Aging alters the vulnerability pattern to amyloid-beta oligomers in wild-type mice: a behavioral and neurobiological study

BACKGROUND: Aging is the primary risk factor for sporadic Alzheimer's disease (AD). While amyloid-beta oligomers (AβOs) accumulation is a key neuropathological process in AD, their specific effects in aged brains and how aging modulates brain response to AβOs remains poorly understood. We investigated how aging contributes to AβO-induced neurotoxicity and cognitive deficits in mice. METHODS: After biochemical and in vitro characterizations on primary cultures of cortical neurons, AβOs or their vehicle were intracerebrally injected into both 3- and 18-month-old wild-type mice. A broad spectrum of assays including synaptic markers, neuroinflammation, apoptosis and cognitive functions was used to establish a preliminary characterization of the interplay between age and AβOs. In vivo data were analyzed using a multifactorial design (Treatment × Age), with two-way ANOVA or other appropriate statistical models. RESULTS: Old mice had significantly reduced synaptic proteins SNAP-25 and PSD-95, elevated neuroinflammatory markers, and increased neuronal apoptosis in hippocampus and cortex, despite showing cognitive performances similar to young mice. All brain biomarkers were worsened after AβO injection in both young and old mice. Age and AβO effects either accumulated or interacted to promote neuroinflammation and apoptosis, depending on brain areas, whereas their effects on synaptic proteins were strictly additive. Moreover, AβO injection induced only mild spatial memory deficits in young mice, in contrast with those observed in old mice in both episodic and spatial memory tests. DISCUSSION: Whereas the young brain showed resilience to maintain memory performances after AβO injection, the coping capacities of the aging brain were exceeded by AβO effects. At the neurobiological level, age and AβO effects were mainly additive, but also acted synergistically in a brain region-dependent vulnerability pattern. This study highlights the value of incorporating aging into preclinical models to improve their translational validity and enhance their relevance for drug testing targeting early stages of sporadic AD.