[Abstract unavailable

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Although the microglial activation is concomitant to the Alzheimer's disease, its precise role (neuroprotection vs neurodegeneration)has not yet been resolved. Here, we show the existence of an age-dependent phenotypic change of microglial activation in the hippocampus of PS1xAPP model, from an alternative activation state with A␤ phagocytic capabilities (at 6 months) to a classic cytotoxic phenotype (expressing TNF-␣ and related factors) at 18 months of age. This switch was coincident with high levels of soluble A␤ oligomers and a significant pyramidal neurodegeneration. In vitro assays, using astromicroglial cultures, demonstrated that oligomeric A␤42 and soluble extracts from 18-month-old PS1xAPP hippocampus produced a potent TNF-␣ induction whereas monomeric A␤42 and soluble extract from 6-or 18-month-old control and 6-month-old PS1xAPP hippocampi produced no stimulation. This stimulatory effect was avoided by immunodepletion using 6E10 or A11. In conclusion, our results show evidence of a switch in the activated microglia phenotype from alternative, at the beginning of A␤ pathology, to a classical at advanced stage of the disease in this model. This change was induced, at least in part, by the age-dependent accumulation of extracellular soluble A␤ oligomers. Finally, these cytotoxic activated microglial cells could participate in the neuronal lost observed in AD.

Dystrophic neurites associated with amyloid plaques precede neuronal death and manifest early in Alzheimer’s disease (AD). In this work we have characterized the plaque-associated neuritic pathology in the hippocampus of young (4- to 6-month-old) PS1M146L/APP751SL mice model, as the initial degenerative process underlying functional disturbance prior to neuronal loss. Neuritic plaques accounted for almost all fibrillar deposits and an axonal origin of the dystrophies was demonstrated. The early induction of autophagy pathology was evidenced by increased protein levels of the autophagosome marker LC3 that was localized in the axonal dystrophies, and by electron microscopic identification of numerous autophagic vesicles filling and causing the axonal swellings. Early neuritic cytoskeletal defects determined by the presence of phosphorylated tau (AT8-positive) and actin–cofilin rods along with decreased levels of kinesin-1 and dynein motor proteins could be responsible for this extensive vesicle accumulation within dystrophic neurites. Although microsomal Aβ oligomers were identified, the presence of A11-immunopositive Aβ plaques also suggested a direct role of plaque-associated Aβ oligomers in defective axonal transport and disease progression. Most importantly, presynaptic terminals morphologically disrupted by abnormal autophagic vesicle buildup were identified ultrastructurally and further supported by synaptosome isolation. Finally, these early abnormalities in axonal and presynaptic structures might represent the morphological substrate of hippocampal dysfunction preceding synaptic and neuronal loss and could significantly contribute to AD pathology in the preclinical stages.

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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 A␤ deposition (3 to 4 months) but late (17 to 18 months) neurodegeneration of pyramidal cells, paralleled to the accumulation of soluble A␤ oligomers. We hypothesized that PI3K/Akt/GSK-3␤ 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-3␤ was increased in the 6-month PS1؋APP hippocampus, whereas in aged PS1؋APP animals (18 months), GSK-3␤ phosphorylation levels displayed a marked decrease. Using N2a and primary neuronal cell cultures, we demonstrated that soluble amyloid precursor protein-␣ (sAPP␣), 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-3␤ activity, and in consequence, exerted a neuroprotective action. On the contrary, several oligomeric A␤ forms, present in the soluble fractions of aged PS1؋APP mice, inhibited the induced phosphorylation of Akt/GSK-3␤ and decreased the neuronal survival. Furthermore, synthetic A␤ oligomers blocked the effect mediated by different neurotrophins (NGF, BDNF, insulin, and IGF-1) and sAPP␣, displaying high selectivity for NGF. In conclusion, the age-dependent appearance of APP-derived soluble factors modulated the PI3K/Akt/GSK-3␤ signaling pathway through the major neurotrophin receptors. sAPP␣ stimulated and A␤ oligomers blocked the prosurvival signaling. Our data might provide insights into the selective vulnerability of specific neuronal groups in Alzheimer disease.The molecular mechanisms underlying the selective neurodegeneration in Alzheimer disease (AD) 5 remain unclear. Amyloid-␤ (A␤) peptides, proteolytically excised from the amyloid precursor protein (APP), are considered the primary pathological agents in AD. However, their precise modes of action, toxic conformational forms, and molecular targets are still controversial (for review see Ref. 1). Transgenic mouse models, overexpressing mutated forms of human APP, are widely used to study AD pathogenesis. These models develop extensive A␤ deposition in the disease-vulnerable brain regions (such as hippocampus); however, neurons are well protected until late ages. The factors and pathways maintaining neuronal integrity at young/middle ages in these AD models and those inducing neurodegeneration in the aged animals remains to be defined.NGF, BDNF, IGF-1, and insulin are trophic factors critical for neuronal survival and plasticity, underlying memory, and learning (2-4) and could be implicated in AD development.

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To determine the kainate receptor subunits that are found in native kainate receptors, we have applied a multiplex PCR of cDNAs reverse transcribed from mRNA harvested from single cultured hippocampal neurons after electrophysiological recording. We found that all the cells showing rapidly desensitizing currents in response to kainate express the GluR6 subunit mRNA, and that some of them also express the GluR5 subunit mRNA. No GluR7, KA-1, or KA-2 subunit mRNAs were detected. Analysis of the editing sites of the GluR6 mRNA demonstrated that the three editing sites present in these subunits are edited to a different extent. Predominant expression of the unedited variant (Q) was observed, but edited and unedited variants may coexist in the same cell. In addition, we show that the Q/R site from the GluR6 subunit controls functional properties of native kainate receptors.