Inability to form new memories is an early clinical sign of Alzheimer’s disease (AD). There is ample evidence that the amyloid-β (Aβ) peptide plays a key role in the pathogenesis of this disorder. Soluble, bio-derived oligomers of Aβ are proposed as the key mediators of synaptic and cognitive dysfunction, but more tractable models of Aβ−mediated cognitive impairment are needed. Here we report that, in mice, acute intracerebroventricular injections of synthetic Aβ _1–42 oligomers impaired consolidation of the long-term recognition memory, whereas mature Aβ _1–42 fibrils and freshly dissolved peptide did not. The deficit induced by oligomers was reversible and was prevented by an anti-Aβ antibody. It has been suggested that the cellular prion protein (PrP ^C ) mediates the impairment of synaptic plasticity induced by Aβ. We confirmed that Aβ _1–42 oligomers interact with PrP ^C , with nanomolar affinity. However, PrP-expressing and PrP knock-out mice were equally susceptible to this impairment. These data suggest that Aβ _1–42 oligomers are responsible for cognitive impairment in AD and that PrP ^C is not required.

To investigate whether and how amyloid-β protein (Aβ) is involved in the neurodegenerative changes characteristic of Alzheimer's disease (AD), primary hippocampal neurones from foetal rat brain were exposed acutely and chronically to micromolar concentrations of a synthetic peptide homologous to residues 25–35 of Aβ (β 25–35). A single application of this peptide (25–100 μM) was ineffective but when the neuronal culture were exposed to β 25–35 (25–100 μM) repeatedly every two days for ten days, cell survival was dramatically reduced. The structural changes and the DNA fragmentation of cells chronically exposed to the peptide suggested that neuronal death occurred by apoptosis. Furthermore, β 25–35 showed the intrinsic ability to polymerize into amyloid-like fibrils in vitro. These results confirm the potential pathogenic role of Aβ in AD, and indicate that amyloid fibrils may induce neuronal death through a specific programmed process.

To identify sequence domains important for the neurotoxic and neuroprotective activities of the prion protein (PrP), we have engineered transgenic mice that express a form of murine PrP deleted for a conserved block of 21 amino acids (residues 105–125) in the unstructured, N‐terminal tail of the protein. These mice spontaneously developed a severe neurodegenerative illness that was lethal within 1 week of birth in the absence of endogenous PrP. This phenotype was reversed in a dose‐dependent fashion by coexpression of wild‐type PrP, with five‐fold overexpression delaying death beyond 1 year. The phenotype of Tg(PrPΔ105–125) mice is reminiscent of, but much more severe than, those described in mice that express PrP harboring larger deletions of the N‐terminus, and in mice that ectopically express Doppel, a PrP paralog, in the CNS. The dramatically increased toxicity of PrPΔ105–125 is most consistent with a model in which this protein has greatly enhanced affinity for a hypothetical receptor that serves to transduce the toxic signal. We speculate that altered binding interactions involving the 105–125 region of PrP may also play a role in generating neurotoxic signals during prion infection.

The cellular mechanisms by which prions cause neurological dysfunction are poorly understood. To address this issue, we have been using cultured cells to analyze the localization, biosynthesis, and metabolism of PrP molecules carrying mutations associated with familial prion diseases. We report here that mutant PrP molecules are delayed in their maturation to an endoglycosidase H-resistant form after biosynthetic labeling, suggesting that they are impaired in their exit from the endoplasmic reticulum (ER). However, we find that proteasome inhibitors have no effect on the maturation or turnover of either mutant or wild-type PrP molecules. Thus, in contrast to recent studies from other laboratories, our work indicates that PrP is not subject to retrotranslocation from the ER into the cytoplasm prior to degradation by the proteasome. We find that in transfected cells, but not in cultured neurons, proteasome inhibitors cause accumulation of an unglycosylated, signal peptide-bearing form of PrP on the cytoplasmic face of the ER membrane. Thus, under conditions of elevated expression, a small fraction of PrP chains is not translocated into the ER lumen during synthesis, and is rapidly degraded in the cytoplasm by the proteasome. Finally, we report a previously unappreciated artifact caused by treatment of cells with proteasome inhibitors: an increase in PrP mRNA level and synthetic rate when the protein is expressed from a vector containing a viral promoter. We suggest that this phenomenon may explain some of the dramatic effects of proteasome inhibitors observed in other studies. Our results clarify the role of the proteasome in the cell biology of PrP, and suggest reasonable hypotheses for the molecular pathology of inherited prion diseases.