The hallmark of
transmissible spongiform encephalopathies (TSEs or
prion diseases) is the accumulation of an abnormally folded, partially
protease-resistant form (
PrP-res) of the normal
protease-sensitive
prion protein (
PrP-sen).
PrP-sen is attached to the cell membrane by a
glycosylphosphatidylinositol (GPI) anchor. In vitro, the anchor and the local membrane environment are important for the conversion of
PrP-sen to
PrP-res. In vivo, however, the anchor is not necessary because transgenic mice expressing anchorless
PrP-sen accumulate
PrP-res and replicate infectivity. To clarify the role of the GPI anchor in TSE
infection, cells expressing GPI-anchored
PrP-sen, anchorless
PrP-sen, or both forms of
PrP-sen were exposed to the mouse
scrapie strain 22L. Cells expressing anchored
PrP-sen produced
PrP-res after exposure to 22L. Surprisingly, while cells expressing anchorless
PrP-sen made anchorless
PrP-res in the first 96 h postinfection, no
PrP-res was detected at later passes. In contrast, when cells expressing both forms of
PrP-sen were exposed to 22L, both anchored and anchorless
PrP-res were detected over multiple passes. Consistent with the in vitro data,
scrapie-infected cells expressing anchored
PrP-sen transmitted disease to mice whereas cells expressing anchorless
PrP-sen alone did not. These results demonstrate that the GPI anchor on
PrP-sen is important for the
persistent infection of cells in vitro. Our data suggest that cells expressing anchorless
PrP-sen are not directly infected with
scrapie. Thus,
PrP-res formation in transgenic mice expressing anchorless
PrP-sen may be occurring extracellularly.