bThe molecular mechanisms of prion-induced cytotoxicity remain largely obscure. Currently, only a few cell culture models have exhibited the cytopathic changes associated with prion infection. In this study, we introduced a cell culture model based on differentiated neurosphere cultures isolated from the brains of neonatal prion protein (PrP)-null mice and transgenic mice expressing murine PrP (dNP0 and dNP20 cultures). Upon exposure to mouse Chandler prions, dNP20 cultures supported the de novo formation of abnormal PrP and the resulting infectivity, as assessed by bioassays. Furthermore, this culture was susceptible to various prion strains, including mouse-adapted scrapie, bovine spongiform encephalopathy, and Gerstmann-Sträussler-Scheinker syndrome prions. Importantly, a subset of the cells in the infected culture that was mainly composed of astrocyte lineage cells consistently displayed late-occurring, progressive signs of cytotoxicity as evidenced by morphological alterations, decreased cell viability, and increased lactate dehydrogenase release. These signs of cytotoxicity were not observed in infected dNP0 cultures, suggesting the requirement of endogenous PrP expression for prion-induced cytotoxicity. Degenerated cells positive for glial fibrillary acidic protein accumulated abnormal PrP and exhibited features of apoptotic death as assessed by active caspase-3 and terminal deoxynucleotidyltransferase nick-end staining. Furthermore, caspase inhibition provided partial protection from prion-mediated cell death. These results suggest that differentiated neurosphere cultures can provide an in vitro bioassay for mouse prions and permit the study of the molecular basis for prion-induced cytotoxicity at the cellular level. P rion diseases comprise a class of transmissible, fatal, neurodegenerative diseases, and they include Creutzfeldt-Jakob disease (CJD) and Gerstmann-Straussler-Scheinker syndrome (GSS) in humans, bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats, and chronic wasting disease in deer. The neuropathological hallmarks of prion diseases are neuronal loss, vacuolation, synaptic alterations, astrogliosis, microglial activation, and the progressive accumulation of a misfolded and protease-resistant isoform (PrPres) of host-encoded proteasesensitive prion protein (protease-sensitive PrP [PrPsen]). The conversion of PrPsen into PrPres and its accumulation are implicated in the pathogenesis of prion diseases (1, 2); however, the molecular basis of neurodegeneration in prion diseases is largely unclear.Several lines of evidence have revealed that animals can harbor high levels of infectivity before or without developing clinical signs (3, 4), indicating the decoupling of prion infectivity from toxicity. One possible explanation for the dissociation is that toxic species of PrPres distinct from the infectious particles are produced after reaching a plateau of infectivity (4). Investigations of the function of putative toxic forms of PrP and PrP in toxic signaling are crucial fo...