Prion Disease Mechanisms

Research is increasingly supporting Prusiner's hypothesis that the BSE and other prion diseases appear to be caused by a "self-replicating protein" or "prion," an aberrant conformation of normal protein (PrP). Prusiner's hypothesis is that the prion protein molecule itself is infective. However, there are several other theories about prion disease. One suggests that the infective form of the prion protein enables a gene to produce additional infective\damaging molecules. Others maintain that there is an infective agent distinct from the prion protein.

The prion protein, PrP, is a normal cell protein present on nerve cell membranes. The gene for PrP is present in most mammals. Althoug its normal function is unclear; knockout mice without the protein appear to be fine at a gross level despite detectable neurological abnormalities.

PrP^sc is a post-transnationally modified PrP. This means PrP^sc is changed after synthesis from the gene. The sequence of amino acids in PrP and PrP^sc is identical. PrP changes conformation, from predominantly helix to predominantly pleated sheet, creating PrP^sc. This protein is extremely resistant to heat and to normal sterilization processes. It does not evoke a detectable immune response or inflammatory reaction in host animals.

The protein is found (predominantly) on the neuronal cell surface attached by a glycosyl phosphatidyl inositol (GPI) anchor. Nguyen et al. (1995) have studied the three dimensional structure of scrapie prion using X-ray diffraction. The infective form has a distinctly different shape from the non-infective form which seems to render it resistant to protease such that it is cleaved incompletely resulting in peptides as well as amino acids.

The altered PrP^sc protein is catalytic or "self-propagating". One PrP^sc causes other PrP molecules to change into PrP^sc. The PrP^sc protein is very resistant to endogenous protease, that would normally destroy the protein. Because the PrP^sc can't be broken down, it builds up, aggregates, then precipitates forming plaques and causing spongiform damage.

Brandner et al (1996) used scrapie PrP^sc in mice to study the role of PrP^sc in the course of disease. Their results suggest that depletion of normal PrP, rather than deposition of infective PrP^sc and their fragments are responsible for neuronal death, spongiform changes and gliosis.

What causes the change in conformation of normal PrP to disease causing PrP^sc? While the spontaneous conversion of PrP to PrP^sc is a thermodynamically unfavorable event because of the activation energy (low incidence of sporadic CJD), a somatic mutation in the PrP gene can make the spontaneous conversion much more thermodynamically favorable (Heaphy, 1996). There are at least 20 known mutations in the PrP gene sequence resulting in "spontaneous" PrP^sc formation. Each mutation results in a slightly different conformation, and slightly different pathology. This results in different prion "strains". Regardless of the mutation the "spontaneous" PrP^sc can also change non-mutated PrP into PrP^sc .

Infectious PrP (ingested or inoculated) appears to replicate in the lymphoid systems (Peyers patches, follicular dendritic cells) at least in sheep. At some point it gains access to the nervous system; once in the brain the infectious PrP accumulates and is deposited in amyloid plaques. The location in the brain where PrP replication and accumulation occurs appears to be "strain specific" (Heaphy, 1996).

The host-cell involved in prion uptake and transport to the nervous system in prion diseases is not known. Perini et al (1996) reported that platelets express normal PrP (bound to the cell surface by GPI-anchor); they shed this protein following agonist stimulation of phosphatidyl inositol phospholipase C which cleaves the GPI anchor. Because platelets express PrP on the surface, it is possible that platelets PrP could be converted to PrP^sc then transported throughout the body.

Animals infected intracerebrally with PrP^sc, in turn, exhibit infectivity in the spleen (the major site of platelet catabolism) 2 week after injection (Perini et al., 1996).