Role of PrPC glycosylation in health and disease
Glycosylation is the most abundant post-translational modification of proteins and has the ability to change the physical properties of the protein and its cell biology. The cellular prion protein (PrPC) is a membrane bound host glycoprotein present in a number of isoforms in vivo due to variable occupancy of the two N-linked glycosylation sites. The function of PrPC is still unclear but it is essential for disease in transmissible spongiform encephalopathies (TSEs). The significance of the PrP glycoforms in the physiological function is unknown. Gene targeted mice have been created with point mutations that selectively abolish the glycosylation sites of PrPC. These GlycoD mutants have been used to study the effect of glycosylation at the different sites on the cell biology of PrPC. This study showed that both glycosylation sites played a role in the cell biology of PrPC. Removal of a single or both glycosylation sites significantly reduced total PrPC protein. The relative amount of the truncated protein produced through proteolytic cleaving was slightly reduced in the GlycoD mutants; however the proportion of truncated to full length PrP was increased, further reducing full length protein. The maintenance of truncated protein levels indicates a potential importance of the fragment in PrPC function. Wild type PrP is predominantly diglycosylated and localised to the cell surface. In this study it was shown that all GlycoD mutants had reduced amounts of cell surface PrPC and an increased proportion of PrPC associated with the secretory pathway. Removal of either the first or the second glycosylation site produced changes in cell biology that were almost indistinguishable from each other whilst disruption of both glycosylation sites produces a more extreme phenotype than removal of a single site. Previous studies have shown an altered susceptibility for TSE disease GlycoD mice. An in vitro conversion assay was used to investigate the ability of the glycoforms to initiate conversion from PrPC to the disease associated PrPSc. Mice which had only the second site abolished were much more efficient at seeding conversion than all other glycoforms. This may reflect the difference in susceptibility between the two monoglycosylated PrPs but does not explain the increased resistance compared to wild type mice. All other GlycoD mutants had similar seeding times to wild type mice despite having increased TSE resistance. The differences observed in the cell biology of PrPC of the GlycoD mutants may go some way to explaining the differences in TSE susceptibilities seen with these mice.