The Prnd-encoded prion protein (PrP)-like protein, Doppel (Dpl), is a homologue of Prnp-encoded PrP, and is N-glycosylated protein with glycosylphosphatidylinositol anchor like PrP. Recently, ectopic expressions of Prnp/Prnd chimeric mRNAs have been identified as the cause of late-onset ataxia observed in several lines of Prnp-knockout mice such as ZrchII, Ngsk, Rcm0, and Rikn mice. However, it remains unclear whether the toxic effect of Dpl expression is a cell-autonomous mechanism but rather reflect a systemic process of heterogeneous cell population in the brain. In this study, the cell-autonomous role of Dpl was estimated by investigating PrP-deficient cells (HpL3-4)-the SV40 large T-antigen immortalized and Rikn Prnp(-/-) mice-derived neuronal cell line expressing Prnp/Prnd chimeric mRNAs. The reverse transcription polymerase chain reaction revealed that serum deprivation did not increase Prnp/Prnd chimeric mRNAs, which in fact was translated into a small amount of Dpl in HpL3-4 cells, whereas serum deprivation induced apoptotic cell death of HpL34 cells. Dpl overexpression enhanced apoptotic cell death, whereas the toxic effect of Dpl on apoptotic cell death was neutralized by PrP expression. These results indicate that Dpl elicited dose-dependently toxic effects on PrP-deficient cells without affecting on PrP-expressing cells, suggesting that the PrP-Dpl interaction can regulate cell death in a cell-autonomous manner. (c) 2005 Elsevier Inc. All rights reserved.