Abstract
BACKGROUND: Alzheimer’s disease (AD) is a progressive neurodegenerative disorder thought to result from complex interactions between genetic and environmental risk factors. The APOE-ε4 allele is the strongest genetic contributor to late-onset AD, while a Western diet - high in saturated fats and refined sugars - is a major lifestyle-related risk factor associated with AD progression. However, how these two factors interact at an early stage of the disease remains unclear. In this study, we examined their combined impact on hippocampal synaptic transmission and plasticity in an AD mouse model and evaluated whether supplementation with d-serine, the key NMDAR co-agonist, could reverse the resulting deficits. METHODS: To assess the combined effects of genetic and dietary risk factors on synaptic function, we crossed APP/PS1 mice with APOE-ε4 KI mice and generated four mouse lines: wild-type, APP/PS1, APOE-ε4, and APP/PS1/APOE-ε4. Hippocampal synaptic transmission and plasticity, NMDAR function and d- and l-serine levels were evaluated using a combination of electrophysiological recordings, pharmacological interventions and capillary electrophoresis in brain slices, under either control or Western diet conditions. RESULTS: A significant impairment of both basal excitatory synaptic transmission and long-term potentiation (LTP) was detected in APP/PS1 mice by 9 months of age. These deficits were significantly more pronounced in APP/PS1/APOE-ε4 mice. Notably, Western diet accelerated these impairments, with significant deficits already present at 7 months in both APOE-ε4 and APP/PS1/APOE-ε4 mice. Mechanistically, these impairments were associated with reduced d-serine availability and NMDAR hypofunction at CA3-CA1 synapses. CONCLUSIONS: This study provides direct evidence of a specific and synergistic interaction between the APOE-ε4 genotype and Western diet in advancing and exacerbating hippocampal synaptic dysfunction in an AD mouse model. These findings highlight d-serine/NMDAR signaling as a key mechanistic pathway through which genetic and environmental risk factors converge in early AD, and underscore the potential of targeting astrocytic d-serine biosynthetic pathways as a promising therapeutic strategy for APOE-ε4 carriers at risk for late-onset AD. TRIAL REGISTRATION: Not applicable. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13195-026-01992-y.