We assessed the contribution of the cellular prion protein (PrPc) in the control of neuronal apoptosis by examining cell death in both human cells and murine primary cultured neurons. We first confirmed our previous finding that staurosporine-induced caspase activation is increased by PrPc overexpression in HEK293 cells. We show here that this phenotype is fully dependent on p53 and that the control of p53 activity by PrPc occurs at both transcriptional and post-transcriptional levels in human cells. Of most interest, we demonstrate that neuronal endogenous PrPc also controls a p53-dependent pro-apoptotic phenotype. Thus, DNA fragmentation and TUNEL ( terminal deoxynucleotidyltransferase- mediated dUTP nick end-labeling)-positive cells were lower in primary cultured neurons derived from Zrch-1 mice embryos in which PrPc has been abrogated than in wildtype neurons. PrPc knock-out neurons also displayed drastically diminished caspase-3-like activity and immunoreactivity together with reduced p53 expression and transcriptional activity, a phenotype complemented in part by PrPc transfection. Interestingly, p53 expression was also reduced in the brain of adult Prnp(-/-) mice. Neuronal PrPc likely controls p53 at a post-transcriptional level because the deletion of cellular prion protein is accompanied by a higher Mdm2-like immunoreactivity and reduced phosphorylated p38 MAPK expression. We therefore propose that the physiological function of endogenous cellular prion could be to regulate p53-dependent caspase-3-mediated neuronal cell death. This phenotype likely occurs through upregulation of p53 promoter transactivation as well as downstream by controlling p53 stability via Mdm2 expression.