observed that reduction in CK2 activity, pharmacologically or genetically, reduced the amount of D1 receptor that was internalized in response to dopamine. Finally, the beta subunit of CK2 was found to interact specifically with the Galpha(s) subunit through protein interaction analyses. Thus CK2 can inhibit G protein-coupled receptor action by enabling faster receptor internalization, possibly through a direct association with Galpha(s).

Dopamine promotes striatal neuronal apoptotic death via ERK signaling cascades

Although the mechanisms underlying striatal neurodegeneration are poorly understood, we have shown that striatal pathogenesis may be initiated by high synaptic levels of extracellular dopamine (DA). Here we investigated in rat striatal primary neurons the mobilization of the mitogen-activated protein kinase (MAPK) signaling pathways after treatment with DA. Instead of observing an elevation of the archetypical pro-cytotoxic MAPKs, p-JNK and p-p38 MAPK, we found that DA, acting through D1 DA receptors, induced a sustained stimulation of the phosphorylated form of extracellular signal-regulated kinase (p-ERK) via a cAMP/protein kinase A (PKA)/Rap1/B-Raf / MAPK/ERK kinase (MEK) pathway. Blockade of D2 DA receptors, beta-adrenergic receptors or N-methyl-D-aspartate receptors with receptor-specific antagonists had no significant effect on this process. Activation of D1 DA receptors and PKA by DA caused phosphorylation and inactivation of the striatal-enriched tyrosine phosphatase, an important phosphatase for the dephosphorylation and subsequent inactivation of p-ERK in the striatum. Interestingly, p-ERK was primarily retained in the cytoplasm, with only low amounts translocated to the nucleus. The scaffold protein beta-arrestin2 interacted with both p-ERK and D1 DA receptor, triggering the cytosolic retention of p-ERK and inducing striatal neuronal apoptotic death. These data provide unique insight into a novel role of p-ERK in striatal neurodegeneration.

NF-kB/NOS cross-talk induced by mitochondrial complex II inhibition: implications for Huntington's disease

Nuclear factor-kB (NF-kB) is a family of DNA-binding proteins that are important regulators involved in immune and inflammatory responses, as well as in cell survival and apoptosis. In the nervous system NF-kB is activated under physiological and pathological conditions including learning and memory mechanisms and neurodegenerative diseases. NF-kB is activated in neurons in response to excitotoxic, metabolic and oxidative stress and there is a body of evidence to suggest that glutamate induces NF-kB by the main ionotropic glutamate receptors. In the present study, 3 nitroproprionic acid (3NP), an irreversible inhibitor of succinate dehydrogenase (SD, complex II) has been employed to provide an experimental model of Huntington's disease (HD). Specifically, we described 3NP-induced activation of NF-kB and of iNOS and nNOS genes in striatal treated slices. To aim to better understand the relationship between these identified dysregulated genes and mitochondrial dysfunction, we investigated in SK-N-MC human neuroblastoma cells following 3NP treatment, whether NF-kB nuclear translocation and activation might be involved in the mechanisms by which 3NP leads to transcriptional activation of NOS genes. These results are relevant to more precisely define the role of NF-kB in neuronal cells and better understand its putative involvement in neurodegeneration.

Receptor regulation of gene expression of axon guidance molecules: implications for adaptation

Axon guidance molecules, critical for neurodevelopment, are also implicated in morphological and other neurodaptative changes mediated by physiological or pharmacological events in adult brain. As an example, the psychostimulant cocaine markedly alters axon guidance molecules in adult brain of cocaine-treated rats. To decipher a potential link between drug-induced activation of G-protein-coupled receptors (GPCRs) and modulation of axon guidance molecules, we investigated whether GPCR activity in a SK-N-MC human neuroepithelioma cell line (which expresses low levels of D1 dopamine receptors) affects gene expression of axon guidance molecules (semaphorins, ephrins, netrins, and their receptors). Using real-time polymerase chain reaction, we identified 17 of 26 axon guidance molecules in these cells, with varying levels of expression. Forskolin, which raised intracellular cAMP levels 340%, increased EphA5, EphB2, and Neuropilin1 expression, paralleling reported changes in the rat hippocampus after cocaine treatment. The dopamine receptor agonist dihydrexidine, which raised cAMP levels 22%, promoted regulatory changes in EphrinA1, EphrinA5, EphB1, DCC, and Semaphorin3C, whereas (+/-)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF81297) altered EphA5, EphrinA1, EphrinA5, and neuropilin1. cAMP and other signal transduction pathways may regulate gene expression of axon guidance molecules, potentially linking monoamine receptor activation to signal transduction cascades, transcriptional regulation of axon guidance molecules, and alterations in neural networks.

Dopamine D1 receptor-mediated toxicity in human SK-N-MC neuroblastoma cells

Striatal degeneration occurs through unknown mechanisms in certain neurodegenerative disorders characterized by increased and sustained synaptic levels of dopamine. In the present studies, we examined the effects of treatment of SK-N-MC neuroblastoma cells with dopamine to understand the participation of dopamine D(1) receptor in postsynaptic cytotoxicity. Treatment of SK-N-MC cells either with dopamine or the D(1) receptor agonist SKF R-38393 resulted in a significant increase in the production of reactive oxygen species (by approximately 2.75-fold) and cell death ( approximately 50%), while antagonism of the D(1) receptor with SCH 23390 significantly reversed (to approximately 75% of control level) these effects. Accumulation of cAMP in dopamine treated cells (t(1/2)=1.5h) preceded changes in ionic gradient (t(1/2)=6.5h), as measured by intracellular potassium concentration and leakage of cytochrome c into the cytosol (t(1/2)=13 h), suggesting a possible staging of toxic events as a result of activation of D(1) receptor by dopamine. Examination of cellular metabolic properties with (13)C NMR spectroscopy showed an inhibitory effect on tricarboxylic acid cycle metabolism via D(1)-mediated receptors after treatment with dopamine, suggesting a direct role for D(1) receptor in dopamine-induced postsynaptic cell death. The present studies provide novel insight into a possible patho-physiological staging of cytotoxic events that are mediated by activation of D(1) receptor.

Integration of G protein signals by extracellular signal-regulated protein kinases in SK-N-MC neuroepithelioma cells

Mammalian cells often receive multiple extracellular stimuli under physiological conditions, and the various signaling inputs have to be integrated for the processing of complex biological responses. G protein-coupled receptors (GPCRs) are critical players in converting extracellular stimuli into intracellular signals. In this report, we examined the integration of different GPCR signals by mitogen-activated protein kinases (MAPKs) using the SK-N-MC human brain neuroepithelioma cells as a neuronal model. Stimulation of the Gi-coupled neuropeptide Y1 and Gq-coupled muscarinic M1 acetylcholine receptors, but not the Gs-coupled dopamine D1 receptor, led to the activation of extracellular signal-regulated kinase (ERK). All three receptors were also capable of stimulating c-Jun NH2-terminal kinases (JNK) and p38 MAPK. The Gi-mediated ERK activation was completely suppressed upon inhibition of Src tyrosine kinases by PP1, while the Gq-induced response was suppressed by both PP1 and the Ca2+ chelator, BAPTA-AM. In contrast, activations of JNK and p38 by Gs-, Gi-, and Gq-coupled receptors were sensitive to PP1 and BAPTA-AM pretreatments. Simultaneous stimulation of Gi- and Gq-coupled receptors resulted in the synergistic activation of ERK, but not JNK or p38 MAPK. The Gi/Gq-induced synergistic ERK activation was PTX-sensitive, and appeared to be a co-operative effect between Ca2+ and Src family tyrosine kinases. Enhanced ERK activation was associated with an increase in CREB phosphorylation, while the JNK and p38-responsive transcription factor ATF-2 was weakly enhanced upon Gi/Gq-induction. This report provides evidence that G protein signals can be integrated at the level of MAPK, resulting in differential effects on ERK, JNK and p38 MAPK in SK-N-MC cells.

Microinfusion of antineuronal antibodies into rodent striatum: failure to differentiate between elevated and low titers

An autoimmune-mediated mechanism has been proposed for several pediatric movement disorders. In a three-center (Brown, Yale, and Johns Hopkins) collaborative effort, serum antineuronal antibodies (ANAb) were measured by use of ELISA or immunohistochemical techniques on 35 children (mean age 11.4 years) with Tourette syndrome, attention deficit hyperactivity disorder, and/or obsessive compulsive disorder. Eight sera, 4 containing the highest and 4 the lowest levels of ANAb, were identified at each institution. Selected sera (total of 9 with elevated and 7 with low ANAb) were re-encoded and sent to each center for infusion into the ventrolateral striatum of 16 male Sprague-Dawley rats. Animals were observed for behavioral abnormalities for 3 days before the start of infusion, during infusion on days 2-4, and for 2 days after infusion. Combined stereotypy scores increased after antibody infusion, but there was no significant effect based on serum titer (p=0.85). Scores differed among centers, but analyses based on individual institutional data again failed to show an effect based on elevated or low ANAb values (Brown, p=0.95; Yale and Johns Hopkins, p=0.81). Post hoc studies with sham surgery and infusion of phosphate-buffered saline support suggestions of nonspecific behavioral effects unrelated to antibody titer. This report emphasizes that any conclusions about antibody-mediated movement disorders that are based upon results from the rodent infusion model must be considered with caution.

The role of D1 dopamine receptors and phospho-ERK in mediating cytotoxicity. Commentary

Striatal neurodegeneration observed in several neurological diseases, occurs through unknown mechanisms. Recent evidence suggests that its pathogenesis may be linked, in part, to high synaptic levels of dopamine (DA), which can then cause neurotoxicity of striatal neurons through mitogen-activated protein kinases (MAPKs). Here we comment on the role of extracellular signal-regulated kinase (ERK) activation in the cytotoxicity mediated upon activation of the D1 DA receptor, and describe a possible mechanism for phospho-ERK (p-ERK) in inducing cytotoxicity.

D1 Dopamine Receptor Mediates Dopamine-induced Cytotoxicity via the ERK Signal Cascade

Postsynaptic striatal neurodegeneration occurs through unknown mechanisms, but it is linked to high extracellular levels of synaptic dopamine. Dopamine-mediated cytotoxicity of striatal neurons occurs through two distinct pathways: autoxidation and the D1 dopamine receptor-linked signaling pathway. Here we investigated the mitogen-activated protein kinase (MAPK) signaling pathways activated upon the acute stimulation of D1 dopamine receptors. In SK-N-MC neuroblastoma cells, endogenously expressing D1 dopamine receptors, dopamine caused activation of phosphorylated (p-)ERK1/2 and of the stress-signaling kinases, p-JNK and p-p38 MAPK, in a time- and dose-dependent manner. Selective stimulation of D1 receptors with the agonist SKF R-38393 caused p-ERK1/2, but not p-JNK or p-p38 MAPK activation, in a manner sensitive to the receptor-selective antagonist SCH 23390, protein kinase A inhibition (KT5720), and MEK1/2 inhibition (U0126 or PD98059). Activation of ERK by D1 dopamine receptors resulted in oxidative stress and cytotoxicity. In cells transfected with a catalytically defective mutant of MEK1, the upstream ERK-specific kinase, both dopamine- and SKF R-38393-mediated cytotoxicity was markedly attenuated, confirming the participation of the ERK signaling pathway. Cell fractionation studies showed that only a small amount of p-ERK1/2 was translocated to the nucleus, with the majority retained in the cytoplasm. From coimmunoprecipitation studies, p-ERK was found to form stable heterotrimeric complexes with the D1 dopamine receptor and beta-arrestin2. In cells transfected with the dominant negative mutant of beta-arrestin2, the formation of such complexes was substantially inhibited. These data provide novel mechanistic insights into the role of ERK in the cytotoxicity mediated upon activation of the D1 dopamine receptor.

Anti-basal ganglia antibodies in PANDAS

An autoimmune-mediated mechanism involving molecular mimicry has been proposed for a variety of pediatric movement disorders that occur after a streptococcal infection. In this study, anti-basal ganglia antibodies (ABGA) were measured in 15 children with the diagnosis of pediatric autoimmune neuropsychiatric disorder associated with streptococcal infection (PANDAS) and compared with those in 15 controls. ELISA and Western immunoblotting (WB) methods were used to detect ABGA against supernatant (S1), pellet (P2), and synaptosomal preparations from adult postmortem caudate, putamen, and globus pallidus. ELISA optical density values did not differ between PANDAS patients and controls across all preparations. Immunoblotting identified multiple bands in all subjects with no differences in the number of bands or their total density. Discriminant analysis, used to assess mean binding patterns, showed that PANDAS patients differed from controls only for the caudate S1 fraction (Wilks' lambda = 0.0236, P < 0.0002), with PANDAS-primarily tic subjects providing the greatest discrimination. Among the epitopes contributing to differences between PANDAS and control in the caudate S1 fraction, mean binding to the epitope at 183 kDa was the most different between groups. In conclusion, ELISA measurements do not differentiate between PANDAS and controls, suggesting a lack of major antibody changes in this disorder. Further immunoblot analyses using a caudate supernatant fraction are required to completely exclude the possibility of minor antibody repertoire differences in PANDAS subjects, especially in those who primarily have tics.

Striatal microinfusion of Tourette syndrome and PANDAS sera: failure to induce behavioral changes

Rodent striatal microinfusions have been suggested as a model for assessing the behavioral effects induced by antineuronal antibodies. We used this approach to evaluate the proposed autoimmune etiology for Tourette syndrome (TS) and pediatric autoimmune neuropsychiatric disorders associated with streptococcal infection (PANDAS). Sera were assessed from patients with TS (n = 9) preselected based on the presence of elevated enzyme-linked immunosorbent assay optical densities against putamen homogenate and sera from patients with PANDAS (n = 8), selected from a larger group assayed for antibodies against a putamen synaptosomal preparation. The effect of antibodies against the streptococcal M5 protein were also studied. A total of 44 Fischer rats received bilateral infusion of sera: 23 ventral striatum (5 PANDAS, 5 TS, 5 anti-M5 protein, and 8 control); 21 ventrolateral striatum (5 PANDAS, 5 TS, 5 anti-M5 protein, and 6 controls). Cannulas were placed bilaterally and symmetrically by stereotactic techniques. After animals were allowed to recover for 1 week, sera were microinfused for 3 days. Animal behavior was then simultaneously quantified by daily observation and monitoring using automated activity boxes for 10 days after infusion. No significant alterations in stereotypic behavior or movement were observed between the PANDAS, TS, or anti-M5 protein and control groups. Our findings are in contrast to previous reports, and suggest the need for further investigations to determine the validity of the model and of autoimmune-mediated hypotheses for pediatric movement disorders.

Bimodal induction of dopamine-mediated striatal neurotoxicity is mediated through both activation of D1 dopamine receptors and autoxidation

Striatal neurodegeneration occurs through unknown mechanisms in certain neurodegenerative disorders characterized by increased and sustained synaptic levels of dopamine (DA). Treatment of rat primary striatal neurons with DA causes profound neurotoxicity, with increased production of free radicals and accelerated neuronal death. DA effects were partly reduced by the antioxidant sodium metabisulfite (SMBS), and the D1 DA receptor antagonist, SCH 23390, and were completely blocked upon co-treatment with SMBS and SCH 23390. Part of DA effects were mimicked by either H(2)O(2), or by the D1 agonist, SKF R-38393, indicating the existence of two distinct signaling pathways through which the neurotoxicity of DA is manifest. DA effects did not proceed through D2-like DA or beta-adrenergic receptor signaling pathways. The D1 receptor-mediated and the autoxidative pathways of DA neurotoxicity converge to cause activation and/or increased synthesis of neuronal and inducible, but not endothelial, nitric oxide synthase (NOS). The reduction of DA striatal neurotoxicity through blockade of D1 DA receptors, suggests novel therapeutic approaches in the management of striatal neurodegeneration.

Orally active oxime derivatives of the dopaminergic prodrug 6-(N,N-di-n-propylamino)-3,4,5,6,7,8-hexahydro-2H-naphthalen-1-one. Synthesis and pharmacological activity

A series of racemic and enantiomerically pure oxime derivatives of the potential anti-Parkinson prodrug 6-(N,N-di-n-propylamino)-3,4,5,6,7,8-hexahydro-2H-naphthalen-1-one (1) were synthesized and pharmacologically evaluated. The oximes induced rotational behavior in the Ungerstedt rat rotation model for Parkinson's disease after oral administration. Especially the unsubstituted oxime ((-)-3) and the acetyl-oxime ((-)-10) induced a pronounced and long lasting effect. In this model, large individual differences were observed in responsiveness to treatment between rats. Though less potent than the parent prodrug, the oxime derivatives of (+/-)-1 and (-)-1 can be orally active, acting as cascade prodrugs.

Chronic stimulation of D1 dopamine receptors in human SK-N-MC neuroblastoma cells induces nitric-oxide synthase activation and cytotoxicity

Elevated synaptic levels of dopamine may induce striatal neurodegeneration in l-DOPA-unresponsive parkinsonism subtype of multiple system atrophy (MSA-P subtype), multiple system atrophy, and methamphetamine addiction. We examined the participation of dopamine and D1 dopamine receptors in the genesis of postsynaptic neurodegeneration. Chronic treatment of human SK-N-MC neuroblastoma cells with dopamine or H2O2 increased NO production and accelerated cytotoxicity, as indexed by enhanced nitrite levels and cell death. The antioxidant sodium metabisulfite or SCH 23390, a D1 dopamine receptor-selective antagonist, partially blocked dopamine effects but together ablated dopamine-mediated cytotoxicity, indicating the participation of both autoxidation and D1 receptor stimulation. Direct activation of D1 dopamine receptors with SKF R-38393 caused cytotoxicity, which was refractory to sodium metabisulfite. Dopamine and SKF R-38393 induced overexpression of the nitric-oxide synthase (NOS) isoforms neuronal NOS, inducible NOS (iNOS), and endothelial NOS in a protein kinase A-dependent manner. Functional studies showed that approximately 60% of total NOS activity was due to activation of iNOS. The NOS inhibitor N(G)-nitro-l-arginine methyl ester and genistein, wortmannin, or NF-kappaB SN50, inhibitors of protein tyrosine kinases phosphatidylinositol 3-kinase and NF-kappaB, respectively, reduced nitrite production by dopamine and SKF R-38393 but were less effective in attenuating H2O2-mediated effects. In rat striatal neurons, dopamine and SKF R-38393, but not H2O2, accelerated cell death through increased expression of neuronal NOS and iNOS but not endothelial NOS. These data demonstrate a novel pathway of dopamine-mediated postsynaptic oxidative stress and cell death through direct activation of NOS enzymes by D1 dopamine receptors and its associated signaling pathways.

Differential cytotoxicity of dopamine and H2O2 in a human neuroblastoma divided cell line transfected with alpha-synuclein and its familial Parkinson's disease-linked mutants

alpha-Synuclein accumulates in Lewy bodies and two missense mutations, A30P and A53T, have been linked to familial Parkinson's disease. Neither the normal function of alpha-synuclein nor the pathomechanism of alpha-synuclein-induced neuropathy are known. SK-N-MC neuroblastoma cells were transiently transfected with either wt alpha-synuclein, or its mutants, and their abilities to protect against oxidative stress were assessed. At low expression levels (1 microg cDNA/10(5) cells), all three synuclein variants were devoid of any effect on dopamine-induced cytotoxicity and nitrite production, whereas at higher expression (5 microg cDNA/10(5) cells), the variants enhanced dopamine-mediated effects. Low levels of wt alpha-synuclein blocked H(2)O(2)-induced cytotoxicity and nitrite production, a protective effect that was partly decreased upon higher expression. Both A30P and A53T increased in a dose-dependent manner H(2)O(2)-induced nitrite production and cell death. These results show an absence of protective effects for the A30P/A53T mutants, and a differential cytoprotective role of alpha-synuclein against oxidants, which varies according to expression levels.

Protein kinase C regulates dopamine D4 receptor-mediated phospholipid methylation

Dopamine D4 receptors (D4 receptors) mediate dopamine-stimulated, folate-dependent phospholipid methylation. To investigate possible regulation of this multi-step D4 receptor-mediated phospholipid methylation cycle by protein kinases, specific kinase activators and inhibitors were studied in SK-N-MC human neuroblastoma cells, using [14C] formate to label folate-derived single-carbon groups. Phorbol dibutyrate (PDB), an activator of protein kinase C, stimulated basal phospholipid methylation and also shifted the dose-response curve for dopamine-stimulated phospholipid methylation to the right by more than an order of magnitude. Calphostin C, an inhibitor of protein kinase C, had little effect on basal phospholipid methylation but significantly inhibited dopamine-stimulated phospholipid methylation and also blocked the stimulatory response to PDB. Chelerythrine, which inhibits protein kinase C and other kinases, strongly inhibited both basal and dopamine-stimulated phospholipid methylation. Forskolin, an activator of protein kinase A, inhibited basal and dopamine-stimulated phospholipid methylation, but only at high concentrations while Rp-cAMP, an inhibitor of protein kinase A, did not block this effect. Inhibition of protein kinase G produced a modest decrease in dopamine-stimulated phospholipid methylation, but neither sodium nitroprusside, which increases nitric oxide (NO) production and activates protein kinase G, nor the NO synthase inhibitor N-nitro-L-arginine had any effect on basal or dopamine-stimulated phospholipid methylation. These observations indicate that protein kinase C is an important regulator of basal and D4 receptor-mediated folate-dependent phospholipid methylation, whereas protein kinase A and protein kinase G have a lesser or minimal role.

Antibodies against a neuron-like (HTB-10 neuroblastoma) cell in children with Tourette syndrome

BACKGROUND

Similar to the model for Sydenham's chorea, antineuronal antibodies (ANAb), which develop in response to a preceding streptococcal infection, have been speculated to have a role in the development of Tourette syndrome (TS).

METHODS

Serum antibodies against the neuron-like HTB-10 neuroblastoma cell were assayed by ELISA methods and Western blot analysis on 41 children with TS (mean age 11.3 years) and 39 control subjects (mean age 12.1 years).

RESULTS

Group comparisons of ELISA assay optical density (OD) showed that mean OD values for serum antibodies were not different [control (mean +/- SEM), .506 +/- .076; and TS, .584 +/- .053 (p = .38)]. In contrast, median values [.353 in control subjects and .477 in TS subjects (p = .012)] were significantly different. Western blots identified numerous bands in all TS and control sera with no difference in identified HTB-10 antigens. There was no relationship between the presence of ANAb and age of tic onset, family history, tic severity, attention deficit hyperactivity disorder, or obsessive compulsive disorder. No relationship existed between positive strep titers (ASO > or = 166 and/or antiDNAaseB > or = 170) and ANAb determinations or the severity of tics.

CONCLUSIONS

Children with TS have higher median, but not mean, levels of ANAb, as measured by the HTB-10 neuroblastoma cell membrane assay. This assay system identified antibodies in both control and clinical groups and failed to identify a relationship between antibodies and clinical phenotype or one-time markers for streptococcal infection. Further studies are required to define a possible immune-mediated hypothesis for TS.

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.

Mechanism of ET(A)-receptor stimulation-induced increases in intracellular Ca2+ in SK-N-MC cells

The mechanism underlying endothelin-1 (ET-1)-induced increases in intracellular Ca2+ concentrations in the human neuroblastoma cell-line SK-N-MC was investigated. ET-receptor agonists increased inositol phosphate (IP)-formation (assessed as accumulation of total [3H]-IPs in [3H]-myo-inositol prelabelled cells) and intracellular Ca2+ (assessed by the FURA-2 method) with an order of potency: ET-1 > sarafotoxin 6b (S6b)> ET-3 = S6c; the ETA-receptor antagonist BQ-123 inhibited both responses with apparent pKi-values of 8.3 and 8.6, respectively, while the ETB-receptor antagonist BQ-788 did not. Pretreatment of the cells with pertussis toxin (PTX, 500 ng ml(-1) overnight) reduced ET-1-induced Ca2+ increases by 46+/-5%, but rather enhanced ET-1-induced IP-formation. Chelation of extracellular Ca2+ by 5 mM EGTA did not affect ET-1-induced IP-formation. However, in the presence of 5 mM EGTA or SKF 96365, an inhibitor of receptor mediated Ca2+ influx (1.0-3.0 x 10(-5) M) ET-1-induced Ca2+ increases were inhibited in normal, but not in PTX-treated cells. [125I]-ET-1 binding studies as well as mRNA expression studies (by RT-PCR) detected only ETA-receptors whereas expression of ETB-receptor mRNA was marginal. ET-1 (10(-8) M) inhibited isoprenaline-evoked cyclic AMP increases; this was antagonized by BQ-123, not affected by BQ-788 and abolished by PTX-treatment. We conclude that SK-N-MC cells contain a homogeneous population of ETA-receptors that couple to IP-formation and inhibition of cyclic AMP formation. Stimulation of these ETA-receptors increases intracellular Ca2+ by at least two mechanisms: a PTX-insensitive IP-mediated Ca2+ mobilization from intracellular stores and a PTX-sensitive influx of extracellular Ca2+.

Coupling of D1 and D5 dopamine receptors to multiple G proteins: Implications for understanding the diversity in receptor-G protein coupling

Dopamine receptors are a subclass of the super family of G protein-coupled receptors, that transduce their effects by coupling to specific G proteins. Within the dopamine receptor family, the adenylyl cyclase stimulatory receptors include the D1 and D5 subtypes. The D1 and D5 dopamine receptors are genetically distinct, sharing >80% sequence homology within the highly conserved seven transmembrane spanning domains, but displaying only 50% overall homology at the amino acid level. When expressed in transfected GH4C1 rat pituitary cells, both D1 and D5 receptors stimulate adenylyl cyclase and have identical affinities toward dopaminergic agonists and antagonists. In order to analyze specific signaling pathways mediated by activation of either D1 or D5 receptors, we have identified the G proteins that are coupled to these receptors. Through functional analyses and competition binding studies, and from immunoprecipitation techniques, using antisera against the various alpha subunits of G proteins, we have established that both D1 and D5 receptors couple to G(s)alpha. In addition, D1 receptors are also coupled to G(o)alpha. Since G(o)alpha has been implicated in the regulation of Ca2+, K+, and Na+ channels, this finding would suggest that D1 receptors can mediate the functional activity of these ion channels. There is also evidence to indicate that D5 receptors couple to G(z)alpha, a novel G protein abundantly expressed in neurons. Thus, despite similar pharmacological properties, such differential coupling of D1 and D5 receptors to G proteins other than G(s)alpha, indicates that dopamine can transduce varied signaling responses upon the simultaneous stimulation of both these receptors.

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.

Regulation and expression of D-1, but not D-5, dopamine receptors in human SK-N-MC neuroblastoma cells

Human SK-N-MC neuroblastoma cells endogenously express functional D-1-like dopamine receptors. The recent identification of at least two distinct type of D-1-like receptors, D-1 and D-5, lead us to investigate the precise molecular identify of the receptors expressed in these cells. By using specific primer sets and amplification of the mRNA by RT-PCR, we show that only D-1, but not D-5, dopamine receptors are expressed in these cells. Treatment of cells with 100 microM dopamine initially caused an upregulation in D-1 mRNA expression, followed by attenuation of the message compared to control, untreated cells. The D-1 receptors in SK-N-MC cells, whose expression is controlled by dopamine in a bimodal manner, may be important in understanding how these receptors are regulated at the molecular level.

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.

The human D1A dopamine receptor gene promoter directs expression of a reporter gene to the central nervous system in transgenic mice

Dopamine receptors are involved in many aspects of dopaminergic neurotransmission including regulation of motor control, cognition, affect and neuroendocrine function. The D1A receptor is the most widely distributed dopamine receptor in the brain and is expressed at high levels in the striatum and nucleus accumbens, but is also found throughout cortical, limbic, hypothalamic and thalamic brain regions. We have cloned a 6.4 kb fragment 5' of the human D1A dopamine receptor gene and shown that this region activates transcription of the chloramphenicol acetyltransferase (CAT) gene in a cell-specific manner. To study the expression of these sequences in vivo we analyzed the expression of the E. coli lac Z gene under the regulation of the 6.4 kb fragment in transgenic mice. Expression of the transgene was primarily detected in the brain, with only low levels detected in peripheral tissues. The 5' flanking sequences were able to direct the tissue-specific expression of lac Z in three different lines of transgenic mice, to a number of brain regions including the caudate-putamen, thalamus, amygdala, cerebral cortex, hippocampus and hypothalamus. Greatest expression of the lac Z gene was detected in areas of the thalamus and amygdaloid complex. In the striatum, beta-galactosidase activity was restricted to neurons within the matrix and was not detected within striosomes. Results of this study demonstrate that the 6.4 kb region upstream of the human D1A receptor gene is sufficient to confer tissue-specific expression in the CNS of transgenic mice. Furthermore, expression of the transgene to neurons within the matrix of the striatum, but not the striosomes suggests that expression of the D1A receptor may be regulated differently within these areas.

Desensitization of D1 dopamine receptors down-regulates the Gs alpha subunit of G protein in SK-N-MC neuroblastoma cells

The effect of long-term (72 h) treatment with dopamine D1 receptor agonists, SKF 38393 and dopamine on D1 dopamine receptor and G-protein (Gs alpha) was investigated in SK-N-MC neuroblastoma cells. The prolonged treatment of cells with 10 microM SKF 38393 or 10 microM dopamine resulted in a decrease in dopamine D1 receptor by 41 and 81%, respectively, as measured by specific antagonist [3H]SCH 23390 binding. Similarly, the prolonged treatment of SK-N-MC cells with 10 microM SKF 38393 or 10 microM dopamine resulted in a reduction of the level of Gs alpha subunit of G-protein. The results indicate that agonist-induced down-regulation of D1 dopamine receptor may also modulate G-proteins.

Presence of dopamine D1 receptors and absence of dopamine D2 receptors in human cerebral meningioma tissue

Preliminary studies have shown that the dopamine D1 receptor is expressed in cerebral meningioma tissue. The current study presents evidence that the iodinated dopamine D1 antagonist [125I]SCH-23982 bound to dopamine binding sites in 33 of the 45 human cerebral meningiomas examined for this. Saturation curves and the linearity of the Scatchard analysis indicate that [125]SCH-23982 binds to a homogeneous population of binding sites. Competition curves reveal the presence of a dopamine D1 receptor by rank order of various dopaminergic and nondopaminergic antagonists ((+)-SCH-23390 greater than (+/-)-SKF-83566 greater than (cis)-flupentixol greater than (+)-butaclamol greater than chlorpromazine greater than 1-sulpiride greater than mianserin greater than (-)-butaclamol). Stereoselectivity was evaluated by (+)- and (-)-butaclamol. The mean (+/- standard deviation) dissociation rate constant was 369 +/- 196 pM with a density of 31.9 +/- 12.5 fmol/mg membrane protein among 33 meningiomas. The dopamine D2 receptor was not present in the 30 meningiomas examined for this. These findings indicate that the dopamine D1 receptor identified is expressed alone and is therefore regulated independent of a D2 receptor in cerebral meningioma tissue. Although the function of the dopamine D1 receptor in cerebral meningiomas has not so far been defined, previous studies have suggested that the D1 receptor might be involved in the control of proliferative growth of meningiomatous tissue.

D1 dopamine receptors of NS20Y neuroblastoma cells are functionally similar to rat striatal D1 receptors

Dopamine or agonists with D1 receptor potency stimulated cyclic AMP (cAMP) accumulation in whole cell preparations of NS20Y neuroblastoma cells. The accumulation of cAMP after D1 stimulation was rapid and linear for 3 min. Both dopamine and the novel D1 receptor agonist dihydrexidine stimulated cAMP accumulation two- to three-fold over baseline. The pseudo-Km for dopamine was approximately 2 microM, whereas for dihydrexidine it was approximately 30 nM. The effects of both drugs were blocked by either the D1-selective antagonist SCH23390 (Ki, 0.3 nM) or the nonselective antagonist (+)-butaclamol (Ki, 5 nM). Both (-)-butaclamol and the D2-selective antagonist (-)-sulpiride were ineffective (Ki greater than 3 microM). Forskolin (10 microM), prostaglandin E1 (1 microM), and adenosine (10 microM) also stimulated cAMP accumulation, but none were antagonized by SCH23390 (1 microM). Finally, muscarinic receptor stimulation (100 microM carbachol) inhibited both D1- and forskolin-stimulated increases in cAMP accumulation by 80%. The present results indicate that NS20Y neuroblastoma cells have D1 receptors that are coupled to adenylate cyclase, and that these receptors have a pharmacological profile similar to that of the D1 receptor(s) found in rat striatum.

Characterization and regulation of beta 1-adrenergic receptors in a human neuroepithelioma cell line

Intact human neuroepithelioma SK-N-MC cells bound the beta-adrenergic antagonist (-)-[3H]-CGP 12177 with a KD of 0.13 nM and a Bmax of 17,500 sites/cell. When the cells were exposed to beta-adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol much greater than norepinephrine greater than epinephrine, which is indicative of a beta 1-subtype receptor. Membranes prepared from the cells bound (-)-3-[125I]iodocyanopindolol with a KD of 11.5 pM. Inhibition of agonist-stimulated cyclic AMP production and competition binding experiments indicated that the beta 1-selective antagonists CGP 20712A and ICI 89,406 were much more potent than the beta 2-selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only beta 1-adrenergic receptors. Northern blot analysis of SK-N-MC mRNA using cDNA probes for the beta 1- and beta 2-adrenergic receptors revealed the presence of a very strong beta 1-adrenergic receptor mRNA signal, while under the same conditions no beta 2-adrenergic receptor mRNA was observed. Thus, SK-N-MC cells appear to express a pure population of beta 1-adrenergic receptors. When the cells were exposed to isoproterenol, there was no observable desensitization during the first hour. After longer exposure, desensitization slowly occurred and the receptors slowly down-regulated to 50% of control levels by 24 h. Other agents that elevate cyclic AMP levels, such as forskolin, cholera toxin, and cyclic AMP analogues, caused no or little substantial receptor loss.