The orphan nuclear receptor Nurr1 promotes the dopaminergic differentiation by regulating the expression of the transcription factor Pitx3

The mesencephalic dopaminergic (mDA) differentiation requires the activation of a cascade of transcription factors, among which play a crucial role the nuclear receptor Nurr1, a member of the steroid-thyroid super family, and the paired-like homeodomain 3, Pitx3. Nurr1 is highly expressed in the ventral midbrain during embryonic development. Interesting only few genes, whose expression is directly regulated by Nurr1, have been identified so far, including Tyrosine hydroxilase and Dopamine Transporter genes. By a genome wide approach we have shown that the gene coding for the “Brain Derived Neurotrophic Factor” (BDNF) is target of Nurr1 (JNC 2007;102:441-53). Moreover by an RNA silencing approach we have hypothesized that Nurr1 may control the level of Pitx3, also required for proper development of the mDA system. By using primary cultures of mDA neurons and a Nurr1-inducible dopaminergic cell line (Exp Cell Res 2003;288:324-34) we further investigated the mechanism by which Nurr1 regulates the Pitx3 gene. We found that Nurr1 overexpression up-regulates that of Pitx3 in a dose dependent manner by binding to a specific nuclear receptor consensus sequence, RXR:VDR, located at the 5' site of the gene. The effect is specific for Nurr1 since other nuclear receptors are unable to promote luciferase expression. Deletion of this motif abolished the ability of Nurr1 to modify transcription of the reporter gene. This data have been further confirmed by chromatin immunoprecipitation assays. We are currently investigating whether potential Nurr1 binding partners are involved. Our findings suggest that Nurr1 may also influence the development and the function of dopaminergic neurons via direct regulation of Pitx3 expression.