Transcription factors specifying dopamine phenotype are decreased in cocaine users

Potential biomarkers of addiction identified by real-time PCR in human peripheral blood lymphocytes: a narrative review

Addiction-related neurobiological factors could be considered as potential biomarkers. The concentration of peripheral biomarkers in tissues like blood lymphocytes may mirror their brain levels. This review is focused on the mRNA expression of potential addiction biomarkers in human peripheral blood lymphocytes (PBLs). PubMed, EMBASE, Web of Science, Scopus and Google Scholar were searched using the keywords 'addiction', 'biomarker', 'peripheral blood lymphocyte', 'gene expression' and 'real-time PCR'. The results showed the alterations in the regulation of genes such as dopamine receptors, opioid receptors, NMDA receptors, cannabinoid receptors, α-synuclein, DYN, MAO-A, FosB and orexin-A as PBLs biomarkers in addiction stages. Such variations could also be found during abstinence and relapse. PBLs biomarkers may help in drug development and have clinical implications.

Nurr1 deficiency shortens free running period, enhances photoentrainment to phase advance, and disrupts circadian cycling of the dopamine neuron phenotype

Neurological and neuropsychiatric disorders, including addiction, schizophrenia, and Parkinson's disease (PD), involve dysfunction in midbrain dopamine (DA) neurotransmission with severity of disease symptoms and progression associated with disrupted circadian rhythms. The nuclear transcription factor Nurr1, essential for DA neuron (DAN) development, survival, and maintenance, is also known to interact with circadian rhythm regulating clock proteins. In the Nurr1-null heterozygous (+/-) mice, a Nurr1 deficient model which reproduces some of the alterations in DA function found in schizophrenia and PD, we measured, using wheel-running activity, the free running period (tau) and photoperiod entrainment. Because Nurr1 has a role in regulating the DA phenotype, we also measured the circadian fluctuations in the number of DANs using tyrosine hydroxylase (TH) immunofluorescence. In Nurr1 +/- mice, tau was significantly shorter and entrainment to a 6 h earlier shift in the dark cycle was accelerated. The Nurr1 wild-type (+/+) mice cycled DAN numbers across time, with a significantly greater number (∼2-fold increase) of DANs at zeitgeber time (ZT) 0 than ZT12. The +/- mice, however, did not cycle the DA phenotype, as no differences in DAN numbers were observed between ZT0 and ZT12. Additionally, the +/- mice had significantly fewer DANs at ZT0 but not at ZT12 as compared to +/+ mice. Based these data, circadian rhythms and fluctuations in the DA phenotype requires normal Nurr1 function. A better understanding is needed of the mechanisms regulating the DA phenotype and subsequent neurotransmission across the circadian cycle and how this is altered in circadian rhythm and DA neurotransmission-associated disorders.

Quantitative Immunohistochemistry to Measure Regional Expression of Nurr1 in the Brain and the Effect of the Nurr1 Heterozygous Genotype

The transcription factor Nurr1 is a member of the steroid hormone nuclear receptor superfamily. Ablation of Nurr1 expression arrests mesencephalic dopamine neuron differentiation while attenuation of Nurr1 in the subiculum and hippocampus impairs learning and memory. Additionally, reduced Nurr1 expression has been reported in patients with Parkinson's disease and Alzheimer's disease. In order to better understand the overall function of Nurr1 in the brain, quantitative immunohistochemistry was used to measure cellular Nurr1 protein expression, across Nurr1 immunoreactive neuronal populations. Additionally, neuronal Nurr1 expression levels were compared between different brain regions in wild-type mice (+/+) and Nurr1 heterozygous mice (+/-). Regional Nurr1 protein was also investigated at various time points after a seizure induced by pentylenetetrazol (PTZ). Nurr1 protein is expressed in various regions throughout the brain, however, a wide range of Nurr1 expression levels were observed among various neuronal populations. Neurons in the parietal and temporal cortex (secondary somatosensory, insular, auditory, and temporal association cortex) had the highest relative Nurr1 expression (100%) followed closely by the claustrum/dorsal endopiriform cortex (85%) and then subiculum (76%). Lower Nurr1 protein levels were found in neurons in the substantia nigra pars compacta and ventral tegmental area (39%) followed by CA1 (25%) and CA3 (19%) of the hippocampus. Additionally, in the parietal and temporal cortex, two distinct populations of high and medium Nurr1 expressing neurons were observed. Comparisons between +/- and +/+ mice revealed Nurr1 protein was reduced in +/- mice by 27% in the parietal/temporal cortex, 49% in the claustrum/dorsal endopiriform cortex, 25% in the subiculum, 33% in substantia nigra pars compacta, 22% in ventral tegmental area, and 21% in CA1 region of the hippocampus. Based on these data, regional mechanisms appear to exist which can compensate for a loss of a Nurr1 allele. Following a single PTZ-induced seizure, Nurr1 protein in the dentate gyrus peaked around 2 h and returned to baseline by 8 h. Since altered Nurr1 expression has been implicated in neurologic disorders and Nurr1 agonists have showed protective effects, understanding regional protein expression of Nurr1, therefore, is necessary to understand how changes in Nurr1 expression can alter brain function.

Nr4a1 suppresses cocaine-induced behavior via epigenetic regulation of homeostatic target genes

Endogenous homeostatic mechanisms can restore normal neuronal function following cocaine-induced neuroadaptations. Such mechanisms may be exploited to develop novel therapies for cocaine addiction, but a molecular target has not yet been identified. Here we profiled mouse gene expression during early and late cocaine abstinence to identify putative regulators of neural homeostasis. Cocaine activated the transcription factor, Nr4a1, and its target gene, Cartpt, a key molecule involved in dopamine metabolism. Sustained activation of Cartpt at late abstinence was coupled with depletion of the repressive histone modification, H3K27me3, and enrichment of activating marks, H3K27ac and H3K4me3. Using both CRISPR-mediated and small molecule Nr4a1 activation, we demonstrated the direct causal role of Nr4a1 in sustained activation of Cartpt and in attenuation of cocaine-evoked behavior. Our findings provide evidence that targeting abstinence-induced homeostatic gene expression is a potential therapeutic target in cocaine addiction.

Epigenetic regulation of immediate-early gene Nr4a2/Nurr1 in the medial habenula during reinstatement of cocaine-associated behavior

Propensity to relapse following long periods of abstinence is a key feature of substance use disorder. Drugs of abuse, such as cocaine, cause long-term changes in the neural circuitry regulating reward, motivation, and memory processes through dysregulation of various molecular mechanisms, including epigenetic regulation of activity-dependent gene expression. Underlying drug-induced changes to neural circuit function are the molecular mechanisms regulating activity-dependent gene expression. Of note, histone acetyltransferases and histone deacetylases (HDACs), powerful epigenetic regulators of gene expression, are dysregulated following both acute and chronic cocaine exposure and are linked to cocaine-induced changes in neural circuit function. To better understand the effect of drug-induced changes on epigenetic function and behavior, we investigated HDAC3-mediated regulation of Nr4a2/Nurr1 in the medial habenula, an understudied pathway in cocaine-associated behaviors. Nr4a2, a transcription factor critical in cocaine-associated behaviors and necessary for MHb development, is enriched in the cholinergic cell-population of the MHb; yet, the role of NR4A2 within the MHb in the adult brain remains elusive. Here, we evaluated whether epigenetic regulation of Nr4a2 in the MHb has a role in reinstatement of cocaine-associated behaviors. We found that HDAC3 disengages from Nr4a2 in the MHb in response to cocaine-primed reinstatement. Whereas enhancing HDAC3 function in the MHb had no effect on reinstatement, we found, using a dominant-negative splice variant (NURR2C), that loss of NR4A2 function in the MHb blocked reinstatement behaviors. These results show for the first time that regulation of NR4A2 function in the MHb is critical in relapse-like behaviors.

Differentially expressed gene networks, biomarkers, long noncoding RNAs, and shared responses with cocaine identified in the midbrains of human opioid abusers

Opioid abuse is now the most common cause of accidental death in the US. Although opioids and most other drugs of abuse acutely increase signaling mediated by midbrain dopamine (DA)-synthesizing neurons, little is known about long-lasting changes in DA cells that may contribute to continued opioid abuse, craving, and relapse. A better understanding of the molecular and cellular bases of opioid abuse could lead to advancements in therapeutics. This study comprises, to our knowledge, the first unbiased examination of genome-wide changes in midbrain gene expression associated with human opioid abuse. Our analyses identified differentially expressed genes and distinct gene networks associated with opioid abuse, specific genes with predictive capability for subject assignment to the opioid abuse cohort, and genes most similarly affected in chronic opioid and cocaine abusers. We also identified differentially expressed long noncoding RNAs capable of regulating known drug-responsive protein-coding genes. Opioid-regulated genes identified in this study warrant further investigation as potential biomarkers and/or therapeutic targets for human substance abuse.

Opposite effects of acute and chronic amphetamine on Nurr1 and NF-κB p65 in the rat ventral tegmental area

Dopamine neurons are overstimulated by drugs of abuse and suffer molecular alterations that lead to addiction behavior. Nurr1 is a transcription factor crucial for dopamine neurons survival and dopamine production, activating the transcription of key genes like tyrosine hydroxylase (TH). Interestingly, nuclear factor-kappa B (NF-κB) has emerged as a new Nurr1 partner in response to inflammatory stimulus. In this study we evaluated the effects of single and repeated amphetamine administration in the expression of Nurr1 and the NF-κB p65 subunit in the rat ventral tegmental area (VTA). We found that acute amphetamine treatment increased Nurr1, p65 and TH protein levels in the VTA. On the other hand, chronic amphetamine treatment decreased Nurr1 and p65 protein levels, but TH was unchanged. Mammalian reporter assays in cell lines showed that p65 represses Nurr1 transcriptional activity in an artificial promoter driven by Nurr1 response elements and in the native rat TH promoter. These results indicate that Nurr1 and NF-κB p65 factors are involved in the adaptive response of dopamine neurons to psychostimulants and that both transcription factors could be regulating Nurr1-dependent transactivation in the VTA.

Using iPSC-derived human DA neurons from opioid-dependent subjects to study dopamine dynamics

INTRODUCTION

The dopaminergic (DA) system plays important roles in addiction. However, human DA neurons from drug-dependent subjects were not available for study until recent development in inducible pluripotent stem cells (iPSCs) technology.

METHODS

In this study, we produced DA neurons differentiated using iPSCs derived from opioid-dependent and control subjects carrying different 3' VNTR (variable number tandem repeat) polymorphism in the human dopamine transporter (DAT or SLC6A3). In addition, the effects of valproic acid (VPA) exposures on iPSC-derived human DA neurons are also examined.

RESULTS

We present the first evidence suggesting that the 3' VNTR polymorphism in the hDAT gene affects DAT expression level in iPSC-derived human DA neurons. In human DA neurons, which provide an appropriate cellular milieu, VPA treatment alters the expression of several genes important for dopaminergic neuron function including DAT, Nurr1, and TH; this might partly explain its action in regulating addictive behaviors. VPA treatment also significantly increased DA D2 receptor (Drd2) expression, especially in the opioid-dependent iPSC cell lines.

CONCLUSIONS

Our data suggest that human iPSC-derived DA neurons may be useful in in vitro experimental model to examine the effects of genetic variation in gene regulation, to examine the underlying mechanisms in neurological disorders including drug addiction, and to serve as a platform for therapeutic development.

A molecular profile of cocaine abuse includes the differential expression of genes that regulate transcription, chromatin, and dopamine cell phenotype

Chronic drug abuse, craving, and relapse are thought to be linked to long-lasting changes in neural gene expression arising through transcriptional and chromatin-related mechanisms. The key contributions of midbrain dopamine (DA)-synthesizing neurons throughout the addiction process provide a compelling rationale for determining the drug-induced molecular changes that occur in these cells. Yet our understanding of these processes remains rudimentary. The postmortem human brain constitutes a unique resource that can be exploited to gain insights into the pathophysiology of complex disorders such as drug addiction. In this study, we analyzed the profiles of midbrain gene expression in chronic cocaine abusers and well-matched drug-free control subjects using microarray and quantitative PCR. A small number of genes exhibited robust differential expression; many of these are involved in the regulation of transcription, chromatin, or DA cell phenotype. Transcript abundances for approximately half of these differentially expressed genes were diagnostic for assigning subjects to the cocaine-abusing vs control cohort. Identification of a molecular signature associated with pathophysiological changes occurring in cocaine abusers' midbrains should contribute to the development of biomarkers and novel therapeutic targets for drug addiction.

Nur transcription factors in stress and addiction

The Nur transcription factors Nur77 (NGFI-B, NR4A1), Nurr1 (NR4A2), and Nor-1 (NR4A3) are a sub-family of orphan members of the nuclear receptor superfamily. These transcription factors are products of immediate early genes, whose expression is rapidly and transiently induced in the central nervous system by several types of stimuli. Nur factors are present throughout the hypothalamus-pituitary-adrenal (HPA) axis where are prominently induced in response to stress. Drugs of abuse and stress also induce the expression of Nur factors in nuclei of the motivation/reward circuit of the brain, indicating their participation in the process of drug addiction and in non-hypothalamic responses to stress. Repeated use of addictive drugs and chronic stress induce long-lasting dysregulation of the brain motivation/reward circuit due to reprogramming of gene expression and enduring alterations in neuronal function. Here, we review the data supporting that Nur transcription factors are key players in the molecular basis of the dysregulation of neuronal circuits involved in chronic stress and addiction.

A genetic network model of cellular responses to lithium treatment and cocaine abuse in bipolar disorder

Background

Lithium is an effective treatment for Bipolar Disorder (BD) and significantly reduces suicide risk, though the molecular basis of lithium's effectiveness is not well understood. We seek to improve our understanding of this effectiveness by posing hypotheses based on new experimental data as well as published data, testing these hypotheses in silico, and posing new hypotheses for validation in future studies. We initially hypothesized a gene-by-environment interaction where lithium, acting as an environmental influence, impacts signal transduction pathways leading to differential expression of genes important in the etiology of BD mania.

Results

Using microarray and rt-QPCR assays, we identified candidate genes that are differentially expressed with lithium treatment. We used a systems biology approach to identify interactions among these candidate genes and develop a network of genes that interact with the differentially expressed candidates. Notably, we also identified cocaine as having a potential influence on the network, consistent with the observed high rate of comorbidity for BD and cocaine abuse. The resulting network represents a novel hypothesis on how multiple genetic influences on bipolar disorder are impacted by both lithium treatment and cocaine use. Testing this network for association with BD and related phenotypes, we find that it is significantly over-represented for genes that participate in signal transduction, consistent with our hypothesized-gene-by environment interaction. In addition, it models related pharmacogenomic, psychiatric, and chemical dependence phenotypes.

Conclusions

We offer a network model of gene-by-environment interaction associated with lithium's effectiveness in treating BD mania, as well as the observed high rate of comorbidity of BD and cocaine abuse. We identified drug targets within this network that represent immediate candidates for therapeutic drug testing. Posing novel hypotheses for validation in future work, we prioritized SNPs near genes in the network based on functional annotation. We also developed a "concept signature" for the genes in the network and identified additional candidate genes that may influence the system because they are significantly associated with the signature.

Correlating human and animal studies of cocaine abuse and gene expression

Gene expression changes resulting from cocaine abuse in both humans and animal models have been studied for several decades. Although human studies have been very useful at illuminating cocaine-related expression changes, there are many factors complicating these studies, including the difficulty of obtaining high-quality postmortem brain tissue and patient comorbidities. Animal models of cocaine abuse have served as valuable additions to human data and allow examination of specific aspects of cocaine abuse, including immediate early gene expression and the molecular effects of abstinence and relapse. In total, human and animal studies of cocaine abuse have uncovered gene expression changes in the brain related to a number of molecular functions, including the extracellular matrix, synaptic communication and neuroplasticity, receptors, ion channels and transporters, oligodendrocytes and myelin, apoptosis and cell death, mitochondrial function, signal transduction, and transcription factors. In addition, the mitogen-activated protein kinase and synaptic long-term potentiation signal transduction pathways are highlighted as pathways in which multiple components are altered by cocaine. Pathways and processes affected by changes in gene expression that overlap among multiple species may be promising pharmacotherapeutic targets for reducing the behavioral effects of cocaine abuse and the relapse potential observed in humans.

Valproate robustly increases Sp transcription factor-mediated expression of the dopamine transporter gene within dopamine cells

Several lines of evidence suggest that valproate, a drug used in the treatment of mania and bipolar disorders, epilepsies, and addictions, may modulate dopamine transporter (DAT) function, yet the effects of valproate on DAT gene expression have not been directly assessed. Utilizing a human dopaminergic cell line and rat midbrain dopamine (DA) neurons in organotypic culture, we found that valproate increased endogenous DAT gene expression in a concentration- and time-dependent manner. Given previous data demonstrating that members of the specificity protein (Sp) family of transcription factors are strong trans-activators of DAT gene transcription, we investigated the Sp-dependence of valproate effects. Valproate-induced transcription of a DAT reporter construct was significantly attenuated by coexpression of a dominant negative form of Sp, mutation of a Sp-responsive cis-element, or expression in a Sp-null cellular background (SL-2 cells). Valproate significantly altered Sp protein abundance in both dopaminergic model systems employed. In summary, valproate treatment significantly increased DAT gene expression in a Sp transcription factor-dependent manner. Some of valproate's therapeutic effects may involve activation of DAT gene expression.

Bdnf gene is a downstream target of Nurr1 transcription factor in rat midbrain neurons in vitro

The transcription factor Nurr1 is essential for the generation of midbrain dopaminergic neurons (mDA). Only a few Nurr1-regulated genes have so far been identified and it remains unclear how Nurr1 influences the development and function of dopaminergic neurons. To identify novel Nurr1 target genes we have used genome-wide expression profiling in rat midbrain primary cultures, enriched in dopaminergic neurons, following up-regulation of Nurr1 expression by depolarization. In this study we demonstrate that following depolarization the hyperexpression of Nurr1 and the brain derived neurotrophic factor (BDNF) are phospholipase C- and protein kinase C-dependent. We show that Bdnf, which encodes a neurotrophin involved also in the phenotypic maturation of mDA neurons, is a novel Nurr1 target gene. By RNA interference experiments we show that a decreased Nurr1 expression is followed by tyrosine hydroxylase and BDNF mRNA and protein down-regulation. Reporter gene assay experiments performed on midbrain primary cultures using four Bdnf promoter constructs show that Bdnf is a direct target gene of Nurr1. Taken together, our findings suggest that Nurr1 might also influence the development and the function of midbrain dopaminergic neurons via direct regulation of Bdnf expression.

A promoter polymorphism (-839 C > T) at the dopamine transporter gene is associated with attention deficit/hyperactivity disorder in Brazilian children

The dopamine transporter (DAT) plays a key role in the regulation of dopaminergic neurotransmission and is also the major site of action for methylphenidate which is one of the main drugs used to treat attention deficit hyperactivity disorder (ADHD). Most association studies with ADHD have concentrated on the 3'-untranslated region of the gene (3'-UTR) mainly in a variable number of tandem repeat (VNTR) polymorphism, but these investigations have reported discordant results. In this study, we tested this VNTR polymorphism and an additional promoter polymorphism -839 C>T (Rs: 2652511) using family-based association analyses in a sample of 243 Brazilian ADHD children and adolescents and their parents. No significant linkage disequilibrium between the two polymorphisms was detected in this sample (D' = 0.56; P = 0.22). No evidence of association with the VNTR polymorphism was found. A significant association (P = 0.03) for biased transmission of the C allele at the -839 C>T polymorphism to ADHD children in the total sample was observed, which was strengthened when the analyses were restricted to the ADHD combined type (P = 0.004). Our results suggest a role for the promoter region of DAT1 gene in ADHD susceptibility in this Brazilian sample.

Chronic cocaine administration modulates the expression of transcription factors involved in midbrain dopaminergic neuron function

Chronic cocaine use leads to pronounced alterations in neuronal functions in brain circuits associated with reward. In the present study, we examined in the rat midbrain the effects of acute, subchronic (5 days) and chronic cocaine treatments (14 days) on the gene expression of transcription factors involved in the development and maintenance of dopaminergic neurons. We show that chronic, but not acute or subchronic, cocaine administration downregulates Nurr1 and Pitx3 transcripts whereas En1 transcripts are upregulated. Conversely, Lmx1b and En2 transcripts are not affected by the drug treatment, indicating that the modulation of the midbrain transcription factors analyzed is highly selective. Interestingly, modification of the gene expression for these transcription factors persists in midbrain as long as two weeks after the last drug administration, suggesting that it may account for some of the enduring alterations in midbrain dopaminergic circuits associated with chronic cocaine use.

Cocaine induces a differential dose-dependent alteration in the expression profile of immediate early genes, transcription factors, and caspases in PC12 cells: a possible mechanism of neurotoxic damage in cocaine addiction

Cocaine is a widely used drug of abuse and psychostimulant that acts on the central nervous system by blocking the dopamine reuptake sites. PC12 cells, a rat pheochromocytoma clonal line, in the presence of nerve growth factor (NGF), multiply and differentiate into competent neurons that can synthesize, store, and secrete the neurotransmitter dopamine (DA). In the present study, we evaluated the effect of increasing doses of cocaine on the expression of immediate early genes (IEGs), c-fos and c-jun, and closely related transcription factors, SP-1 and NF-kbeta, at 24 h after the exposure to cocaine (50, 100, 200, 500, 1000, 2500 microM) in NGF-differentiated PC12 cells. Cocaine (50-500 microM) resulted in significant induction of the expression of c-fos, c-jun, SP-1, and NF-kbeta. However, higher concentrations of cocaine (1000 and 2500 microM) resulted in the downregulation of these expressions after 24 h. To further understand the role of dose-dependent changes in the mechanisms of cell death, we evaluated the protein expression of apoptotic markers. A concentration-dependent increase in the expression of caspase-9 and -3 was observed up to 500 microM cocaine. However, the higher dose did not show any expression. We also evaluated the effect of increasing doses of cocaine on DA concentration and the expression of dopamine transporter (DAT). A significant dose-dependent decrease in the concentration of DA as well as the expression of DAT was observed 24 h after the exposure of PC12 cells to cocaine. Therefore, in the present study, we reported that cocaine has both upstream and downstream regulatory actions on some IEGs and transcription factors that can regulate the mechanism of cell death, and these effects on gene expression are independent of its action on the dopaminergic system.

Engrailed: complexity and economy of a multi-functional transcription factor

Engrailed is a homeodomain-containing transcription factor with numerous, overlapping roles in neural development. Its multifunctional nature depends upon an extremely diverse group of molecular functions including the ability to regulate both transcription and translation, and to be released from and internalized by cells. Recent findings have shown how some of these functions relate to specific roles in development and disease.

Age-associated changes in mRNA levels of Phox2, norepinephrine transporter and dopamine beta-hydroxylase in the locus coeruleus and adrenal glands of rats

Age-related changes in the gene expression of the transcription factors, Phox2a and 2b, and two marker proteins, norepinephrine transporter (NET) and dopamine beta-hydroxylase (DBH), of noradrenergic neurons were characterized in the locus coeruleus (LC) and adrenal glands using in situ hybridization. Analysis of changes was performed in rats that were 1-23 months of age. Compared to 1-month-old rats, there was a 62% increase of Phox2a messenger RNA (mRNA) in the LC of 3-month-old rats, and a decline of 37% in 23-month-old rats. In contrast, levels of Phox2b mRNA in the LC remained unchanged in 3-month-old rats, but declined to a 30% reduction in 23-month-old rats. Interestingly, mRNA levels of NET in the LC decreased with increasing age to a reduction of 29%, 30% and 43% in 3-, 8- and 23-month-old rats, respectively. Similarly, DBH mRNA in the LC declined with increasing age to a 56% reduction in 23-month-old rats. mRNA levels of Phox2a, Phox2b, NET and DBH in the adrenal medulla of 23-month-old rats were significantly lower than those of 1-month-old rats. Semi-quantitative reverse transcription assays of the same genes yielded data similar to in situ hybridization experiments, with beta-actin mRNA levels being unchanged across the ages. Taken together, these data reveal that reduced Phox2 mRNAs in the LC and adrenal medulla of aging rats are accompanied by a coincidental decline in mRNA levels of NET and DBH and suggest a possible relationship between Phox2 genes and the marker genes in noradrenergic neurons after birth.

Dopamine neurons express multiple isoforms of the nuclear receptor nurr1 with diminished transcriptional activity

Nurr1 (NR4A2) is an orphan nuclear receptor required for the development and maintenance of the dopaminergic phenotype in neurons of the ventral midbrain. This study demonstrates that multiple splice variants of nurr1 are produced in rat and human dopamine neurons. Formed by alternative RNA splicing in exon 7, nurr1a has a truncated carboxy-terminus, nurr1b has an internal deletion in the ligand-binding domain and nurr1c, newly identified in this study, has a novel carboxy-terminus produced by a frame shift downstream of the splice junction. Alternative RNA splicing in exon 3 produces the isoform known as the transcriptionally-inducible nuclear receptor (TINUR), lacking the amino-terminus. Nurr2 and the newly identified nurr2c are produced by utilization of both exon 3 and exon 7 alternative splice sites. In rat midbrain, variants other than full-length nurr1 constitute 20-35% of NR4A2 transcripts. Transfection studies in dopaminergic SK-N-AS cells demonstrate that nurr1a, nurr1b, nurr1c and TINUR have significantly reduced transcriptional activities compared with full-length nurr1, while nurr2 and nurr2c are inactive. Furthermore, in these experiments, nurr2 and nurr2c both act as dominant negatives. Production of these nurr1 variants in vivo as demonstrated here could represent a novel regulatory mechanism of nurr1 transcriptional activity and therefore, dopaminergic phenotype.

The dopamine transporter: role in neurotoxicity and human disease

The dopamine transporter (DAT) is a plasma membrane transport protein expressed exclusively within a small subset of CNS neurons. It plays a crucial role in controlling dopamine-mediated neurotransmission and a number of associated behaviors. This review focuses on recent data elucidating the role of the dopamine transporter in neurotoxicity and a number of CNS disorders, including Parkinson disease, drug abuse, and attention deficit hyperactivity disorder (ADHD).