Age-dependent Accumulation of Soluble Amyloid β (Aβ) Oligomers Reverses the Neuroprotective Effect of Soluble Amyloid Precursor Protein-α (sAPPα) by Modulating Phosphatidylinositol 3-Kinase (PI3K)/Akt-GSK-3β Pathway in Alzheimer Mouse Model

Marisa Vizuete,Cristina Caballero,Raquel Sanchez-Varo,Antonia Gutierrez,Javier Vitorica,Manuel Torres,Sebastian Jimenez,Elisabeth Sanchez-Mejias,Diego Ruano,Laura Trujillo-Estrada,Irene Carmona-Cuenca

doi:10.1074/jbc.m110.209718

Abstract

Neurotrophins, activating the PI3K/Akt signaling pathway, control neuronal survival and plasticity. Alterations in NGF, BDNF, IGF-1, or insulin signaling are implicated in the pathogenesis of Alzheimer disease. We have previously characterized a bigenic PS1×APP transgenic mouse displaying early hippocampal Aβ deposition (3 to 4 months) but late (17 to 18 months) neurodegeneration of pyramidal cells, paralleled to the accumulation of soluble Aβ oligomers. We hypothesized that PI3K/Akt/GSK-3β signaling pathway could be involved in this apparent age-dependent neuroprotective/neurodegenerative status. In fact, our data demonstrated that, as compared with age-matched nontransgenic controls, the Ser-9 phosphorylation of GSK-3β was increased in the 6-month PS1×APP hippocampus, whereas in aged PS1×APP animals (18 months), GSK-3β phosphorylation levels displayed a marked decrease. Using N2a and primary neuronal cell cultures, we demonstrated that soluble amyloid precursor protein-α (sAPPα), the predominant APP-derived fragment in young PS1×APP mice, acting through IGF-1 and/or insulin receptors, activated the PI3K/Akt pathway, phosphorylated the GSK-3β activity, and in consequence, exerted a neuroprotective action. On the contrary, several oligomeric Aβ forms, present in the soluble fractions of aged PS1×APP mice, inhibited the induced phosphorylation of Akt/GSK-3β and decreased the neuronal survival. Furthermore, synthetic Aβ oligomers blocked the effect mediated by different neurotrophins (NGF, BDNF, insulin, and IGF-1) and sAPPα, displaying high selectivity for NGF. In conclusion, the age-dependent appearance of APP-derived soluble factors modulated the PI3K/Akt/GSK-3β signaling pathway through the major neurotrophin receptors. sAPPα stimulated and Aβ oligomers blocked the prosurvival signaling. Our data might provide insights into the selective vulnerability of specific neuronal groups in Alzheimer disease.

Highlights

Neurotrophins (NGF, BDNF, insulin, and IGF-1) and sAPP␣, displaying high selectivity for NGF
Because GSK-3␤ activity was inhibited by phosphorylation on Ser-9, we have first quantified, by Western blots, the Ser-9 phospho-GSK-3␤ levels in our PS1ϫAPP tg model
The most relevant discrepancies between Alzheimer disease (AD) and the transgenic models were the absence of Tau phosphorylation and late, if any, neuronal degeneration, even in the presence of large A␤ accumulation since early ages

Summary

Introduction

Neurotrophins (NGF, BDNF, insulin, and IGF-1) and sAPP␣, displaying high selectivity for NGF. We have first determined in vitro using N2a cell cultures the effect of soluble brain extracts, derived from 6- and 18-monthold WT, PS1 (data not shown), and PS1ϫAPP mice on the Ser-9 phosphorylation of GSK-3␤. The activation of the Akt and phosphorylation of GSK-3␤ were specific for the factors present in PS1ϫAPP mice because the addition of a similar amount of soluble proteins from WT or PS1 transgenic mice (data not shown) was devoid of any effect.

Results

Conclusion