Dopamine neurons express multiple isoforms of the nuclear receptor nurr1 with diminished transcriptional activity

Nurr1 (NR4A2) is an orphan nuclear receptor required for the development and maintenance of the dopaminergic phenotype in neurons of the ventral midbrain. This study demonstrates that multiple splice variants of nurr1 are produced in rat and human dopamine neurons. Formed by alternative RNA splicing in exon 7, nurr1a has a truncated carboxy-terminus, nurr1b has an internal deletion in the ligand-binding domain and nurr1c, newly identified in this study, has a novel carboxy-terminus produced by a frame shift downstream of the splice junction. Alternative RNA splicing in exon 3 produces the isoform known as the transcriptionally-inducible nuclear receptor (TINUR), lacking the amino-terminus. Nurr2 and the newly identified nurr2c are produced by utilization of both exon 3 and exon 7 alternative splice sites. In rat midbrain, variants other than full-length nurr1 constitute 20-35% of NR4A2 transcripts. Transfection studies in dopaminergic SK-N-AS cells demonstrate that nurr1a, nurr1b, nurr1c and TINUR have significantly reduced transcriptional activities compared with full-length nurr1, while nurr2 and nurr2c are inactive. Furthermore, in these experiments, nurr2 and nurr2c both act as dominant negatives. Production of these nurr1 variants in vivo as demonstrated here could represent a novel regulatory mechanism of nurr1 transcriptional activity and therefore, dopaminergic phenotype.