NR4A2 (Nuclear Receptor Subfamily 4, Group A, Member 2)
HZF-3; Immediate-early response protein NOT; NGFI-B/Nur77 beta-type transcription factor homolog; NOT; Nuclear receptor related 1; Nuclear receptor subfamily 4 group A member 2; Nurr1; Orphan nuclear receptor NURR1; RNR1; T-cell nuclear receptor NOT; TINUR; Transcriptionally inducible nuclear receptor; Transcriptionally inducible nuclear receptor related
Orphan nuclear receptor Nurr1, also known as nuclear receptor subfamily 4, group A, member 2 (NR4A2), together with Nur77 (NR4A1) and Nor1 (NR4A3), is a member of the steroid/thyroid hormone nuclear receptor superfamily. Unlike the other nuclear receptors, Nurr1 is an immediate early gene, and its transcription can be induced by various stimuli such as depolarization, cAMP, inflammation, hormones, calcium, and growth factors (Volpicelli et al. 2004). Structurally, Nurr1 lacks a hydrophobic pocket for ligand binding and might function as ligand-independent nuclear receptor (Wang et al. 2016). The transcriptional regions of Nurr1 (AF1 and AF2, respectively) are localized at the N- and C-terminal, and the DNA-binding domain (DBD) is localized centrally, while the ligand-binding domain (LBD) is in the C-terminal part of the protein. The DBD is highly conserved among the nuclear receptor family members and is composed of two zinc finger modules able to bind nerve growth factor-inducible ß-binding response element (NBRE) as monomer or homodimer, Nurr response element as homodimer (NurRE), or DNA response elements composed of direct repeats spaced by five nucleotides (DR5) as heterodimer with retinoid X receptor (RXR).
In mice, Nurr1 expression is detected in the ventral midbrain at embryonic day 10 (E10), and its expression is reduced in the postnatal stages but is maintained into adulthood. It has been shown to regulate several aspects of post-mitotic development. It regulates key genes involved in dopaminergic (DA) neurotransmission, including tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2), DA transporter (DAT), bicoid-related Pitx3 (Volpicelli et al. 2012) RET, which encodes for the coreceptor for the glial cell line-derived neurotrophic factor (GDNF) family associated to the GDNF family receptor alpha (GFRas), neuropilin, and brain-derived neurotrophic factor (Fig.1b; BDNF, Volpicelli et al. 2007).
Nurr1 null mice die soon after birth for agenesis of mesDA neurons (Zetterstrom et al. 1997). Brains of heterozygous animals (Nurr1+/−), apparently healthy, contained reduced dopamine levels and seem to have an increased susceptibility to toxic stress including mitochondrial toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the proteasome inhibitor lactacystin and methamphetamine. Conditional gene targeting of Nurr1 in mice, both in late-differentiating DA neurons and in adult brain, has provided definitive evidence of a Nurr1 role in the maintenance and survival of DA neurons. Briefly, Nurr1 ablation at late stages of mDA neuron development by crossing Nurr1 with mice carrying Cre under control of the DAT locus showed a rapid loss of striatal DA, loss of mDA neuron markers, and neuron degeneration. Instead, Nurr1 mice, conditionally ablated in adult DA neurons by tamoxifen treatment of conditional Nurr1 gene-targeted mice expressing the CreERT2 enzyme under the DAT gene regulatory sequences, exhibit progressive DA pathology associated with modest reduction of DA neuronal markers in ventral midbrain and striatum, reduced striatal DA, impaired motor behaviors, and dystrophic axons and dendrites, a hall-marker in early human PD (Cheng et al. 2010). Thus, in mice the adult ablation of Nurr1 recapitulates early features of PD and supports the idea that loss of function of Nurr1 might contribute to PD.
Nurr1 and Parkinson’s Disease
Parkinson’s disease (PD) is the second most common neurodegenerative disorder, characterized by the progressive loss of DA neurons in the substantia nigra pars compacta and striatal deficiency (Decressac et al. 2013). The main motor and non-motor symptoms of PD, including bradykinesia, rigidity, resting tremor, and postural instability, seriously impair the patient’s quality of life. Although the pathogenesis of PD is not still clearly understood, it is known that environmental and genetic factors contribute to PD. It is already known that the exposure to environmental toxins, such as MPTP, paraquat, and rotenone has been found to increase the risk of developing PD. Until now, at least 15 causal genes have been identified to be related to PD, such as α-synuclein, parkin, LARRK2, PINK, and DJ1 and Nurr1 (Verstraeten et al. 2015). PD is characterized by the presence of Lewy bodies containing aggregated and misfolded α-synuclein in patients’ nigral DA neurons. In addition, mitochondrial dysfunction, reactive oxygen species, neuroinflammation, and autophagy or proteasome system impairment are considered as major pathogenic contributors to PD.
Even if mutations in Nurr1 have not been identified as major genetic risk factors for PD, patients with familiar and sporadic PD polymorphism (7048-7049insG) in intron 6 of Nurr1, affecting Nurr1 splicing, have been found (Zheng et al. 2003; Liu et al. 2012). In addition, in sporadic PD patients, three other variations in exon 1 (−253C>T, −223C>T, and –309C>T) were found. Even in familial PD, two mutations (−291delT and –245 T>G) in the noncoding exon 1 within the 5′untranslated region of NURR1 have been reported. Only one coding missense mutation in exon 3 of Nurr1 (709C>G) has been identified in a patient with nonfamilial PD. This mutation markedly attenuates Nurr1-induced transcriptional activation.
Downregulation of Nurr1 expression has been found in postmortem human brain tissue of sporadic PD patients with α-synuclein inclusions specifically in SN of DA neurons. In addition, an age-related decline of the number of Nurr1-expressing DA neurons has been noted in the aging human brain. In addition, data from conditional knockout mice seem to indicate Nurr1 expression could be linked to development and progression of PD pathology and, in particular, in the development of early functional and degenerative changes that are seen in affected DA neurons. Interestingly, Nurr1 was also identified as a potential peripheral PD biomarker downregulated in peripheral blood lymphocytes of PD patients, independent of medication, disease severity, or duration. These observations deserve further exploration in large cohorts of well-characterized patients, particularly those in early stage of PD.
Recent work shows that the modulation of Nurr1 expression could be used as an effective treatment in PD. Given the dual role of Nurr1 in both the development and maintenance of mDA neurons and protection from death induced by inflammation, Kim and collaborators (Kim et al. 2015) found that two antimalarial drugs, amodiaquine and chloroquine, stimulate the transcriptional function of Nurr1 through physical interaction with its ligand-binding domain. The two compounds, administered to 6-hydroxydopamine lesioned rats, a model of PD, significantly improved behavioral deficits without any detectable signs of dyskinesia. However, since these antimalaric drugs are widely used, a survey on PD patients that have used these drugs is still lacking, to confirm if the benefits in humans are similar to those reported in the rat model.
A new Nurr1 transactivator, IRX4204 retinoid X receptor (RXR) agonist, activates cellular RXR-Nurr1 signaling and promotes substantia nigra DA neuron survival in vitro, suggesting that it could be used in PD prevention and/or treatment.
Nurr1 Function in Axon Genesis
Topoisomerase IIβ (Top IIβ) is downregulated in Nurr1 null mice. Top IIβ promoter shows two binding sites for Nurr1. Suppression of Top IIβ expression in mesencephalic cultures affects DA neuron neurite elongation and collapses the growth cone. Therefore, Nurr1 might influence the processes of axon genesis in dopaminergic neurons via the regulation of Top IIβ expression (Xin Heng et al. 2012).
Nurr1 Neuroprotection and Inflammation
Excitotoxicity, oxidative stress, and mitochondrial dysfunction have been implicated in the pathology of many neurodegenerative disorders including PD (Decressac et al. 2013). An important strategy for neuroprotection is the regulation of the transcriptional expression of neuroprotective genes, including anti-apoptotic factors and scavengers of reactive oxygen species (ROS). In response to pathophysiological effectors such as hypoxia, oxidative stress, excitotoxicity, and ischemia, the transcription factor cyclic AMP-responsive element-binding protein (CREB) is activated. CREB-induced transcription factors or cofactors may contribute to activate a gene expression program essential for neuronal protection.
In the CNS, pathological stimuli such as ischemia, seizures, and focal brain injury, associated with CREB activation, robustly induce Nurr1 expression. Thus, in neurons exposed to excitotoxic and oxidative stress Nurr1 is a mediator of CREB-dependent neuroprotective responses and is also a regulator of neuroprotective genes. Moreover, synaptic N-methyl-d-aspartate (NMDA) receptor activation induces CREB-mediated neuroprotection and leads to upregulation of Nurr1.
Thus, Nurr1 might be important for DA neuron maintenance and survival via neurotrophic signaling regulation. Indeed, NMDA receptor-induced Nurr1 expression promotes survival effect by binding BDNF promoter (Barneda-Zahonero et al. 2012; Volpicelli et al. 2007). In addition, the expression of the GDNF tyrosine kinase receptor RET also depends on Nurr1 during development. Conditional ablation of GDNF, RET, or Nurr1 in mice results in a progressive pathological changes resembling to early stages of PD, suggesting that the preservation of GDNF-RET-Nurr1 pathway is important for nigral DA neuron integrity and function. Data published by Decressac and co-workers demonstrate that α-synuclein might interfere with GDNF signaling via downregulation of Nurr1 and its transcriptional target RET, not only in α-synuclein overexpression models but also in human PD. Nurr1 overexpression was able to restore GDNF signaling, affected in α-synuclein-overexpressing DA neurons and provides protection of nigral DA neurons against α-synuclein toxicity, also in the absence of exogenous GDNF (Decressac et al. 2012).
Nurr1 seems to have both constitutive and inducible anti-inflammatory activity in monocyte/macrophage lineage immune cells, as well as in astrocytes and microglia, where it functions to mitigate the release of pro-inflammatory cytokines and neurotoxic factors. The anti-inflammatory activity is regulated toward the NFkB signaling pathway in response to inflammatory stimuli such as tumor necrosis factor (TNF-α) and bacterial lipopolysaccharide (LPS; Saijo et al. 2009).
In brain glial cells, the anti-inflammatory effects of Nurr1 are mediated by docking to NFkB-p65 on target inflammatory gene promoters, followed by recruitment of the CoREST corepressor complex, resulting in clearance of NFkB-p65 and transcriptional repression. Since microglia activation and the increased levels of pro-inflammatory mediators might contribute to PD pathology, these studies suggest that Nurr1 protects against neuronal loss by limiting the production of neurotoxic mediators by microglia and astrocytes.
However, Nurr1 also may play a pro-inflammatory role. It is expressed at elevated levels in inflamed joint tissues from patients with arthritis. Nurr1 expression can be induced by inflammatory mediators in resident and infiltrating immune cells and promotes synovial hyperplasia by increasing proliferation of synoviocytes and inducing transcription of matrix-degrading enzymes and pro-inflammatory mediators (McCoy et al. 2015). In fact, an increased expression of Nurr1 in the synovium of patients with rheumatoid arthritis has been shown, suggesting a pro-inflammatory role for Nurr1 in the pathogenesis of rheumatoid arthritis (Murphy et al. 2001).
Nurr1 also influences maturation and differentiation of Th17 T-cells and plays a role in autoimmunity and in resolution of infections (Raveney et al. 2013). Thus, Nurr1, according to the context and to the pathological model, appears to play a dual role (i.e. pro- or anti-inflammatory) in the resolution of inflammatory signaling both in activated immune cells and glial cells.
Nurr1 and Cancer
Nurr1 and Cardiovascular Disease
Atherosclerosis is characterized by hardening of arteries with the formation of plaques that compromise normal blood flow. Atherosclerotic plaques contain cholesterol, calcium deposits, and fat and stimulate a proliferative response in smooth muscle cells within the media of arteries with further constrictions to blood flow leading to myocardial infarction, stroke, and death. The macrophages release cytokines and growth factors and aggravate the local inflammation and lead to excessive uptake of lipids and the transition of macrophages into lipid-laden foam cells that remain resident in the atherosclerotic lesion. Nurr1 seems to have an anti-mitogenic effect in smooth muscle cells antagonizing atherosclerotic plaques formation by inhibition of NFκB-dependent expression of inflammatory genes in macrophages. In addition, Nurr1 is negatively regulated by miR-145 in smooth muscle cells, and mice lacking miR-145 are resistant to the development of atherosclerotic plaques, owing to their high expression of Nurr1. Studies in human macrophages using lentiviral-mediated overexpression or silencing of Nurr1 demonstrated that Nurr1 inhibited the uptake of oxidized LDL by macrophages and reduced the expression of pro-inflammatory cytokines and chemokines supporting the role of Nurr1 in protection against cardiovascular disease (Safe et al. 2016) (Fig. 2).
Nurr1 is a member of the steroid-thyroid hormone nuclear receptor superfamily widely expressed throughout the brain. Its expression is detectable in telencephalic structures such as cortex and hippocampus, although it is well studied in midbrain dopaminergic neurons. In fact, Nurr1 regulates key genes involved in DA neurotransmission such as TH, DAT, VMAT2, Pitx3, and RET, and via the regulation of Top IIβ expression, it might influence the processes of axon genesis in dopaminergic neurons. Mutations in this gene have been associated with disorders related to dopaminergic dysfunction, including Parkinson’s disease, schizophrenia, and manic depression. In addition to its role in DA neurotransmission, Nurr1 appears to be important in the regulation of inflammation and resolution of inflammatory signaling both in activated immune cells and glial cells. Furthermore, Nurr1 was found to be a prognostic factor for high tumor grade, decreased survival, and increased distant metastasis in bladder cancer, breast cancer, and prostatic cancer. It seems to have an anti-mitogenic effect in smooth muscle cells antagonizing atherosclerotic plaques formation.
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