Requirements for heterodimerization between the orphan nuclear receptor Nurr1 and retinoid X receptors

Mechanisms of NURR1 Regulation: Consequences for Its Biological Activity and Involvement in Pathology

NURR1 (Nuclear receptor-related 1 protein or NR4A2) is a nuclear protein receptor transcription factor with an essential role in the development, regulation, and maintenance of dopaminergic neurons and mediates the response to stressful stimuli during the perinatal period in mammalian brain development. The dysregulation of NURR1 activity may play a role in various diseases, including the onset and progression of neurodegenerative diseases, and several other pathologies. NURR1 is regulated by multiple mechanisms, among which phosphorylation by kinases or SUMOylation are the best characterized. Both post-translational modifications can regulate the activity of NURR1, affecting its stability and transcriptional activity. Other non-post-translational regulatory mechanisms include changes in its subcellular distribution or interaction with other protein partners by heterodimerization, also affecting its transcription activity. Here, we summarize the currently known regulatory mechanisms of NURR1 and provide a brief overview of its participation in pathological alterations.

Medicinal Chemistry and Chemical Biology of Nurr1 Modulators: An Emerging Strategy in Neurodegeneration

Nuclear receptor related 1 (Nurr1) is a transcription factor with neuroprotective and antineuroinflammatory properties. Observations from genetic studies and human patients support potential of Nurr1 as a therapeutic target in neurodegeneration, but due to a lack of high-quality chemical tools for pharmacological control of Nurr1, its target validation is pending. Nevertheless, considerable progress has recently been made in elucidating structural and functional characteristics of Nurr1, and several ligand scaffolds have been discovered. Here, we analyze Nurr1's structure and mechanisms compared to other nuclear receptors, summarize the known small molecule Nurr1 ligands, and discuss the available evidence for the therapeutic potential of Nurr1 in neurodegeneration.

Nuclear Receptors as Regulators of Pituitary Corticotroph Pro-Opiomelanocortin Transcription

The hypothalamic–pituitary–adrenal (HPA) axis plays a critical role in adaptive stress responses and maintaining organism homeostasis. The pituitary corticotroph is the central player in the HPA axis and is regulated by a plethora of hormonal and stress related factors that synergistically interact to activate and temper pro-opiomelanocortin (POMC) transcription, to either increase or decrease adrenocorticotropic hormone (ACTH) production and secretion as needed. Nuclear receptors are a family of highly conserved transcription factors that can also be induced by various physiologic signals, and they mediate their responses via multiple targets to regulate metabolism and homeostasis. In this review, we summarize the modulatory roles of nuclear receptors on pituitary corticotroph cell POMC transcription, describe the unique and complex role these factors play in hypothalamic–pituitary–adrenal axis (HPA) regulation and discuss potential therapeutic targets in disease states.

N-3 (Omega-3) Fatty Acids: Effects on Brain Dopamine Systems and Potential Role in the Etiology and Treatment of Neuropsychiatric Disorders

BACKGROUND & OBJECTIVE

A number of neuropsychiatric disorders, including Parkinson's disease, schizophrenia, attention deficit hyperactivity disorder, and, to some extent, depression, involve dysregulation of the brain dopamine systems. The etiology of these diseases is multifactorial, involving genetic and environmental factors. Evidence suggests that inadequate levels of n-3 (omega- 3) polyunsaturated fatty acids (PUFA) in the brain may represent a risk factor for these disorders. These fatty acids, which are derived from the diet, are a major component of neuronal membranes and are of particular importance in brain development and function. Low levels of n-3 PUFAs in the brain affect the brain dopamine systems and, when combined with appropriate genetic and other factors, increase the risk of developing these disorders and/or the severity of the disease. This article reviews the neurobiology of n-3 PUFAs and their effects on dopaminergic function.

CONCLUSION

Clinical studies supporting their role in the etiologies of diseases involving the brain dopamine systems and the potential of n-3 PUFAs in the treatment of these disorders are discussed.

Transcriptomic Analysis Shows Decreased Cortical Expression of NR4A1, NR4A2 and RXRB in Schizophrenia and Provides Evidence for Nuclear Receptor Dysregulation

Many genes are differentially expressed in the cortex of people with schizophrenia, implicating factors that control transcription more generally. Hormone nuclear receptors dimerize to coordinate context-dependent changes in gene expression. We hypothesized that members of two families of nuclear receptors (NR4As), and retinoid receptors (RARs and RXRs), are altered in the dorsal lateral prefrontal cortex (DLPFC) of people with schizophrenia. We used next generation sequencing and then qPCR analysis to test for changes in mRNA levels for transcripts encoding nuclear receptors: orphan nuclear receptors (3 in the NR4A, 3 in the RAR, 3 in the RXR families and KLF4) in total RNA extracted from the DLPFC from people with schizophrenia compared to controls (n = 74). We also correlated mRNA levels with demographic factors and with estimates of antipsychotic drug exposure (schizophrenia group only). We tested for correlations between levels of transcription factor family members and levels of genes putatively regulated by these transcription factors. We found significantly down regulated expression of NR4A1 (Nurr 77) and KLF4 mRNAs in people with schizophrenia compared to controls, by both NGS and qPCR (p = or <0.01). We also detected decreases in NR4A2 (Nurr1) and RXRB mRNAs by using qPCR in the larger cohort (p<0.05 and p<0.01, respectively). We detected decreased expression of RARG and NR4A2 mRNAs in females with schizophrenia (p<0.05). The mRNA levels of NR4A1, NR4A2 and NR4A3 were all negative correlated with lifetime estimates of antipsychotic exposure. These novel findings, which may be influenced by antipsychotic drug exposure, implicate the orphan and retinoid nuclear receptors in the cortical pathology found in schizophrenia. Genes down stream of these receptors can be dysregulated as well, but the direction of change is not immediately predictable based on the putative transcription factor changes.

Intestinal nuclear receptors in HDL cholesterol metabolism

The intestine plays a pivotal role in cholesterol homeostasis by functioning as an absorptive and secretory organ in the reverse cholesterol transport pathway. Enterocytes control cholesterol absorption, apoAI synthesis, HDL biogenesis, and nonbiliary cholesterol fecal disposal. Thus, intestine-based therapeutic interventions may hold promise in the management of diseases driven by cholesterol overload. Lipid-sensing nuclear receptors (NRs) are highly expressed in the intestinal epithelium and regulate transcriptionally the handling of cholesterol by the enterocytes. Here, we discuss the NR regulation of cholesterol fluxes across the enterocytes with special emphasis on NR exploitation as a bona fide novel HDL-raising strategy.

Nuclear import and export signals control the subcellular localization of Nurr1 protein in response to oxidative stress

Orphan receptor Nurr1 participates in the acquisition and maintenance of the dopaminergic cell phenotype, modulation of inflammation, and cytoprotection, but little is known about its regulation. In this study, we report that Nurr1 contains a bipartite nuclear localization signal (NLS) within its DNA binding domain and two leucine-rich nuclear export signals (NES) in its ligand binding domain. Together, these signals regulate Nurr1 shuttling in and out of the nucleus. Immunofluorescence and immunoblot analysis revealed that Nurr1 is mostly nuclear. A Nurr1 mutant lacking the NLS failed to enter the nucleus. The Nurr1 NLS sequence, when fused to green fluorescent protein, led to nuclear accumulation of this chimeric protein, indicating that this sequence was sufficient to direct nuclear localization of Nurr1. Furthermore, two NES were characterized in the ligand binding domain, whose deletion caused Nurr1 to accumulate predominantly in the nucleus. The Nurr1 NES was sensitive to CRM1 and could function as an independent export signal when fused to green fluorescent protein. Sodium arsenite, an agent that induces oxidative stress, promoted nuclear export of ectopically expressed Nurr1 in HEK293T cells, and the antioxidant N-acetylcysteine rescued from this effect. Similarly, in dopaminergic MN9D cells, arsenite induced the export of endogenous Nurr1, resulting in the loss of expression of Nurr1-dependent genes. This study illustrates that Nurr1 shuttling between the cytosol and nucleus is controlled by specific nuclear import and export signals and that oxidative stress can unbalance the distribution of Nurr1 to favor its cytosolic accumulation.

Direct regulation of Pitx3 expression by Nurr1 in culture and in developing mouse midbrain

Due to their correlation with major human neurological diseases, dopaminergic neurons are some of the most studied neuronal subtypes. Mesencephalic dopaminergic (mDA) differentiation requires the activation of a cascade of transcription factors, among which play a crucial role the nuclear receptor Nurr1 and the paired-like homeodomain 3, Pitx3. During development the expression of Nurr1 precedes that of Pitx3 and those of typical dopaminergic markers such as tyrosine hydroxylase (TH) and dopamine Transporter (DAT) that are directly regulated by Nurr1. Interestingly we have previously demonstrated that Nurr1 RNA silencing reduced Pitx3 transcripts, leading to the hypothesis that Nurr1 may control Pitx3 expression.

Here we show that Nurr1 overexpression up-regulates that of Pitx3 in a dose-dependent manner by binding to a non-canonical NBRE consensus sequence, located at the 5′ site of the gene. Interestingly, this sequence shows the same effect as the canonical one in promoting gene translation, and its deletion abolishes the ability of Nurr1 to sustain reporter gene expression. Moreover, we show that there is a direct interaction between Nurr1 and the Pitx3 gene promoter in dopaminergic cell cultures and midbrain embryonic tissue. Altogether, our results suggest that the regulation of Pitx3 by Nurr1 may be an essential event controlling the development and function of mDA neurons.

General molecular biology and architecture of nuclear receptors

Nuclear receptors (NRs) regulate and coordinate multiple processes by integrating internal and external signals, thereby maintaining homeostasis in front of nutritional, behavioral and environmental challenges. NRs exhibit strong similarities in their structure and mode of action: by selective transcriptional activation or repression of cognate target genes, which can either be controlled through a direct, DNA binding-dependent mechanism or through crosstalk with other transcriptional regulators, NRs modulate the expression of gene clusters thus achieving coordinated tissue responses. Additionally, non genomic effects of NR ligands appear mediated by ill-defined mechanisms at the plasma membrane. These effects mediate potential therapeutic effects as small lipophilic molecule targets, and many efforts have been put in elucidating their precise mechanism of action and pathophysiological roles. Currently, numerous nuclear receptor ligand analogs are used in therapy or are tested in clinical trials against various diseases such as hypertriglyceridemia, atherosclerosis, diabetes, allergies and cancer and others.

Control of nuclear receptor activities in metabolism by post-translational modifications

Nuclear receptors (NRs) are molecular transducers of endocrine and dietary signals allowing tissues to adapt their transcriptional responses to endogenous or exogenous cues. These signals act in many cases as specific ligands, converting of NRs into transcriptionally active molecules. This on-off mechanism needs, however, to be finely tuned with respect to the tissue environment and adjusted to the organism needs. These subtle adjustments of NR transcriptional activity are brought about by post-translational modifications (PTMs), which can be, in the case of orphan NRs, the sole regulatory mechanism. The role of PTMs, with a more specific focus on phosphorylation, affecting the functions of NR controlling metabolic events is described in this review.

The role of NR4A transcription factors in memory formation

In various physiological contexts, Nr4a genes are transcribed in response to external stimuli as part of an immediate early response that initiates a cascade of gene expression ultimately leading to distinct physiological outcomes in each of these contexts. The signaling pathway that initiates Nr4a gene expression in most of these contexts consists of elevated intracellular cAMP activating PKA, which in turn leads to phosphorylation of CREB and new gene synthesis. This cAMP-PKA-CREB pathway is a central molecular pathway in the formation of a long-term memory. Indeed, learning induces Nr4a family gene expression, and long-term memory formation requires at least two waves of transcription after learning, suggesting that NR4A nuclear receptors may contribute to the second of these waves of gene expression. In this article, we review insights gained in other physiological contexts regarding Nr4a function and regulation, and highlight how these lessons can be applied to the study of memory formation.

Retinoid X receptors: common heterodimerization partners with distinct functions

Retinoid X receptors (RXRs) have been implicated in a diversity of cellular processes ranging from cellular proliferation to lipid metabolism. These pleiotropic effects stem not only from the ability of RXRs to dimerize with diverse nuclear receptors, which exert transcriptional control on specific aspects of cell biology, but also because binding of RXR ligands to heterodimers can stimulate transcriptional activation by RXR partner receptors. This signaling network is rendered more complex by the existence of different RXR isotypes (RXRα, RXRβ, RXRγ) with distinct properties that thereby modulate the transcriptional activity of RXR-containing heterodimers. This review discusses the emerging roles of RXR isotypes in the RXR signaling network and possible implications for our understanding of nuclear receptor biology and pharmacology.

Stem cell potential in Parkinson's disease and molecular factors for the generation of dopamine neurons

Parkinson's disease (PD) involves the loss of dopamine (DA) neurons, making it the most expected neurodegenerative disease to be treated by cell replacement therapy. Stem cells are a promising source for cell replacement therapy due to their ability to self-renew and their pluripotency/multipotency that allows them to generate various types of cells. However, it is challenging to derive midbrain DA neurons from stem cells. Thus, in this review, I will discuss the molecular factors that are known to play critical roles in the generation and survival of DA neurons. The developmental process of DA neurons and functions of extrinsic soluble factors and homeodomain proteins, forkhead box proteins, proneural genes, Nurr1 and genes involved in epigenetic control are discussed. In addition, different types of stem cells that have potential for future cell replacement therapy are reviewed.

NR4A orphan nuclear receptors: transcriptional regulators of gene expression in metabolism and vascular biology

Members of the nuclear hormone receptor superfamily, including the peroxisome proliferator-activated receptor and the liver X receptor subfamilies, orchestrate transcriptional networks involved in the control of metabolism and the development of vascular disease. In addition to these well-characterized ligand-activated transcription factors, the nuclear receptor (NR) superfamily comprises many orphan receptors, whose ligands and physiological functions remain unknown. Among this group of orphan receptors is the NR4A subfamily, including Nur77 (NR4A1), Nurr1 (NR4A2), and NOR1 (NR4A3). These orphan NRs constitute an evolutionary ancient and highly conserved group of transcription factors. In contrast to other members of the superfamily, NR4A receptors function as ligand-independent transcription factors and immediate- or early-response genes, which are rapidly induced by a pleiotropy of environmental cues. Early functional studies have pointed to a critical role of NR4A receptors in regulating differentiation, proliferation, and apoptosis. More recent research has characterized NR4A receptors as key transcriptional regulators of glucose and lipid homeostasis, adipogenesis, inflammation, and vascular remodeling. In this review, we will summarize recent advances in understanding the molecular biology and physiological functions of NR4A receptors and discuss their role in the transcriptional control of metabolism and vascular remodeling.

Proteasomal degradation of retinoid X receptor alpha reprograms transcriptional activity of PPARgamma in obese mice and humans

Obese patients have chronic, low-grade inflammation that predisposes to type 2 diabetes and results, in part, from dysregulated visceral white adipose tissue (WAT) functions. The specific signaling pathways underlying WAT dysregulation, however, remain unclear. Here we report that the PPARgamma signaling pathway operates differently in the visceral WAT of lean and obese mice. PPARgamma in visceral, but not subcutaneous, WAT from obese mice displayed increased sensitivity to activation by its agonist rosiglitazone. This increased sensitivity correlated with increased expression of the gene encoding the ubiquitin hydrolase/ligase ubiquitin carboxyterminal esterase L1 (UCH-L1) and with increased degradation of the PPARgamma heterodimerization partner retinoid X receptor alpha (RXRalpha), but not RXRbeta, in visceral WAT from obese humans and mice. Interestingly, increased UCH-L1 expression and RXRalpha proteasomal degradation was induced in vitro by conditions mimicking hypoxia, a condition that occurs in obese visceral WAT. Finally, PPARgamma-RXRbeta heterodimers, but not PPARgamma-RXRalpha complexes, were able to efficiently dismiss the transcriptional corepressor silencing mediator for retinoid and thyroid hormone receptors (SMRT) upon agonist binding. Increasing the RXRalpha/RXRbeta ratio resulted in increased PPARgamma responsiveness following agonist stimulation. Thus, the selective proteasomal degradation of RXRalpha initiated by UCH-L1 upregulation modulates the relative affinity of PPARgamma heterodimers for SMRT and their responsiveness to PPARgamma agonists, ultimately activating the PPARgamma-controlled gene network in visceral WAT of obese animals and humans.

The glucocorticoid receptor is a co-regulator of the orphan nuclear receptor Nurr1

Nurr1 (NR4A2) is an atypical nuclear receptor (NR) because of its inability to bind a ligand and to activate transcription following canonical NR rules. An affinity chromatography-based screen identified the glucocorticoid receptor (GR) as an interactant of Nurr1. The co-localization of these two NRs in the hippocampus and the substantia nigra, as well as their involvement in similar neurological processes led us to investigate the functional consequences of such a physical interaction. GR interfered with Nurr1 transcriptional activity, and Nurr1 association to GR confers glucocorticoid regulation to this orphan receptor. The N-terminal domain of Nurr1 interacts directly with GR, whereas several domains of GR can associate to Nurr1. The GR-mediated increase in Nurr1 transcriptional activity requires the N-terminal domain of GR, but not a functional DNA binding domain. Finally, SMRT and SRC2, two co-regulators of GR, modulated the transcriptional activity of the Nurr1-GR complex, but not that of Nurr1 alone. Our results therefore establish GR as a transcriptional regulator of Nurr1, and open new opportunities in the pharmacological regulation of Nurr1 by glucocorticoids in the CNS.

Manipulation of reciprocal salt bridges at the heterodimerization interface alters the dimerization properties of mouse RXRalpha and PPARgamma1

Heterodimerization with RXR is essential for the high-affinity specific binding of multiple nuclear receptors to their cognate DNA sequences. NR dimerization is a two-step process, initiated in solution by interaction between amino acid residues with helices 9 and 10 of the ligand binding domains of RXR and its NR partners. Studies of the orphan nuclear receptor HNF4alpha, which forms homodimers exclusively, have indicated that two charged residues in this region, HNF4alpha(K300) and HNF4alpha(E327), are key mediators of dimerization. We have analyzed the contribution of the homologous residues in RXRalpha (RXRalpha(E395), RXRalpha(K422)) and PPARgamma (PPARgamma(E405), PPARgamma(K432)) to the formation of the RXRalpha-PPARgamma heterodimer. Charge reversal mutants of RXRalpha (RXRalpha(E395K), RXRalpha(K422E)) and PPARgamma (PPARgamma(E405K), PPARgamma(K432E)) show impaired ability to form heterodimers with wild-type PPARgamma and RXRalpha, respectively. However, pairs of mutants with balanced charge changes, i.e., RXRalpha(E395K) with PPARgamma(K432E) and RXRalpha(K422E) with PPARgamma(E405K), are able to form dimers. Ligand response is preserved in the PPARgamma mutants, indicating the mutation does not result in major structural derangement of the protein. These results establish the importance of salt bridges between these residues in the heterodimerization of nuclear receptors, and offer a technical approach to generating functional NR mutants with directed heterodimerization specificity. Such mutants will be valuable tools in the genetic analysis of NR function.

Regulation of GTP cyclohydrolase I expression by orphan receptor Nurr1 in cell culture and in vivo

Nurr1 is an orphan nuclear transcription factor essential for the terminal differentiation of dopamine (DA) neurons in the ventral midbrain (VM). To identify the Nurr1-target genes, we carried out microarray and quantitative real-time PCR analyses of Nurr1 null and wild-type mice in VM at embryonic day (E) 12.5 and shortly after birth (P0). In addition to the absence of mRNAs of DA synthesizing enzymes, the guanosine 5'-triphosphate (GTP) cyclohydrolase I (GTPCH) was also substantially reduced in the VM of Nurr1-null mice. GTPCH is the first enzyme in the synthesis pathway of tetrahydrobiopterin (BH4), an essential cofactor for tyrosine hydroxylase in DA synthesis. In the mouse, Nurr1 and GTPCH mRNA were first detected at E10.5, and GTPCH transcription paralleled that of Nurr1. Small interfering RNA targeted against Nurr1 decreases GTPCH expression in MC3T3-E1 osteoblasts in cell culture. Cotransfection of Nurr1 and the GTPCH-luciferase (luc) reporter increased the luc activity by about threefold in N2A cells. Additional analysis using 5'-deletions and mutants revealed that Nurr1 activates GTPCH transcription indirectly through the proximal promoter region, in the absence of the nerve growth factor-induced clone B (NGFI-B) responsive element-like sites, similarly, as recently reported for DA transporter regulation by Nurr1.

Transcriptional repression of matrix metalloproteinase gene expression by the orphan nuclear receptor NURR1 in cartilage

The NR4A orphan receptors (Nur77, NURR1, and NOR-1) are emerging as key regulators of cytokine and growth factor action in chronic inflammatory diseases. In this study, we address the role of these receptors in cartilage homeostasis during inflammatory joint disease. We document for the first time expression of the NR4A receptors in osteoarthritic cartilage. Relative to Nur77 and NOR-1, NURR1 is expressed at the highest level and correlates with cyclooxygenase-2 levels in cartilage. Consistent with this observation, cyclooxygenase-2-derived prostaglandin E(2) (PGE(2)) rapidly and potently induces NURR1 expression in chondrocytes, suggesting that this receptor may regulate PGE(2)-mediated processes in cartilage. We demonstrate that PGE(2) represses interleukin-1beta-induced matrix metalloproteinase (MMP)-1 and that transient overexpression of NURR1 is sufficient to antagonize expression of this gene. Furthermore, MMP-1 promoter activity is potently suppressed by NURR1, resulting in a significant reduction in endogenous MMP-1 mRNA and secreted pro-MMP-1 protein. In addition, NURR1 selectively antagonizes cytokine-induced MMP-3 and -9 expression with minimal effects on MMP-2 and -13 and tissue inhibitor of matrix metalloproteinases-1 and -2. To explore the molecular mechanisms of NURR1 transrepression, we reveal that this receptor targets a critical region of the MMP-1 promoter (-1772 to -1546 bp) and that repression does not require consensus binding sites for NURR1. We confirm that NURR1 targets a 40-bp promoter sequence that is also positively regulated by ETS transcription factors. Finally, functional studies indicate that transcriptional antagonism exists between NURR1 and ETS1 on the MMP-1 promoter. We propose a protective function for NURR1 in cartilage homeostasis by selectively repressing MMP gene expression during inflammation.

CAR2 displays unique ligand binding and RXRalpha heterodimerization characteristics

The constitutive androstane receptor (CAR; NR1I3) regulates the expression of genes involved in xenobiotic metabolism. Alternative splicing of the human CAR gene yields an array of mRNAs that encode structurally diverse proteins. One form of CAR, termed CAR2, contains an additional four amino acids (SPTV) that are predicted to reshape the ligand-binding pocket. The current studies show a marked, ligand-independent, CAR2-mediated transactivation of reporters containing optimal DR-3, DR-4, and DR-5 response elements, and reporters derived from the natural CYP2B6 and CYP3A4 gene promoters. Overexpression of the RXRalpha ligand binding domain was critical for achieving these effects. CAR2 interaction with SRC-1 was similarly dependent on the coexpression of RXRalpha. Mutagenesis of Ser233 (SPTV) to an alanine residue yielded a receptor possessing higher constitutive activity. Alternatively, mutating Ser233 to an aspartate residue drastically reduced the transactivation capacity of CAR2. The respective abilities of these mutagenized forms of CAR2 to transactivate a DR-4 x 3 reporter element correlated with their ability to interact with RxRalpha and to recruit SRC-1 in a ligand-regulated manner. Together, these results demonstrate a robust RXRalpha-dependent recruitment of coactivators and transactivation by CAR2. In addition, CAR2 displays novel dose responses to clotrimazole and androstanol compared with the reference form of the receptor while at the same time retaining the ability to bind CITCO. This result supports a hypothesis whereby the four-amino-acid insertion in CAR2 structurally modifies its ligand binding pocket, suggesting that CAR2 is regulated by a set of ligands distinct from those governing the activity of reference CAR.

Nuclear receptor 4A2 and C/EBPbeta regulate the parathyroid hormone-mediated transcriptional regulation of the 25-hydroxyvitamin D3-1alpha-hydroxylase

1Alpha-hydroxylase is the enzyme responsible for the production of the active form of vitamin D, 1,25-dihydroxyvitamin D3. 1Alpha-hydroxylase, found largely in the kidney, is known to be up-regulated by parathyroid hormone (PTH), however the mechanism of action of PTH and any required transcription factors have not been clearly identified. During gene array analysis we observed that NR4A2, a nuclear orphan receptor, is markedly up-regulated in a porcine kidney cell line (AOK-B50) following PTH stimulation. NR4A2 over-expression increases the endogenous induction of 1alpha-hydroxylase mRNA in the absence of PTH, however optimal stimulation is achieved when both NR4A2 and PTH are present. An unconventional site of action of NR4A2 was localized to a fragment comprising the sequence from -35/+22 of the 1alpha-hydroxylase promoter at a C/EBP consensus site. Study of the involvement of C/EBPbeta in the 1alpha-hydroxylase regulation revealed that the transcriptional enhancement by NR4A2 on the 1alpha-hydroxylase promoter is inhibited by C/EBPbeta. In addition, C/EBPbeta over-expression decreases the endogenous levels of both NR4A2 and 1alpha-hydroxylase mRNA.

Multiple signaling pathways regulate the transcriptional activity of the orphan nuclear receptor NURR1

The orphan nuclear receptor nurr1 (NR4A2) is an essential transcription factor for the acquisition and maintenance of the phenotype of dopamine (DA)-synthesizing neurons in the mesencephalon. Although structurally related to ligand-regulated nuclear receptors, nurr1 is functionally atypical due to its inability to bind a cognate ligand and to activate transcription following canonical nuclear receptor (NR) rules. Importantly, the physiological stimuli that activate this NR and the signaling proteins that regulate its transcriptional activity in mesencephalic neurons are unknown. We used an affinity chromatography approach and CSM14.1 cells of mesencephalic origin to isolate and identify several proteins that interact directly with nurr1 and regulate its transcriptional activity. Notably, we demonstrate that the mitogen-activated protein kinases, ERK2 and ERK5, elevate, whereas LIM Kinase 1 inhibits nurr1 transcriptional activity. Furthermore, nurr1 recruits ERK5 to a NBRE-containing promoter and is a potential substrate for this kinase. We have identified amino acids in the A/B domain of nurr1 important for mediating the ERK5 activating effects on nurr1 transcriptional activity. Our results suggest that nurr1 acts as a point of convergence for multiple signaling pathways that likely play a critical role in differentiation and phenotypic expression of dopaminergic (DAergic) neurons.

VIP is a transcriptional target of Nurr1 in dopaminergic cells

The orphan nuclear receptor Nurr1 is required for the development of the ventral mesencephalic dopaminergic neurons. These are the same neurons that are invariantly lost in patients with Parkinson's disease. Nurr1 mRNA expression is not confined to the developing midbrain, and yet Nurr1 appears to be essential for either the maturation of progenitors into fully post-mitotic dopaminergic neurons and/or once formed, their survival. The function of Nurr1 in the transactivation of gene(s) important for neuronal development and/or maintenance is uncharacterized. To characterize potential downstream target genes of Nurr1, we sought to identify mRNAs that are differentially affected by Nurr1 expression. Using a dopaminergic cell line in which Nurr1 content was tightly regulated, differential display analysis identified transcripts altered by Nurr1 expression, including the mRNA encoding vasoactive intestinal peptide (VIP). Herein, we demonstrate that Nurr1 regulates VIP mRNA and protein levels, and transactivates the VIP promoter through Nurr1-responsive cis elements. In addition, dopaminergic cells release and utilize VIP to mediate survival when challenged with paraquat. Nurr1 regulation of VIP is also demonstrated in vivo as loss of Nurr1 function results in diminished VIP mRNA levels within the developing midbrain.

Nurr1 dependent regulation of pro-inflammatory mediators in immortalised synovial fibroblasts

Background

Nurr1 is an orphan member of the nuclear receptor superfamily; these orphan receptors are a group for which a ligand has yet to be identified. Nurr1 has been shown to regulate the expression of a small number of genes as a monomeric, constitutively active receptor. These Nurr1 regulated genes are primarily associated with dopamine cell maturation and survival. However, previous reports have shown an increased expression of Nurr1 in the synovium of patients with rheumatoid arthritis (RA) suggesting a pro-inflammatory role for Nurr1 in RA. In this study we investigate the potential pro-inflammatory role of Nurr1 by monitoring Nurr1 dependent gene expression in an immortalised synoviocyte cell line, K4IM.

Methods

We overexpressed the wild type and a dominant negative form of the orphan nuclear receptor Nurr1, in a model synoviocyte cell line. Using the Affymetrix HG-U133 Genechips we demonstrate the effects on the transcriptome by the receptor. Further evidence of gene expression change was demonstrated using quantitative RT-PCR and ELISA analysis.

Results

We show that Nurr1 regulates transcription of a small number of genes for pro-inflammatory modulators of which the most significant is interleukin-8 (IL-8). We also demonstrate increased synthesis and secretion of IL-8 further supporting a role for Nurr1 in inflammatory signalling pathways.

Conclusion

Using microarray analysis we show that elevated levels of Nurr1 leads to increased gene expression of pro-inflammatory genes: IL-8, Amphiregulin and Kit ligand in a model cell line. This data provides further evidence for an additional role for Nurr1 in inflammation and may play a role in the pathogenesis of rheumatoid arthritis.

The role of Nurr1 in the development of dopaminergic neurons and Parkinson's disease

Nurr1, a transcription factor belonging to the orphan nuclear receptor superfamily, is critical in the development and maintenance of the dopaminergic system and as such it may have role in the pathogenesis of Parkinson' disease (PD). Human Nurr1 gene has been mapped to chromosome 2q22-23 and Nurr1 protein is predominantly expressed in central dopaminergic neurons. Nurr1 interacts with other factors critical for the survival of mensencephalic dopaminergic neurons and it appears to regulate the expression of tyrosine hydroxylase (TH), dopamine transporter (DAT), vesicular monoamine transporter 2 (VMAT2), and l-aromatic amino acid decarboxylase (AADC), all of which are important in the synthesis and storage of dopamine. Experimental studies in Nurr1 knock-out mice indicate that Nurr1 deficiency results in impaired dopaminergic function and increased vulnerability of those midbrain dopaminergic neurons that degenerate in PD. Decreased Nurr1 expression is found in the autopsied PD midbrains, particularly in neurons containing Lewy bodies, as well as in peripheral lymphocytes of patients with parkinsonian disorders. Several variants in Nurr1 gene have been reported in association with PD. All these studies suggest that Nurr1 is not only essential in the development of mensencephalic dopaminergic neurons and maintenance of their functions, but it may also play a role in the pathogenesis of PD.

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.

Cell cycle regulation of breast cancer cells through estrogen-induced activities of ERK and Akt protein kinases

The proliferative effect of estrogens on breast cancer cell (BCC) is mainly mediated through estrogen receptors (ER). Non-transcriptional effects of estrogens, exerted through activation of several protein kinases, may also contribute to BCC proliferation. However, the relative contribution of these two responses to BCC proliferation is not known. We characterized a novel estrogenic receptor ligand which possess Akt and ERK activating properties distinct from that of 17beta-estradiol. Early and delayed waves of activation of these kinases were detected upon estrogenic challenge of BCC, but only molecules able to promote a significant, delayed activation of ERK-induced BCC proliferation. Estrogen-induced cell cycle progression was not sensitive to the inhibition of ERK-regulating kinases MEK1 and 2. ERalpha was found to be necessary, but not sufficient for kinases activation. Thus, estrogens elicit a distinct pattern of early and delayed activation of ERK and Akt, and early protein kinase activation is probably not involved in BCC proliferation. Structural variations in the estrogen molecule may confer novel biological properties unrelated to estrogen-dependent transcriptional activation.

The monomeric orphan nuclear receptor Schistosoma mansoni Ftz-F1 dimerizes specifically and functionally with the schistosome RXR homologue, SmRXR1

In an attempt to understand development and differentiation processes of the parasitic blood fluke Schistosoma mansoni, several members of the nuclear receptor superfamily were cloned, including SmFtz-F1 (S. mansoni Fushi Tarazu-factor 1). The Ftz-F1 nuclear receptor subfamily only contains orphan receptors that bind to their response element as monomers. Whereas SmFtz-F1 displays these basic functional properties, we have identified an original and specific interaction between SmFtz-F1 and the schistosome RXR homologue, SmRXR1. The mammalian two-hybrid assay showed that the D, E, and F domains of SmFtz-F1 were capable of interacting specifically with the E domain of SmRXR1 but not with that of mouse RXRalpha. Using three-dimensional LBD homology modelling and structure-guided mutagenesis, we were able to demonstrate the essential role of exposed residues located in the dimerization interfaces of both receptors in the maintenance of the interaction. Cotransfection experiments with constructions encoding full-length nuclear receptors show that SmRXR1 potentiates the transcriptional activity of SmFtz-F1 from various promoters. Nevertheless, the lack of identification of a dimeric response element for this SmFtz-F1/SmRXR1 heterodimer seems to indicate a "tethering" mechanism. Thus, our results suggest for the first time that a member of the Ftz-F1 family could heterodimerize functionally with a homologue of the universal heterodimerization partner of nuclear receptors. This unique property confirms that SmFtz-F1 may be involved in the development and differentiation of schistosome-specific structures.

Retinoid X receptor regulates Nur77/TR3-dependent apoptosis [corrected] by modulating its nuclear export and mitochondrial targeting

Retinoid X receptor (RXR) plays a central role in the regulation of intracellular receptor signaling pathways by acting as a ubiquitous heterodimerization partner of many nuclear receptors, including the orphan receptor Nur77 (also known as TR3 [corrected] or NGFI-B), which translocates from the nucleus to mitochondria, where it interacts with Bcl-2 to induce apoptosis. Here, we report that RXRalpha is required for nuclear export and mitochondrial targeting of Nur77 through their unique heterodimerization that is mediated by dimerization interfaces located in their DNA-binding domain. The effects of RXRalpha are attributed to a putative nuclear export sequence (NES) present in its carboxyl-terminal region. RXRalpha ligands suppress NES activity by inducing RXRalpha homodimerization or altering RXRalpha/Nur77 heterodimerization. The RXRalpha NES is also silenced by RXRalpha heterodimerization with retinoic acid receptor or vitamin D receptor. Consistently, we were able to show that the mitochondrial targeting of the RXRalpha/Nur77 heterodimer and its induction of apoptosis are potently inhibited by RXR ligands. Together, our results reveal a novel nongenotropic function of RXRalpha and its involvement in the regulation of the Nur77-dependent apoptotic pathway [corrected]

Nuclear orphan receptor Nurr1 directly transactivates the osteocalcin gene in osteoblasts

Nurr1, an NGFI-B nuclear orphan receptor, which transactivates promoters through an NGFI-B response element (NBRE), is strongly induced by parathyroid hormone through the cAMP-protein kinase A signaling pathway in osteoblasts. Here, we demonstrate that multiple agents activating diverse signaling pathways in osteoblasts induce Nurr1. The strongest Nurr1 inducers were activators of cAMP-protein kinase A-coupled signaling, followed by protein kinase C- and calcium-coupled signaling activators. Receptor tyrosine kinase activators had minimal effect, whereas serine/threonine kinase activators had no effect on basal Nurr1 mRNA levels. Computer analysis of osteoblastic promoters indicated two potential NBREs in the rat osteocalcin (Ocn) promoter. Intriguingly, the proximal site maps to the cAMP-responsive cis-element. We tested whether Nurr1 induces Ocn expression through the NBRE-like site. Recombinant and endogenous Nurr1 protein from primary mouse osteoblasts bound to a consensus NBRE in EMSAs. Nurr1 induced a consensus 3 x NBRE-luciferase reporter construct in mouse osteoblasts. Recombinant and endogenous Nurr1 protein bound to the proximal NBRE-like site in the Ocn promoter in EMSAs. Endogenous Nurr1 protein bound to this site as a monomer, because neither retinoid X receptor alpha nor retinoid X receptor beta antibody supershifted the protein-DNA complex. Ocn promoter-luciferase constructs lacking or containing a mutated proximal NBRE-like site had markedly blunted responses to Nurr1 overexpression. Finally, adenovirally expressed Nurr1 protein bound to the proximal NBRE-like site in chromatin immunoprecipitation assays and induced Ocn mRNA in primary rat osteoblasts. We conclude that Ocn is a Nurr1 target gene, which positions Nurr1 in the core of transcriptional factors regulating osteoblastic gene expression.

Making connections: the development of mesencephalic dopaminergic neurons

The disorders of two adjacent sets of mesencephalic dopaminergic (MDNs) are associated with two significant health problems: Parkinson's disease and drug addiction. Because of this, a great deal of research has focused on understanding the growth, development and maintenance of MDNs. Many transcription factors and signaling pathways are known to be required for normal MDNs formation, but a unified model of MDN development is still unclear. The long-term goal is to design therapeutic strategies to: (i) nurture and/or heal endogenous MDNs, (ii) replace the affected tissue with exogenous MDNs from in vitro cultivated stem cells and (iii) restore normal connectivity. Recent developmental biology studies show great promise in understanding how MDNs develop both in vivo and in vitro. This information has great therapeutic value and may provide insight into how environmental and genetic factors increase vulnerability to addiction.