Subcellular distribution of the prion protein in sickness and in health

•The subcellular distribution of PrP affects its function and pathogenic conversion.•Endogenous proteolysis may regulate function and pathogenic conversion.•Cytosolic forms of PrP may have pleiotropic effects. The cellular prion protein (PrPC) is an ubiquitously expressed glycoprotein that is most abundant in the central nervous system. It is thought to play a role in many cellular processes, including neuroprotection, but may also contribute to Alzheimer's disease and some cancers. However, it is best known for its central role in the prion diseases, such as Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE), and scrapie. These protein misfolding diseases can be sporadic, acquired, or genetic and are caused by refolding of endogenous PrPC into a beta sheet-rich, pathogenic form, PrPSc. Once prions are present in the central nervous system, they increase and spread during a long incubation period that is followed by a relatively short clinical disease phase, ending in death. PrP molecules can be broadly categorized as either ‘good’ (cellular) PrPC or ‘bad’ (scrapie prion-type) PrPSc, but both populations are heterogeneous and different forms of PrPC may influence various cellular activities. Both PrPC and PrPSc are localized predominantly at the cell surface, with the C-terminus attached to the plasma membrane via a glycosyl-phosphatidylinositol (GPI) anchor and both can exist in cleaved forms. PrPC also has cytosolic and transmembrane forms, and PrPSc is known to exist in a variety of conformations and aggregation states. Here, we discuss the roles of different PrP isoforms in sickness and in health, and show the subcellular distributions of several forms of PrP that are particularly relevant for PrPC to PrPSc conversion and prion-induced pathology in the hippocampus.