Cellular and sub-cellular pathology of animal prion diseases: relationship between morphological changes, accumulation of abnormal prion protein and clinical disease

The transmissible spongiform encephalopathies (TSEs) or prion diseases of animals are characterised by CNS spongiform change, gliosis and the accumulation of disease-associated forms of prion protein (PrPd). Particularly in ruminant prion diseases, a wide range of morphological types of PrPd depositions are found in association with neurons and glia. When light microscopic patterns of PrPd accumulations are correlated with sub-cellular structure, intracellular PrPd co-localises with lysosomes while non-intracellular PrPd accumulation co-localises with cell membranes and the extracellular space. Intracellular lysosomal PrPd is N-terminally truncated, but the site at which the PrPd molecule is cleaved depends on strain and cell type. Different PrPd cleavage sites are found for different cells infected with the same agent indicating that not all PrPd conformers code for different prion strains. Non-intracellular PrPd is full-length and is mainly found on plasma-lemmas of neuronal perikarya and dendrites and glia where it may be associated with scrapie-specific membrane pathology. These membrane changes appear to involve a redirection of the predominant axonal trafficking of normal cellular PrP and an altered endocytosis of PrPd. PrPd is poorly excised from membranes, probably due to increased stabilisation on the membrane of PrPd complexed with other membrane ligands. PrPd on plasma-lemmas may also be transferred to other cells or released to the extracellular space. It is widely assumed that PrPd accumulations cause neurodegenerative changes that lead to clinical disease. However, when different animal prion diseases are considered, neurological deficits do not correlate well with any morphological type of PrPd accumulation or perturbation of PrPd trafficking. Non-PrPd-associated neurodegenerative changes in TSEs include vacuolation, tubulovesicular bodies and terminal axonal degeneration. The last of these correlates well with early neurological disease in mice, but such changes are absent from large animal prion disease. Thus, the proximate cause of clinical disease in animal prion disease is uncertain, but may not involve PrPd.