We have established stable transfectants expressing -synuclein in TSM1 neurons. We show that in basal and staurosporine-induced conditions the number of terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling (TUNEL)-positive -synuclein-expressing neurons was drastically lower than in mock-transfected TSM1 cells. This was accompanied by a lower DNA fragmentation as evidenced by the reduction of propidium iodide incorporation measured by fluorescence-activated cell sorter analysis. -Synuclein strongly reduces staurosporine-induced caspase 3 activity and immunoreactivity. We establish that -synuclein triggers a drastic reduction of p53 expression and transcriptional activity. This was accompanied by increased Mdm2 immunoreactivity while p38 expression appeared enhanced, indicating that -synuclein-induced p53 downregulation likely occurs at a post-transcriptional level. We showed previously that ␣-synuclein displays an antiapoptotic function that was abolished by the dopaminergic derived toxin 6-hydroxydopamine (6OHDA). Interestingly, -synuclein retains its ability to protect TSM1 neurons even after 6OHDA treatment. Furthermore, -synuclein restores the antiapoptotic function of ␣-synuclein in 6OHDA-treated neurons. Altogether, our data document for the first time that -synuclein protects neurons from staurosporine and 6OHDA-stimulated caspase activation in a p53-dependent manner. Our observation that -synuclein contributes to restoration of the ␣-synuclein antiapoptotic function abolished by 6OHDA may have direct implications for Parkinson's disease pathology. In this context, the cross-talk between these two parent proteins is discussed.Parkinson's disease (PD) 1 is one of the most common and devastating diseases in the elderly (1, 2). This pathology is characterized by intracellular aggregates, called Lewy bodies (LB) (3, 4) that are thought to be responsible for final dementia occurring not only in PD but also in Lewy body diseases. The main component of LB was identified as ␣-synuclein (5), a 140-amino acid-long synaptic protein (6) that accumulates within these neuropathological hallmarks (7-9). The central role of ␣-synuclein in PD pathology has been emphasized by the observation that familial forms of PD were due to two mutations borne by ␣-synuclein (10, 11). Interestingly, these mutations trigger alterations of the biophysical properties of ␣-synuclein, leading to an exacerbation of misfolding and aggregation (12). Therefore, as with many other neurodegenerative diseases such as Alzheimer's and prion diseases, among others (13-15), PD can be documented as a disease associated with protein misfolding. The fact that such aggregates of proteins were recently shown to exhibit an intrinsic toxic potential (16) could lead to a reunifying theory linking misfolding and neurodegeneration.Interestingly, we have shown that ␣-synuclein displays an antiapoptotic phenotype that was abolished by PD-related mutations (17). This antiapoptotic function was also prevented when neuronal cells were expo...