Onset of accumulation of PrPres in murine ME7 scrapie in relation to pathological and PrP immunohistochemical changes

In a murine scrapie model, three different methods (immunohistochemistry, Western blotting and histoblotting) fur determining disease-specific PrP accumulation were compared. The incubation period of ME7 scrapie in the Fl cross of C57 BL and VM/Dk mice is about 230 days, Mice show hippocampal neuronal loss from 160-180 days post-inoculation (dpi), CAl neuron dendritic spine atrophy at 126 dpi, and axon terminal degeneration and synaptic loss from 84-98 dpi. Infectivity titres of at least 100 are present from LO dpi. PrP was detected immunohistochemically at 60 dpi in the hippocampus and in the thalamus. Thus, PrP accumulation in the hippocampus precedes even the earliest neurodegenerative changes. Low amounts of PrP immunolabelling were found between 60 dpi and 126 dpi, after which the intensity increased markedly. The histoblot method detected PrPres ill one of four mice at 100 dpi. Western blotting of whole brains fil st identified the PrPres at 80 dpi. Thus, in our hands. the most sensitive method for detecting disease-specific accumulations of PrP was immunohistochemical examination. However, immunohistochemical methods are unable to distinguish the normal and abnormal isoforms of PrP. It is therefore possible that the initial accumulation of PrP takes place as PrPsen and that the translation of PrPsen to PrPres does nor take place until the later stages of the disease process. The accumulation of disease-specific PrP lags behind the development of infectivity titres. The relative rates of increase of infectivity titre and PrP accumulation are different, suggesting that these parameters may be measures of different biological events. (C) 2001 Harcourt Publishers Ltd.