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.

α-Synuclein Induces the GSK-3-Mediated Phosphorylation and Degradation of NURR1 and Loss of Dopaminergic Hallmarks.

In Parkinson's disease, the dysfunction of the dopaminergic nigrostriatal tract involves the loss of function of dopaminergic neurons of the substantia nigra pars compacta followed by death of these neurons. The functional recovery of these neurons requires a deep knowledge of the molecules that maintain the dopaminergic phenotype during adulthood and the mechanisms that subvert their activity. Previous studies have shown that transcription factor NURR1, involved in differentiation and maintenance of the dopaminergic phenotype, is downregulated by α-synuclein (α-SYN). In this study, we provide a mechanistic explanation to this finding by connecting α-SYN-induced activation of glycogen synthase kinase-3 (GSK-3) with NURR1 phosphorylation followed by proteasomal degradation. The use of sequential deletion mutants and single point mutants of NURR1 allowed the identification of a domain comprising amino acids 123-PSSPPTPSTPS-134 that is targeted by GSK-3 and leads to subsequent ubiquitination and proteasome degradation. This study provides a detailed analysis of the regulation of NURR1 stability by phosphorylation in synucleinopathies such as Parkinson's disease.

Tuning melatonin receptor subtype selectivity in oxadiazolone-based analogues: Discovery of QR2 ligands and NRF2 activators with neurogenic properties.

New multi-target indole and naphthalene derivatives containing the oxadiazolone scaffold as a bioisostere of the melatonin acetamido group have been developed. The novel compounds were characterized at melatonin receptors MT1R and MT2R, quinone reductase 2 (QR2), lipoxygenase-5 (LOX-5), and monoamine oxidases (MAO-A and MAO-B), and also as radical scavengers. We found that selectivity within the oxadiazolone series can be modulated by modifying the side chain functionality and co-planarity with the indole or naphthalene ring. In phenotypic assays, several oxadiazolone-based derivatives induced signalling mediated by the transcription factor NRF2 and promoted the maturation of neural stem-cells into a neuronal phenotype. Activation of NRF2 could be due to the binding of indole derivatives to KEAP1, as deduced from surface plasmon resonance (SPR) experiments. Molecular modelling studies using the crystal structures of QR2 and the KEAP1 Kelch-domain, as well as the recently described X-ray free-electron laser (XFEL) structures of chimeric MT1R and MT2R, provided a rationale for the experimental data and afforded valuable insights for future drug design endeavours.

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.