APOE genotype affects the pre‐synaptic compartment of glutamatergic nerve terminals

Apolipoprotein E (APOE) genotype affects outcomes of Alzheimer's disease and other conditions of brain damage. Using APOE knock‐in mice, we have previously shown that APOE‐ε4 Targeted Replacement (TR) mice have fewer dendritic spines and reduced branching in cortical neurons. As dendritic spines are post‐synaptic sites of excitatory neurotransmission, we used APOE TR mice to examine whether APOE genotype affected the various elements of the glutamate–glutamine cycle. We found that levels of glutamine synthetase and glutamate uptake transporters were unchanged among the APOE genotypes. However, compared with APOE‐ε3 TR mice, APOE‐ε4 TR mice had decreased glutaminase levels (18%, p < 0.05), suggesting decreased conversion of glutamine to glutamate. APOE‐ε4 TR mice also had increased levels of the vesicular glutamate transporter 1 (20%, p < 0.05), suggesting that APOE genotype affects pre‐synaptic terminal composition. To address whether these changes affected normal neurotransmission, we examined the production and metabolism of glutamate and glutamine at 4–5 months and 1 year. Using high‐frequency ¹³C/¹H nuclear magnetic resonance spectroscopy, we found that APOE‐ε4 TR mice have decreased production of glutamate and increased levels of glutamine. These factors may contribute to the increased risk of neurodegeneration associated with APOE‐ε4, and also act as surrogate markers for Alzheimer's disease risk.