Identification of a new brain-specific transcription factor, NURR1

Nurr1-RXR heterodimers mediate RXR ligand-induced signaling in neuronal cells

The retinoid X receptor (RXR) is essential as a common heterodimerization partner of several nuclear receptors (NRs). However, its function as a bona fide receptor for endogenous ligands has remained poorly understood. Such a role would depend on the existence of RXR activating ligands in vivo and on the ability of such ligands to influence relevant biological functions. Here we demonstrate the presence of endogenous RXR ligands in the embryonic central nervous system (CNS) and show that they can activate heterodimers formed between RXR and the orphan NR Nurr1 in vivo. Moreover, RXR ligands increase the number of surviving dopaminergic cells and other neurons in a process mediated by Nurr1-RXR heterodimers. These results provide evidence for a role of Nurr1 as a ligand-independent partner of RXR in its function as a bona fide ligand-activated NR. Finally, our findings identify RXR-Nurr1 heterodimers as a potential target in the treatment of neurodegenerative disease.

Molecular mechanisms underlying midbrain dopamine neuron development and function

The mesencephalic dopaminergic system is involved in the control of multiple brain functions including movement control and emotion and is of clinical importance because it is implicated in several psychiatric disorders, of which many are considered to have a neurodevelopmental origin. Studies into the developmental pathways of these neurons have led to the identification of the transcription factors En1, Pitx3, Nurr1 and Lmx1b, all shown to be important for the development of the mesencephalic dopaminergic system. In this paper, we discuss the consequences of genetic ablation of essential developmental genes. Furthermore, we discuss the consequences of changes in dopamine homeostasis for the function of the mesencephalic dopaminergic system. Finally, we analyse the potential of the mesencephalic dopaminergic system to adapt to gene dysfunction.

The transcription factor Nurr1 in human NT2 cells and hNT neurons

Human, neuronally committed hNT or NT2-N cells, originally derived from the Ntera2/D1 (NT2) clone after exposure to retinoic acid (RA), represent a potentially important source of cells to treat neurodegenerative diseases. Our previous in vitro experiments showed that hNT cells possess immunocytochemically detectable markers typical of dopaminergic (DA) ventral mesencephalic (VM) neurons, including tyrosine hydroxylase (TH), dopamine transporter (DAT), dopamine receptor (D2), and aldehyde dehydrogenase (AHD-2). In the current study, we sought to examine whether Nurr1, an orphan receptor of the nuclear receptor superfamily shown to be essential for the development, differentiation and survival of midbrain DA neurons, would be expressed in 3, 4, or 5 week RA-induced hNT neurons and their NT2 precursors. Our immunocytochemical analyses indicate that NT2 cells as well as hNT neurons independent of the length of RA-driven differentiation were Nurr1-immunoreactive. RT-PCR analysis confirmed the expression of Nurr1-specific mRNA in both NT2 precursors and the hNT neurons. Furthermore, immunocytochemical co-expression of Nurr1 and TH was detected in hNT neurons. The findings of this study suggest that Nurr1 may be important during the development of hNT neurons and involved in their differentiation into the dopaminergic phenotype.

Hzf-3 expression in the amygdala after establishment of conditioned taste aversion

We studied the regulation of the expression of the inducible orphan nuclear receptor known as HZF-3 (or Nurr1) in acquisition of conditioned taste aversion in rats. Our results show that HZF-3 expression in the lateral/basolateral (LA/BLA) amygdala complex was significantly up-regulated when both the conditioned and the unconditioned stimuli were paired, but not when either of the stimuli was presented alone. Induction in the LA/BLA had a faster onset than induction in the central nucleus of the amygdala. The results implicate HZF-3 in the acquisition of associative aversive experiences.

Genetic analysis of Nurr1 haplotypes in Parkinson's disease

Nurr1 gene plays an important role in the development of the mesencephalic dopaminergic system. Genetic variability of Nurr1 gene may be associated with risk of Parkinson's disease (PD). We found three polymorphic loci (c.-2922(C)2-3, IVS6+18insG and EX8+657 (9-10CA)) of the Nurr1 gene in our PD patients and a novel intron 7+33 C-->T variant in one PD patient. We proceeded to perform a haplotype analysis in a case control study. A total of 202 PD patients (mean age 65.04+/-9.44 years, 55.4% men) and 202 age, gender and race matched controls (mean age 64.33+/-10.12 years, 54.0% men) were studied. The intron 7+33 C-->T variant was present in only one of the PD patients (0.5%) but in none of the controls. The Nurr1 mRNA levels in the lymphocytes did not significantly differ between the affected patient and controls. We found complete linkage disequilibrium between c.-2922(C)2-3 and IVS6+18insG polymorphic loci (D=0.25). Analysis of the three loci haplotype frequencies did not demonstrate any significant difference between PD and controls. There were also no significant differences in the haplotype frequencies between young and late onset PD patients. In conclusion, we demonstrated a large common haplotype block spanning the Nurr1 gene in our population. The intron 7+33 C-->T variant most likely represents either a non-functional mutation or a rare polymorphism in our study population. Our study suggests that Nurr1 variability is unlikely to play a major role in the majority of our PD patients.

Regulation of NGFI-B expression during the ovulatory process

NGFI-B is an immediate-early gene that encodes an orphan nuclear receptor. In the rat ovary, the preovulatory surge of LH induces NGFI-B expression in granulosa cells of preovulatory follicles, reaching a peak within 1 h and declining to control levels at 6 h. The LH-stimulated NGFI-B expression is abolished by alpha-amanitin, but superinduced by cycloheximide. Similarly, treatment of human luteinized granulosa cells with LH causes a rapid and transient stimulation of NGFI-B expression. Interestingly, the induction of NGFI-B expression in response to LH stimulation in preovulatory granulosa cells requires signaling through protein kinase Czeta. Furthermore, two other NGFI-B family members, Nurr1 and Nor1, are also rapidly stimulated by LH in granulosa cells of preovulatory follicles through the activation of protein kinase Czeta. The cell-type specific expression and LH induction of NGFI-B suggests a potential role of NGFI-B in the ovulatory process.

Overexpression of midbrain-specific transcription factor Nurr1 modifies susceptibility of mouse neural stem cells to neurotoxins

Nurr1 is a member of the nuclear receptor superfamily of transcription factors that is highly expressed in midbrain dopaminergic (DA) neurons, the cells primarily lost in human Parkinson's disease (PD), and in Nurr1-null mice selective agenesis of midbrain DA neurons is found. To investigate possible correlation between the expression of Nurr1 gene and neurotoxin-induced cell death of DA neurons, a neural stem cell line (NSC, A3) and Nurr1-overexpressing NSC (A3.Nurr1) were exposed to DA neurotoxins 6-hydroxydopamine (6-OHDA) and methyl phenylpyridinium (MPP(+)). Although both neurotoxins were shown to induce cell death in A3 and A3.Nurr1 cells, patterns of cell deaths were different. A3.Nurr1 cells showed increased vulnerability to 6-OHDA cytotoxicity, but increased resistance to MPP(+)-induced cell death when compared to A3 cells. To investigate the differential vulnerability to neurotoxins by Nurr1 protein correlates with biochemical features that discriminate between apoptosis and necrosis, we carried out a nucleosomal DNA fragmentation assay and electron microscopy. While 6-OHDA treatment induced shrinkage of cytoplasmic membrane, condensation of nuclei and generation of apoptotic bodies in both cell lines, cells treated with MPP(+) showed mitochondrial swelling, indicating that 6-OHDA- but not MPP(+)-mediated cell death was apoptotic. These results suggest that DA neuronal cell death in response to 6-OHDA and MPP(+) may progress through separate signaling pathways differentially regulated by the Nurr1 protein. Our observations indicated that Nurr1 may play a role in the manifestation of DA neurotoxicity and that variations in Nurr1 expression might be a susceptibility factor for DA neurodegeneration in PD.

Activation of nuclear orphan receptor NURR1 transcription by NF-kappa B and cyclic adenosine 5'-monophosphate response element-binding protein in rheumatoid arthritis synovial tissue

Modulation of the NURR subfamily of nuclear receptors may be an important mechanism regulating pathways associated with inflammatory joint disease. We examined the signaling mechanisms through which inflammatory mediators, produced by rheumatoid arthritis (RA) synovial tissue, contribute to the regulation of the NURR subfamily. Markedly enhanced expression of NURR1 is observed in synovial tissue of patients with RA compared with normal subjects. Modulation by proinflammatory mediators in primary RA and normal synoviocytes shows that PGE(2), IL-1beta, and TNF-alpha markedly enhance NURR1 mRNA and protein levels in contrast to other subfamily members, NUR77 and NOR-1. We have established that transcriptional activation of the NURR1 gene by IL-1beta and TNF-alpha requires a proximal promoter region that contains a consensus NF-kappaB DNA-binding motif. IL-1beta- and TNF-alpha-induced NF-kappaB binding to this site is due predominantly to p65-p50 heterodimer and p50 homodimer subunit protein complexes. We further demonstrate a direct CREB-1-dependent regulation by PGE(2) situated at promoter region -171/-163. Moreover, analyses confirm the presence of CREB-1 and NF-kappaB p50 and p65 subunit binding to the NURR1 promoter under basal conditions in freshly explanted RA synovial tissue. In summary, enhanced NF-kappaB- and CREB-1-binding activity on the NURR1 promoter by inflammatory mediators delineates novel mechanisms in the regulation of NURR1 transcription. PGE(2)-, TNF-alpha-, and IL-1beta-dependent stimulation of the NURR1 gene implies that NURR1 induction represents a point of convergence of at least two distinct signaling pathways, suggesting an important common role for this transcription factor in mediating multiple inflammatory signals.

Generation of tyrosine hydroxylase-immunoreactive neurons in ventral mesencephalic tissue of Nurr1 deficient mice

Nurr1 is an orphan nuclear receptor belonging to the family of evolutionary conserved steroid/thyroid hormone receptors. It has been shown that Nurr1 is required for development of ventral mesencephalic dopaminergic cells in vivo and that Nurr1 regulates the tyrosine hydroxylase (TH) gene. The aim of this study was to investigate the possibility of finding ventral mesencephalic TH-positive neurons in Nurr1 deficient tissue when developed in the presence of wild type (WT) striatum. Therefore, fetal ventral mesencephalic tissue from embryonic day (E) 9.5-10.5 fetuses from Nurr1 mutant mice was co-cultured with lateral ganglionic eminence (LGE) from WT fetuses using the 'roller-drum' culture technique. TH-immunohistochemistry revealed similar number of positive neurons in WT, heterozygous, and Nurr1 deficient tissue, respectively. When ventral mesencephalon, dissected from E10.5 fetuses, was cultured alone without the presence of LGE, significantly more TH-immunoreactive neurons were found in WT and Nurr1 +/- than that seen in Nurr1 -/- cultures. In single ventral mesencephalic cultures dissected from E15.5, TH-positive neurons were found in all tissue cultures derived from knockout animals. Interestingly, the formation of TH-positive nerve fiber bundles was obvious in WT cultures while not observed in cultures of knockout tissue. When ventral mesencephalon was cultured alone in serum-free medium, almost no TH-positive neurons were found in cultures of knockout tissue. The addition of the growth factors epidermal growth factor and fibroblast growth factor-8 did not induce TH-immunoreactivity in serum-free Nurr1 -/- tissue cultures. In conclusion, TH-positive neurons may be generated in ventral mesencephalic tissue of Nurr1 deficient mice, suggesting that Nurr1 is not required for TH gene expression in ventral midbrain in vitro.

Kainate-induced genes in the hippocampus: lessons from expression patterns

Kainate, the analog of the excitatory amino acid L-glutamate, upon binding to non-NMDA glutamate receptors, causes depolarization of neurons followed by severe status epilepticus, neurodegeneration, plasticity and gliosis. These events are best observed in hippocampus, the limbic structure implicated in learning and long-term memory formation. Neurons in all hippocampal structures undergo hyper-activation, however, whereas the cells in the CA subfields degenerate within 2--3 days following the application of kainate, the granule cells of the dentate gyrus are resistant to any form of neurodegeneration and even initiate new synaptic contacts. These physiological and histological changes are modulated by short-term and long-term alterations in gene expression. Perhaps close examination of the changing spatio-temporal patterns of mRNAs of various genes may help in generating a clearer picture of the molecular events leading to complex cognitive functions.

Genetic pathways in the developmental specification of hypothalamic neuropeptide and midbrain catecholamine systems

The neuropeptide concept concerns the diverse and broad physiological functions of neuropeptides in behavioral adaptation. Neuropeptides like vasopressin and corticotropin-releasing hormone can coordinate multiple brain functions due to the anatomical organization of the neurons producing them. The cell bodies are focally positioned in the hypothalamus and send long-reaching efferents to limbic and brainstem areas. Likewise, midbrain dopamine systems coordinate emotional behaviors and movement control by specific connectivity of neurons in the midbrain to limbic and striatal centers, respectively. The fundament of the functions of these signalling molecules is laid out during development when transmitter identity and connectivity are specified. This is a highly controlled process involving multiple transcription factors and growth factors acting together in genetic pathways. Here, the genetic pathways enrolling in developing vasopressin, corticotropin-releasing hormone, and midbrain dopamine neurons are discussed.

Selective increase of Nurr1 mRNA expression in mesencephalic dopaminergic neurons of D2 dopamine receptor-deficient mice

The orphan nuclear receptor Nurr1 is critical for the survival of mesencephalic dopaminergic precursor neurons. Little is known about the mechanisms that regulate Nurr1 expression in vivo. Other members of this receptor family have been shown to be activated by dopamine. We sought to determine if Nurr1 expression is also regulated by endogenous dopamine through dopamine receptors. Consequently, we investigated the expression of Nurr1 mRNA in genetically modified mice lacking both functional copies of the D2 dopamine receptor gene and in their congenic siblings. Quantitative in situ hybridization demonstrated a significant increased expression of Nurr1 mRNA in the substantia nigra pars compacta and the ventral tegmental area of D2 dopamine receptor -/- mice. No change in Nurr1 expression was detected in other brain regions, such as the habenular nuclei and temporal cortex. Among the cell groups studied, mesencephalic dopaminergic neurons are unique in that they express both Nurr1 and the D2 dopamine receptor, and synthesize dopamine. Thus, it seems plausible that the selective increase in Nurr1 expression observed in D2 receptor-deficient mice is the consequence of an impaired dopamine autoreceptor function.

Early induction of the orphan nuclear receptor NOR-1 during cell death of the human breast cancer cell line MCF-7

The neuron-derived orphan receptor (NOR-1) is a member of the NGFI-B subfamily within the nuclear receptor superfamily. In T-cell apoptosis, where NGFI-B plays an essential role, a functional redundancy between NGFI-B and NOR-1 has been demonstrated. Here, we examined the regulation and expression of the NOR-1 gene during cell death induced by a calcium ionophore A23187 in the human breast cancer cell line MCF-7. A23187 caused a transient increase in NOR-1 mRNA levels within 6 h after treatment. To delineate the sequences required for the transitional response to A23187, a series of promoter deletion mutants were constructed. From the transient transfection experiments, the element responsive to A23187 was identified between -94 and -42 base pairs upstream from the transcription initiation site. This 53-base pairs region contains three copies of the cAMP response element (CRE). Furthermore, phosphorylation of the CRE-binding protein (CREB), which affects the transcription of the CRE dependent-genes, was detected 30 min after A23187 stimulation. Our findings are consistent with NOR-1 involvement in A23187-induced cell death via the CRE-CREB signaling pathway.

Expression of the orphan receptor TR4 during brain development of the rat

The orphan receptor TR4, member of the nuclear hormone receptor family, is related to the orphan receptors TR2, COUP-TFI and ARP-1, and was originally cloned from the adult rat brain. The latter two orphan receptors have been implicated in central nervous system (CNS) development. To investigate a possible role for TR4 in brain development, expression of TR4 was studied in rat embryos. At embryonic days 14.5 and 19.5, high expression of TR4 was found in the CNS, while low expression was detected throughout the embryo. In postnatal rats, TR4 was mainly expressed in the hippocampus and cerebellum, resembling the expression pattern found in adult brain. These data show that like COUP-TFI and ARP-1, expression of TR4 becomes restricted to distinct areas. In adult brain, TR4 is predominantly expressed in granule cells of both hippocampus and cerebellum. The data suggest a possible role for TR4 during proliferation and maturation of brain structures.

Epigenetic cues in midbrain dopaminergic neuron development

Midbrain dopaminergic (DA) neurons subserve complex and varied neural functions in vertebrate CNS. Their progenitors give rise to DA neurons by the action of two extracellular inducers, Sonic Hedgehog and FGF8. After this first commitment, the function of selectively activated transcription factors, like the orphan steroid nuclear receptor Nurr1, is required for DA final determination. Subsequently, DA function is selectively modulated by specific interaction with the developing striatal target tissue. Committed and determined DA neurons express the key genes involved in DA neurotransmission at different times in development. Synthesis and intracellular accumulation of DA is achieved shortly after expression of Nurr1, while high affinity uptake, responsible for ending the neurotransmission, takes place after a few days. Cell contacts between the presynaptic DA neurons and target striatal neurons are apparently necessary for the fine modulation of DA function, in vivo and in vitro.

Characterization of the 5'-flanking region of the human dopamine transporter gene

The dopamine transporter (DAT) plays a major role in modulating dopamine (DA) neurotransmission by controlling the levels of this neurotransmitter in the extracellular space. We have isolated 8.3 kb of the 5'-flanking regulatory region of the human DAT (hDAT) gene and identified numerous potential elements involved in transcriptional control of the DAT. A series of hDAT-luciferase reporter constructs encompassing increasing amounts of 5'-flanking sequence was utilized in transient transfection assays assessing basal activity and response to selected stimuli. Our results suggest that the proximal hDAT 5'-flanking region displays a strong, nonselective promoter activity that is silenced through regulatory elements present in the distal portion of the 5'-flanking sequence. Although potential cyclic AMP responsive elements (CRE) were identified on the sequence, hDAT constructs were unresponsive to cyclic AMP induction. The transcription factor nurr1 increases the transcriptional activity of several larger hDAT constructs, consistent with the presence of several putative NGFI-B response elements (NBRE). The cloning and functional analysis of an extensive portion of the 5'-flanking regulatory region of the hDAT gene provides further insights into the factors involved in the regulation of this gene.

4.5 kb of the rat tyrosine hydroxylase 5' flanking sequence directs tissue specific expression during development and contains consensus sites for multiple transcription factors

To delineate DNA sequences responsible for developmentally correct expression of the rat tyrosine hydroxylase (TH) gene, we analyzed a line of transgenic mice expressing high levels of human placental alkaline phosphatase (AP) under control of 4.5 kb of 5' flanking DNA from the rat TH gene in embryos and adults. Several regions, such as the accessory olfactory bulb, which were not thought to synthesize TH protein or do so only transiently, were shown to express TH protein using an improved method of antigen retrieval for TH immunohistochemistry. Many of these regions had been shown to express TH-driven reporter genes in transgenic mice. In the central nervous system, AP was detected in essentially all TH-expressing cell groups throughout development and in adults. In the peripheral nervous system, transgene expression paralleled endogenous TH expression in the developing adrenal medulla and sympathetic ganglia but not in transiently TH-positive cells in dorsal root ganglia. Peripheral expression in the adult adrenal medulla was very weak and absent in sympathetic ganglia. The specificity with which the 4.5 kb region directs transgene expression in embryos is comparable to that observed with longer 5' flanking promoter regions, implying that this region contains the control elements for appropriate expression during development. Sequence analysis of the region demonstrates a GT dinucleotide repeat, an element that resembles the neural restrictive silencer element (NRSE), which restricts transcription of neuronal genes in non-neuronal cells, and consensus sites for three families of transcription factors, Ptx1/3, Nurr1 and Gli1/2, which are required for the early differentiation of mesencephalic neurons.

Differential expression of tyrosine hydroxylase in catecholaminergic neurons of neonatal wild-type and Nurr1-deficient mice

The orphan nuclear receptor Nurr1 is a transcription factor that belongs to the steroid/thyroid hormone receptor superfamily and is expressed in many regions of the brain. To determine the physiological role of Nurr1, we previously generated mice with a null mutation in the Nurr1 gene. Nurr1-null mice appear to develop normally but die within 12 h after birth. Subsequent analysis revealed the absence of neurotransmitter dopamine and tyrosine hydroxylase immunoreactivity in the central dopaminergic area of newborn pups. Herein, using in situ hybridization histochemistry, we show that Nurr1 is expressed only in subset of catecholamine producing neurons (A2 partly, A8-A10 and A11 catecholaminergic cell groups), and is excluded from the norepinephrine producing neurons (A1, A2, A5-A6 catecholaminergic cell groups). Nurr1 was not expressed in the dopamine synthesizing cell groups (A12-A16 catecholaminergic cell groups) of the diencephalon and the olfactory bulb. As previously shown and confirmed in this study, tyrosine hydroxylase immunoreactivity was absent in the substantia nigra and ventral tegmental area of Nurr1-deficient mice. However, the loss of Nurr1 expression in A2 and A11 dopaminergic neurons did not affect their tyrosine hydroxylase immunoreactivity. This study begins to dissect cues necessary for understanding the complex regulation of the catecholaminergic biosynthetic pathway with regard to local, chemical and developmental changes in the brain.

Expression of nerve growth factor-induced clone B subfamily and pro-opiomelanocortin gene in lung cancer cell lines

Nerve growth factor-induced clone B (NGFI-B), Nur-related factor 1, and neuron-derived orphan receptor-1 have structural features of ligand-activated transcriptional regulators and constitute the NGFI-B subfamily within the nuclear receptor superfamily. The NGFI-B subfamily is highly expressed in neuroendocrine organs and regulates the pro-opiomelanocortin (POMC) gene. Small-cell lung cancer (SCLC) is considered to be a neuroendocrine tumor that produces large numbers of polypeptide hormones. In this study we measured the NGFI-B subfamily and POMC messenger RNA (mRNA) levels in various lung-cancer cell lines by means of the quantitative reverse transcription-polymerase chain reaction, and evaluated the correlations between expression of these genes and polypeptide hormone productions. We also examined the effect of antisense oligonucleotide to NGFI-B mRNA on the expression of POMC mRNA. The NGFI-B subfamily and POMC mRNAs were highly expressed in SCLC cell lines. In addition, there were strong correlations between the NGFI-B, POMC genes, and the adrenocorticotropin hormone (ACTH) level. Further, the antisense oligonucleotide significantly suppressed POMC gene expression. We conclude that the NGFI-B subfamily was a significant molecule in SCLC and that the NGFI-B was a positive transcriptional factor for ACTH production.

Organization of the human orphan nuclear receptor Nurr1 gene

Recent reports have revealed that Nurr1 (also known as NOT/TINUR/RNR-1/HZF-3), a member of the steroid/thyroid hormone nuclear receptor superfamily, is predominantly expressed in the midbrain; substantia nigra (SN) and ventral tegmental area (VTA). Nurr1 null mice are born lethal, lacking the midbrain dopamine (DA) neurons, suggesting that Nurr1 is essential for the development and differentiation of midbrain DA neurons. Human Nurr1 gene has been mapped on chromosome 2q22-23, which is reported to associate weakly with schizophrenia. We cloned and sequenced the human Nurr1 gene, which is approximately 8.3kb long, consisting of eight exons and seven introns. Comparisons of the human Nurr1 with the mouse Nurr1, mouse Nur77 and human NOR-1 revealed that their genomic structures were highly conserved. The 5'-flanking region of the human Nurr1 included three transcriptional regulatory elements, cAMP-response element (CRE), CArG-like element and Sp-1 site, which were surrounded by CpG island, and showed a strong homology with the mouse Nurr1. We performed a primer extension analysis using mRNA from HeLa S3 cells stimulated with phorbol 12-myristate 13-acetate (PMA), Ca2+ ionophore A23187 and cycloheximide (CHX) in order to induce the Nurr1 mRNA expression, and determined one transcription initiation site within CRE. The transient transfection assay indicates that the regulatory elements in the 5'-flanking region are robust for mitogen-induced expression of the human Nurr1. Further analysis of the polymorphism of the human Nurr1 gene may reveal the association with diseases characterized by changes of the DA system, such as Parkinson's disease and schizophrenia.

Molecular cloning of the human Nurr1 gene: characterization of the human gene and cDNAs

Nurr1 is a member of the nuclear receptor superfamily of transcription factors that is expressed predominantly in the central nervous system, including developing dopaminergic neurons. Recently, it was demonstrated that Nurr1 is critical for midbrain dopaminergic cell differentiation. In order to investigate a possible relation of Nurr1 with the pathogenesis of Parkinson's disease or other neuropsychiatric disorders, we have cloned and characterized the human Nurr1 gene. The gene exists as a single copy in the human genome and comprises eight exons spanning 8kb. We determined the complete nucleotide sequence and flanking regions of the gene. Potential regulatory regions included consensus binding sites for NF-kappaB, CREB, and Sp1. Isolation of human Nurr1 cDNAs from fetal brain suggested the presence of a new splicing variant of Nurr1 in the human brain.

An isoform of Nurr1 functions as a negative inhibitor of the NGFI-B family signaling

NGFI-B, Nurr1 and NOR-1 constitute a distinct subfamily within the nuclear receptor superfamily. To clarify the transcriptional regulation by the NGFI-B family, we searched for other components that can bind to the NBRE response element, a known target sequence for these transcription factors. By low stringency hybridization using the DNA binding domain of NOR-1 as a probe, a C-terminal truncated Nurr1 isoform, named Nurr2, was isolated from a mouse MC3T3-E1 cell cDNA library. Nurr2 had a novel cryptic exon located upstream in the Nurr1 promoter region, and was generated by alternative splicing at exons 1, 2 and 6. The C-terminal region was encoded by frame-shifted exon 6, and so Nurr2 lacked the C-terminal sequences corresponding to the putative ligand binding domain or dimerization domain. Quantitative reverse transcriptase-PCR experiments confirmed the presence of the Nurr2 isoform in mouse, rat and human. It was, like Nurr1, highly expressed in the pituitary and the cerebral cortex. Nurr2 and Nurr1 were also concomitantly induced by forskolin in NIH3T3 cells. Functional analysis using a reporter gene, containing NBRE response elements, indicated that while the isoform was inactive by itself, it could inhibit transactivation by the members of the NGFI-B family. These results indicate that the C-terminal truncated isoform, Nurr2, may act as a negative regulator of the NGFI-B family signaling.

Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid-induced seizure activity

Nurr1 is an immediate early gene encoding a member of the steroid-thyroid hormone receptor family. In PC12 cells, Nurr1 is readily induced by membrane depolarization, but not by growth factors. Nurr1 is predominantly expressed in the brain, and is essential to the differentiation of midbrain dopaminergic neurons. However, Nurr1 is also expressed in brain regions unrelated to dopaminergic neurons, e.g., hippocampus and cerebral cortex, and its immediate induction following seizure activity suggests a potential involvement of this transcription factor in modulating gene expression in the nervous system. To investigate the response of Nurr1 to neuronal activation, we analyzed Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid (KA)-induced seizure. In P7 animals, systemic injection of KA increased Nurr1 mRNA levels in a few hilar cells of the dentate gyrus and some pyramidal cells of the CA3 region of the hippocampus. In older animals, Nurr1 induction progressively expanded to all hippocampal regions (P14, P21) and eventually to cortical regions (adult). The increase was rapid and transient in the dentate gyrus, a structure resistant to the neurotoxic effect of KA, and was more prolonged in other regions more susceptible to KA toxicity. Induction of Nurr1 at early postnatal stages and rapid increase in the dentate gyrus following KA-induced seizure, suggest that Nurr1 expression is modulated by neuronal activity. On the other hand, prolonged Nurr1 induction in regions sensitive to KA toxicity indicates a possible involvement of Nurr1 in selective neuronal vulnerability.

On CNS repair and protection strategies: novel approaches with implications for spinal cord injury and Parkinson's disease

In the adult mammalian central nervous system lost nerve cells are not replaced and there is no regeneration of injured axons in white matter. Together, these two facts mean that there are no spontaneous reparative mechanisms in operation. Instead, the adult central nervous system copes with the risks of injuries and diseases by protective encapsulation in bone, by a multitude of neuroprotective mechanisms, and finally by the fact that many important functions are represented by a much larger number of neurons than minimally needed. The long life expectancy of a human being nevertheless means that the risk that the central nervous system is affected by disease, injury or other forms of insults for which it cannot fully compensate is relatively high. Experimentally, two strategies are being pursued in order to develop ways of minimizing various forms of CNS damage, namely neuroprotective and reparative strategies. Here we present a possible reparative intervention applicable to spinal cord injury based on multiple white-to-gray matter peripheral nerve bridge grafts and work based on the specific role of Nurr1 for dopamine neuron development, suggesting that development of ligands to transcription factor might be a new inroad to neuroprotective treatments in Parkinson's disease.

Novel mechanism of action for Nur77 and antagonism by glucocorticoids: a convergent mechanism for CRH activation and glucocorticoid repression of POMC gene transcription

Whereas orphan nuclear receptors of the Nur77 (NGFI-B) subfamily were previously known to act on transcription as monomers or as heterodimers with RXR, we have recently shown that Nur77 homodimers potently activate transcription upon interaction with a novel palindromic response element, the NurRE. In fact, reporter plasmids containing the NurRE respond to physiological stimuli in conditions where the NBRE, a binding site for Nur77 monomers, does not. Nur77 and its related receptors were shown to be important mediators for control of apoptosis induced by the T-cell receptor, and they also mediate the effect of the hypothalamic hormone CRH on transcription of the pituitary pro-opiomelanocotin (POMC) gene. In both systems, glucocorticoids antagonize the stimulatory effects of Nur77 on transcription by a mechanism that involves protein:protein interactions. Thus, the Nur77 signalling pathway appears to be a point of convergence for stimulatory signals and glucocorticoid repression in both endocrine and lymphoid systems.

Cell death in the regulation of immune responses

Cell death is an integral part of the functioning of the immune system. For T cells, potentially autoreactive or 'useless' cells are removed through apoptosis in response to signals (or lack of signals) from their T cell receptor complex. A myriad of proteins that can initiate or protect cells from cell death have recently been identified.

Distribution of messenger RNAs for the orphan nuclear receptors Nurr1 and Nur77 (NGFI-B) in adult rat brain using in situ hybridization

Nurr1 and Nur77 (NGFI-B) are orphan nuclear receptors, belonging to the steroid/thyroid hormone receptor gene superfamily. They have conserved amino acid sequence in the zinc-finger DNA binding domains and similar COOH-terminal regions, but have no known ligands. However, different expression patterns during brain development and tissue distributions of these messenger RNAs imply that they might reflect a different transcriptional role in the brain. In this study, the regional and cellular expression of messenger RNAs encoding these two proteins in rat brain has been determined by in situ hybridization. Nurr1 messenger RNA is highly expressed in the piriform and entorhinal cortices, hippocampus, medial habenular and paraventricular thalamic nuclei. Moderate labeling was detected in layers II-V of most of the cerebral cortex, and in the dorsal lateral geniculate nucleus, substantia nigra (pars compacta and reticularis) and interpeduncular nucleus. No Nurr1 hybridization signal was seen in the rhombencephalon. In the cerebellum, Nurr1 messenger RNA is present in the internal granular cell layer and Purkinje cell layer. In contrast, Nur77 has a widespread distribution, with the highest level of expression in the cerebral cortex. Moderate hybridization signals were detected in the hippocampus, the lateral dorsal and posterior nuclei, reuniens thalamic nuclei, and paraventricular and supraoptic hypothalamic nuclei. In the rhombencephalon, higher signals were present in the medial and lateral vestibular, dorsal cochlear and facial, and raphe magnus nuclei. Nur77 signal was also detected in the nucleus of the spinal tract of the trigeminal nerve. In the cerebellum, Nur77 messenger RNA is highly expressed in the Purkinje cell layer and lateral deep nucleus of the cerebellum. Our results show that Nurr1 and Nur77 messenger RNAs have both overlapping and different distribution patterns within the brain, suggesting that they might regulate different sets of responsive genes.

Structure, mapping and expression of a human NOR-1 gene, the third member of the Nur77/NGFI-B family

We identified a human homologue of NOR-1 (neuron-derived orphan receptor) from the fetal brain. There are two transcripts for human NOR-1, encoding 626 amino acid residues with a calculated molecular mass of 68 kDa. The high homology between hNOR-1, mNur77/rNGFI-B/hTR3, and mNurr1/rRNR-1/hNOT indicated that these three orphan receptors form a distinct subfamily within the steroid/thyroid receptor superfamily. Human NOR-1 mRNA was detected in the adult heart and skeletal muscle as well as in the fetal brain, indicating that its expression is not restricted to events that occur during neural development. The hNOR-1 gene is more than 35 kilobases long and interrupted by seven introns. The exon-intron structure of the gene is generally conserved when compared with the steroid/thyroid receptor superfamily and is remarkably similar to that of the Nur77/NGFI-B genes. This suggests that the Nur77/NGFI-B family has evolved from a common ancestral gene. Fluorescence in situ hybridization (FISH) revealed that the gene is located on chromosome 9q.

Cellular expression of the immediate early transcription factors Nurr1 and NGFI-B suggests a gene regulatory role in several brain regions including the nigrostriatal dopamine system

Nurr1 and NGFI-B are closely related orphan members of the steroid-thyroid hormone receptor family involved in immediate early responses to stimuli such as growth factors. In-situ hybridization in the developing and adult mouse and rat demonstrated Nurr1 mRNA in several regions during early central nervous system (CNS) development. Expression persisted through the pre- and postnatal periods and was also found in several areas in the adult CNS. Positive areas include the olfactory bulb, parts of the cortex, the hippocampal formation and substantia nigra where Nurr1 and tyrosine hydroxylase mRNAs were co-expressed. 6-Hydroxydopamine-induced degeneration of mesencephalic dopamine neurons led to a corresponding loss of Nurr1 mRNA, demonstrating a link between Nurr1 and dopaminergic neurons. NGFI-B mRNA was not found in the prenatal CNS but was highly expressed in the adult brain in many areas including the olfactory bulb, cortex, basal ganglia and hippocampus. The spatiotemporal distribution of Nurr1 and NGFI-B mRNAs suggests that these transcription factors are involved in the development and maturation of specific sets of CNS neurons. The experimental data imply that one of these functions may be to control gene regulatory events important for development and function of those neurons that degenerate in patients with Parkinson's disease.

HZF-3, an immediate-early orphan receptor homologous to NURR1/NOT: induction upon membrane depolarization and seizures

Nuclear hormone receptors are thought to play important roles in the regulation of gene expression in the hippocampus. We report here the isolation of an additional member of this superfamily, HZF-3, which is expressed preferentially in the rat brain and therein within the hippocampus. HZF-3 cDNA encodes a 66.6-kDa protein with high homology to the inducible nuclear orphan receptors NURR1/NOT and NGFI-B. Like NURR1/NOT and NGFI-B, HZF-3 mRNA accumulates in PC12 cells subsequent to membrane depolarization. In the rat brain, HZF-3 expression is induced following seizures elicited by the chlorinated insecticide lindane. The time course of HZF-3 induction by membrane depolarization in PC12 cells and seizures in animals is more prolonged than that observed for other immediate-early genes, such as NGFI-B. Electrophoretic mobility shift assays demonstrated HZF-3 to interact specifically with the same DNA target element as NGFI-B: NBRE. These results suggest HZF-3 is a member of a distinct family of inducible orphan receptors which are likely to display synergistic and/or antagonistic regulatory functions in mediating signal-induced transcriptional responses in the nervous system.

Antisense oligonucleotide to NOR-1, a novel orphan nuclear receptor, induces migration and neurite extension of cultured forebrain cells

We previously identified a novel orphan nuclear receptor referred to as NOR-1 from rat forebrain cells. This study examined the role of NOR-1 in primary cultured forebrain cells by selectively inhibiting NOR-1 expression by addition of antisense oligonucleotide to the culture media. Treating cells with the antisense oligomer resulted in the following dramatic morphological changes: (i) cell migration, (ii) extension of processes, and (iii) formation of cellular aggregates. Immunocytochemistry for microtubule-associated protein 2 revealed that the processes were filled with neurites growing from neuronal cells. These findings suggest that NOR-1 may be involved in the molecular mechanisms regulating neural differentiation.

Gene expression of NOR-1, a neuron-derived orphan receptor, is inducible in neuronal and other cell lineages in culture

We recently isolated a cDNA of NOR-1, a novel member of the steroid/thyroid hormone receptor superfamily, from a rat neuronal cell cDNA library. NOR-1 has strong amino acid sequence homology to orphan receptors NGFI-B and Nurr1, and is expressed predominantly in the brain tissue in vivo. In the present study, we examined NOR-1 gene expression in various cultured cells with distinct properties of neuronal cells (PC12), glial cells (C6), endocrine cells (GH3, AtT-20 and Y-1), or fibroblasts (NIH3T3 and L), by reverse transcription-polymerase chain reaction and nucleotide sequencing. NOR-1 transcripts were induced with forskolin and 12-O-tetradecanoylphorbol-13-acetate in all cell lines examined. The nerve growth factor induced not only NGFI-B but also NOR-1 and Nurr1 transcripts in PC12 cells. These findings suggest that NOR-1 gene expression is induced by various stimuli in diverse cell types.

The nuclear hormone-receptor family in the brain: classics and orphans

Nuclear hormone receptors comprise a superfamily of over 40 transcription factors. About half of them are classical receptors for lipophilic ligands such as steroids and vitamins. Almost all of these true receptors are present in the brain, where they transduce chemical signals from endocrine organs or signals of nutritional origin into cellular responses. The other members resemble the classical receptors in structure, but have no known ligands, and are hence called 'orphan receptors'. The issue of whether ligands for nuclear orphan receptors exist is controversial. Evidence is emerging that orphan receptors might be activated by signal transduction pathways or might be constitutive enhancers or repressors that interact with the classical receptors. Thus, nuclear orphan receptors are placed in strategic positions in the regulation of gene expression in the nervous system.

Expression of NOR-1 and its closely related members of the steroid/thyroid hormone receptor superfamily in human neuroblastoma cell lines

Previously, we isolated a cDNA of neuron derived orphan receptor (NOR-1), a putative transcription factor with strong homologies to the orphan nuclear receptors NGFI-B and NURR1. In the present study, we examined the gene expression of NOR-1 as well as NGFI-B and NURR1 in human neuroblastoma cell lines by reverse transcription-polymerase chain reaction and nucleotide sequencing. Although the mRNAs of these orphan receptors were detected in all six neuroblastoma cell lines examined, basal expression levels of these genes varied among cell lines. Treatment with forskolin and 12-O-tetradecanoylphorbol-13-acetate rapidly increased the expression of all these genes in neuroblastoma NB-OK-1 cells. This induction did not require de novo protein synthesis, indicating that the NOR-1 gene as well as NGFI-B and NURR1 genes is an immediate-early gene. This is the first demonstration of NOR-1 gene expression in tumor cell lines.

A novel pathway for vitamin A signaling mediated by RXR heterodimerization with NGFI-B and NURR1

In addition to its role as a 9-cis retinoic acid receptor, RXR has an important role in the regulation of multiple hormonal pathways through heterodimerization with nuclear receptors. Here, we show that two orphan receptors, NGFI-B and NURR1, which have been shown previously to interact with DNA as monomers, also can heterodimerize with RXR. These heterodimers bind selectively to a class of retinoic acid response elements composed of direct repeats spaced by 5 nucleotides. In this respect they are similar to heterodimers formed between RXR and the receptor for all-trans retinoic acid, RAR. However, whereas RXR is inhibited in the RXR-RAR heterodimer, NGFI-B/NURR1 promote efficient activation in response to RXR ligands and therefore shift RXR from a silent to an active heterodimerization partner. These data show that NGFI-B and NURR1 can increase the potential of RXR to modulate gene expression in a ligand-dependent manner by allowing a distinct class of direct repeats to serve as specific RXR response elements. Because expression of both NGFI-B and NURR1 is rapidly induced by various growth factors, these findings also suggest a novel mechanism for convergence between vitamin A or retinoid and growth factor signaling pathways.

Focal brain injury induces multiple immediate early genes encoding zinc finger transcription factors

Focal brain injury is known to markedly induce the fos and jun families of immediate early genes (IEGs). This study employed in situ hybridization to examine the effects of focal brain injury, produced by the intracerebral injection of saline or ibotenic acid on the expression of IEGs encoding zinc finger transcription factors. Thirty minutes after the injections, NGFI-A, NGFI-B, NGFI-C, and egr-3 mRNAs were induced in dentate gyrus, hippocampal pyramidal cells, cerebral cortex, caudate-putamen and piriform cortex of the injured hemisphere. Nurr1 was induced in hippocampal pyramidal cells and dentate granule cells. After three hours the induction of NGFI-A, NGFI-B, NGFI-C and Nurr1 persisted in all brain regions except for the dentate granule cells. By six hours after injection mRNAs for most of the zinc finger genes had returned to control levels. However, the expression of egr-3 3 and 6 h after the injection was identical to that observed at 30 min after the injection and it was the only gene the expression of which persisted 6 h following the injections. Twenty-four hours after the injection, the expression of all five IEGs returned to control levels. In general, no gross differences in the IEG induction were observed between the animals injected with saline and ibotenic acid. Since these zinc finger genes were expressed in the same regions where fos and jun family members are induced by similar types of brain injury, it is suggested that these transcription factors may act in concert with Fos/Jun family members.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal regulation of hypothalamic gene expression: identification of multiple novel estrogen induced genes

Estrogen (E) has been shown to play a major role in hypothalamic function and is a prerequisite for progesterone (P) induced sexual behavior in female rats. In the course of studies in search of steroid induced hypothalamic genes, we discovered a surprisingly large number of E-induced genes (21 mRNAs in total). This is the largest number of E-induced genes ever identified in a single organ. Many of these mRNAs exhibit considerable magnitudes of induction and their levels were maintained typically during subsequent P treatment. Among the induced genes, several encode metabolic enzymes and may account for some of the morphological changes observed in hypothalamic neurons in response to E. Since E appears to play a major role in defining the pattern of hypothalamic gene expression in conjunction with its capacity for behavioral modulation, these newly identified cDNAs may serve as genetic markers for correlative studies of E-induced central nervous system behavior.

Induction of multiple immediate early genes in rat hypothalamic paraventricular nucleus after stress

We have previously demonstrated that stress causes a rapid and transient elevation in the expression of the immediate early genes (IEGs) c-fos and the members of the jun gene family in the brain. Here we demonstrate the effect of stress on the expression of fra-1, fra-2 and recently characterized IEGs encoding zinc finger containing proteins. Capsaicin-induced stress caused a rapid and transient induction of NGFI-A, NGFI-B, fra-2 and TIS11 in the hypothalamic paraventricular nucleus. The NGFI-A mRNA levels were also slightly increased in the cerebral cortex and striatum. The expression of fra-1, NGFI-C, egr-3 and Nurr1 did not show any change. The time course of the induction of NGFI-A, NGFI-B, fra-2 and TIS11 was similar to that previously observed with c-fos and the members of the jun family. The present results suggest that NGFI-A, NGFI-B, fra-2 and TIS11 mediate some of the stress-induced changes of the PVN at the level of gene expression. Although the genes of the NGFI/egr family encode structurally similar proteins, they seem to be regulated differentially and thus have diverse roles in regulating gene expression in the brain.

Expression of nuclear hormone receptors within the rat hippocampus: identification of novel orphan receptors

Within the hippocampus, stimulus-transcriptional coupling plays an important role in post-seizure neuronal adaptation, post-ischemic cell death and the induction of long-term potentiation. To identify additional mediators of hippocampal transcriptional responses a targeted approach was developed and used to characterize the spectrum of nuclear hormone receptors expressed within this brain region. cDNAs encoding the DNA-binding domains of six different members of the nuclear hormone receptor superfamily were isolated. A majority were identical or closely related to receptors known to be expressed within the hippocampus. Two additional isolates, HZF-2 and HZF-3, encode the DNA-binding domain of novel members of the nuclear hormone receptor superfamily.

Peroxisome proliferator-activated receptors A link between endocrinology and nutrition?

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily like the steroid, thyroid, or retinoid hormone receptors, which are ligand-activated transcription factors regulating gene expression. PPARs mediate the induction of the enzymes of the peroxisomal and microsomal fatty-acid oxidation pathways by hypolipidemic drugs such as clofibrate and are probably also involved in the gene expression of other lipid-metabolism-associated proteins that are controlled by fibrate hypolipidemic drugs. That PPARs play an important role in the regulation of lipid metabolism is reinforced by the discovery of their activation by physiologic concentrations of fatty acids. This observation raises the question of whether fatty acids are ligands of PPARs, which would imply that nutritional fatty acids can act like hormones.