Overexpression of midbrain-specific transcription factor Nurr1 modifies susceptibility of mouse neural stem cells to neurotoxins.

Nurr1 is a member of the nuclear receptor superfamily of transcription factors that is highly expressed in midbrain dopaminergic (DA) neurons, the cells primarily lost in human Parkinson's disease (PD), and in Nurr1-null mice selective agenesis of midbrain DA neurons is found. To investigate possible correlation between the expression of Nurr1 gene and neurotoxin-induced cell death of DA neurons, a neural stem cell line (NSC, A3) and Nurr1-overexpressing NSC (A3.Nurr1) were exposed to DA neurotoxins 6-hydroxydopamine (6-OHDA) and methyl phenylpyridinium (MPP(+)). Although both neurotoxins were shown to induce cell death in A3 and A3.Nurr1 cells, patterns of cell deaths were different. A3.Nurr1 cells showed increased vulnerability to 6-OHDA cytotoxicity, but increased resistance to MPP(+)-induced cell death when compared to A3 cells. To investigate the differential vulnerability to neurotoxins by Nurr1 protein correlates with biochemical features that discriminate between apoptosis and necrosis, we carried out a nucleosomal DNA fragmentation assay and electron microscopy. While 6-OHDA treatment induced shrinkage of cytoplasmic membrane, condensation of nuclei and generation of apoptotic bodies in both cell lines, cells treated with MPP(+) showed mitochondrial swelling, indicating that 6-OHDA- but not MPP(+)-mediated cell death was apoptotic. These results suggest that DA neuronal cell death in response to 6-OHDA and MPP(+) may progress through separate signaling pathways differentially regulated by the Nurr1 protein. Our observations indicated that Nurr1 may play a role in the manifestation of DA neurotoxicity and that variations in Nurr1 expression might be a susceptibility factor for DA neurodegeneration in PD.