The nuclear receptor related protein Nurr
The nuclear receptor related-1 protein, Nurr1 (NR4A2), is a transcription factor that regulates the Darifenacin HBr of genes critical for the development, maintenance, and survival of dopaminergic neurons (Alavian et al., 2014, Decressac et al., 2013, Dong et al., 2016, Jankovic et al., 2005, Johnson et al., 2011, Kadkhodaei et al., 2009, Luo, 2012, Zetterstrom et al., 1997). In particular, Nurr1 plays a fundamental role in maintaining dopamine homeostasis by regulating transcription of the genes governing dopamine synthesis (TH, tyrosine hydroxylase; DDC, dopa decarboxylase), packaging (SLC18A2, vesicular monoamine transporter 2, VMAT2), and reuptake (DAT, dopamine transporter, also known as SLC6A3) (Hermanson et al., 2003, Iwawaki et al., 2000, Johnson et al., 2011, Sacchetti et al., 2001) (Figure 1A). Nurr1 also regulates the survival of dopaminergic neurons by stimulating the transcription of genes coding for neurotrophic factors (brain-derived neurotrophic factor, nerve growth factor), anti-inflammatory responses (glial cell-derived neurotrophic factor receptor c-Ret), and oxidative stress management (SOD1), as well as repressing the transcription of pro-inflammatory genes (tumor necrosis factor α, inducible nitric oxide synthase, interleukin-1β) (Galleguillos et al., 2010, Johnson et al., 2011, Kadkhodaei et al., 2013, Kim et al., 2003, Saijo et al., 2009, Sakurada et al., 1999, Volpicelli et al., 2007). Validation of Nurr1 as a PD therapeutic is primarily derived from mouse models and human data. Homozygous mice lacking Nurr1 fail to generate midbrain dopaminergic neurons and die shortly after birth, heterozygous mice have motor impairments analogous to parkinsonian deficits, and conditional ablation of Nurr1 in adult animals recapitulates early features of PD with progressive dopaminergic neuropathology (Jiang et al., 2005, Kadkhodaei et al., 2009, Kadkhodaei et al., 2013, Zetterstrom et al., 1997, Zhang et al., 2012a). In patients with PD, the expression of Nurr1 is reduced compared with age-matched controls (Chu et al., 2006, Le et al., 2008, Montarolo et al., 2016, Moran et al., 2007), although only a few rare polymorphisms in Nurr1 appear to be associated with the disease (Grimes et al., 2006, Le et al., 2003). Stimulation of Nurr1 activity may combat both the reduced dopamine levels and the increased oxidative stress associated with PD. Efforts to identify Nurr1 agonists have been hampered by major gaps in our understanding of the receptor\'s structure and regulation. In particular, the only reported crystal structure of the receptor (apo Nurr1), published over 15 years ago, reveals the canonical nuclear receptor (NR) ligand-binding pocket is filled by bulky amino acid side chains (Wang et al., 2003). Subsequent efforts to identify ligand-binding sites within Nurr1, utilizing NMR studies of the isolated ligand-binding domain (LBD), have suggested that small molecules may bind to the receptor in regions corresponding to both canonical and non-canonical ligand-binding pockets (de Vera et al., 2016, Kim et al., 2015, Poppe et al., 2007). Phenotypic assays have identified a small number of synthetic ligands that reportedly up-regulate transcription and protein levels of Nurr1 target genes, provide some degree of neuroprotection, and improve behavioral deficits in mouse models (Dong et al., 2016, Kim et al., 2015, McFarland et al., 2013, Smith et al., 2015, Zhang et al., 2012b). However, there is little evidence that any of these ligands directly activate endogenous Nurr1, with the exception of the antimalarial drug amodiaquine (Kim et al., 2015). Endogenous ligands for Nurr1 have yet to be reported, further limiting our understanding of how this receptor is regulated. Efforts to drug Nurr1 indirectly by targeting the RXR LBD in Nurr1:RXR heterodimers have demonstrated enhanced expression of Nurr1 target genes by RXR agonists (McFarland et al., 2013, Spathis et al., 2017, Volakakis et al., 2015). This approach may, however, be limited by the established promiscuous association of RXR with other NRs (e.g., RAR, VDR, TR, PPAR, LXR, FXR), and complicated by the apparent repression of Nurr1 transcriptional activity upon complexation with RXR (Perez et al., 2012, Perlmann and Jansson, 1995). The receptor also lacks the canonical coregulator-binding groove, although some reports suggest that alternative interaction surfaces for regulatory proteins may be present on the Nurr1 LBD (Codina et al., 2004, de Vera et al., 2016, Volakakis et al., 2006).