β-amyloid oligomers and prion protein: Fatal attraction?

The relationship between Alzheimer disease (AD) and prion-related encephalopathies (TSE) has been proposed by different points of view. Recently, the scientific attention has been attracted by the results proposing the possibility that PrP ^c, the protein whose pathologic form is responsible of TSE, can mediated the toxic effect of β amyloid (Aβ) oligomers. The oligomers are considered the culprit of the neurodegenerative process associated to AD, although the pathogenic mechanism activated by these small aggregates remain to be elucidated. In the initial study based on the binding screening, PrP ^c was identified as ligand/receptor of Aβ oligomers, while long-term potentiation (LTP) analysis in vitro and behavioral studies in vivo demonstrated that the absence of PrP ^c abolished the damage induced by Aβ oligomers. The high affinity binding Aβ oligomers-PrP ^c has been confirmed, whereas a functional role of this association has been excluded by three different studies. We approached this issue by the direct application of Aβ oligomers in the brain followed by the behavioral examination of memory deficits. Our data using PrP knockout mice suggest that Aβ 1-42 oligomers are responsible for cognitive impairment in AD but PrP ^c is not required for their effect. Similarly, in two other studies the LTP alterations induced by Aβ 1-42 oligomers was not influenced by the absence of PrP. Possible explanations of these contradictory results are discussed.