Prions are infectious proteins consisting mainly of PrP Sc , a β sheet-rich conformer of the normal host protein PrP C , and occur in different strains. Strain identity is thought to be encoded by PrP Sc conformation. We found that biologically cloned prion populations gradually became heterogeneous by accumulating "mutants", and selective pressures resulted in the emergence of different mutants as major constituents of the evolving population. Thus, when transferred from brain to cultured cells, "cell-adapted" prions outcompeted their "brain-adapted" counterparts, and the opposite occurred when prions were returned from cells to brain. Similarly, the inhibitor swainsonine selected for a resistant substrain, while in its absence the susceptible substrain outgrew its resistant counterpart. Prions, albeit devoid of a nucleic acid genome, are thus subject to mutation and selective amplification.

Prions are thought to consist mainly or entirely of misfolded PrP, a constitutively expressed host protein. Prions associated with the same PrP sequence may occur in the form of different strains; the strain phenotype is believed to be encoded by the conformation of the PrP. Some cell lines can be persistently infected by prions and, interestingly, show preference for certain strains. We report that a cloned murine neuroblastoma cell population, N2a-PK1, is highly heterogeneous in regard to its susceptibility to RML and 22L prions. Remarkably, sibling subclones may show very different relative susceptibilities to the two strains, indicating that the responses can vary independently. We have assembled four cell lines, N2a-PK1, N2a-R33, LD9 and CAD5, which show widely different responses to prion strains RML, 22L, 301C, and Me7, into a panel that allows their discrimination in vitro within 2 weeks, using the standard scrapie cell assay (SSCA).standard scrapie cell assay ͉ infectivity ͉ response index ͉ PrP

Sc, an aggregated conformer of the host protein PrP C . Prions come in different strains, all based on the same PrP C sequence, but differing in their conformations. The efficiency of prion transmission between species is usually low, but increases after serial transmission in the new host, suggesting a process involving mutation and selection. Even within the same species, the transfer of prions between cell types entails a selection of favoured 'substrains', and propagation of prions in the presence of an inhibitory drug can result in the appearance of drug-resistant prion populations. We propose that prion populations are comprised of a variety of conformers, constituting 'quasi-species', from which the one replicating most efficiently in a particular environment is selected.