β-amyloid monomers are neuroprotective

The 42-aa-long β-amyloid protein - Aβ_1-42 - isthought to play a central role in the pathogenesis of Alzheimer's disease (AD) (Walsh and Selkoe, 2007). Data from AD brain (Shankar et al., 2008), transgenic APP (amyloid precursor protein)-overexpressing mice (Lesne et al., 2006), and neuronal cultures treated with synthetic Aβpeptides (Lambert et al., 1998) indicate that self-association of Aβ_1-42 monomers into soluble oligomers is required for neurotoxicity. The function of monomeric Aβ_1-42 is unknown. The evidence that Aβ_1-42 is present in the brain and CSF of normal individuals suggests that the peptide is physiologically active (Shoji, 2002). Here we show that synthetic Aβ_1-42 monomers support the survival of developing neurons under conditions of trophic deprivation and protect mature neurons against excitotoxic death, a process that contributes to the overall neurodegeneration associated with AD. The neuroprotective action of Aβ_1-42 monomers was mediated by the activation of the PI-3-K (phosphatidylinositol-3- kinase) pathway, and involved the stimulation of IGF-1 (insulin-like growth factor-1) receptors and/or other receptors of the insulin superfamily. Interestingly, monomers of Aβ_1-42 carrying the Arctic mutation (E22G) associated with familiar AD (Nilsberth et al., 2001) were not neuroprotective. We suggest that pathological aggregation of Aβ_1-42 may also cause neurodegeneration by depriving neurons of the protective activity of Aβ_1-42 monomers. This "loss-of-function" hypothesis of neuronal death should be taken into consideration when designing therapies aimed at reducing Aβ burden.