Activity of the Nur1 carboxy-terminal domain depends on cell type and integrity of the activation function 2

Nurr1, a member of the nuclear hormone receptor superfamily, was recently d emonstrated to be of critical importance in the developing central nervous system, where it is required for the generation of midbrain dopamine cells. Nuclear receptors encompass a transcriptional activation function (activat ion function 2; AF2) within their carboxyl-terminal domains important for l igand-induced transcriptional activation. Since a Nurr1 ligand remains to b e identified, the role of the Nurr1 AF2 region in transcriptional activatio n is unclear. However, here we show that the Nurr1 AF2 contributes to const itutive activation independent of exogenously added ligands in human embryo kidney 293 cells and in neural cell lines. Extensive mutagenesis indicated a crucial role of the AF2 core region for transactivation but also identif ied unique features differing from previously characterized receptors, In a ddition, Nurr1 did not appear to interact with, and was not stimulated by, several previously identified coactivators such as the steroid receptor coa ctivator 1. In contrast, adenovirus protein E1A, stably expressed in 293 ce lls, was shown to contribute to AF2-dependent activation. Finally, while th e AF2 core of RXR is required for ligand-induced transcriptional activation by Nurr1-RXR heterodimers, the functional integrity of Nurr1 AF2 core is n ot critical. These results establish that the ligand binding domain of Nurr 1 has intrinsic capacity for transcriptional activation depending on cell t ype and mode of DNA binding. Furthermore, these results are consistent with the possibility that gene expression in the central nervous system can be modulated by an as yet unidentified ligand interacting with the ligand bind ing domain of Nurr1.