Neuronal age drives β-sheet accumulation and, together with ApoE4, enhances synaptic Aβ localization in APPNL-F neurons

Amyloid-{beta} (A{beta}) accumulation and aggregation are defining features of Alzheimers disease (AD), but the earliest cellular events driving these processes remain poorly understood. Here, we investigate A{beta} dynamics in primary neurons derived from APPNL-F knock-in mice, which express human APP and A{beta} at endogenous (non-overexpressed) levels. Using correlative optical photothermal infrared (OPTIR) spectromicroscopy and immunofluorescence, we show that physiological APPNL-F expression is sufficient to drive intraneuronal A{beta} accumulation at synapses. Modelling neuronal aging through prolonged culture reveals a marked increase in synaptic A{beta} burden accompanied by {beta}-sheet-rich structural transitions Supplementation with astrocyte-derived apolipoprotein E (ApoE) isoforms demonstrates that ApoE4 markedly increases synaptic A{beta} accumulation while reducing {beta}-sheet content relative to untreated cultures, suggesting a potential shift toward less fibrillar species. Together, these findings establish APPNL-F neurons as a physiologically relevant system for dissecting early A{beta} pathology and show that both A{beta} quantity (accumulation) and quality (structural maturation) are regulated by aging and ApoE genotype. This work provides mechanistic insight into the earliest molecular events that may underlie synaptic vulnerability in AD.