Functional studies on the nuclear receptor Nurr1

Abstract: The nuclear receptor (NR) superfamily comprises the largest family of ligand-activated transcription factors in eukaryotes, and it plays critical roles in many aspects of vertebrate development and physiology. The NR family is well conserved throughout evolution, and is found in organisms as diverse as nematodes, flies and mammals. The family includes receptors for small lipophilic ligands such as the steroid receptors, thyroid hormone, vitamin D3 and retinoids. These molecules can easily pass through the plasma membrane, enter the cell and bind their cognate receptor, thereby regulating the transcription of target genes. The ligands thus provide a direct link between signaling molecules and transcriptional response. The superfamily contains receptors for known ligands and it contains a large number of socalled "orphan receptors", for which ligands do not exist or have not been identified. Nurr1 is an orphan receptor that is mainly expressed in the central nervous system where it plays a critical role in the development of the midbrain dopamine neurons. These are the cells that degenerate in patients with Parkinson's disease. Nurr1, together with two-subfamily members NGFI-B and Nor1, is unique within the NR family by being encoded by immediate early genes that are rapidly induced by various stimuli, for example by growth factors. Nurr1 can bind to DNA as monomers and promotes constitutive transcriptional activation that is dependent on two distinct activation functions (AF1 and AF2), located in the amino- and carboxyterminal regions of Nurr1, respectively. The present studies aimed at functionally characterizing Nurr1 and determing the domains that are important for the activity of the receptor. We showed that the Nurr1 ligand binding domain (LBD) contributes to constitutive activation in a manner that depends on cell type, in the absence of exogenously added ligands. The unusual structured features of Nurr1 AF2 suggested an alternative mechanism for coactivator recruitment, subsequently supported by the crystal structure of the Nurr1 LBD. This domain has a distinctly different surface for binding of potential coactivators. These results demonstrated that Nurr1 AF2 has an intrinsic ability to activate transcription, which suggests that other factors that can modulate the Nurr1 activity remain to be identified. By characterizing the function of AF1, we identified a short core activation region located close to the amino-terminal end of Nurr1. In addition, we showed that the amino-terminal activation domain is a potential target for crosstalk via MAPK-dependent phosphorylation events. Gene-targeting of the Nurr1 gene leads to agenesis of the midbrain dopamine neurons. The highly homologous gene of Nurr1 in humans is thus a good candidate gene for treating disorders with an involvement of the dopamine neuron system, such as Parkinson's disease, schizophrenia (SZ) and manic depression. By sequencing genomic DNA, three different unique missense mutations were identified in patients suffering from psychosis, two in SZ and one in manic-depressive disorder. All three mutations were located in exon 3 and resulted in a decrease in transcriptional activity, when analyzed in vitro.