No abstract

Proteolytic processing of the amyloid precursor protein (APP) by ␤-secretase, ␤-site APP cleaving enzyme (BACE1), is the initial step in the production of the amyloid ␤ (A␤) peptide, which is involved in the pathogenesis of Alzheimer's disease. The normal cellular function of the prion protein (PrP C ), the causative agent of the transmissible spongiform encephalopathies such as CreutzfeldtJakob disease in humans, remains enigmatic. Because both APP and PrP C are subject to proteolytic processing by the same zinc metalloproteases, we tested the involvement of PrP C in the proteolytic processing of APP. Cellular overexpression of PrP C inhibited the ␤-secretase cleavage of APP and reduced A␤ formation. Conversely, depletion of PrP C in mouse N2a cells by siRNA led to an increase in A␤ peptides secreted into the medium. In the brains of PrP knockout mice and in the brains from two strains of scrapieinfected mice, A␤ levels were significantly increased. Two mutants of PrP, PG14 and A116V, that are associated with familial human prion diseases failed to inhibit the ␤-secretase cleavage of APP. Using constructs of PrP, we show that this regulatory effect of PrP C on the ␤-secretase cleavage of APP required the localization of PrP C to cholesterol-rich lipid rafts and was mediated by the N-terminal polybasic region of PrP C via interaction with glycosaminoglycans. In conclusion, this is a mechanism by which the cellular production of the neurotoxic A␤ is regulated by PrP C and may have implications for both Alzheimer's and prion diseases.lipid raft ͉ proteolysis ͉ scrapie ͉ glycosaminoglycan A lzheimer's disease (AD) is characterized by the presence of extracellular senile plaques and intracellular neurofibrillary tangles within the afflicted brain. The major constituents of senile plaques are the amyloid ␤ (A␤) peptides, which are derived from the proteolytic processing of the amyloid precursor protein (APP) (1). In the amyloidogenic pathway, ␤-secretase cleavage of APP yields a soluble N-terminal fragment sAPP␤, along with a short membrane-bound C-terminal fragment that is subsequently cleaved by ␥-secretase to release the A␤ peptides. In the alternative, nonamyloidogenic pathway, ␣-secretase cleaves APP within the A␤ sequence, thus precluding the formation of A␤, and releases a soluble N-terminal fragment sAPP␣. The transmembrane aspartyl protease, ␤-site APP cleaving enzyme (BACE1), has been identified as ␤-secretase (2), members of the ADAM (a disintegrin and metalloprotease) family, particularly ADAM10 and ADAM17, are responsible for ␣-secretase cleavage (3), while a complex of at least four proteins, the presenilins, nicastrin, Aph-1, and Pen-2, constitutes the ␥-secretase (2).The prion protein (PrP) is the causative agent of the transmissible spongiform encephalopathies (TSEs) that include CreutzfeldtJakob disease (CJD), Gerstmann-Scheinker-Straussler (GSS) disease, kuru and fatal familial insomnia in humans, bovine spongiform encephalopathy in cattle, and scrapie in sheep (4). In these diseases, the normal ce...

A mutation equivalent to P102L in the human PrP gene, associated with Gerstmann-Straussler syndrome (GSS), has been introduced into the murine PrP gene by gene targeting. Mice homozygous for this mutation (101LL) showed no spontaneous transmissible spongiform encephalopathy (TSE) disease, but had incubation times dramatically different from wild-type mice following inoculation with different TSE sources. Inoculation with GSS produced disease in 101LL mice in 288 days. Disease was transmitted from these mice to both wild-type (226 days) and 101LL mice (148 days). In contrast, 101LL mice infected with ME7 had prolonged incubation times (338 days) compared with wild-type mice (161 days). The 101L mutation does not, therefore, produce any spontaneous genetic disease in mice but significantly alters the incubation time of TSE infection. Additionally, a rapid TSE transmission was demonstrated despite extremely low levels of disease-associated PrP.

The expression of the prion protein (PrP) is essential for transmissible spongiform encephalopathy (TSE) or prion diseases to occur, but the underlying mechanism of infection remains unresolved. To address the hypothesis that glycosylation of host PrP is a major factor influencing TSE infection, we have inoculated gene-targeted transgenic mice that have restricted N-linked glycosylation of PrP with three TSE strains. We have uniquely demonstrated that mice expressing only unglycosylated PrP can sustain a TSE infection, despite altered cellular location of the host PrP. Moreover we have shown that brain material from mice infected with TSE that have only unglycosylated PrPSc is capable of transmitting infection to wild-type mice, demonstrating that glycosylation of PrP is not essential for establishing infection within a host or for transmitting TSE infectivity to a new host. We have further dissected the requirement of each glycosylation site and have shown that different TSE strains have dramatically different requirements for each of the glycosylation sites of host PrP, and moreover, we have shown that the host PrP has a major role in determining the glycosylation state of de novo generated PrPSc.